Molybdenum Oxotransferase Active Site ... - Brown University
Transcript of Molybdenum Oxotransferase Active Site ... - Brown University
Molybdenum Oxotransferase Active Site Models and Their
Oxygen Atom Transfer Reactivity
By Lee Taylor Elrod
B.S. University of Vermont 2010
A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy in the Department of Chemistry at Brown University
Providence, Rhode Island, May 2018
© Copyright 2018 Lee Taylor Elrod
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This dissertation by Lee Taylor Elrod is accepted in its present form by the Department
of Chemistry as satisfying the dissertation requirement for the degree of Doctor of
Philosophy
Recommended to the Graduate Council
Date____________ _____________________________
Eunsuk Kim, Ph.D. Advisor
Date____________ _____________________________
Jerome Robinson, Ph.D. Reader
Date____________ _____________________________
Paul Williard, Ph.D. Reader
Approved by the Graduate Council
Date____________ _____________________________
Andrew G. Campbell, Ph.D.
Dean of the Graduate School
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Curriculum Vitae
Lee Taylor Elrod
Education:
PhD Inorganic Chemistry with Dr. Eunsuk Kim Brown University, Providence, RI
2018
B.S. Degree in Chemistry University of Vermont, Burlington, VT
2010
High School Diploma Libertyville High School, Libertyville, IL
2006
Academic Accomplishments:
• Dissertation Fellowship, Brown University • Recipient Charles E. Braun Award, University of Vermont • Recipient Clinton D. Cook Award in Chemistry, University of Vermont • Recipient Presidential Scholarship (2006-2010), University of Vermont • Recipient Academic Excellence Scholarship (2006-2010), University of Vermont
Presentations:
Elrod, L. T.; Kim, E. “Oxygen Atom Transfer Mediated By Molybdenum Oxo Complexes and Lewis Acid”. Poster presentation at 252nd ACS National Meeting & Exposition, Philadelphia, PA, August 2016
Publications:
• Elrod, L. T.; Robinson, J. R.; Victor, E.; Kim, E. “Lewis Acid Enhanced Nitrate and Perchlorate reduction by Mo(µ-O) Dimer” Manuscript in preparation.
• Elrod, L. T.; Kim, E. “Lewis Acid Assisted Nitrate Reduction with Biomimetic Molybdenum Oxotransferase Complex” Inorg. Chem. 2018, 57, 2594 -2602.
• Cao, R.; Elrod, L. T.; Lehane, R. L.; Kim, E.; Karlin, K.D. “A Peroxynitrite Dicopper Complex: Formation via Cu–NO and Cu–O2 Intermediates and Reactivity via O–O Cleavage Chemistry” J. Am. Chem. Soc., 2016, 138, 16148-16158.
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• Roering, A. J.; Elrod, L. T.; Pagano, J. K.; Guillot, S. L.; Chan, S. M.; Tanski, J. M.; Waterman, R. “A General Synthesis of Phosphaalkenes at Zirconium with Liberation of Phosphaformamides” Dalton Trans. 2013, 42, 1159-1167.
• Elrod, L. T.; Boxwala, H.; Haq, H.; Zhao, A. W.; Waterman, R. “As-As Bond Formation Via Reductive Elimination from a Zirconocene Bis(dimesitylarsenide) Compound” Organometallics, 2012, 31, 5204-5207.
• Roering, A. J.; Maddox, A. F.; Elrod, L. T.; Chan, S. M.; Ghebreab, M. B.; Donovan, K. L.; Davidson, J. J.; Hughes, R. P.; Shalumova, T.; MacMillan, S. N.; Tanski, J. M.; Waterman, R. "General Preparation of (N3N)ZrX (N3N = N(CH2CH2NSiMe3)3
3–) Complexes from a Hydride Surrogate" Organometallics, 2009, 28, 573-581.
Teaching Experience:
2012 to Present Brown University, Department of Chemistry Providence, RI
Fall 2015:
• Head teaching assistant advanced undergraduate inorganic lab, 1 section
Spring 2014:
• Head teaching assistant undergraduate bioinorganic lab, 2 sections
Fall 2013:
• Head teaching assistant advanced undergraduate inorganic lab, 1 section per
Spring 2013:
• Teaching assistant undergraduate bioinorganic lab, 2 sections
Fall 2012:
• Teaching assistant undergraduate general chemistry laboratory, 1 section
2011-2012 University of Vermont Department of Chemistry Burlington, VT
• Teaching assistant undergraduate general chemistry laboratory, 5 sections per semester
• Teaching assistant undergraduate inorganic chemistry, one section Spring 2012
• Oversaw and assisted undergraduate and high school researchers in lab affiliated with NOYCE scholarship/ACS Project SEED
2010 Indiana University Bloomington, IN
• Graduate teaching assistant for organic chemistry laboratory
2009 University of Vermont Department of Chemistry Burlington, VT
• Teaching assistant for general chemistry laboratory, 2 sections
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Acknowledgments
I would like to thank the following people for their help and contributions to this
work. First, I would like to thank Dr. Eunsuk Kim. Over the past five and half years
she has help me continue to develop as a scientist. Her continued support and insight
into my project is why I can present my work here. Her enthusiasm for bioinorganic
chemistry and research has had a profound impact on how I approach research. Her
encouragement has helped me overcome the day to day obstacles of research, and I am
thankful for her support, guidance, and patience.
I would also like to thank Dr. Jerome Robinson for his continued help with
experimental techniques, discussions about science, and serving on my thesis
committee. The X-ray crystallography and structure determination of the Mo2VO3
complex presented here was solved by Dr. Robinson and was critical to the preparation
of this thesis. Thank you to Dr. Paul Williard and Dr. Wesley Bernskoetter for serving
as my committee members. Their insights and comments have fostered helpful
discussions about my research. I would also like to thank Dr. Eric Victor for
computational work performed on the Mo2VO3 complex.
Thank you to all the current and former Kim Group members.
Last, and certainly not least. I would like to thank my parents Lee and Jackie
Elrod. Without their love and support this would not have been possible. Through the
wonderful and terrible times, they have always been there for me, and that makes me
truly blessed.
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Table of Contents
Signature Page iii
Curriculum Vitae iv
Acknowledgments vi
Table of Contents vii
List of Figures ix
List of Schemes xv
List of Equations xvi
Chapter 1. Introduction 1
1.1. Introduction 2
1.2. Nitrate and Perchlorate 3
1.3. Nitrate and Perchlorate Reducing Enzymes 5
1.4. Biomimetic Nitrate Reducing Molybdenum Complexes 8
1.5. Perchlorate Reducing Complexes 11
1.6. Lewis Acid Additives and Oxygen Atom Transfer 13
1.7. References 16
Chapter 2: Lewis Acid Assisted Nitrate Reduction with Biomimetic Molybdenum Oxotransferase Complex
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2.1. Abstract 28
2.2. Introduction 29
2.3. Experimental Section 34
2.4. Results and Discussion 40
2.5. Conclusions 58
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2.6. References 59
Chapter 3: Structure and Oxygen Atom Transfer Reactivity of Dinuclear (µ-O)Molybdenum(V) Complex
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3.1. Abstract 71
3.2. Introduction 72
3.3. Experimental Section 76
3.4. Results and Discussion 82
3.5. Conclusions 105
3.6. References 106
Chapter 4: Lewis Acid Assisted Perchlorate Reduction with Dinuclear Molybdenum(V)(µ-Oxo) Complex
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4.1. Abstract 113
4.2. Introduction 114
4.3. Experimental Section 117
4.4. Results and Discussion 120
4.5. Conclusions 125
4.6. References 125
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List of Figures
Figure 1.1. Reduction of DMSO by DMSOR from Rhodobacter sphaeroides.
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Figure 1.2. The global nitrogen cycle.
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Figure 1.3. Reduced active sites of a) DMSOR from Rhodobacter sphaeroides b) Nar from Escherichia coli and PcrAB from Azopira suillum c) Nap from D. desulfuricans , where Asp = aspartate, Cys = cysteine, and Ser = serine.
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Figure 1.4. OAT with Holm dithiolene DMSOR structural and functional model complexes.
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Figure 1.5. Model complexes a) Mo2O3(5-SO3ssp)2(sol)2 b) Mo2O3(L-NS2)2(sol)2, where sol = DMF.
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Figure 1.6. Proposed associative mechanism for nitrate reduction by [WIV(SC6H2-2,4,6-Pri
3)(S2C2Me2)2](Et4N).
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Figure 1.7. Catalytic nitrate reduction with [Et4N][Mo(SPh)(PPh3)(mnt)2] and triphenylphosphine.
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Figure 1.8. Catalytic perchlorate reduction with ReV(O)(hoz)2Cl or [ReV(O)(hoz)2(OH2)]OTf and organic sulfide.
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Figure 1.9. Nitrate reduction by N(afaCy)3FeII(OTf)](OTf).
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Figure 1.10. Perchlorate reduction by N(afaCy)3FeII(OTf)](OTf).
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Figure 1.11. OAT from (TBP8Cz)Mnv(O) to aryl phosphine
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Figure 1.12. Generation of valence tautomer from (TBP8Cz)Mnv(O) and OAT with [(TBP8Cz•+)MnIV(O)-Zn2+].
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Figure 1.13. Proposed binding of Sc3+ to [MnV(O)(TAML)][PPh4].
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Figure 2.1. Active site structures of the oxidized forms of periplasmic nitrate reductase (Nap) from D. desulfuricans and formate dehydrogenases (Fdhs) from E. coli (X = SeCys) or R.capsulatus (X = Cys).
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Figure 2.2. MoVI(O)2(SN)2 (1) and MoIV(O)(SN)2 (SN=bis(4-t-butylphenyl)-2-pyridylmethanethiolate)
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Figure 2.3. KBr IR of Mo(O)2(SN)2 (1) (yellow trace) and Mo(O)(SN)2 (2)
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Figure 2.4a. Room temperature 1H NMR Mo(O)2(SN)2 (1) in CD2Cl2
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Figure 2.4b. Room temperature 1H NMR Mo(O)2(SN)2 (1) in CD2Cl2. 43
Figure 2.5a. Room temperature 1H NMR Mo(O)(SN)2 (2) in CD2Cl2.
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Figure 2.5b. Room temperature 1H NMR Mo(O)(SN)2 (2) in CD2Cl2.
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Figure 2.6. Room temperature UV-vis of Mo(O)2(SN)2 (1) and Mo(O)(SN)2 (2) in DCM.
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Figure 2.7. Room temperature UV-vis of Mo(O)(SN)2 (2) and Bu3N(NO3) (10 equiv.) after 24 hours in DCM.
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Figure 2.8. Room temperature UV-vis of Mo(O)2(SN)2 (1) generated from Mo(O)(SN)2 (2) (1 equiv.), Sc(OTf)3 (2 equiv.) and Bu4N(NO3) (10 equiv.) in DCM.
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Figure 2.9. KBr of Mo(O)2(SN)2 (1) generated from Mo(O)(SN)2 (2) (1 equiv.), Sc(OTf)3 (2 equiv.) and Bu4N(NO3) (10 equiv.) in DCM.
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Figure 2.10. 1H NMR of Mo(O)2(SN)2 (1) generated from Mo(O)(SN)2 (2) (1 equiv.), Sc(OTf)3 (2 equiv.) and Bu4N(NO3) (10 equiv.) and authentic Mo(O)2(SN)2 (1).
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Figure 2.11. Reaction of Mo(O)(SN)2 (2) (0.8 mM), [Bu4N][NO3] (8.0 mM), and Sc(OTf)3 (1.7 mM) followed by UV-Vis spectroscopy at –40 ºC in dichloromethane for 1.5 hours. The spectral changes correspond to the conversion of Mo(O)(SN)2 (2) (λmax = 328, 430 nm) to Mo(O)2(SN)2 (1) (λmax = 370 nm).
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Figure 2.12. IR spectra (KBr) of Mo(16O)2(SN)2 (1) and Mo(18O)2(SN)2 (118/18), along with Mo(16/18O)2(SN)2 (118/16) generated from 218 with [Bu4N][NO3] in the presence of Sc(OTf)3 (red solid), and Mo(16/18O)2(SN)2 (118/16) generated from 218 with trimethylamine n-oxide.
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Figure 2.13. Room temperature UV-vis in H2O for positive Griess reagent test for nitrite generated from Mo(O)(SN)2 (2) (1 equiv.), Sc(OTf)3 (2 equiv.) and Bu4N(NO3) (10 equiv.) and prepared nitrite calibration curve (inset).
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Figure 2.14. Room temperature 15N NMR (in CD2Cl2) spectra of a) the reaction mixture of Mo(O)(SN)2 (2) (1equiv.), Sc(OTf)3 (2 equiv.) and Bu4N(15NO3) (10 equiv.) showing excess nitrate signal at 6.46 ppm, b)
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authentic [Bu4N][15NO2], and c) an equimolar mixture of [Bu4N][15NO2] and Sc(OTf)3. Figure 2.15. IR spectra (gas cell) of headspace from the reaction mixtures of Mo(O)(SN)2 (2) (1 equiv.) and Bu4N(NO3) (10 equiv.) before and after addition of Sc(OTf)3 (2 equiv.), along with the spectrum from the reaction of [Bu4N][NO2] (1 equiv.) and Sc(OTf)3 (1 equiv.) and that of authentic N2O.
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Figure 2.16. UV-vis of pyrogallol in 50% KOH (aq) before and after head space transfer from reaction of Sc(OTf)3 (3.5 x 10-5
mol) and Bu4N(NO2) (3.5 x 10-5 mol).
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Figure 2.17. Room temperature UV-vis in DCM of Mo(O)(SN)2 (2) before and after addition of Sc(OTf)3 .
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Figure 2.18. 1H NMR of Mo(O)(SN)2 (2) with Sc(OTf)3 and authentic Mo(O)(SN)2 (2) in CD2Cl2.
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Figure 2.19. 1H NMR of Mo(O)2(SN)2 (1) with Sc(OTf)3 (top) and authentic Mo(O)2(SN)2 (1) (bottom) in CD2Cl2.
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Figure 3.1. Active site structures of the oxidized forms of (a) respiratory nitrate reductase (Nar) from Escherichia coli and (b) periplasmic nitrate reductase (Nap) from D. desulfuricans where Asp = aspartate and Cys = cysteine.
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Figure 3.2. cis-dioxomolybdenum(VI) thiosemicarbazone (X = Me, H, I, Br, OCF3, NO2).
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Figure 3.3. KBr IR of Mo(O)2(LBr)(MeOH) (1).
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Figure 3.4. 1H NMR Mo(O)2(LBr)(MeOH) (1) in DMSO-d6 (400 MHz).
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Figure 3.5. Room temperature UV-vis of Mo(O)2(LBr)(MeOH) (1) in THF.
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Figure 3.6. Room temperature 1H NMR (in acetone-d6, 400MHz) spectra of a) of Mo(O)2(LBr)(MeOH) (1) and b) Mo2O3(LBr)2(THF)2·2THF (2).
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Figure 3.7. Thermal ellipsoid plot of Mo2O3(LBr)2(THF)2·2THF (2) projected at the 50% probability level.
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Figure 3.8. Room temperature 1H NMR (in THF-d8, 400MHz) spectra of a) of Mo(O)2(LBr)(MeOH) (1) and b) Mo2O3(LBr)2(THF)2·2THF (2).
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Figure 3.9. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) in THF.
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Figure 3.10. KBr IR of Mo2O3(LBr)2(THF)2·2THF (2).
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Figure 3.11. Overlay of 1H NMR Mo(O)2(LBr)(MeOH) (1) in DMSO-d6 (bottom) and 1H NMR Mo2O3(LBr)2(THF)2·2THF (2) in DMSO-d6 (top) (400 MHz) (5-0 ppm).
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Figure 3,12. Overlay of 1H NMR Mo(O)2(LBr)(MeOH) (1) in DMSO-d6 (bottom) and 1H NMR Mo2O3(LBr)2(THF)2·2THF (2) in DMSO-d6 (top) (400 MHz) (10-5 ppm).
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Figure 3.13 Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) before (blue trace) and after the addition of 100 equivalents DMSO (orange trace) in THF.
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Figure 3.14. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) with [Bu4N][NO3] (1 equivalent) in THF.
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Figure 3.15. Overlay of a) Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] (0.5 equiv.) b) Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] (1 equiv.) c) Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO2] (1 equiv.) d) Mo(O)2(LBr)(MeOH) (1) in acetone-d6 (400 MHz).
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Figure 3.16. KBr IR of reaction mixture of Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] (1 equivalent).
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Figure 3.17. Mo2O3(LBr)2(THF)2·2THF (2) (0.15 mM) and [Bu4N][NO3] (0.15 mM) followed by UV-Vis spectroscopy at room temperature in THF for 2 hours. The spectral changes correspond to the conversion of Mo2O3(LBr)2(THF)2·2THF (2) to Mo(O)2(LBr)(THF) (3).
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Figure 3.18. Mo2O3(LBr)2(THF)2·2THF (2) (1.6 mM) and [Bu4N][NO3] (1.6 mM) followed by UV-Vis spectroscopy at room temperature in THF for 2 hours. The spectral changes correspond to the conversion of Mo2O3(LBr)2(THF)2·2THF (2) to Mo(O)2(LBr)(THF) (3).
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Figure 3.19. Room temperature UV-vis in H2O for positive Griess reagent test for nitrite generated from Mo2O3(LBr)2(THF)2·2THF (2) (1 equiv.) [Bu4N][NO3] (1 equiv.) Prepared nitrite calibration curve (inset).
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Figure 3.20. Room temperature 15N NMR (in acetone-d6) spectra of a) [Bu4N][15NO3] showing nitrate signal at 6.46 ppm, b) reaction of Mo2O3(LBr)2(THF)2·2THF (2) and [Bu4N][15NO3].
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Figure 3.21. Room temperature 15N NMR (in acetone-d6) spectra of a) Bu4N[15NO2] showing nitrate signal at 243.67 ppm, b) reaction of Mo(O)2(LBr)(MeOH) (1) and [Bu4N][15NO2 ] (2 equivalents).
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Figure 3.22. Room temperature UV-vis of Mo(O)2(LBr)(MeOH) (1) (1 equivalent) with [Bu4N][NO2] (2 equivalent) in THF.
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Figure 3.23. Overlay of a) Mo(O)2(LBr)(MeOH) (1) b) Mo(O)2(LBr)(MeOH) (1) (1 equivalent) with [Bu4N][NO2] (2 equivalents) in acetone-d6 (400 MHz).
