Reaction of a Biomimetic Molybdenum Complex

with Carbon DioxideBy Michael Yanagisawa

Brown University ‘13

• Global levels of carbon dioxide (CO2) are rising, giving way to global climate change

• A goal worldwide: reduce CO2 levels› Less CO2 means less global warming

Ref. 1

• The challenge: CO2 is very stable› CO2 reduction gets rid of favorable linear

structure

› CO2 reduction is a thermodynamic uphill reaction

• Nature effectively reduces CO2

› Photosynthesis produces sugars from CO2

• Bio-inspired solutions seem promising

• The enzyme formate dehydrogenase (FDH) reversibly converts formate (HCO2

-) to CO2

› CO2 + H+ + 2e- → HCO2-

• Our goal: to mimic the active site of FDH to convert carbon dioxide into formate

› Formate can be used as a fuel source

• Recycling CO2 from a harmful product into a useful substrate could solve the CO2 problem

Ref. 2,3

• The active site of FDH consists of a molybdenum center with two pterin ligands

MoIV

O

HN

NH

HN

NH

O

S

SO

NH

HN NH

HN

O

S

S

SeH

Cys

+ formate

- CO2 and H+

MoVI

O

HN

NH

HN

NH

O

S

SO

NH

HN NH

HN

O

S

S

SeHCys

O

C O

H

Ref. 4

• Biomimetic complexes have been synthesized by other groups

• Characteristics› Mo metal center› Ditholene ligands

• For our complex, R = phenyl

MoIVS

S

S

SR

R

R

R

O

Ref. 5

• Jun Seo of our group reacted a tungsten analogue with carbon dioxide› W and Mo have very similar properties

• The reaction formed a tungsten dimer

WS

S

S

S

O

CO2 (2 atm)

MeCN, 90°C

WS

S

O

WS

S

O

S

S

S

Ref. 6

• Our compound is [MoO(pdt)2]2-

• pdt = phenyl dithiolene

• What is the product of a reaction with CO2?

Research Question

MoIVS

S

S

SPh

Ph

Ph

Ph

O

CO2 (2 atm)

heat?

• The dream: our complex reduces CO2 to formate like its inspiration, FDH

• The hope: CO2 binds onto the metal center, revealing an intermediate to forming formate

• Another possibility: a product analogous to Jun's product i.e. a Mo dimer

+

OH

O

P4S10

NiCl2

dioxaneNi

S

S

S

S

Ph

Ph

Ph

Ph

Mo(CO)3(MeCN)3 + Ni

S

S

S

S

Ph

Ph

Ph

Ph

DCM

MoS

S

S

S

Ph

Ph

Ph

Ph

OC CO

MoS

S

S

S

Ph

Ph

Ph

Ph

OC CO

MoIVS

S

S

S

Ph

Ph

Ph

Ph

O

Ref. 7

• 50 mL Schlenk flask filled with

• 20 mg [MoO(pdt)2]2- and

• 2 mL MeCN

• attached to a 100 mL CO2 bulb (2 atm)

• Reaction heated to 90°C and stirred 2 days

• After two days, we dried the product

• The crude product was rinsed with diethyl ether and collected (the “ether layer”)

• The remaining product (the “metal layer”) was then redried

Spectroscopy

To test the reaction, we used IR, UV-Vis, EPR, 1H and 13C NMR, GC/MS, and ESI

We also tried to crystallize the product for x-ray crystallography, but the product did not crystallize

We report IR, EPR, GC/MS, and ESI and their interesting interpretations

Characteristic Mo=O stretching visible Starting material νMoO = 886 cm-1

After reaction, stretching shifted left to product νMoO = 924 cm-1

For [MoVO(pdt)2]-, νMoO = 924 cm-1

Interpretation:› Our product has a Mo(V) center

Ref. 5b

No peaks present

Interpretation:› Our product has no unpaired electrons› IR shows evidence of Mo(V)› Conclusion: antiferromagnetic Mo(V) dimer

GC/MS of ether layer› Peaks at 51, 77, and 105› Matches up with diphenyl ethanedione

Interpretation:› In the reaction, some dithiolene ligand is

falling off the metal center and is part of the organic product

ESI of metal layer (still preliminary)› Peaks at 427 and 757

› Mass of [MoO(pdt)2]2- = 596.66

Interpretation:› The 757 peak again suggests some sort of

dimer. The 427 peak suggests a complementary metal center; 427 and 757 average to around the 596.

Conclusion

Our product is a Mo(V) antiferromagnetic dimer

MoIVS

S

S

SPh

Ph

Ph

Ph

O

CO2 (2 atm)

heat?

Conclusion

Predicted product (MW ~ 757)

Where R1 + R2 may be:› A CO2