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Figure 3.24. IR spectra (gas cell) of headspace from the Mo(O)2(LBr)(MeOH)
(1) (1 equivalent) with [Bu4N][NO2] (2 equivalents) (orange solid) and that of authentic N2O (blue dotted).
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Figure 3.25. UV-vis of pyrogallol in 50% KOH (aq) before (blue trace) and after head space transfer (orange trace) from the reaction of Mo(O)2(LBr)(MeOH) (1) (1 equivalent) with [Bu4N][NO2] (2 equivalents).
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Figure 3.26. Mo2O3(LBr)2(THF)2·2THF (2) (0.15 mM), Sc(OTf)3 (0.15 mM), and [Bu4N][NO3] (0.15 mM) followed by UV-Vis spectroscopy at room temperature in THF for 2 hours.
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Figure 3.27. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) with [Bu4N][NO2] (1 equivalent) in THF.
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Figure 3.28. Overlay of a) Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO2] (1 equiv.) and b) Mo(O)2(LBr)(MeOH) (1) (1 equivalent) with [Bu4N][NO2] (2 equivalents) in acetone-d6 (400 MHz).
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Figure 3.29. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) with [Bu4N][NO3] (0.5 equivalent) in THF.
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Figure 3.30. Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] (0.5 equivalents) in acetone-d6 (400 MHz).
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Figure 3.31. IR spectra (gas cell) of headspace from the Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) with [Bu4N][NO3] (0.5 equivalents) (orange solid) and that of authentic N2O (blue dotted).
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Figure 4.1. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) and [Bu4N][ClO4] (10 equivalents) in THF.
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Figure 4.2. 1H NMR overlay of a) Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent), Sc(OTf)3 (1 equivalent) and [Bu4N][ClO4] (0.25
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equivalents) b) Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) and [Bu4N][ClO4] (10 equivalents) c) Mo(O)2(LBr)(MeOH) (1) in acetone-d6 Figure 4.3. 35Cl NMR overlay of a) Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) and [Bu4N][ClO4] (10 equivalents) b) Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent), Sc(OTf)3 (1 equivalent) and [Bu4N][ClO4] (0.25 equivalents) in acetone-d6.
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Figure 4.4. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent), Sc(OTf)3 (1 equivalent), and [Bu4N][ClO4] (1 equivalent) in THF.
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Figure 4.5. Mo2O3(LBr)2(THF)2·2THF (2) (0.13 mM), Sc(OTf)3 (0.13 mM), and [Bu4N][NO3] (1.3mM) followed by UV-Vis spectroscopy at room temperature in THF for 5 minutes.
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Figure 4.5. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent), Sc(OTf)3 (1 equivalent), and [Bu4N][ClO4] (1/4 equivalent) in THF.
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List of Schemes
Scheme 2.1. Nitrate reduction by R. capsulatus Fdh
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Scheme 2.2 Reactivity of Mo(O)(SN)2 (2) and [Bu4N][NO3] in absence (top) and presence (bottom) of Sc(OTf)3 (SN = bis(4-t-butylphenyl)-2-pyridylmethanethiolate).
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Scheme 2.3. Mo(18/16O)2(SN)2 (118/16) preparation from Mo(18O)(SN)2 (218) via Sc(OTf)3 assisted nitrate reduction (I) or trimethylamine n-oxide (II).
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Scheme 2.4. Lewis acid assisted nitrate reduction by Mo(O)(SN)2 (2) to form Mo(O)2(SN)2 (1) by oxygen atom transfer.
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Scheme 3.1. OAT with dimer disproportionation.
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Scheme 3.2 OAT without dimer disproportionation.
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Scheme 3.3. Proposed Mo2O3(LBr)2(THF)2·2THF (2) reactivity with 1.0 and 0.5 equivalents NO3−.
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Scheme 4.1. Perchlorate reduction by A. suilum PcrAB.
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Scheme 4.2. Reduction of perchlorate (ClO4−) to chloride (Cl−) and molecular oxygen by perchlorate reductase (PcrAB) and chlorite dismutase (Cld).
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Scheme 4.3. Catalytic reduction of perchlorate (ClO4−) to chloride (Cl−) by ReV-oxo complexes.
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List of Equations
Equation 1.1
6
Equation 1.2
6
Equation 1.3
6
Equation 1.4
6
Equation 1.5
6
Equation 1.6
6
Equation 1.7
11
Equation 2.1
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Equation 3.1
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Equation 3.2
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Equation 3.3
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Equation 3.4
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1
Chapter 1: Introduction
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1.1. Introduction
Remediation of ground water contamination associated with industrialization will
continue to be a challenge in the 21st century. As the world’s population grows, it’s use of
nitrate (NO3−) and perchlorate (ClO4−) in agricultural and industrial processes will
continue to grow with demand. With the increase in use there will be an inevitable
increase in ground water contamination associated with run off from agricultural and
industrial use. While the negative health effects and environmental impact of nitrate
contamination is well known, the long-term effects of perchlorate contamination are still
largely unknown. Both oxyanions are pervasive ground water contaminants owning to
their high solubility and mobility in water. Owing to the difficulties associated with
remediation of perchlorate and nitrate, the likely negative impacts to the environment and
health, and the likely increase in ground water contamination with population growth, the
development of new systems that can remediate perchlorate and nitrate contamination is
an important ongoing process.
Presented here are efforts to further develop bioinspired systems that can reduce
nitrate and perchlorate by oxygen atom transfer (OAT) using high valent Mo(IV/VI)
metal complexes. I will briefly discuss the environmental/health impacts and difficulties
in remediation associated with perchlorate and nitrate contamination. The enzymes that
utilize perchlorate and nitrate for metabolic processes will be presented and their active
site structures discussed. Complexes that have been successfully utilized for the reduction
of nitrate and perchlorate through OAT are presented. The effect of Lewis acid additives
on OAT will be briefly discussed.
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1.2. Nitrate and Perchlorate
The environmental impact of nitrate is well known. The use of nitrate containing
fertilizers makes agricultural run off a common source for nitrate. Introduction of nitrate
into water sources leads to eutrophication that can result in harmful algal blooms and fish
kills in aquatic environments.1-2 While modern water chemistry quickly identified nitrate
as the one of the sources of eutrophication and lead to the development of remediation
technologies, with the increase of the global population the problem of eutrophication
will remain an issue. Disastrous environmental consequences could arise from the
combined increased waste production associated with population growth, the growing
dependence on nitrate containing fertilizers, and higher water temperatures associated
with global warming.3 The high solubility of nitrates in water complicates its removal
from ground and surface water.4
Nitrate and related nitrite (NO2−) found in the human body are from both external
and internal sources. The largest sources of nitrate are from external exposure through
consumption of contaminated water and food. Vegetables grown in nitrate contaminated
soils and water have increased nitrate levels. Nitrate and nitrite are also commonly used
as food additives commonly found in cured meats and cheeses.5 The negative health
effects of nitrate consumption are attributed related to its conversion to nitrite by bacterial
nitrite reductases found in the gastrointestinal tract and mouths.6 Consumption of nitrates
is associated with the potentially fatal blood disorder methemoglobinemia, also known as
the “blue baby syndrome”, that arises from nitrite oxidation of hemoglobin from Fe(II) to
Fe(III) producing methemoglobin. The reduced ability of the blood to carry oxygen
4
results in hypoxia.5-6 Intake of high levels of nitrate is also associated with spontaneous
abortion, premature birth, birth defects, and hypertension.4-5 While the direct link
between nitrate/nitrite and cancer has not been established, the formation of reactive N-
nitroso compounds associated with cancer have been linked to consumption of nitrate.7
Like nitrate, perchlorate (ClO4−) is a pervasive ground and surface water
contaminant owning to its high solubility in water. However, the long term environmental
effects of perchlorate are not yet known. Prior to the late 1990s, perchlorate could not be
detected in ground water below 100 µg/L. Only with better methods of detection was the
extent of perchlorate contamination in the United States realized.8-9 Like nitrate,
perchlorate contamination of ground water is linked to its use in fertilizers. The largest
source of perchlorate contamination is from the use of perchlorate salts in a wide variety
of industrial processes, including manufacture of industrial lubes, dyes, rubbers,
fertilizers, paints, in electroplating processes, and battery manufacturing. Ammonium
perchlorate is widely used in solid rocket fuels and in the production of pyrotechnics and
munitions owing to its high oxygen content.8, 10 The long term effects of perchlorate,
chlorate (ClO3−) and related chlorite (ClO2−) consumption are largely unknown. Chlorate
and chlorite can cause hemolytic anemia. Thyroid function is impacted by perchlorate
consumption, with potassium perchlorate is used to treat hyperthyroidism.8 Perchlorate
competitively inhibits the uptake of iodide by the thyroid gland, altering the production of
hormones in the body, and is a potential source of hypothyroidism.9, 11
Perchlorates physical properties make remediation difficult. As a solid perchlorate
salts are reactive and potentially explosive, and a commonly used solid oxidant for jet
propulsion systems and explosives. While perchlorate is reasonably strong oxidant
5
thermodynamically (equation 1), in solution rates of reaction are slow due to high kinetic
barriers.10 These kinetic barriers prevent reaction with reducing agents commonly used in
remediation technologies, such as sulfite and zero valent iron. Perchlorate is generally a
non-complexing anion, a poor nucleophile, and kinetically inert to reduction. These
properties make perchlorate ideal for use in the laboratory as an electrolyte in
electrochemical studies or as a non-coordinating anion in crystallizations, but makes
remediation from ground water difficult.10, 12
1.3. Nitrate and Perchlorate Reducing Enzymes
Molybdenum enzymes containing pyranopterin cofactors are utilized for a wide of
biological functions. Many eukaryotic and prokaryotic organisms utilize mononuclear
oxotransferase enzymes, capable of two-electron oxidation-reduction reactions, for
metabolic functions.13-14 The dimethylsulfoxide reductase (DMSOR) family of enzymes
utilize high valent Mo(VI-VI) metal centers bound to two pyranopterin cofactors to
catalyze the two-electron oxygen atom transfer to and from substrates (Figure 1.1).15-16
The DMSOR family of enzymes is one of the largest and most diverse family of
molybdenum containing enzymes. In addition to reduction of DMSO to dimethyl sulfide
(equation 4) performed by DMSOR, the DMSOR family contains enzymes that reduce
perchlorate and chlorate (equations 1 and 2, potentials w/respect to standard hydrogen
electrode at pH ~7), nitrate (equation 3) and trimethylamine oxide (equation 5). Transfer
of the oxygen atom to from substrate to the molybdenum center results in the formation
of a two-electron oxidized metal center. The reduced molybdenum center is reformed by
use of a redox partner, such as NADH (equation 6) and the oxygen atom transferred is
then released as water after the addition of protons.
6
Figure 1.1. Reduction of DMSO by DMSOR from Rhodobacter sphaeroides.15-16
Figure 1.2. The global nitrogen cycle. ANAMMOX = anaerobic ammonium oxidation.
Nitrate reductase enzymes utilize nitrogen in its highest oxidation state, and play a
critical role in denitrification, assimilatory and dissimilatory nitrate reduction steps of the
global nitrogen cycle (Figure 1.2). Nitrate reductase enzymes are further classified by
7
their role. Assimilatory nitrate reductase (Nas) incorporates nitrogen from nitrate into
biomass and is utilized for growth, while dissimilatory nitrate reductase (Nap) does not
incorporate nitrogen from nitrate into biomass. Nitrate reductase (Nar) generates
metabolic energy by using nitrate as the terminal electron acceptor in nitrate respiration
and is utilized denitrification.17 Respiratory nitrate reductase (Nar) from Escherichia
coli18 and (b) periplasmic nitrate reductase (Nap) from D. desulfuricans19 are two well
studied nitrate reductase enzymes (Figure 1.3). 19-22
Figure 1.3. Oxidized active sites of a) DMSOR from Rhodobacter sphaeroides16 b) Nar from Escherichia coli18 and PcrAB from Azopira suillum11 c) Nap from D. desulfuricans19 , where Asp = aspartate, Cys = cysteine, and Ser = serine.
Perchlorate reductase (PcrAB) has an identical active site as Nar, yet has different
amino acid residues in the secondary coordination environment, and likely evolved from
a common ancestor11 Perchlorate reductase utilize perchlorate and chlorate (equations 1
and 2) as terminal electron acceptors during anaerobic respiration.23 Perchlorate reducing
organisms often contain chlorite dismutase, a heme enzyme that further converts the
chlorite (ClO2−) generated from perchlorate reduction to molecular oxygen and
chloride.23-24
8
1.4. Biomimetic Nitrate Reducing Molybdenum Complexes
Interest in the molybdenum mediated oxygen atom transfer exhibited by the
mononuclear oxotransferase enzymes, including nitrate reductase, led to the development
of numerous biomimetic molybdenum systems capable of oxygen atom transfer.25-27
Early biomimetic systems consisted of thiocarbamate (S2CNR2)28-32 and Schiff base
ligand systems, that were complicated by MoV2O3 (µ-O) dimers.33-35 While not desirable
for mononuclear enzyme models, these systems were capable of oxygen atom transfer.
Later generation biomimetic systems utilized bulky ligand systems36-37 or charged
complexes38 to prevent dimerization. Numerous structural and functional models of
DMSOR enzymes, including nitrate reductase, trimethylamine oxide reductase, and
selenate reductase, utilizing dithiolene ligand systems with molybdenum and tungsten
were reported by Holm in the late 1990s and early 2000s (Figure 1.4).39-43 In addition to
developing numerous biomimetic Mo and W complexes, Holm and coworkers
extensively studied the kinetics and thermodynamics of the OAT reactions.44-46
Development of Mo systems that are capable of OAT reactivity continues to be an active
area of research, with systems capable of light induced OAT47 and polymer supported
systems48 demonstrating varied new directions the field may take.
Figure 1.4. OAT with Holm dithiolene DMSOR structural and functional model
complexes.
9
While many molybdenum systems demonstrate stochiometric and even catalytic
oxidation of phosphines and sulfides utilizing dimethylsulfoxide, few have demonstrated
nitrate reduction capability. Binuclear Mo2O3(5-SO3ssp)2(sol)2 (ssp = 2-
(saliclideneamino)benzenethiolato(2-), sol = DMF) from Holm35 and Mo2O3(L-
NS2)2(sol)2 ( L-NS2 = 2,6-bis(2,2-diphenyl-2-thioethyl)pyridinate(2-), sol = DMF)
(Figure 1.5), originally reported as a MoIV mono-oxo complex by Holm35, 49 and later
shown to be a dimer by Young50 both demonstrate nitrate reduction. Nitrate reduction by
W(IV) and Mo(IV) bis(dithiolene) complexes, containing sterically encumbered axial
sulfido ligands were reported by Holm and coworkers, with the tungsten complex product
being isolated, due to instability of the molybdenum complex.42 The reaction was found
to obey a second-rate law and proposed to take place by an associative reaction
mechanism, suggesting nitrate reduction occurs through direct oxo transfer to the metal
center (Figure 1.6) .
Figure 1.5. Model complexes a) Mo2O3(5-SO3ssp)2(sol)2 b) Mo2O3(L-NS2)2(sol)2, where sol = DMF.
10
Figure 1.6. Proposed associative mechanism for nitrate reduction by [WIV(SC6H2-2,4,6-Pri
3)(S2C2Me2)2](Et4N).
Catalytic nitrate reduction utilizing [Et4N][Mo(SPh)(PPh3)(mnt)2] (mnt = 1,2-
dicyanoethylenedithiolate(2-)) (Figure 1.7) was reported by Sarkar and coworkers.51 The
active catalyst was proposed to be formed through loss of triphenylphosphine, resulting
in the pentacoordinate MoIV thiolate complex. After nitrate reduction occurs
triphenylphosphine reduces the MoVIO species generating triphenylphosphine oxide and
reforming the MoIV species that can further undergo reactivity with nitrate or reform the
catalytically inactive [Et4N][Mo(SPh)(PPh3)(mnt)2].
Figure 1.7. Catalytic nitrate reduction with [Et4N][Mo(SPh)(PPh3)(mnt)2] and triphenylphosphine.
11
1.5. Perchlorate Reducing Complexes
Complexes that reduce perchlorate to chloride under mild conditions are rare.
Reduction of perchlorate to chloride through oxygen atom transfer was reported by
Espenson and coworkers. Methylrhenium dioxide, (MDO = Ch3ReO2) generated in situ
from methylrhenium trioxide (MTO = Ch3ReO3) and the powerful reducing agent
hypophosphorus acid (H3PO2) at pH = 0 with 1.0 M trifluoromethanesulfonic acid
(HOTf) reacts with perchlorate to regenerate MTO (equation 7).52-53
Catalytic reduction of perchlorate to chloride under mild conditions utilizing
organic sulfides with the air and water stable ReV(O)(hoz)2Cl and
[ReV(O)(hoz)2(OH2)]OTf (hoz = [2-(2’-hydroxyphenyl)-2-oxazoline], OTf=
trifluoromethanesulfonate) (Figure 1.8) was reported by Abu-Omar in 2000.54
ReV(O)(hoz)2Cl and [ReV(O)(hoz)2(OH2)]OTf show little decomposition after hundreds
of turnovers, react cleanly to produce sulfoxide and chloride, are readily prepared from
inexpensive and commercially available starting materials. and have been utilized in the
preparation of hybrid heterogenous catalyst systems.55 Re(V) complexes containing
tetradentate iminophenolate ligands have also been utilized in the successful reduction of
perchlorate to chloride with organic sulfides.56
12
Figure 1.8. Catalytic perchlorate reduction with ReV(O)(hoz)2Cl or [ReV(O)(hoz)2(OH2)]OTf (above) with organic sulfide.
A Fe(II) azafulvene-amine ([N(afaCy)3Fe(OTf)](OTf)) reported by Fout and
coworkers is an example of a synthetic system that can reduce both nitrate and
perchlorate.57 Utilizing ligands that incorporate non-covalent interactions, inspired by
Borovik’s non-heme Fe(III)-oxo complexes58, [N(afaCy)3FeII(OTf)](OTf) was shown to
react with nitrite to form [N(afaCy)3FeIII(O)](OTf) and [N(afaCy)3FeII(NO)](OTf)2.59
Nitrate reduction was achieved with 3 equivalents of [N(afaCy)3FeII(OTf)](OTf) and 1
equivalent of [Bu4N][NO3] in the presence of triethylamine, yielding
[N(afaCy)3FeIII(O)](OTf) and [N(afaCy)3FeII(NO)](OTf)2 in a 2:1 ratio, indicating that the
nitrite generated by reduction is further reduced to NO (Figure 1.9). Perchlorate reduction
to chloride was also achieved with 5 equivalents of [N(afaCy)3FeII(OTf)](OTf) and 1
equivalent of [Bu4N][ClO4]s to form [N(afaCy)3FeIII(O)](OTf) and
[N(afaCy)3FeII(Cl)](OTf) in a 4:1 ratio (Figure1.10). Catalytic reduction of both nitrate
and perchlorate was achieved through the use of the diphenylhydrazine, a 2H+/2e− source,
and decamethylferrocenium triflate as a sacrificial oxidant.57
13
Figure 1.9. Nitrate reduction by N(afaCy)3FeII(OTf)](OTf).
Figure 1.10. Perchlorate reduction by N(afaCy)3FeII(OTf)](OTf).
1.6. Lewis Acid Additives and Oxygen Atom Transfer
The ability to alter the redox reactivity of metal oxo complexes with redox-
inactive Lewis acid metal cations has been well established. Redox-inactive Lewis acid
additives have been shown to alter redox potentials60-63, electron transfer rates60, 64-66, and
reactivity62, 67-72 of metal oxo complexes. The ability to alter oxygen atom transfer
reactivity with Lewis acids has also been demonstrated.
The MnV-oxo corrolazine complex (TBP8Cz)Mnv(O) developed by Goldberg and
coworkers are capable of OAT (Figure 1.11) to aryl phosphines to generate the
(TBP8Cz)MnIII.62, 73 The electron transfer and hydrogen atom transfer activity of
Goldberg’s high valent MnV-oxo species were enhanced with the addition of the Zn2+.
The addition of the Lewis acid binds to the diamagnetic corrolazine complex, possibly
14
through the terminal oxo, with a high affinity and induces valence tautomerization
resulting in a paramagnetic MnIV species [(TBP8Cz•+)MnIV(O)-Zn2+] (Figure 1.12).
Addition of 1,10 phenanthroline regenerates the MnV-oxo species demonstrating that the
tautomerization is reversible.69 Both (TBP8Cz)Mnv(O) and the MnIV valence tautomer are
capable of OAT. However, addition of the Lewis acid results in dramatically slower rates
of oxygen atom transfer and is attributed to a less electrophilic terminal oxo of the MnIV
valence tautomer, demonstrating the ability to tune OAT reactivity through the addition
of a Lewis acid metal cation.62
Figure 1.11. OAT from (TBP8Cz)Mnv(O) to aryl phosphine.
15
Figure 1.12. Generation of valence tautomer from (TBP8Cz)Mnv(O) and OAT with [(TBP8Cz•+)MnIV(O)-Zn2+].
A [MnV(O)(TAML)][PPh4] complex reported by Nam and coworkers
demonstrated enhanced OAT reactivity with the addition of Sc(OTf)3. The addition of
Sc3+ results in a 0.7 V increase in the one-electron reduction potential compared to the
Lewis acid free complex, and enhances the oxidizing power of the complex. Interestingly
the binding site of the Sc3+ was proposed to take place on the macrocyclic TAML ligand
and not the terminal Mn-oxo (Figure 1.13). This was attributed to the low basicity of the
oxo group compared to the amide carbonyl of the ligand. This work demonstrated the
ability to alter the oxidizing power a metal oxo system by altering the secondary
coordination sphere.
16
Figure 1.13. Proposed binding of Sc3+ to [MnV(O)(TAML)][PPh4].
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61. Park, Y. J.; Cook, S. A.; Sickerman, N. S.; Sano, Y.; Ziller, J. W.; Borovik, A. S.,
Heterobimetallic complexes with MIII-([small mu ]-OH)-MII cores (MIII = Fe, Mn, Ga;
MII = Ca, Sr, and Ba): structural, kinetic, and redox properties. Chemical Science 2013, 4
(2), 717-726.
62. Zaragoza, J. P. T.; Baglia, R. A.; Siegler, M. A.; Goldberg, D. P., Strong
Inhibition of O-Atom Transfer Reactivity for MnIV(O)(π-Radical-Cation)(Lewis Acid)
versus MnV(O) Porphyrinoid Complexes. J. Am. Chem. Soc. 2015, 137 (20), 6531-6540.
63. Hong, S.; Lee, Y.-M.; Sankaralingam, M.; Vardhaman, A. K.; Park, Y. J.; Cho,
K.-B.; Ogura, T.; Sarangi, R.; Fukuzumi, S.; Nam, W., A Manganese(V)–Oxo Complex:
Synthesis by Dioxygen Activation and Enhancement of Its Oxidizing Power by Binding
Scandium Ion. J. Am. Chem. Soc. 2016, 138 (27), 8523-8532.
25
64. Fukuzumi, S.; Ohkubo, K., Metal ion-coupled and decoupled electron transfer.
Coord. Chem. Rev. 2010, 254 (3), 372-385.
65. Fukuzumi, S.; Ohkubo, K.; Lee, Y.-M.; Nam, W., Lewis Acid Coupled Electron
Transfer of Metal–Oxygen Intermediates. Chemistry – A European Journal 2015, 21
(49), 17548-17559.
66. Lee, Y.-M.; Bang, S.; Kim, Y. M.; Cho, J.; Hong, S.; Nomura, T.; Ogura, T.;
Troeppner, O.; Ivanovic-Burmazovic, I.; Sarangi, R.; Fukuzumi, S.; Nam, W., A
mononuclear nonheme iron(iii)-peroxo complex binding redox-inactive metal ions.
Chemical Science 2013, 4 (10), 3917-3923.
67. Park, J.; Morimoto, Y.; Lee, Y.-M.; You, Y.; Nam, W.; Fukuzumi, S., Scandium
Ion-Enhanced Oxidative Dimerization and N-Demethylation of N,N-Dimethylanilines by
a Non-Heme Iron(IV)-Oxo Complex. Inorg. Chem. 2011, 50 (22), 11612-11622.
68. dPark, Y. J.; Ziller, J. W.; Borovik, A. S., The Effects of Redox-Inactive Metal
Ions on the Activation of Dioxygen: Isolation and Characterization of a Heterobimetallic
Complex Containing a MnIII–(μ-OH)–CaII Core. J. Am. Chem. Soc. 2011, 133 (24),
9258-9261.
69. Leeladee, P.; Baglia, R. A.; Prokop, K. A.; Latifi, R.; de Visser, S. P.; Goldberg,
D. P., Valence Tautomerism in a High-Valent Manganese–Oxo Porphyrinoid Complex
Induced by a Lewis Acid. J. Am. Chem. Soc. 2012, 134 (25), 10397-10400.
70. Hong, S.; Pfaff, F. F.; Kwon, E.; Wang, Y.; Seo, M.-S.; Bill, E.; Ray, K.; Nam,
W., Spectroscopic Capture and Reactivity of a Low-Spin Cobalt(IV)-Oxo Complex
Stabilized by Binding Redox-Inactive Metal Ions. Angew. Chem. Int. Ed. 2014, 53 (39),
10403-10407.
26
71. Park, J.; Morimoto, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S., Metal Ion Effect on
the Switch of Mechanism from Direct Oxygen Transfer to Metal Ion-Coupled Electron
Transfer in the Sulfoxidation of Thioanisoles by a Non-Heme Iron(IV)−Oxo Complex. J.
Am. Chem. Soc. 2011, 133 (14), 5236-5239.
72. Bang, S.; Lee, Y.-M.; Hong, S.; Cho, K.-B.; Nishida, Y.; Seo, M. S.; Sarangi, R.;
Fukuzumi, S.; Nam, W., Redox-inactive metal ions modulate the reactivity and oxygen
release of mononuclear non-haem iron(III)–peroxo complexes. Nat Chem 2014, 6 (10),
934-940.
73. Prokop, K. A.; Neu, H. M.; de Visser, S. P.; Goldberg, D. P., A Manganese(V)–
Oxo π-Cation Radical Complex: Influence of One-Electron Oxidation on Oxygen-Atom
Transfer. J. Am. Chem. Soc. 2011, 133 (40), 15874-15877.
27
Chapter 2: Lewis Acid Assisted Nitrate Reduction with
Biomimetic Molybdenum Oxotransferase Complex
28
2.1 Abstract
The reduction of nitrate (NO3−) to nitrite (NO2−) is of significant biological and
environmental importance. While MoIV(O) and MoVI(O)2 complexes that mimic the
active site structure of nitrate reducing enzymes are prevalent, few of these model
complexes can reduce nitrate to nitrite through oxygen atom transfer (OAT) chemistry.
We present a novel strategy to induce nitrate reduction chemistry of a previously known
catalyst MoIV(O)(SN)2 (2), where SN = bis(4-tert-butylphenyl)-2-pyridylmethanethiolate,
that is otherwise incapable of proceeding OAT with nitrate. Addition of nitrate together
with the Lewis acid Sc(OTf)3 (OTf = trifluoromethanesulfonate) to 2 results in an
immediate and clean conversion of 2 to MoVI(O)2(SN)2 (1). The Lewis acid additive
further reacts with the OAT product, nitrite, to form N2O and O2. This work highlights
the ability of Sc3+ additives to expand the reactivity scope of an existing MoIV(O)
complex together with which Sc3+ can convert nitrate to stable gaseous molecules.
29
2.2. Introduction. Nitrate reduction to nitrite is of significant biological and
environmental importance. Due to widespread use of nitrate in agricultural applications
and high solubility in water, it is a pervasive contaminant in ground water and can lead to
eutrophication.1 Nitrate consumed can be reduced to nitrite by nitrate reducing bacteria in
the mouth and gut, and further reduced to NO in the body, and is linked to negative
health effects including the formation of reactive N-nitroso compounds associated with
cancer and the blood disorder methemoglobinemia.2-3 Nitrate reductases found in
prokaryotic and eukaryotic microorganisms catalyze the reduction of nitrate to nitrite for
metabolic processes, and play an important role in denitrification and nitrate assimilation
steps within the global nitrogen cycle.2, 4
One of the well-studied nitrate reductases is periplasmic nitrate reductase (Nap)
from Desulfovibrio desulfuricans that belongs to the dimethylsulfoxide reductase
(DMSOR) family of enzymes. The active site of Nap for the reduction of nitrate to nitrite
contains a MoIV/VI metal center bound by two pyranopterin cofactors (Figure 2.1).5 The
two-electron reduction of nitrate (NO3–) to nitrite (NO2
–) is coupled with oxygen atom
transfer (OAT) from NO3– to MoIV to form the MoVI-oxo species which subsequently
releases water upon the addition of two external electrons and protons (equation 1)6-8.
Formate dehydrogenase (Fdh), another DMSOR family enzyme which catalyzes the
oxidation of formate to CO2 and has a very similar active site structure as Nap (Figure 1),
has also been shown to be capable of reducing nitrate to nitrite.9
NO3– + 2 H+ + 2 e– → NO2
– + H2O (1)
30
Figure 2.1. Active site structures of the oxidized forms of periplasmic nitrate reductase (Nap) from D. desulfuricans5 and formate dehydrogenases (Fdhs) from E. coli (X = SeCys)10 or R.capsulatus (X = Cys).11
Artificial systems that remediate nitrate contamination are desirable to combat
against ground water contamination associated with increased use of fertilizers and global
industrialization. Bioinspired catalysts could play an important role in the reduction of
nitrate to nitrite in the remediation processes. There are a number of biomimetic model
complexes that replicate the active site structure and reactivity of DMSOR, utilizing a
variety of ligand scaffolds including dithiolenes12-13, tris(pyrazolyl)borates14-15, salan16,
pyridylmethanethiolate17-18, and Schiff base derivatives19. However, few are known to
reduce nitrate to nitrite.20-24 Binuclear (µ-oxo)molybdenum(V) and complexes reported
by Holm and Young have demonstrated nitrate reduction capability.20-22, 25 Catalytic
nitrate reduction to nitrite coupled to triphenyl phosphine oxidation using a MoIV
dithiolene complex was reported by Sarkar.24 Nitrate reduction by a W(IV)
bis(dithiolene) complex containing a sterically encumbered axial sulfido ligand was
reported by Holm and coworkers.23
31
One of the reasons very few complexes out of many structural biomimetic models
can replicate the activity of nitrate reductases might be the absence of secondary
coordination environment found in the metalloenzyme. Recent studies by Moura and
Cerqueira emphasize that the small differences in the amino acid residues in the
secondary coordination spheres dictate the type of catalytic reactions between Fdh and
Nap who share remarkably similar structures.26-27 The latest study by Leimkühler showed
that the highly conserved, positively charged arginine residue in the secondary
coordination environment of Fdh and Nap is important in nitrate reduction by facilitating
proper binding and stabilization of the substrate during the catalytic cycle (Scheme 2.1).9
Scheme 2.1. Nitrate reduction by R. capsulatus Fdh (adapted from Ref. 9)
The importance of the secondary coordination environment in enzyme catalysis is
not limited to Nap and Fdh. Noncovalent interactions in the secondary coordination
sphere of metalloenzymes, such as H-bonding networks present in cytochrome P45028,
32
horseradish peroxidase29, and hemoglobin30, play a critical role in the chemical
transformations they take part in. Strategies to incorporate non-covalent interactions into
synthetic model complexes through ligand design have been developed in recognition of
the importance of the secondary coordination sphere.31-35 Seminal examples include
Collman’s picket fence porphyrins36-37 and Borovik’s non-heme iron scaffold bearing an
amide microenvironment.38 More recently, the research group of Fout applied this
strategy to successfully achieve nitrate reduction with a bioinspired iron catalyst.39
Addition of redox inactive Lewis acid metal cations can offer an alternative
approach to influence reaction environment without directly tailoring the ligand
backbone. Nature employs redox-inactive metals in conjunction with high valent metal-
oxo species in metalloenzymes, such as the Mn4CaO4 cluster found in the oxygen
evolving complex (OEC) found in photosystem II (PSII).40 While the role of Ca2+ in the
oxidation of water is not completely consensus its presence is essential to the observed
reactivity of OEC.41-42 Interest in PSII and the role of Ca2+ in OEC has led to the
investigation of the effects of redox-inactive Lewis acid metal cations on reactivity of
metal-oxo species.43-44 Redox-inactive Lewis acid additives have been shown to alter
redox potentials45-48, electron transfer rates45, 49-51, and reactivity47, 52-57 of metal oxo
complexes. Oxygen atom transfer (OAT) reactivity can be influenced by the addition of
Lewis acids. The rate of OAT reactivity of Goldberg’s MnV(O) porphyrinoid complexes
is dramatically decreased with the addition of Zn2+.47 Another MnV(O) system reported
by Nam demonstrates the ability of Sc3+ to enhance OAT activity by increasing the
oxidizing power of MnV(O) through binding to the ligand.48
33
A biomimetic complex MoIV(O)(SN)2 (2), where SN = bis(4-tert-butylphenyl)-2-
pyridylmethanethiolate), reported by the research group of Holm17 is a very well
documented system that facilitates efficient OAT from an organic/inorganic substrate to
yield MoVI(O)2(SN)2 (1) (Figure 2.2). Complex 2 mediates OAT chemistry with a wide
variety of substrates including various N- and S-oxides.17 However, the original report
indicated that nitrate reduction to nitrite by 2 was unsuccessful. Inspired by recent reports
demonstrating the ability of redox-inactive metal cations to alter the OAT reactivity of
MnV(O) systems47-48, we investigated the effect of Lewis acid additives on the OAT
activity of 2. Herein, we report a strategy to induce novel nitrate reduction with 2 through
addition of Sc(OTf)3 (OTf = trifluoromethanesulfonate), demonstrating the ability of
Lewis acids additives to expand the reactivity scope of previously developed metal-oxo
systems.
Figure 2.2. MoVI(O)2(SN)2 (1) and MoIV(O)(SN)2 (SN=bis(4-t-butylphenyl)-2-pyridylmethanethiolate)
34
2.3. Experimental Section
General Considerations. Unless otherwise specified all reactions and manipulations
were carried out under an inert nitrogen atmosphere using a MBraun Labmaster SP
gloveblox or under argon using standard Schlenck line techniques. 3Å molecular sieves
were dried under vacuum for 24 hours at 250 °C prior to use. THF, pentane, and
acetonitrile were degassed then dried using an MBraun solvent purification systems
under an Ar atmosphere, and stored over activated 3 Å molecular sieves. Anhydrous
dichloromethane and methanol were purchased from Aldrich, degassed by sparging with
Ar for 30 minutes, and stored over activated 3 Å molecular sieves for 48 hours prior to
use. Ultrapure, deionized water was obtained from a Millipore Direct-Q 3 UV Water
Purification System. Tetrabutylammonium nitrate, triphenylphosphine, trimethylamine n-
oxide, 99% nitrous oxide, Amberlite IRA-400 chloride form resin, potassium hydroxide,
and pyrogallol were purchased from Sigma-Aldrich and used as received. Scandium
triflate was purchased from Strem and used as received. Tetrabutylammonium nitrite was
purchased from Sigma-Aldrich and recrystallized from THF at –35 °C prior to use. H218O
(98%), Na15NO3, and Na15NO2 were purchased from Cambridge Isotopes and used as
received. CD2Cl2 was purchased from Cambridge Isotopes, degassed via freeze-pump-
thaw cycle (5x) and dried over 3 Å molecular sieves for 48 hours prior to use. Griess
Reagent Kit for Nitrite Determination (G-7921) was purchased from Molecular Probes.
Samples were lyophilized using a Labcono FreeZone 4.5 freeze dry system. Mo(O)2(SN)2
(1), and Mo(18/18O)2(SN)2 (118/18) were prepared according to the published procedure.17
Physical Methods. All samples for spectroscopic analysis were prepared inside a
nitrogen glovebox unless otherwise noted. Infrared spectra were recorded on a Bruker
35
Tensor 27 FTIR spectrometer. Headspace detection of N2O was performed with a Bruker
A131 IR gas cell equipped with quartz windows. Room temperature UV-vis spectra were
recorded on a Varian Cary 50 Bio spectrometer. Low temperature UV-Vis spectra were
recorded on a Varian Cary 50 Bio spectrometer equipped with a 2 mm Hellma All-Quartz
Immersion probe fitted in a 24/40 Schlenk tube with a 14/20 female joint sealed with a
rubber septum. 1H and 15N NMR were recorded with a Bruker 400MHz Avance III
ultrashield spectrometer. 1H NMR was referenced to CD2Cl2 residual solvent signal (δ
5.32). 15N NMR was externally referenced to nitromethane in CD2Cl2. Elemental analyses
were carried out by Intertek Pharmaceutical Services (Whitehouse, NJ).
Synthesis of Mo(O)2(SN)2 (1). Mo(O)2(SN)2 was synthesized following the published
procedure.17 An additional recrystallization from THF/pentane (1:9) at -35 °C is was
needed to obtain pure compound, resulting in a lower yield than previously reported
(85%). UV-vis (dichloromethane): λmax (εM) 370 nm (7399). IR (KBr): 901, 936 cm-. 1H
NMR (CD2Cl2, 400 MHz): δ 9.42 (d, 1 H, J = 4.7 Hz, py), 7.57 (t, 1 H, J = 8.0 Hz, py),
7.28-7.24 (m, 4H, ph), 7.17 (d, 2H, J = 8.7 Hz, ph), 7.00 (d, 2H, J = 8.7 Hz, ph), 6.96 (t, 1
H, J = 5.7 Hz, py), 6.90 (d, 1H, J = 8.2 Hz, py), 1.32 (s, 9H, t-Bu), 1.28 (s, 9H, t-Bu).
Anal. Calcd for C52H60MoN2O2S2 • 0.6 CH2Cl2: C, 66.08; H, 6.45; N, 2.83. Found: C,
65.61; H, 6.53; N, 2.86.
Synthesis of Mo(O)(SN)2 (2). Mo(O)2(SN)2 (1) (500.0 mg, 0.552 mmol) and
triphenylphosphine (290.0 mg, 1.10 mmol) were dissolved in 20 mL of THF. The yellow
solution was refluxed under N2 for 5 h at 70 °C. The resulting dark brown solution was
dried to a brown oil and washed with 20 mL of acetonitrile (3x) resulting in the
precipitation of a dark brown solid. Recrystallization from dichloromethane/pentane
36
(1:15) at -35 °C yielded 314 mg (64%) of 2 as a semi-crystalline dark brown solid.
Additional recrystallization from dichloromethane/pentane (1:15) at -35 °C afforded
brown/black crystalline material. UV-vis (dichloromethane): λmax (εM) 328 (5926), 430
(4676), 518 (892), 700 (546). IR (KBr): 947 cm-. 1H NMR (CD2Cl2, 400 MHz): δ 9.37 (d,
1 H, J = 5.6 Hz, py), 7.86 (t, 1 H, J = 7.8 Hz, py), 7.47 (d, 1H, J = 8.1 Hz, py), 7.37 (t,
1H, J = 6.5 Hz, py), 7.32 (d, 2H, J = 7.8 Hz, ph), 7.24 (d, 2H, J = 7.6 Hz, ph), 7.16 (q,
4H, J = 6.2 Hz, ph), 1.32 (s, 9H, t-Bu), 1.26 (s, 9H, t-Bu). Anal. Calcd for
C52H60MoN2OS2 • 0.5 CH2Cl2: C, 67.69; H, 6.60; N, 3.01. Found: C, 68.04; H, 6.64; N,
2.98.
Reaction of Mo(O)(SN)2 (2), Sc(OTf)3 and [Bu4N][NO3]. Sc(OTf)3 (11.1 mg, 22.5
μmol) dissolved in 2.0 mL dichloromethane was added to a stirring solution of
Mo(O)(SN)2 (2) (10.0 mg, -11.2 μmol), and [Bu4N][NO3] (34.2 mg, 112 μmol) in 3.0 mL
of dichloromethane resulting in the rapid formation of a bright yellow solution. The
solution was dried to a bright yellow oil after an additional 30 min of stirring. MeOH (10
mL x 2) was added to remove excess Sc(OTf)3 and [Bu4N][NO3] and resulted in the
precipitation of a yellow solid. Recrystallization of the yellow solid dissolved in minimal
dichloromethane and layered pentane at –35 °C resulted in 8.2 mg (80%) of Mo(O)2(SN)2
(1) as a yellow microcrystalline solid whose spectroscopic characterizations match those
from independently synthesized 1.17
Detection and Quantification of Nitrite. The methanol washes of above were dried and
extracted with 1.00 mL of ultrapure, deionized H2O and filtered through celite to remove
any insoluble material. Detection of nitrite with Griess reagent was performed according
37
to manufacturer’s instructions. Nitrite concentration was determined using a NaNO2
calibration curve prepared in ultrapure, deionized H2O.
Low temperature UV-Vis monitoring of the reaction of Mo(O)(SN)2 (1), Sc(OTf)3
and [Bu4N][NO3] at –40 °C. After blanking spectrometer with 4.0 mL dichloromethane
at –40°C in a dry ice/acetonitrile cold bath, Mo(O)(SN)2 (2) (5.0 mg, 5.6 μmol) in 2.0 mL
dichloromethane was injected and cooled to –40 °C, and the spectrum of 2 was recorded.
A mixture of Sc(OTf)3 (6.0 mg, 12.2 μmol) and [Bu4N][NO3] (17.1 mg, 56.2 μmol) in 1.0
mL dichloromethane was injected to the solution of 2, after which a series of spectra were
measured for 90 minutes at –40°C.
Synthesis of Mo(18O)(SN)2 (218). Compound 218 was prepared by following the same
procedure for 2 using an 18O-labled precursor, Mo(18O)2(SN)2 (118/18) (80.0 mg, 0.088
mmol) and triphenylphosphine (46.1 mg, 0.176 mmol). The reaction resulted in 43.1 mg
(55%) of 218. UV-vis (dichloromethane): λmax (εM) 328 (5926), 430 (4676), 518 (892),
700 (546). IR (KBr): 901 cm–1
Reaction of Mo(18O)(SN)2 (218), Sc(OTf)3 and [Bu4N][NO3). The reaction was followed
by the same procedure for 2/Sc(OTf)3/[Bu4N][NO3] with slightly different amounts of
reagents. A mixture of Sc(OTf)3 (10.2 mg, 0.21 μmol) and [Bu4N][NO3] (31.5 mg, 1.03
μmol) dissolved in 1 mL of dichloromethane was added to a stirring solution of 218 (9.2
mg, 0.10 μmol) in 4 mL dichloromethane resulting in the rapid formation of a bright
yellow solution. Removal of dichloromethane yielded the 5.5 mg (59%) of
Mo(18/16O)2(SN)2 (118/16) as a yellow powder. UV-vis (dichloromethane): λmax (εM) 370
nm (4389). IR (KBr): 924, 867 cm–1.
38
Synthesis of Mo(18/16O)2(SN)2 (118/16) from Mo(18O)(SN)2 (218) and trimethylamine n-
oxide (TMAO). 1.01 mL of a 13.3 mM stock solution of TMAO in DMF (0.13 μmol)
was added to a stirring solution of 218 (12.0 mg, 0.13 μmol) in dichloromethane. After 30
min the volatiles were removed in vacuum to afford a yellow oil that was washed with
MeOH (10 mL x 2) resulting in the precipitation of a yellow solid. Recrystallization of
the yellow solid dissolved in minimal dichloromethane and layered pentane at –35 °C
resulted in 9.3 mg (76%) of Mo(18/16O)2(SN)2 (118/16) as a yellow powder. UV-vis
(dichloromethane): λmax (εM) 370 nm (4389). IR (KBr): 924, 867 cm–1.
Synthesis of [Bu4N][15NO3]. 4 mL of Amberlite IRA-400 chloride form resin (5.6 mEq)
was packed into a 1 x 10 cm column and washed with 200 mL deionized H2O. A solution
of Na(15NO3) (525 mg, 6.17 mmol) dissolved in 2 mL of deionized H2O was slowly
eluted through the column. The column was then washed with additional 200 mL
deionized H2O. Subsequently, a solution of Bu4NI (750 mg, 2.03 mmol) dissolved in 20
mL of deionized H2O was slowly eluted through the column and the fractions were
saved. Removal of water under vacuum afforded a clear oil. The clear oil was then
lyophilized for 24 hours resulting in 0.5081 g (71.8 %) of [Bu4N][15NO3] as white
powder. 1H NMR(CD2Cl2, 400 MHz): δ 3.22 (t, 2 H, J = 8.6), 1.66 (m, 2H), 1.45 (m, 2H),
1.04 (t, 3H, J = 7.4). 15N NMR(CD2Cl2) δ 6.46.
Synthesis of [Bu4N][15NO2]. [Bu4N][15NO2] was prepared analogously to
[Bu4N][15NO3] using 450 mg Na(15NO2) (6.52 mmol) and 750 mg Bu4NI (2.03 mmol)
resulting in 0.4441 g (75.8 %) of white powder 1H NMR(CD2Cl2, 400 MHz): δ 3.22 (t, 2
H, J = 8.6), 1.66 (m, 2H), 1.45 (m, 2H), 1.04 (t, 3H, J = 7.4). 15N NMR(CD2Cl2) δ
243.67.
39
Detection of N2O by IR spectroscopy. In a glovebox, Mo(O)(SN)2 (2) (50.0 mg, 56.2
μmol) and [Bu4N][NO3] (171.2 mg, 0.562 mmol) were dissolved in 5.0 mL of
dichloromethane in a 10 mL Schlenk flask. Separately, a solution of Sc(OTf)3 (55.0 mg,
0.112 mmol) in 2.0 mL of dichloromethane was prepared and loaded into an air-tight
syringe inside glovebox. After recording the background of the evacuated IR gas cell, the
Schlenk flask containing 2 and [Bu4N][NO3] was connected to the IR gas cell with a
three-way stopcock and Tygon tubing. After evacuating the tubing connecting the gas
cell and flask, the system was left under static vacuum. The Sc(OTf)3 solution was
quickly injected into the flask containing 2/[Bu4N][NO3] via syringe resulting in a bright
yellow solution. After 5 min the headspace from the reaction flask was transferred by
opening the stopcocks of the flask and IR gas cell. The stopcocks were then closed, and
the IR spectrum was recorded. The same procedure was employed for the detection of
N2O from the reaction of Sc(OTf)3 and [Bu4N][NO2] using a solution of [Bu4N][NO2]
(32.2 mg, 0.112 mmol) in 5.0 mL dichloromethane and a solution of Sc(OTf)3 (55.0 mg,
0.112 mmol) in 2.0 mL dichloromethane.
Detection of O2 from the reaction of Sc(OTf)3 and [Bu4N][NO2] with alkaline
pyrogallol. In a 25 mL Schlenk flask Pyrogallol (500.0 mg) was dissolved with stirring
in 10.0 mL of deoxygenated 50% KOH (aq) solution. A Schleck cuvette (1 cm
pathlength) fitted with a 14/20 male connector was connected to the 25 mL flask containg
the alkaline pyrogallol solution under positive Ar flow. The UV-vis spectrum of the faint
yellow alkaline pyrogallol solution was recorded. The flask was then reconnected to the
Schlenck line through a three-way stopcock. In a glovebox [Bu4N][NO2] (10.0 mg, 34.7
μmol) was dissolved in 3.0 mL of dichloromethane in a 10 mL Schlenk flask. Separately,
40
a solution of Sc(OTf)3 (17.2 mg, 34.7 μmol) dissolved in 2.0 mL methane was prepared
and loaded into an air-tight syringe inside glovebox. The Schlenck flask containing
[Bu4N][NO2] was connected to the pyrogallol flask with Tygon tubing through the 3-way
stopcock. The system was left under static vacuum after evacuating the pyrogallol
containg flask and tubing connecting the two flasks. The solution of Sc(OTf)3 was
quickly injected into the nitrite containing flask via syringe. After 1 min the headspace
was transferred by opening the stopcocks on both Schlenck flasks. The light yellow
solution rapidly turned light brown. After 30 mins the stopcocks were resealed and the
UV-vis of the darkened pyrogallol solution was recorded.
2.4. Results and Discussion
Preparation of Mo(O)2(SN)2 (1) and Mo(O)(SN)2 (2). Complex Mo(O)2(SN)2 (1) was
prepared following the reported procedure17 with minor modifications. Additional
recrystallization from THF/pentane was necessary to obtain the pure yellow
microcrystalline solid in 85% yield. The IR (νMo=O = 901, 936 cm–1) (Figure 2.3) and
diamagnetic 1H NMR spectral features (Figure 2.4a-b) are in good agreement with the
previously published values.17 Complex Mo(O)(SN)2 (2) was prepared using excess
triphenylphosphine in place of previously used triethylphosphine17 and was isolated as a
dark brown solid in 64% yields. Additional recrystallizations from CH2Cl2/pentane
afforded brown/black crystalline material and were found to aid the long-term stability of
2 at –35 °C under N2. The IR spectral features including a characteristic νMo=O frequency
at 947 cm–1 (Figure 2.3) and well resolved diamagnetic 1H NMR features of 2 (Figure
2.5a-b) matched the previously published values, with minor discrepancies in the
chemical shifts probably due to different instruments employed. The UV-Vis absorption
41
spectra of 1 (λmax = 370 nm) and 2 (λmax = 328 nm) originally reported in DMF, are
nearly identical when dichloromethane is used as the solvent (Figure 2.6). Elemental
analysis was performed on 1 and 2 to ensure purity of the bulk material. 1H NMR
samples of 1 and 2 in CDCl3 indicates that 0.6 and 0.5 equivalents of dichloromethane
from recrystallization are present in crystalline samples of 1 and 2, respectively, and is
accounted for in the elemental analysis.
Figure 2.3. IR spectra (KBr) of Mo(O)2(SN)2 (1) (yellow trace) and Mo(O)(SN)2 (2) (brown trace).
42
Figure 2.4a. Room temperature 1H NMR Mo(O)2(SN)2 (1) in CD2Cl2.
Figure 2.4b. Room temperature 1H NMR Mo(O)2(SN)2 (1) in CD2Cl2.
43
Figure 2.5a. Room temperature 1H NMR Mo(O)(SN)2 (2) in CD2Cl2.
Figure 2.5b. Room temperature 1H NMR Mo(O)(SN)2 (2) in CD2Cl2.
44
Figure 2.6. Room temperature UV-vis of Mo(O)2(SN)2 (1) (yellow trace) and Mo(O)(SN)2 (2) (brown trace) in DCM.
Lewis Acid requirement for nitrate reduction. Complex Mo(O)(SN)2 (2) carries out
an efficient OAT reaction with a record number of substrates.17 However, there is a limit
in the substrate scope. Holm and coworkers reported that the reaction between 2 and
nitrate does not yield 1 without providing further experimental details and
characterization. With fresh insights on the substrate activation mechanisms for Fdh and
Nap (Section 1), we thought that altering the secondary coordination environment by
Lewis acid may induce previously unseen OAT activity with 2. The OAT activity with
nitrate can be readily examined by UV-Vis spectroscopy from pronounced spectroscopic
differences of 1 and 2 (Figure 2.6). Consistent with the previous report,17 we did not
observe the formation of 1 from the reaction of Mo(O)(SN)2 (2) and [Bu4N][NO3] at
room temperature in CH2Cl2 up to 24 hours (Figure 2.7, Scheme 2.2).
45
Scheme 2.2 Reactivity of Mo(O)(SN)2 (2) and [Bu4N][NO3] in absence (top) and presence (bottom) of Sc(OTf)3 (SN = bis(4-t-butylphenyl)-2-pyridylmethanethiolate).
Figure 2.7. Room temperature UV-vis of Mo(O)(SN)2 (2) and [Bu3N][NO3] (10 equiv.) after 24 hours in DCM.
The reactivity of 2 and nitrate is remarkably altered with the addition of Lewis acid
(Scheme 2.2). Addition of Sc(OTf)3 (2 equiv.) to a CH2Cl2 solution of 2 (1 equiv.) and
[Bu4N][NO3] (10 equiv.) results in a rapid color change from brown to bright yellow. The
UV-Vis spectrum of the of the reaction mixture contains a single absorption band at 370
nm and matches that of authentic Mo(O)2(SN)2 (1) (Figure 2.8). Complex 1 generated
from the Sc3+ assisted OAT from nitrate by 2 was isolated in good yields (80%) following
46
recrystallization from dichloromethane/pentane after the removal of byproducts with
MeOH. The formation of 1 was further confirmed by IR (Figure 2.9) and 1H NMR
spectroscopy (Figure 2.10). The generation of nitrite (NO2–), the other OAT product, was
confirmed through positive Griess reagent tests58 on the MeOH soluble material.
Figure 2.8. Room temperature UV-vis of Mo(O)2(SN)2 (1) generated from Mo(O)(SN)2
(2) (1 equiv.), Sc(OTf)3 (2 equiv.) and [Bu4N][NO3] (10 equiv.) in DCM
Figure 2.9. IR spectra (KBr) of Mo(O)2(SN)2 (1) generated from Mo(O)(SN)2 (2) (1 equiv.), Sc(OTf)3 (2 equiv.) and [Bu4N][NO3] (10 equiv.) in DCM.
47
Figure 2.10. 1H NMR of Mo(O)2(SN)2 (1) generated from Mo(O)(SN)2 (2) (1 equiv.), Sc(OTf)3 (2 equiv.) and [Bu4N][NO3] (10 equiv.) (top) and authentic Mo(O)2(SN)2 (1) (bottom) in CD2Cl2.
No intermediates were observed for the novel OAT reactivity even at low temperatures.
Instead, monitoring the reaction at –40 °C shows a clean conversion of Mo(O)(SN)2 (2)
to Mo(O)2(SN)2 (1) (Figure 2.11). Addition of Sc(OTf)3 (2 equiv.) to the dichloromethane
solution of 2 (1 equiv.) and [Bu4N][NO3] (10 equiv.) results in the loss of the absorption
bands 328 and 430 nm and the emergence of a single absorption band at 370 nm with the
tight isosbestic points at 340 and 407 nm (Figure 2.11). The observed isosbestic points
match those reported for the OAT from Ph3AsO to 2 to form 1 at room temperature,17
which indicates that Sc3+ enables 2 to undergo OAT with nitrate in the same manner with
other known substrates.
48
Figure 2.11. Reaction of Mo(O)(SN)2 (2) (0.8 mM), [Bu4N][NO3] (8.0 mM), and Sc(OTf)3 (1.7 mM) followed by UV-Vis spectroscopy at –40 ºC in dichloromethane for 1.5 hours. The spectral changes correspond to the conversion of Mo(O)(SN)2 (2) (λmax = 328, 430 nm) to Mo(O)2(SN)2 (1) (λmax = 370 nm).
Nitrate as oxygen atom source: 18O labeling experiments. Isotopic labeling studies
were conducted to confirm nitrate as the oxygen atom source for the observed conversion
of Mo(O)(SN)2 (2) to Mo(O)2(SN)2 (1). Attempts at monitoring the reaction using N18O3–
were unsuccessful due to insufficient purity of commercially available 18O-labeled
nitrate. As an alternative approach, we have prepared 18O labeled 2 for the reaction with
NO3–. Doubly 18O labeled Mo(18O)2(SN)2 (118/18) was prepared through the treatment of 1
with H218O by adopting a known procedure.17 Upon 18O substitution, the molybdenum
oxygen stretches were shifted from 901 and 936 cm–1 to 887 and 858 cm–1 in the IR
spectra, consistent with the known report17 and are in good agreement with theoretical
values from reduced mass calculations (890 and 856 cm–1) (Figure 2.12). As reported for
the synthesis of 2 (see above), the 18O labeled 2, Mo(18O)(SN)2 (218) was prepared from
118/18 and triphenylphosphine.
49
Treatment of Mo(18O)(SN)2 (218) with [Bu4N][NO3] in the presence of Sc(OTf)3
was accompanied by the brown to yellow color change observed for the analogous
unlabeled reaction. The IR spectrum of the recrystallized reaction product shows major
molybdenum-oxo stretches at 867 and 924 cm–1 in between the values for 116/16 and 118/18,
suggesting the formation of the mixed labeled di-oxo species Mo(18/16O)2(SN)2 (118/16)
(Figure 2.12). Holm and co-workers reported 218 reacts with Ph2SO to yield the mixed
labeled bis-oxo compound 118/16 that was characterized by mass spectrometry,17 but the
IR features of 118/16 were not reported. To confirm the product generated from 218/NO3–
1/Sc(OTf)3 is the same O-atom transferred product obtained from 218 with other known
substrates, the reaction of 218 with trimethylamine n-oxide (TMAO) was carried out
(Scheme 2.3). The product obtained from 218/TMAO resulted in the identical IR spectrum
as the one from 218/NO3–1/Sc(OTf)3 (Figure 2.12), indicating that nitrate is the source of
the oxygen atom to generate Mo(18/16O)2(SN)2 (118/16) from Mo(18O)(SN)2 (218).
Scheme 2.3. Mo(18/16O)2(SN)2 (118/16) preparation from Mo(18O)(SN)2 (218) via Sc(OTf)3 assisted nitrate reduction (I) or trimethylamine n-oxide (II).
50
Figure 2.12. IR spectra (KBr) of Mo(16O)2(SN)2 (1, black dashed) and Mo(18O)2(SN)2
(118/18, black solid), along with Mo(16/18O)2(SN)2 (118/16) generated from 218 with [Bu4N][NO3] in the presence of Sc(OTf)3 (red solid), and Mo(16/18O)2(SN)2 (118/16) generated from 218 with trimethylamine n-oxide (blue dotted).
Fate of Nitrate. The Lewis acid assisted OAT from nitrate by Mo(O)(SN)2 (2) would
generate nitrite (NO2–) as a reaction product. The formation of nitrite was first probed by
the Griess reagent test,58 the most commonly used method of detection of nitrite. All the
reaction products and byproducts excluding 1 were extracted with MeOH, on which the
Griess test was conducted. Treatment of the MeOH soluble material with the Griess
reagent resulted in the formation of the azo dye with an absorbance band at 548 nm
(Figure 2.13) indicative of the presence of nitrite. However, only a trace amount (<1%) of
nitrite was obtained based off the prepared nitrite calibration curve although a higher
yield of nitrite was expected from the high conversion yield of 1 from 2/NO3–/Sc3+,
which indicates that there must be a secondary reaction for nitrite.
51
Figure 2.13. Room temperature UV-vis in H2O for positive Griess reagent test for nitrite generated from Mo(O)(SN)2 (2) (1 equiv.), Sc(OTf)3 (2 equiv.) and Bu4N(NO3) (10 equiv.) Prepared nitrite calibration curve (inset)
To further investigate the fate of nitrite, 15N NMR studies were carried out, for
which [Bu4N][15NO3] and [Bu4N][15NO2] were independently prepared (see section 2).
The formation or consumption of nitrate or nitrite can be readily identified by the 15N
chemical shifts for [Bu4N][15NO3] at 6.46 ppm and [Bu4N][15NO2] at 243.67 ppm in
CD2Cl2. There was no reaction between [Bu4N][15NO3] and Sc(OTf)3 (1:1 ratio), from
which only unreacted nitrate signal at 6.46 ppm was observed in 15N NMR spectrum
(data not shown). Likewise, when Mo(O)(SN)2 (2) was reacted with [Bu4N][15NO3] (1-10
equiv.) in the absence of Sc(OTf)3, only the unreacted starting reagents, 2 and
[Bu4N][15NO3] were observed in 15N- and 1H-NMR, IR, and UV-vis spectroscopy. When
the reaction of 2 (1 equiv.) and [Bu4N][15NO3] (10 equiv.) was carried out in the presence
of with Sc(OTf)3 (2 equiv.) in a sealed J Young NMR tube, the clean formation of the O-
52
atom abstracted metal product, Mo(O)2(SN)2 (1), was detected by 1H NMR spectroscopy,
which was further confirmed by UV-Vis and IR spectroscopy. However, the 15N NMR
spectrum of the reaction products did not show a signal for 15NO2– nor for any other 15N-
containing products beside excess unreacted substrate, 15NO3–. (Figure 2.14a).
Figure 2.14. Room temperature 15N NMR (in CD2Cl2) spectra of a) the reaction mixture of Mo(O)(SN)2 (2) (1equiv.), Sc(OTf)3 (2 equiv.) and Bu4N(15NO3) (10 equiv.) showing excess nitrate signal at 6.46 ppm, b) authentic [Bu4N][15NO2], and c) an equimolar mixture of [Bu4N][15NO2] and Sc(OTf)3.
The lack of 15N- signal for the anticipated product, nitrite, led us to consider a
secondary reaction for nitrite involving gas evolution. Detection of NO by utilizing
established chemical method59 and IR spectroscopy was negative. However, when the
headspace of the reaction mixture of Mo(O)(SN)2 (2)/Sc(OTf)3/[Bu4N][NO3] was
analyzed by IR spectroscopy, the characteristic signals60 reported for N2O was observed
53
at 2236 and 2212 cm–1 (Figure 2.15). N2O can be potentially generated from nitroxyl
(NO–) that may be produced from OAT reactivity of 2 with NO2–, but the presence of
large excess nitrate (10 equiv.) in the reaction mixture would make this scenario unlikely.
This led us to investigate the reaction between [Bu4N][NO2] and Sc(OTf)3. To our
surprise, gas evolution was observed upon mixing of Sc(OTf)3 and [Bu4N][NO2], in the
absence of molybdenum complexes. Addition of Sc(OTf)3 to [Bu4N][15NO2] (1:1 ratio) in
CD2Cl2 results in the complete loss of the nitrite signal at 243 ppm in the 15N NMR
(Figure 2.14c). The gas generated from the reaction of [Bu4N][NO2] with Sc(OTf)3 was
examined and identified through headspace analysis using an IR gas cell. Two features
present at 2236 and 2212 cm–1 in the IR spectrum from the headspace match the N-N
stretches of authentic N2O (figure 2.15). In addition to N2O, the headspace for the
reaction of [Bu4N][NO2]with Sc(OTf)3 was tested positive for O2 generation.
Transferring the headspace to an alkaline solution of pyrogallol, a known dioxygen
scavenger that does not react with N2O61-62, resulted in the rapid formation of a light
brown solution. The color change and appearance of an absorbance band at 409 nm in the
UV-vis spectrum (Figure 2.16) are in agreement with the alkaline pyrogallic O2 detection
reported by Karlin and coworkers.63-64 The presence of O2 and N2O in the head space of
the reaction of [Bu4N][NO2] with Sc(OTf)3 suggests that Sc(OTf)3 carries out the
unexpected disproportionation of nitrite to form O2 and NO– in which the latter dimerizes
to N2O. Although the conversion of nitrite to O2 and NO– by Sc3+ is an interesting
chemical transformation in its own right, the mechanism for the reaction is beyond the
scope of this manuscript.
54
Figure 2.15. IR spectra (gas cell) of headspace from the reaction mixtures of Mo(O)(SN)2 (2) (1 equiv.) and [Bu4N][NO3] (10 equiv.) before (blue solid) and after (red solid) addition of Sc(OTf)3 (2 equiv.), along with the spectrum from the reaction of [Bu4N][NO2] (1 equiv.) and Sc(OTf)3 (1 equiv.) (green solid) and that of authentic N2O (black dotted).
Figure 2.16. UV-vis of pyrogallol in 50% KOH (aq) before (blue trace) and after head space transfer (red trace) from reaction of Sc(OTf)3 (3.5 x 10-5
mol) and Bu4N(NO2) (3.5 x 10-5 mol).
409
55
Role of Sc3+ in observed nitrate reduction. The addition of the Lewis acidic Sc(OTf)3 is
crucial for the observed nitrate reduction by Mo(O)(SN)2 (2). It is also important how
Sc(OTf)3 is introduced to the reaction. The direct interaction between Sc(OTf)3 and
Mo(O)(SN)2 (2) is disruptive and needs to be avoided for nitrate reduction to occur.
Addition of Sc(OTf)3 to 2 in the absence of nitrate results in fast decomposition of 2 as
judged by UV-vis (Figure 2.17) and 1H NMR (Figure 2.18) spectroscopy. Likewise,
addition of Sc(OTf)3 to Mo(O)2(SN)2 (1) also leads to decomposition in the absence of
nitrate (Figure 2.19). When excess nitrate (5-10 equivalents) is present, the
decomposition of 1 or 2 by Sc(OTf)3 is no longer observed and a clean OAT chemistry
can be achieved.
Figure 2.17. Room temperature UV-vis in DCM of Mo(O)(SN)2 (2) (solid trace) before and after addition of Sc(OTf)3 (dashed).
56
Figure 2.18. 1H NMR of Mo(O)(SN)2 (2) with Sc(OTf)3 (top) and authentic Mo(O)(SN)2
(2) (bottom) in CD2Cl2.
Figure 2.19. 1H NMR of Mo(O)2(SN)2 (1) with Sc(OTf)3 (top) and authentic Mo(O)2(SN)2 (1) (bottom) in CD2Cl2.
57
Effects of Lewis acid on OAT have been reported in two different MnV(O)
systems. Nam and coworkers reported48 an advantageous effect of Lewis acid in OAT
chemistry of a non-heme Mn(V)-oxo complex in which the binding Sc3+ to the ligand
scaffold enhances the oxidizing power of Mn(V)-oxo and the OAT activity. Goldberg
and coworkers reported47 an inhibitory effect of Lewis acid in OAT reaction with a
Mn(V)-oxo porphyrinoid complex, in which the binding of Zn2+ to the ligand periphery
leads to a valence tautomerization to form a Mn(IV)-oxo π-radical cation which proceeds
OAT dramatically slower than the Mn(V)-oxo valence tautomer. In our system, the
acquired OAT activity of Mo(O)(SN)2 (2) with nitrate by Lewis acid is not likely due to
the binding of Sc3+ to the ligand frame of 2 because the SN ligand does not have basic
sites outside of the Mo chelating donor atoms, and the direct interaction between 2 and
Sc3+ leads to decomposition. However, the binding of Sc3+ to the substrate would likely
pull electron density from nitrate and induce the weakening of N-O bond which would
favor the OAT by 2 (Scheme 2.4). The proposed role for Sc3+ in the activation of NO3– is
similar to the role of Lewis acid for N2 activation on a recently reported Fe system by
Szymczak and coworkers, in which secondary sphere Lewis acids weaken and polarize
the N−N bond via ‘push-pull mechanism’.65 The importance of the substrate bond
strength in the OAT chemistry of DMSOR has been thoroughly discussed by Solomon
and coworkers.66-67 X-ray absorption spectroscopic and DFT calculation studies on the
DMSOR model complexes suggest that OAT requires the S-O bond elongation which
then subsequently facilitates electron transfer from MoIV to the sulfur atom.66 Although
molybdenum oxotranferase enzymes do not utilize Lewis acidic metal ions for the
58
substrate activation, our study shows that application of Sc3+ as an additive can emulate
the substrate bond activation, one of the key effects of the secondary environment of the
enzyme.
Scheme 2.4. Lewis acid assisted nitrate reduction by Mo(O)(SN)2 (2) to form
Mo(O)2(SN)2 (1) by oxygen atom transfer.
2.5. Conclusions. Nitrate is one of the most common contaminants in the groundwater
and poses a number of health problems. Reduction of nitrate (NO3–) to nitrite (NO2
–) is
biologically and environmentally important but is difficult to achieve outside of the
enzymatic systems. Complex Mo(O)(SN)2 (2)17 is one of the best known biomimetic
complexes possessing the oxygen atom transfer (OAT) reactivity with a wide variety of
substrates, yet 2 is not capable of reducing nitrate. In this work, we present a new
strategy that expands the reactivity of 2. Addition of a small amount of Sc3+ to the
mixture of 2/NO3– results in the activation of nitrate through a secondary coordination
interaction, which leads to an immediate and clean formation of Mo(O)2(SN)2 (1) and
nitrite (NO2–). In addition to activating nitrate for reduction, the Sc3+ additive can further
convert nitrite to gaseous molecules, N2O and O2, which can be easily removed from the
reaction solution. This work demonstrates the ability of Lewis acid additives to expand
59
the reactivity scope for existing metal complexes and bring forth novel reactivity without
the need to develop new ligand systems.
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69
Chapter 3: Structure and Oxygen Atom Transfer
Reactivity of Dinuclear (µ-O)Molybdenum(V) Complex
70
3.1. Abstract
Nature utilizes mononuclear molybdenum oxotransferase enzymes for a wide
variety of oxygen atom transfer (OAT) reactions, including the biologically and
environmentally important reduction of nitrate (NO3−) to nitrite (NO2–) achieved by
nitrate reductase enzymes. While an abundance of MoIV(O) and MoVI(O)2 complexes
mimic the OAT reactivity displayed by dimethylsulfoxide reductase (DMSOR) enzymes,
few examples utilize nitrate as the oxygen atom donor, replicating the reactivity of the
nitrate reductase enzymes. The reactivity of early model systems was often complicated
by the rapid and sometimes irreversible formation of the dinuclear MoV2O3 species
through comproportionation of MoIV(O) and MoVIO2 species. While synthetic systems
that form MoV2O3 species are avoided as active site models, they are capable of
stochiometric and catalytic OAT reactivity. Presented here is the synthesis and crystal
structure of a MoV2O3 thiosemicarbazone species previously utilized in the catalytic
oxidation of triphenylphosphine by DMSO. Additionally, nitrate and nitrite reduction to
gaseous products is presented, demonstrating that the often maligned MoV2O3 complexes
can achieve difficult OAT chemistry.
71
3.2. Introduction. Nature utilizes mononuclear molybdenum oxotransferase enzymes for
a wide variety of oxygen atom transfer (OAT) reactions.1 The biologically and
environmentally relevant two-electron reduction of nitrate (NO3−) to nitrite (NO2−) is
carried out by nitrate reductase enzymes including periplasmic nitrate reductase (Nap)
from Desulfovibrio desulfuricans2 and membrane-bound respiratory nitrate reductase
(Nar) from Escherichia coli.3 Nap and Nar, both members of the dimethylsulfoxide
reductase (DMSOR) family of enzymes, utilize a high valent MoIV/VI metal center bound
by two pyranopterin cofactors (Figure 3.1). During the reduction of nitrate an oxygen
atom is transferred to the MoIV center generating a MoVI-oxo species. The MoIV center is
regenerated upon addition of externally provided protons and electrons completing the
catalytic cycle with the release of water resulting in the overall reactivity in equation 1.1, 4
Figure 3.1. Active site structures of the oxidized forms of (a) respiratory nitrate reductase (Nar) from Escherichia coli3 and (b) periplasmic nitrate reductase (Nap) from D. desulfuricans2 where Asp = aspartate and Cys = cysteine.
72
Interest in oxotransferase enzymes and molybdenum mediated OAT led to the
development of numerous synthetic systems that take advantage of the MoIV/VI redox
couple, utilizing a wide variety ligand scaffolds and oxygen atom donors, that are capable
of oxygen atom transfer to (equation 2) and from (equation 3) substrates.5-7 While the
oxygen atom transfer from dimethylsulfoxide (DMSO) to phosphine oxygen atom
acceptors is a hallmark of DMSOR model complexes, with systems capable of catalytic
OAT8-9, there are far less examples of oxotransferase model systems capable of OAT
from nitrate.10-13 The further development of bioinspired molybdenum complexes that
can reduce nitrate could be a useful tool in the remediation of nitrate, a pervasive
groundwater contaminant linked to adverse health effects.14
Unlike the mononuclear active sites of oxotransferase enzymes, the reactivity of
early model systems was often complicated by the formation of µ-oxo molybdenum (V)
dimers.15-17 The rapid and sometimes irreversible formation of the dinuclear MoV2O3
species through comproportionation of MoIV(O) and MoVIO2 species (equation 4) has
been well studied.5
Strategies to prevent dimer formation were developed, including the use of sterically
bulky ligands and charged complexes, and have been successfully implemented in later
73
generation oxotransferase model complexes. While dimer formation is not desired for
model systems of the mononuclear oxotransferase enzymes, its formation does not
necessarily prevent oxygen atom transfer from occurring. Stoichiometric and catalytic
OAT are possible if the dimer formation is reversible and equilibrates rapidly (scheme
3.1)16, 18-20. There are also examples of MoV2O3 species proposed to directly interact with
oxygen atom donors to generate MoVIO2 species21, with a catalytic example utilizing
Mo2O3(dtc)2I2(THF)2 (dtc = S2CNEt2) reported by Baird and co-workers (Scheme 3.2)22-
23. Schiff base and 2,6-bis(2,2-diphenyl-2-thioethyl)-pyridinate containing MoV2O3
complexes reported by Holm10, 21 and Young12 are also capable of nitrate reduction,
further demonstrating µ-oxo molybdenum (V) dimers OAT capabilities.
Scheme 3.1. OAT with dimer disproportionation.
Scheme 3.2 OAT without dimer disproportionation.
74
Recent work by Duhme-Klair and co-workers demonstrated the ability to tune the
oxygen atom transfer of a series of di-oxo molybdenum(VI) thiosemicarbazone
complexes with ligand modification (Figure 3.2)9. The rate of catalytic oxidation of
triphenylphosphine with DMSO increased with the strength of electron withdrawing
substituents of the thiosemicarbazone ligand. Absent from the work was information on
the reduced molybdenum species in the catalytic cycle, however the authors alluded to
formation of Mo(V) dimers as a possible source of catalyst deactivation in the oxygen
atom transfer activity. Our interest in the identity of the reduced molybdenum species of
the catalytic cycle and the possibility to expand the substrate scope of the system to
include nitrate led us to further develop the thiosemicarbazone system reported by
Duhme-Klair and co-workers. The isolation and characterization of the MoV2O3
thiosemicarbazone complex and its nitrate reduction ability is presented.
Figure 3.2. cis-dioxomolybdenum(VI) thiosemicarbazone (X = Me, H, I, Br, OCF3,
NO2).9
75
3.3. Experimental Section
General Considerations. Unless otherwise specified all reactions and manipulations
were carried out under an inert nitrogen atmosphere using a MBraun Labmaster SP
gloveblox or under argon using standard Schlenk line techniques. 3Å molecular sieves
were dried under vacuum for 24 hours at 250 °C prior to use. THF and pentane were
degassed then dried using an MBraun solvent purification systems under an Ar
atmosphere, and stored over activated 3 Å molecular sieves. Anhydrous dimethyl
sulfoxide was purchased from Aldrich, degassed by sparging with Ar for 30 minutes, and
stored over activated 3 Å molecular sieves for 72 hours prior to use. Ultrapure, deionized
water was obtained from a Millipore Direct-Q 3 UV Water Purification System.
Tetrabutylammonium nitrate, polymer-bound triphenylphosphine (100-200 mesh, ~3.0
mmol/g loading), trimethylamine n-oxide, 99% nitrous oxide, potassium hydroxide,
THF-d8 and pyrogallol were purchased from Sigma-Aldrich and used as received.
Scandium triflate was purchased from Strem and used as received. Tetrabutylammonium
nitrite was purchased from Sigma-Aldrich and recrystallized from THF at –35 °C prior to
use. Acetone-d6 was purchased from Cambridge Isotopes, degassed via freeze-pump-
thaw cycle (5x) and dried over 3 Å molecular sieves for 24 hours prior to use. DMSO-d6
and THF-d8 were purchased from Sigma-Aldrich and used as received. Griess Reagent
Kit for Nitrite Determination (G-7921) was purchased from Molecular Probes.
[Bu4N][15NO3] and [Bu4N][15NO2] were prepared according following the previously
reported procedure (Section 2.3).
Physical Methods. All samples for spectroscopic analysis were prepared inside a
nitrogen glovebox unless otherwise noted. Infrared spectra were recorded on a Bruker
76
Tensor 27 FTIR spectrometer. Headspace detection of N2O was performed with a Bruker
A131 IR gas cell equipped with calcium fluoride windows. Room temperature UV-Vis
spectra were recorded on a Varian Cary 50 Bio spectrometer with using screw cap UV-
Vis cuvettes, Schlenk cuvette, or a 2 mm Hellma All-Quartz Immersion probe fitted in a
24/40 Schlenk tube with a 14/20 female joint sealed with a rubber septum. NMR were
recorded with a Bruker 400MHz Avance III ultrashield spectrometer. 1H NMR were
referenced to acetone residual solvent signal (δ 2.05), DMSO residual signal (δ 2.50), or
THF residual solvent signal (δ 1.72). 15N NMR was externally referenced to nitromethane
in CD2Cl2. Elemental analysis was carried out by Intertek Pharmaceutical Services
(Whitehouse, NJ). X-ray crystallographic data was collected with a Bruker Smart Apex I
diffractometer. The structure was solved and refined using Bruker SHELXTL Software
Package.
Synthesis of Mo(O)2(LBr)(MeOH) (1). Mo(O)2(LBr)(MeOH) was synthesized following
the published procedure.9 UV-Vis (THF): λmax (εM) 233 nm (32000), 252 nm (35400),
320 nm (18000), 414 nm (5860). IR (KBr): 906, 940 cm-. 1H NMR (Acetone-d6, 400
MHz): δ 8.65 (s, 1 H), 7.83 (d, 1H, J= 2.5 Hz) 7.59 (dd, 1 H, J = 8.6 Hz, 2.5 Hz), 6.90 (d,
1H, J = 8.8 Hz ), 6.84 (br, 1H), 3.45 (q, 2H), 3.33 (s, 3H), 1.24 (t, 3H).
Synthesis of Mo2O3(LBr)2(THF)2·2THF (2). Polymer supported triphenylphosphine
(125 mg, ~0.375 mmol) was added to a stirring solution of Mo(O)2(LBr)(MeOH) (1)
(115.0 mg, 0.250 mmol) in 10.0 mL THF. The resulting dark brown solution was stirred
for 6 h then filtered through a pad of celite and concentrated to approximately 5 mL.
Recrystallization by vapor diffusion of pentane into the THF solution yielded 115.9 mg
(82.2 %) of 2 as blue-black crystals. UV-Vis (THF): λmax (εM) 236 nm (22200), 252 nm
77
(19700), 316 nm (13300), 454 nm (12200). IR (KBr): 970 cm-. 1H NMR (Acetone-d6, 400
MHz): δ 9.00 (s, 1 H), 7.93 (dd, 1 H, J = 5.9 Hz, 2.5 Hz) 7.83 (d, 1H, J= 2.5 Hz) 7.64
(dd, 1 H, J = 8.9 Hz, 2.4 Hz), 7.09 (br, 2H), 3.64 (m, 8H), 3.53 (m, 2H), 1.80 (m, 8H),
1.27 (q, 3H). Anal. Calcd. for C36H52Br2Mo2N6O9S2: C, 38.10; H, 4.64; N, 7.45. Found:
C, 38.21; H, 4.65; N, 7.50.
Reaction of Mo2O3(LBr)2(THF)2·2THF (2) and DMSO. Method 1:
Mo2O3(LBr)2(THF)2·2THF (2) (5.0 mg, 4.4 μmol) was dissolved in 0.5 mL of DMSO-d6
in a screw cap NMR tube. After half an hour the dark brown solution turned orange and
the 1H NMR was recorded. Method 2: In a screw cap UV-Vis cuvette, 100.0 uL of a 0.89
mM stock solution of 2 was added to 2.9 mL of THF and 50 μL of DMSO and the UV-
Vis was recorded after a half an hour (85% yield by UV-Vis).
Reaction of Mo2O3(LBr)2(THF)2·2THF (2) and trimethylamine n-oxide (TMAO).
TMAO (330 μL of 13.3 mM stock solution in THF/MeOH 95:5, 4.4 μmol) was added to
a stirring solution of 2 (5.0 mg, 4.4 μmol) dissolved in 5.0 mL THF. The resulting bright
yellow solution was stirred for an additional 5 minutes. After recording UV-Vis spectrum
the solution was dried to a yellow residue, then dissolved in acetone-d6. (97% yield by
UV-Vis)
Catalytic Oxygen atom transfer reactivity of Mo2O3(LBr)2(THF)2·2THF (2) with
DMSO and triphenylphosphine. Following the procedure from Duhme-Klair and
coworkers9 0.4 mL of a 255 mM triphenylphosphine in CD2Cl2 (25.5 equivalents) and 0.4
mL of 5mM 2 (0.5 equivalents) in DMSO-d6 were mixed in a J-Young NMR tube. 31P
NMR were recorded periodically for 70 hours. The concentration of triphenylphosphine
at time t , [PPh3]t, was determined from integrating the triphenylphosphine signal at -6
78
ppm and triphenylphosphine oxide signal at 26 ppm. The pseudo-first-order rate
constants (kobs) was determined from the slope of the equation ln([PPh3]t/[PPh3]0) = -kobst
where [PPh3]o was the starting triphenylphosphine concentration. Calculated kobs 107[s-] =
18. Reported kobs 107[s-] = 21(1).
Reaction of Mo2O3(LBr)2(THF)2·2THF (2) and [Bu4N][NO3]. 680 μL of a 13.1 mM
stock solution of [Bu4N][NO3] (8.86 μmol) in THF was added to a stirring solution of 2
(10.0 mg, 8.86 μmol) in 5.0 mL THF. The dark brown solution gradually turned orange-
brown over 2 hours. After recording a UV-Vis of an aliquot of the reaction mixture, the
sample was dried to an oily brown residue that was then dissolved in acetone-d6 and 1H
NMR was recorded. After recording the 1H NMR, the sample was dried onto KBr and the
IR recorded. (89% yield by UV-Vis)
Reaction of Mo2O3(LBr)2(THF)2·2THF (2) and 0.5 equivalents of [Bu4N][NO3]. 340
μL of a 13.1 mM stock solution of [Bu4N][NO3] (4.43 μmol) in THF was added to a
stirring solution of 2 (10.0 mg, 8.86 μmol) in 5.0 mL THF. The dark brown solution
gradually turned orange-brown over 2 hours. After recording a UV-Vis of an aliquot of
the reaction mixture, the sample was dried to an oily brown residue that was then
dissolved in acetone-d6. (93% yield by UV-Vis)
Reaction of Mo2O3(LBr)2(THF)2·2THF (2) and [Bu4N][NO2]. 640 μL of a 13.9 mM
stock solution of [Bu4N][NO2] (8.86 μmol) in THF was added to a stirring solution of 2
(10.0 mg, 8.86 μmol) in 5.0 mL THF. The dark brown reaction turned orange with 5
minutes of stirring. The reaction was dried to a yellow/orange residue after taking aliquot
for UV-Vis following an additional 30 minutes of stirring. The residue was dissolved in
acetone-d6 and 1H NMR was recorded.
79
Reaction of Mo2O3(LBr)2(THF)2·2THF (2) and [Bu4N][15NO3]. In a J-Young NMR
tube 2 (18.5 mg, 34.8 μmol) and [Bu4N][NO3] (5.0 mg, 16.4 μmol) were dissolved in 0.4
mL acetone-d6. The dark brown solution gradually turned orange over 2 hours. 15N NMR
was then recorded.
Reaction of Mo(O)2(LBr)(MeOH) (1) and 2 equivalents [Bu4N][NO2]. [Bu4N][NO2]
(5.0 mg, 16.4 μmol) in 1.0 mL THF was added to a stirring solution of 1 (16.0 mg, 16.4
μmol) in 4.0 mL THF. The dark brown/orange solution rapidly turned bright orange and
generated yellow precipitate. After recording a UV-Vis of the reaction mixture, the
orange solution was filtered, dried, and dissolved in acetone-d6 and 1H NMR recorded.
Reaction of Mo(O)2(LBr)(MeOH) (1) and 2 equivalents [Bu4N][15NO2]. In a J-Young
NMR tube 1 (16.0 mg, 16.4 μmol) and [Bu4N][NO2] (5.0 mg, 16.4 μmol) were dissolved
in 0.6 mL acetone-d6. The dark brown solution rapidly turned orange, and developed
yellow precipitate over the time it took to record the 15N NMR.
Detection and Quantification of Nitrite. The above reaction mixtures were dried and
extracted with 1.00 mL of ultrapure, deionized H2O with sonication then filtered through
celite to remove any insoluble material. Detection of nitrite with Griess reagent was
performed according to manufacturer’s instructions. Nitrite concentration was determined
using a NaNO2 calibration curve prepared in ultrapure, deionized H2O.
UV-Vis monitoring of the reaction of Mo2O3(LBr)2(THF)2·2THF (2) and
[Bu4N][NO3]. After blanking spectrometer with 4.0 mL THF, 1.0 mL of a stock solution
of 2 (0.86 μmol) was injected and the spectrum of 2 was recorded. A stock solution of
80
[Bu4N][NO3] was diluted to 1.0 mL (0.86 μmol) in THF was injected into the solution of
2, after which a series of spectra were measured for 3 hours.
UV-Vis monitoring of the reaction of Mo2O3(LBr)2(THF)2·2THF (2) and
[Bu4N][NO3], synthetic scale. [Bu4N][NO3] (5.0 mg, 16.3 μmol) were dissolved in 1.0
mL of THF was added to a stirring solution of MoO3(LBr)2(THF)2·2THF (2) (18.5 mg,
16.3 μmol) in 9.0 mL THF. The reaction progress was monitored for 3 hours by
periodically recording UV-Vis spectra of 50 μL aliquots of the reaction mixture diluted to
3 mL of THF.
UV-Vis monitoring of the reaction of Mo2O3(LBr)2(THF)2·2THF (2), Sc(OTf)3 and
[Bu4N][NO3] (1:1:10). ]. After blanking spectrometer with 5.0 mL THF, 1.0 mL of a
stock solution of 2 (0.86 μmol) was injected and the spectrum of 2 was recorded. A stock
solution of [Bu4N][NO3] (8.6 μmol) was mixed with a stock solution of Sc(OTf)3 (0.86
μmol) and diluted to 2.0 mL in THF then injected into the solution of 2, after which a
series of spectra were measured for 2 hours.
Detection of N2O by IR spectroscopy. In a glovebox, of Mo(O)2(LBr)(MeOH) (1) (50.0
mg, 108 μmol) and [Bu4N][NO2] (15.7 mg, 54.0 μmol) were dissolved in 10.0 mL of
THF in a 25 mL Schlenk flask. After recording the background of the evacuated IR gas
cell, the Schlenk flask containing the reaction was connected to the IR gas cell with a
three-way stopcock and Tygon tubing. After evacuating the tubing connecting the gas
cell and flask, the system was left under static vacuum. The headspace from the reaction
flask was transferred by opening the stopcocks of the flask and IR gas cell. The stopcocks
were then closed, and the IR spectrum was recorded. The same procedure was employed
for the detection of N2O from the reaction of [Bu4N][NO3] and MoO3(LBr)2(THF)2·2THF
81
(2) and using a solution of [Bu4N][NO3] (6.7 mg, 22.0 μmol) and 2 (50.0 mg, 44.0 μmol)
in 25.0 mL of THF.
Detection of O2 from the reaction of of Mo(O)2(LBr)(MeOH) (1) and [Bu4N][NO2]
with alkaline pyrogallol. In a 25 mL Schlenk flask Pyrogallol (500.0 mg) was dissolved
with stirring in 10.0 mL of deoxygenated 50% KOH (aq) solution. A Schleck cuvette (1
cm pathlength) fitted with a 14/20 male connector was connected to the 25 mL flask
containg the alkaline pyrogallol solution under positive Ar flow. The UV-Vis spectrum of
the faint yellow alkaline pyrogallol solution was recorded. The flask was then
reconnected to the Schlenck line through a three-way stopcock. In a glovebox
[Bu4N][NO2] (10.0 mg, 34.7 μmol) and 1 (32.0 mg, 69.3 μmol) were dissolved in 10.0
mL of THF in a 25 mL Schlenk flask. Once the reaction was completed the Schlenck
flask connected to the pyrogallol flask with Tygon tubing through the 3-way stopcock.
The system was left under static vacuum after evacuating the pyrogallol containg flask
and tubing connecting the two flasks. The headspace was transferred by opening the
stopcocks on both Schlenck flasks. The light yellow solution rapidly darkened. After 30
mins the stopcocks were resealed and the UV-Vis of the darkened pyrogallol solution
was recorded.
3.4. Results Discussion
Preparation of Mo(O)2(LBr)(MeOH) (1) and Mo2O3(LBr)2(THF)2·2THF (2). Complex
Mo(O)2(LBr)(MeOH) (1) (Figure 3.2, X = Br) was prepared following the reported
procedure and isolated as an orange brown powder in 95% yield (99% reported).9 The IR
82
(νMo=O = 906, 940 cm–1) (Figure 3.3) and diamagnetic 1H NMR in DMSO-d6 spectral
features (Figure 3.4) are in good agreement with the published values.9, 24 The UV-Vis
absorption spectra of 1 was not previously reported and is presented in THF (λmax = 252
nm) (Figure 3.5). The 1H NMR of 1 is also presented in Acetone-d6 (Figure 3.6a).
Figure 3.3. KBr IR of Mo(O)2(LBr)(MeOH) (1).
Figure 3.4. 1H NMR Mo(O)2(LBr)(MeOH) (1) in DMSO-d6 (400 MHz).
83
Figure 3.5. Room temperature UV-vis of Mo(O)2(LBr)(MeOH) (1) in THF.
Initial attempts to prepare a MoIVO or MoV2O3 species from 1 utilizing a variety
of phosphines, including triphenyl-, trimethyl-, and trioctyl phosphine, were hindered by
the inability to remove the phosphine oxides generated in the reaction, due to similar
solubility of the phosphine oxides as the molybdenum containing product. The phosphine
oxide free product was obtained by stirring a solution of 1 in THF with polymer
supported triphenylphosphine. Following the reaction, the polymer bound phosphine
oxide is removed from solution by filtration. Vapor diffusion of pentane into the dark
brown THF solution over two days resulted in the formation of a blue-black diffraction-
quality crystals identified as Mo2O3(LBr)2(THF)2·2THF (2) through X-ray structure
determination (Figure 3.7).
84
Figure 3.6. Room temperature 1H NMR (in acetone-d6, 400MHz) spectra of a) of Mo(O)2(LBr)(MeOH) (1) and b) Mo2O3(LBr)2(THF)2·2THF (2).
The crystal structure of 2 contains bridging and terminal molybdenum oxygen
bonds and molybdenum oxygen angles that are in good agreement with structurally
similar Mo2VO3 dimers22, 25-26. In addition to the two THF molecules bound to the N-H of
the ligand in the solid state, two THF molecules are bound trans to the terminal
molybdenum oxygen bonds. The Mo-O(THF) bond lengths of 2 are similar in length to
the long Mo-O(THF) bond lengths of Mo2O3(dtc)2I2(THF)2 (dtc = S2CNEt2) reported by
Baird and co-workers.25 Like Mo2O3(dtc)2I2(THF)2, when 2 is dissolved in poorly
coordinating, such as acetone-d6, only unbound THF is observed in the 1H NMR (δ 3.63
85
and 1.79), suggesting that the THF is dissociated from 2. A diamagnetic 1H NMR spectra
is obtained for 2 in acetone-d6 (Figure 3.6b) indicative of antiferromagnetic coupling of
the MoV centers and appears as one species is in solution. However, when
Mo2O3(LBr)2(THF)2·2THF (2) is dissolved in THF-d8 the peaks in the 1H NMR are split
into two sets of signals with ~7:3 intensity (Figure 3.8) with unbound THF signals
located at δ 3.62 and δ 1.79. This could be attributed to disproportionation of
Mo2O3(LBr)2(THF)2·2THF (2) into MoIV(O) and MoVI(O)2 species (equation 4) that does
not occur in acetone but is possible in THF. Mo(O)2(LBr)(MeOH) (1) 1H NMR was
recorded in THF-d8 in attempts to identify peaks present in the NMR of
Mo2O3(LBr)2(THF)2·2THF (2) to a dioxo species in solution. While some of the signals
split signals of Mo2O3(LBr)2(THF)2·2THF (2) match the signals present in 1 in THF-d8,
the presence of MeOH in 1 complicates the comparison. Diffusion ordered spectroscopy
(DOSY) NMR experiments will be undertaken to approximate the molecular weight of
the species or specie in solution. The split signals could also be caused by dynamic
processes in THF. Separate signals could arise from ligand exchange of THF, resulting in
bound and unbound THF species. Exchange of bridging and terminal oxos through a low
energy intramolecular process, resulting in syn and anti complexes in equilibrium has
also been reported for MoV2O3 27. Future work will include variable temperature NMR
experiments to see if dynamic processes are occurring for 2 in THF.
The UV-Vis of 2 in THF (λmax = 252 nm) is reminiscent of previously reported
Mo2VO3 Schiff base dimers with a strong visible band near 450 nm (Figure 3.9). KBr IR
of 2 shows a single strong absorption at 970 cm-1 assigned to ν(Mo=O) (Figure 3.10).
86
Complex 2 is readily soluble in THF, acetone, dimethylformamide, has poorer solubility
in dichloromethane, toluene, and benzene, and is insoluble in pentane.
Figure 3.7. Thermal ellipsoid plot of Mo2O3(LBr)2(THF)2·2THF (2) projected at the 50% probability level. Selected bond distances (Å): Mo(1)–O(1) 1.8722(2), Mo(1)–O(2) 1.6724(18), Mo(1)–O(3) 2.0092(18), Mo(1)–O(4) 2.4262(18), Mo(1)–N(1) 2.178(2), Mo(1)–S(1) 2.4025(7), N(3)–O(5) 2.8961. Selected bond angles (°): O(2)–Mo(1)–O(1) 102.66(7), O(2)–Mo(1)–O(3) 101.58(9), O(2)–Mo(1)–S(1) 101.75(7), O(2)–Mo(1)–N(1) 96.27(9), O(2)–Mo(1)–O(4) 173.78(8). H-atoms except H(3) (H attached to N(3)) have been omitted for clarity.
Figure 3.8. Room temperature 1H NMR (in THF-d8, 400MHz) spectra of a) of Mo(O)2(LBr)(MeOH) (1) and b) Mo2O3(LBr)2(THF)2·2THF (2).
a
b
87
Figure 3.9. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) in THF.
Figure 3.10. KBr IR of Mo2O3(LBr)2(THF)2·2THF (2).
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Mo2O3(LBr)2(THF)2·2THF (2) with O-Atom Donors. The ability of 2 to take part in
oxygen atom transfer was investigated using several commonly used O-atom donors.
Dissolution of Mo2O3(LBr)2(THF)2·2THF (2) in DMSO-d6 resulted in the formation of an
orange solution whose 1H NMR (Figures 3.11 and 3.12) matches authentic
Mo(O)2(LBr)(MeOH) (1) without MeOH, where the vacant coordination site is occupied
by THF resulting in of Mo(O)2(LBr)(THF) (3). Di-oxo formation is further evident from
the UV-Vis spectra of 2 in THF with added DMSO (Figure 3.13), which results in the
loss of the 454 nm (εM =12200) absorbance band, and the increase and shift of the 316
nm (εM = 13300) absorbance band to 320 nm (εM = 18000), that matches the spectra of
authentic 1 in THF (85% yield by UV-Vis). Reaction of 2 with trimethylamine n-oxide
(TMAO) in THF resulted in the rapid formation of a yellow/orange solution and
formation of the molybdenum dioxo product as demonstrated in the UV-Vis in THF (97
% yield by UV-Vis) and 1H NMR in acetone-d6.
89
Figure 3.11. Overlay of 1H NMR Mo(O)2(LBr)(MeOH) (1) in DMSO-d6 (bottom) and 1H NMR Mo2O3(LBr)2(THF)2·2THF (2) in DMSO-d6 (top) (400 MHz) (5-0 ppm).
Figure 3,12. Overlay of 1H NMR Mo(O)2(LBr)(MeOH) (1) in DMSO-d6 (bottom) and 1H NMR Mo2O3(LBr)2(THF)2·2THF (2) in DMSO-d6 (top) (400 MHz) (10-5 ppm).
90
Figure 3.13 Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) before (blue trace) and after the addition of 100 equivalents DMSO (orange trace) in THF.
The possible role of 2 in the catalytic OAT chemistry reported by Duhme-Klair9
and co-workers was investigated. Replacing the one equivalent of Mo(O)2(LBr)(MeOH)
(1) ([cat] = 5mM) with 0.5 equivalents of Mo2O3(LBr)2(THF)2·2THF (2) ([cat]=2.5mM)
under the catalytic conditions reported for the OAT reaction between triphenylphosphine
and DMSO yielded a similar pseudo-first order rate constant (kobs) of 18s- that is similar
to the reported kobs of 21s-. Dissolution of 0.5 equivalents of 2 in DMSO results in the
formation of 1 equivalent of a dioxo species that then follows the previously reported
catalytic cycle. These findings suggest that under the catalytic conditions reported that
the dinuclear 2 directly participates in the OAT reaction (Scheme 3.2), or an equilibrium
with a MoIVO and MoVIO2 species and 2 exists allowing for reactivity with the MoIV
mono-oxo species (Scheme 3.1). Regardless of the route, 2 has been shown to participate
in the catalytic cycle, and further demonstrates MoV2O3 ability to take part in useful OAT
reactions.
91
The OAT substrate scope for Mo2O3(LBr)2(THF)2·2THF (2) was further expanded
to include biologically and environmentally relevant nitrate (NO3−). Treatment of
Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] in THF was accompanied by the
gradual dark brown to orange color change reminiscent of the reaction of 2 with DMSO.
The UV-Vis of the of the reaction mixture (Figure 3.14) features the loss of the 450 nm
absorption band and contains the absorption bands at 320 and 414 nm found in authentic
Mo(O)2(LBr)(MeOH) (1) and Mo(O)2(LBr)(THF) (3). 1H NMR of the reaction product in
acetone-d6 (Figure 3.15b) further confirms the formation of 3 from nitrate reduction with
2. KBr IR of the reaction mixture (Figure 3.16) contain two stretches at 938 and 900 cm−
attributed molybdenum-oxo stretches (νMo=O (1) = 906, 940 cm− ), however the spectrum
is dominated by tetrabutylammonium features (~1500 cm−). Attempts to remove
tetrabutylammonium products from the nitrate reduction reaction mixture were
unsuccessful due to similar solubility of the di-oxo molybdenum product.
Figure 3.14. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) with [Bu4N][NO3] (1 equivalent) in THF.
92
Figure 3.15. Overlay of a) Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] (0.5 equiv.) b) Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] (1 equiv.) c) Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO2] (1 equiv.) d) Mo(O)2(LBr)(MeOH) (1) in acetone-d6 (400 MHz).
Figure 3.16. KBr IR of reaction mixture of Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] (1 equivalent).
a
b
c
d
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Monitoring the reaction of Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] in
THF by UV-Vis demonstrates the concentration dependence of the reaction. At UV-Vis
concentrations (0.15 mM) the loss of the 454 nm absorption band is accompanied with
the growth of the 320 nm absorption band, analogous to the reaction of 2 and DMSO, and
the emergence of new absorption features at 365 and 400 nm (Figure 3.17). Monitoring
the reaction of 2 and [TBA][NO3] at the synthetic scale concentration (1.6 mM) (Figure
3.18) by UV-Vis over the same time results in the loss of the 454 nm absorption band
accompanied with the growth of the 320 and 414 nm absorption bands observed in 1 and
3.
Figure 3.17. Mo2O3(LBr)2(THF)2·2THF (2) (0.15 mM) and [Bu4N][NO3] (0.15 mM) followed by UV-Vis spectroscopy at room temperature in THF for 2 hours. The spectral changes correspond to the conversion of Mo2O3(LBr)2(THF)2·2THF (2) to Mo(O)2(LBr)(THF) (3).
94
Figure 3.18. Mo2O3(LBr)2(THF)2·2THF (2) (1.6 mM) and [Bu4N][NO3] (1.6 mM) followed by UV-Vis spectroscopy at room temperature in THF for 2 hours. The spectral changes correspond to the conversion of Mo2O3(LBr)2(THF)2·2THF (2) to Mo(O)2(LBr)(THF) (3).
The fate of the nitrite (NO2−) generated from the OAT from nitrate (NO3−) by
Mo2O3(LBr)2(THF)2·2THF (2) was examined. After extracting the dried reaction mixture
with ultrapure water, Griess reagent test,28 the most commonly used method of detection
of nitrite, was then used to detect the nitrite in the reaction products. Treatment of the
water-soluble material with the Griess reagent resulted in the formation of the azo dye
with an absorbance band at 548 nm (Figure 3.19) indicative of the presence of nitrite.
However, only a trace amount (<1% yield) of nitrite was obtained based off the prepared
nitrite calibration curve.
95
Figure 3.19. Room temperature UV-vis in H2O for positive Griess reagent test for nitrite generated from Mo2O3(LBr)2(THF)2·2THF (2) (1 equiv.) [Bu4N][NO3] (1 equiv.) Prepared nitrite calibration curve (inset).
15N NMR studies were carried out to further investigate the fate of nitrite. The
consumption of nitrate with generation of nitrite can be readily identified by the 15N
NMR chemical shifts for [Bu4N][15NO3] at 6.46 ppm and [Bu4N][15NO2] at 243.67 ppm
in acetone-d6. Addition of [Bu4N][15NO3] to 2 in acetone-d6 results in the complete loss
of the 6.46 ppm signal in 15NMR without the formation of the 243.67 ppm signal for
nitrite (Figure 3.20). In previous work by Holm11, 21, clean nitrate reduction via MoV2O3
complexes required nitrite scavenging sulfamic acid to prevent further reactivity of the
generated nitrite with the di-oxo molybdenum products. The loss of the nitrite signal in
the 15N NMR and low concentrations of nitrite as determined by the Griess reagent test
suggests that Mo(O)2(LBr)(THF) (3) generated through OAT with
Mo2O3(LBr)2(THF)2·2THF (2) and [Bu4N][NO3] can further react with nitrite. This was
confirmed by reacting two equivalents of Mo(O)2(LBr)(MeOH) (1) with [Bu4N][15NO2] in
96
acetone-d6. The rapid reaction was accompanied by precipitation of yellow solid with the
loss of the 243.67 ppm signal in 15N NMR (Figure 3.21) and the formation of a broad
shoulder in at 360 nm in the UV-Vis spectrum (Figure 3.22) and new peaks in the 1H
NMR (Figure 3.23).
Figure 3.20. Room temperature 15N NMR (in acetone-d6) spectra of a) [Bu4N][15NO3] showing nitrate signal at 6.46 ppm, b) reaction of Mo2O3(LBr)2(THF)2·2THF (2) and [Bu4N][15NO3].
97
Figure 3.21. Room temperature 15N NMR (in acetone-d6) spectra of a) Bu4N[15NO2] showing nitrate signal at 243.67 ppm, b) reaction of Mo(O)2(LBr)(MeOH) (1) and [Bu4N][15NO2 ] (2 equivalents).
Figure 3.22. Room temperature UV-vis of Mo(O)2(LBr)(MeOH) (1) (1 equivalent) with [Bu4N][NO2] (2 equivalent) in THF.
243.67 a
b
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Figure 3.23. Overlay of a) Mo(O)2(LBr)(MeOH) (1) b) Mo(O)2(LBr)(MeOH) (1) (1 equivalent) with [Bu4N][NO2] (2 equivalents) in acetone-d6 (400 MHz).
When the headspace of the reaction mixture of Mo(O)2(LBr)(MeOH) (1) with two
equivalents of [Bu4N][NO2] in THF was analyzed by IR spectroscopy, the characteristic
signals29 reported for nitrous oxide (N2O) were observed at 2236 and 2212 cm–1 (Figure
3.24).
a
b
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Figure 3.24. IR spectra (gas cell) of headspace from the Mo(O)2(LBr)(MeOH) (1) (1 equivalent) with [Bu4N][NO2] (2 equivalents) (orange solid) and that of authentic N2O (blue dotted).
Transferring the headspace of the reaction to an alkaline solution of pyrogallol30, a
commonly used oxygen scavenger, resulted in darkening of the solution, and the
appearance of an absorbance band at 409 nm in the UV-Vis spectrum (Figure 3.25)
indicating that O2 is generated in addition to N2O. The absorption band at 365 nm is
attributed to decomposition of Mo(O)2(LBr)(THF) (3) with nitrite generated from the
reaction of Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] (Figure 3.17). Previous
work by our group has demonstrated the ability of Sc(OTf)3 to act as a nitrite scavenger,
resulting in the formation of nitrous oxide and O2 (Section 2.4). Monitoring the reaction
of Mo2O3(LBr)2(THF)2·2THF (2) (0.15 mM) with [Bu4N][NO3] (10 equivalents) and
Sc(OTf) (2 equivalents) in THF by UV-Vis demonstrates the ability cleanly convert 2 to
3 when the NO2− is scavenged by the Lewis acid additive (Figure 3.26).
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Figure 3.25. UV-vis of pyrogallol in 50% KOH (aq) before (blue trace) and after head space transfer (orange trace) from the reaction of Mo(O)2(LBr)(MeOH) (1) (1 equivalent) with [Bu4N][NO2] (2 equivalents).
Figure 3.26. Mo2O3(LBr)2(THF)2·2THF (2) (0.15 mM), Sc(OTf)3 (0.15 mM), and [Bu4N][NO3] (1.5 mM) followed by UV-Vis spectroscopy at room temperature in THF for 2 hours. The spectral changes correspond to the conversion of Mo2O3(LBr)2(THF)2·2THF (2) to Mo(O)2(LBr)(THF) (3).
101
The ability to generate Mo(O)2(LBr)(THF) (3) from OAT from nitrite to
Mo2O3(LBr)2(THF)2·2THF (2) was also examined. Addition of [Bu4N][NO2] to 2
generated an orange solution in shorter reaction times (~ 5 mins) than the reaction of 2
and [Bu4N][NO3] under analogous reaction conditions. The UV-Vis spectrum of the
reaction mixture in THF (Figure 3.27) is remarkably similar to the UV-Vis spectrum of
Mo(O)2(LBr)(MeOH) (1) with two equivalents of [Bu4N][NO2] (Figure 3.22), with both
exhibiting the loss of the 454 nm absorption band of 2 with the appearance of a broad
shoulder around 360-370 nm. The 1H NMR of the reaction of 2 with [Bu4N][NO2] does
not show clean formation of 3 (Figure 3.15), but is reminiscent of the reaction of 1 with
[Bu4N][NO2] (Figure 3.28). Generation of 3 from NO2− followed by decomposition
seems plausible, due to the observed decomposition of 1 with nitrite.
Figure 3.27. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) with [Bu4N][NO2] (1 equivalent) in THF.
102
Figure 3.28. Overlay of a) Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO2] (1 equiv.) and b) Mo(O)2(LBr)(MeOH) (1) (1 equivalent) with [Bu4N][NO2] (2 equivalents) in acetone-d6 (400 MHz).
To further investigate nitrite reactivity, 2 was reacted with 0.5 equivalents of
[Bu4N][NO3]. The formation of Mo(O)2(LBr)(THF) (3) is shown in the UV-Vis spectrum
of the reaction product in THF (Figure 3.29) and matches the UV-Vis of authentic
Mo(O)2(LBr)(MeOH) (1), and Mo(O)2(LBr)(THF) (3) generated by OAT from DMSO or
trimethylamine n-oxide and 2 (see above). 1H NMR of 2 and 0.5 equivalents of
[Bu4N][NO3] (Figures 3.15 and 3.30) further indicates the consumptions of 2 with the
formation of 4 equivalents of dioxo species. The formation of the dioxo species 3 from 2
and 0.5 equivalents of [Bu4N][NO3], suggests that 2 reacts with nitrate forming two
equivalents of 3 and the nitrite formed is then consumed with the remaining 2 in solution
forming an additional two equivalents 3 and a nitroxyl (NO−) product (Scheme 3.3). It is
possible that nitroxyl (NO−) generated from the nitrite reduction could further react to
form N2O through formation of HNO with trace water or H+ followed by rapid
103
dehydrative dimerization31-32. Formation of the proposed nitroxyl product from the
reaction of 2 with 0.5 equivalents of [Bu4N][NO3] is supported by the appearance of N2O
in the headspace of the reaction mixture as identified by IR spectroscopy (Figure 3.31).
Figure 3.29. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) with [Bu4N][NO3] (0.5 equivalent) in THF.
Figure 3.30. Mo2O3(LBr)2(THF)2·2THF (2) with [Bu4N][NO3] (0.5 equivalents) in acetone-d6 (400 MHz) demonstrating the ~4:1 ratio of Mo(O)2(LBr)(sol) (3) to tetrabutylammonium. −CH3 of 3 at 1.22 ppm. −CH3 of tetrabutylammonium at 0.99 ppm and −CH2−CH3 of tetrabutylammonium at 1.44 ppm.
104
Figure 3.31. IR spectra (gas cell) of headspace from the Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) with [Bu4N][NO3] (0.5 equivalents) (orange solid) and that of authentic N2O (blue dotted).
Scheme 3.3. Proposed Mo2O3(LBr)2(THF)2·2THF (2) reactivity with 1.0 and 0.5 equivalents NO3−.
3.5. Conclusion
Nature utilizes high valent MoIV/VI metal centers in mononuclear oxotransferase
enzymes that are capable of a wide variety of oxygen atom transfer reactions. While
105
molybdenum(V) oxo formation is precluded in biological systems, it is pervasive in
oxotransferase model complexes, and is widely viewed as undesirable when attempting to
model the enzyme active sites. Strategies to prevent dimer formation have been
developed and are widely utilized. While MoV2O3 formation can complicate reactivity,
it’s formation does not necessarily prevent stoichiometric or catalytic OAT product
formation. We have demonstrated that Mo2O3(LBr)2(THF)2·2THF (2) can reduce nitrate,
a pervasive ground water contaminant, to gaseous products, in addition to being involved
in the catalytic OAT activity previously reported by Duhme-Klair9.
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Chimica Acta 1995, 237 (1), 117-122.
24. ussein, M. A.; Guan, T. S.; Haque, R. A.; Khadeer Ahamed, M. B.; Abdul Majid,
A. M. S., Mononuclear dioxomolybdenum(VI) thiosemicarbazonato complexes:
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27. Thompson, R. L.; Lee, S.; Geib, S. J.; Cooper, N. J., Intramolecular
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111
Chapter 4: Lewis Acid Assisted Perchlorate Reduction
with Dinuclear Molybdenum(V)(µ-Oxo) Complex
112
4.1. Abstract
Perchlorate is a pervasive groundwater contaminant. The long term environmental
impact and health effects from perchlorate contamination is still largely unknown.
Remediation is challenging due to the high solubility of perchlorate salts in water and
their relative kinetic inertness to reduction. Nature utilizes the high valent molybdenum
oxotransferase enzyme perchlorate reductase (PcrAB) to reduce perchlorate (ClO4− ) and
chlorate (ClO3−) to chlorite (ClO2−). While structural perchlorate reductase active site
models exist, they are not capable of the difficult reduction of perchlorate. Synthetic
systems capable of perchlorate reduction to chloride have been reported but are rare.
Presented is the reduction of perchlorate by a MoV2O3 thiosemicarbazone complex.
Addition of the Lewis acid Sc(OTf)3 dramatically enhances the rate of perchlorate
reduction to the proposed chloride product.
113
4.2. Introduction.
Perchlorate (ClO4−) salts are used in a wide variety of industrial and agricultural
applications including manufacture of industrial lubes, dyes, rubbers, fertilizers, paints, in
electroplating processes, and battery manufacturing. Over 90% of all perchlorates salts
are manufactured as the strong oxidant ammonium perchlorate. Ammonium perchlorate
is widely used in solid rocket fuels and in the production of pyrotechnics and munitions
owing to its high oxygen content.1-2 Perchlorate salts are a pervasive ground water
contaminant due to their high solubility in water and organic solvents. Even though
perchlorates are strong oxidants and can be highly reactive solids, when dissolved
perchlorate salts become nonreactive and stable due to high kinetic barriers.1, 3 The extent
of perchlorate contamination of groundwater was not realized prior to 1997. Only after
the development of better techniques for perchlorate detection in groundwater has the
extent of perchlorate contamination been established.3 While long-term health effects of
consumption of perchlorate contaminated water are still largely unknown, it is known
that perchlorate competitively inhibits the uptake of iodide by the thyroid gland, altering
the production of hormones in the body, and is a potential source of hypothyroidism.4-5
Chlorate and chlorite formed from the reduction of perchlorate can lead to hemolytic
anemia in mammals, and have been shown to be taken up by plants grown in
contaminated soils.4-5 Remediation of perchlorate is needed and remains a challenge.
Typical water treatments, such as carbon adsorption, is complicated by the low tendency
of perchlorate to adsorb to surfaces and its kinetic inertness to reduction.2-3
Like nitrate (NO3−), perchlorate is utilized by some bacteria for metabolic
processes. Perchlorate reductase (PcrAB) is a member of the dimethylsulfoxide reductase
114
(DMSOR) family of enzymes and uses perchlorate and chlorate as the terminal electron
acceptors during anaerobic respiration.6-7 The active site of perchlorate reductase from A.
suilum is closely related to the active site of the membrane-bound respiratory nitrate
reductase NarG found in E. coli, with both featuring two pyranopterin cofactors and
aspartate (Asp) residue bound to a MoIV/VI metal center (Scheme 4.1). Perchlorate
reductase (PcrAB) reduces perchlorate (ClO4− ) and chlorate (ClO3−) to chlorite
(ClO2−) through oxygen atom transfer (OAT) to the MoIV center to form a MoVI-oxo
species that releases water on the addition of two external electrons and protons.6, 8
Perchlorate reducing bacteria, such as A. oryzae, often contain chlorite dismutase (Cld), a
Fe heme containing enzyme, that further converts chlorite (ClO2−) to molecular oxygen
and chloride (Cl−) for the overall reduction of chlorine shown in Scheme 4.2.9-10
Scheme 4.1. Perchlorate reduction by A. suilum PcrAB (adapted from Ref. 6).
115
Scheme 4.2. Reduction of perchlorate (ClO4−) to chloride (Cl−) and molecular oxygen by perchlorate reductase (PcrAB) and chlorite dismutase (Cld).
New remediation technologies have taken advantage of the enzymes utilized by
nature. Perchlorate reduction to chloride and molecular oxygen utilizing free and lipid
encapsulated perchlorate reductase and chlorite dismutase isolated from A. oryzae has
been achieved, and highlights the potential of biocatalysts in perchlorate remediation.9, 11
In addition to biocatalysis, the reduction of perchlorate to chloride through hypochlorite
(ClO−) has been achieved using transition metal complexes. Catalytic reduction of
perchlorate to chloride under mild conditions utilizing organic sulfides with the air and
water stable ReV(O)(hoz)2Cl or [ReV(O)(hoz)2(OH2)]OTf (hoz = [2-(2’-hydroxyphenyl)-
2-oxazoline], OTf= trifluoromethanesulfonate) was reported by Abu-Omar in 200012 .
Recently Mosch-Zanetti reported similar reduction of perchlorate to chloride with Rev-
oxo complexes with tetradentate iminophenolate ligands (Scheme 4.3).13 In addition to
nitrate reduction, catalytic perchlorate reduction to chloride and oxide products with a
FeII azafulvene-amine complex was recently reported by Fout and co-workers.14
Interestingly, a molybdenum bis(dithiolene) structural and functional DMSOR model
complex reported by Holm15 that was capable of nitrate reduction was not capable of
perchlorate reduction.
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Scheme 4.3. Catalytic reduction of perchlorate (ClO4−) to chloride (Cl−) by ReV-oxo complexes (adapted from Ref. 13).
Inspired by Fout and co-workers’ ability to reduce perchlorate with a complex
that also demonstrated nitrate reduction capabilities, we investigated the ability to reduce
perchlorate using biomimetic Mo2O3(LBr)2(THF)2·2THF (2). The effect of the Lewis acid
additive Sc(OTf)3 on the reactivity was also investigated. Preliminary results indicate the
successful reduction of perchlorate to chloride with 2. The addition of Sc(OTf)3 results in
a dramatic decrease in the reaction time needed to convert 2 to the proposed di-oxo
product Mo(O)2(LBr)(sol) (3).
4.3. Experimental Section
General Considerations. Unless otherwise specified all reactions and manipulations
were carried out under an inert nitrogen atmosphere using a MBraun Labmaster SP
gloveblox or under argon using standard Schlenk line techniques. 3Å molecular sieves
were dried under vacuum for 24 hours at 250 °C prior to use. THF was degassed then
dried using an MBraun solvent purification systems under an Ar atmosphere, and stored
over activated 3 Å molecular sieves. Tetrabutylammonium perchlorate was purchased
from Sigma-Aldrich and used as received. Scandium triflate was purchased from Strem
and used as received. Acetone-d6 was purchased from Cambridge Isotopes, degassed via
freeze-pump-thaw cycle (5x), stored over 3 Å molecular sieves for 6 hours, then filtered
117
from sieves before use. Mo(O)2(LBr)(MeOH) (1) was prepared following the published
procedure.16
Physical Methods. All samples for spectroscopic analysis were prepared inside a
nitrogen glovebox unless otherwise noted. Room temperature UV-Vis spectra were
recorded on a Varian Cary 50 Bio spectrometer with using screw cap UV-Vis cuvettes,
Schlenk cuvette, or a 2 mm Hellma All-Quartz Immersion probe fitted in a 24/40 Schlenk
tube with a 14/20 female joint sealed with a rubber septum. 1H and 35Cl NMR were
recorded with a Bruker 400MHz Avance III ultrashield spectrometer. 1H NMR was
referenced to acetone-d6 residual solvent signal (δ 2.05).
CATUION: Perchlorate salts pose a significant risk when used. The risk of explosion
was reduced by performing the reactions on the smallest scale possible. A PTFE coated
spatula was used to weigh out [Bu4N][ClO4] and care was taken to not crush or grind
the material.
Synthesis of Mo2O3(LBr)2(THF)2·2THF (2). Polymer supported triphenylphosphine
(125 mg, ~0.375 mmol) was added to a stirring solution of Mo(O)2(LBr)(MeOH) (1)
(115.0 mg, 0.250 mmol) in 10.0 mL THF. The resulting dark brown solution was stirred
for 6 h then filtered through a pad of celite and concentrated to approximately 5 mL.
Recrystallization by vapor diffusion of pentane into the THF solution yielded 115.9 mg
(82.2 %) of 2 as blue-black crystals. UV-Vis (THF): λmax (εM) 236 nm (22200), 252 nm
(19700), 316 nm (13300), 454 nm (12200). IR (KBr): 970 cm-. 1H NMR (Acetone-d6, 400
MHz): δ 9.00 (s, 1 H), 7.93 (dd, 1 H, J = 5.9 Hz, 2.5 Hz) 7.83 (d, 1H, J= 2.5 Hz) 7.64
(dd, 1 H, J = 8.9 Hz, 2.4 Hz), 7.09 (br, 2H), 3.64 (m, 8H), 3.53 (m, 2H), 1.80 (m, 8H),
118
1.27 (q, 3H). Anal. Calcd for C36H52Br2Mo2N6O9S2: C, 38.10; H, 4.64; N, 7.45. Found:
C, 38.21; H, 4.65; N, 7.50.
Reaction of Mo2O3(LBr)2(THF)2·2THF (2) and [Bu4N][ClO4] (1:10). [Bu4N][ClO4]
(15.1 mg, 44.3 μmol) was added to a stirring solution of Mo2O3(LBr)2(THF)2·2THF (2)
(5.0 mg, 4.43 μmol) The dark brown solution turned orange/brown after 72 hours of
stirring. Following an additional day of stirring, the reaction was dried to an orange
residue, dissolved in acetone-d6 and 1H and 35Cl NMR were recorded.
Reaction of Mo2O3(LBr)2(THF)2·2THF (2), Sc(OTf)3 and [Bu4N][ClO4] (1:1:1). Stock
solutions of Sc(OTf)3 and [Bu4N][ClO4] were prepared in THF. Sc(OTf)3 (4.43 μmol) and
[Bu4N][ClO4] (4.43 μmol) were added to a stirring solution of 2 (5.0 mg, 4.43 μmol) in 5
mL THF. The solution turned orange with 5 minutes of stirring. After an additional 30
mins of stirring the UV-Vis in THF was recorded and the reaction was dried to an orange
residue. The residue was dissolved in acetone-d6 and 1H and 31Cl NMR were recorded.
Reaction of Mo2O3(LBr)2(THF)2·2THF (2), Sc(OTf)3 and [Bu4N][ClO4] (1:1:0.25).
Reaction run analogously to above reaction with of 2 (5.0 mg ,4.43 μmol), Sc(OTf)3 (4.43
μmol), and [Bu4N][ClO4] (1.11 μmol).
UV-Vis monitoring of the reaction of Mo2O3(LBr)2(THF)2·2THF (2), Sc(OTf)3 and
[Bu4N][ClO4] (1:1:10). After blanking spectrometer with 5.0 mL THF, 1.0 mL of a stock
solution of 2 (0.86 μmol) was injected and the spectrum of 2 was recorded. 1.5 mL of a
stock solution of [Bu4N][ClO4] (8.6 μmol) was diluted mixed with a 0.5 mL of a stock
solution of Sc(OTf)3 (0.86 μmol) in THF was injected into the solution of 2, after which
a series of spectra were measured for 5 minutes.
119
4.4. Results and Discussion
Addition of excess [Bu4N][ClO4] to Mo2O3(LBr)2(THF)2·2THF (2) in THF
resulted in the dark brown solution gradually turning orange brown over the course of
three days. UV-Vis of reaction aliquots at 12, 24 and 36 hours (data not shown) showed
little change from the starting spectrum of 2 in THF. After three days of stirring at
ambient temperature the solution was noticeably orange. UV-Vis of the reaction mixture
(Figure 4.1) was reminiscent of Mo(O)2(LBr)(sol) (3) generated by the reaction of 2 with
[Bu4N][NO3]. The absorption band at 454 nm was significantly diminished and shifted to
444 nm, suggesting that oxygen atom transfer (OAT) from perchlorate to 2 was
successful but likely not complete. After an additional day of stirring the reaction was
dried to an orange brown residue. 1H NMR of the residue in acetone-d6 (Figure 4.2b) is
similar to 1H NMR of authentic Mo(O)2(LBr)(MeOH) (1) and Mo(O)2(LBr)(sol) (3)
generated through OAT with NO3 and 2, further suggesting that perchlorate reduction
was successful. Attempts to detect Cl containing products through 35Cl NMR were
unsuccessful (Figure 4.3a). The only chlorine product detected in the reaction mixture by
35Cl NMR was the excess with only the excess [Bu4N][ClO4] located at 1000 ppm.
120
Figure 4.1. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) and [Bu4N][ClO4] (10 equivalents) in THF.
Figure 4.2. 1H NMR overlay of a) Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent), Sc(OTf)3 (1 equivalent) and [Bu4N][ClO4] (0.25 equivalents) b) Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) and [Bu4N][ClO4] (10 equivalents) c) Mo(O)2(LBr)(MeOH) (1) in acetone-d6.
c
b
a
121
Figure 4.3. 35Cl NMR overlay of a) Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent) and [Bu4N][ClO4] (10 equivalents) b) Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent), Sc(OTf)3 (1 equivalent) and [Bu4N][ClO4] (0.25 equivalents) in acetone-d6.
Encouraged by the preliminary perchlorate reduction results, and previous success
inducing OAT from nitrate with Lewis acid additives, the effect of Sc(OTf)3 on the
perchlorate reactivity was investigated. Addition of Sc(OTf)3 (1 equivalent) to a stirring
solution of 2 (1 equivalent) and [Bu4N][ClO4] (1 equivalent) resulted in the formation of
an orange solution in under 5 minutes. UV-Vis of the reaction mixture (Figure 4.4) shows
the loss of the absorption band at 454 nm and formation of the 414 nm absorption band
found in the UV-Vis spectrum of 1 and 3, suggesting that oxygen atom transfer (OAT)
from perchlorate to 2 to generate a dioxo species was successful. Monitoring the reaction
of 2 (1 equivalent) and [Bu4N][ClO4] (10 equivalent) with Sc(OTf)3 (1 equivalent) by
UV-vis (Figure 4.5) indicates that the UV-Vis spectrum in Figure 4.1, featuring the 444
nm absorption band, was from the incomplete reaction of perchlorate and 2. Sc(OTf)3
greatly reduces the reaction time for 2 and [Bu4N][ClO4] from days to minutes.
a
b
122
Figure 4.4. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent), Sc(OTf)3 (1 equivalent), and [Bu4N][ClO4] (1 equivalent) in THF.
Figure 4.5. Mo2O3(LBr)2(THF)2·2THF (2) (0.13 mM), Sc(OTf)3 (0.13 mM), and [Bu4N][ClO4] (1.3mM) followed by UV-Vis spectroscopy at room temperature in THF for 5 minutes. The spectral changes correspond to the conversion of Mo2O3(LBr)2(THF)2·2THF(2) to Mo(O)2(LBr)(sol) (3).
The ability to reduce [Bu4N][ClO4] to chloride was investigated through the
reaction of 2 (1 equivalent) and [Bu4N][ClO4] (0.25 equivalent). Without the addition of
123
Sc(OTf)3 no change in the UV-vis spectrum of the reaction was observed, even with
reaction times of up to 1.5 weeks. However, addition of Sc(OTf)3 (1 equivalent) to 2 (1
equivalent) and [Bu4N][ClO4] (0.25 equivalent) in THF results in the formation of an
orange solution in under 5 minutes. The UV-Vis of the reaction mixture (Figure 4.5)
matches the spectrum of the reaction of 2 (1 equivalent) with [Bu4N][ClO4] (1
equivalent) and Sc(OTf)3 (1 equivalent) (Figure 4.4). The 1H NMR of the dried reaction
mixture dissolved in acetone-d6 (Figure 4.2c) further indicates that 2 has been converted
to the di-oxo species 3. The complete loss of the perchlorate signal (1000 ppm) in the
35Cl NMR (Figure 4.3b) and the apparent consumption of Mo2O3(LBr)2(THF)2·2THF (2)
by UV-vis and 1H NMR, suggests that the reduced Cl products chlorate (ClO3−), chlorite
(ClO2−) and hypochlorite (ClO−) are further utilized for OAT to 2 generating the
proposed dioxo product Mo(O)2(LBr)(sol) (3). 35Cl NMR of authentic [Bu4N][Cl] has a
single broad feature located at ~ 2.0 ppm in acetone-d6 (data not shown). To date
detection of chloride by 35Cl NMR has been unsuccessful. In previous work by Abu-
Omar the chloride (Cl−) generated through perchlorate reduction was precipitated as
AgCl and isolated upon the addition of AgBF4 at the end of the reaction.12 Identification
and isolation of the proposed chloride (Cl−) product is ongoing, but future plans include
the addition of AgBF4 at the end of the reaction of 2 (1 equivalent), Sc(OTf)3 (1
equivalent) and [Bu4N][ClO4] (0.25 equivalent) in attempts to isolate AgCl.
124
Figure 4.5. Room temperature UV-vis of Mo2O3(LBr)2(THF)2·2THF (2) (1 equivalent), Sc(OTf)3 (1 equivalent), and [Bu4N][ClO4] (1/4 equivalent) in THF.
4.5. Conclusions.
Perchlorate is a pervasive contaminant in groundwater due to the high solubility
and stability in water. New remediation technologies are required to combat
contamination and lessen the potential health risks associated with the perchlorate
consumption. Taking inspiration from nature, where perchlorate reductase enzymes
utilize high valent molybdenum active sites to achieve the difficult reduction of
perchlorate, we have investigated the ability of Mo2O3(LBr)2(THF)2·2THF (2) to reduce
perchlorate. While perchlorate reduction with 2 is successful it is very slow. Addition of
the Lewis acid additive greatly accelerates the reduction of perchlorate. Conversion of 2
to the proposed di-oxo product Mo(O)2(LBr)(sol) (3) was achieved using 0.25 equivalents
of [Bu4N][ClO4] suggesting that perchlorate is reduced to chloride. This works
demonstrates the ability of Lewis acid additives to enhance reactivity associated with
difficult chemical transformations, such as perchlorate reduction.
125
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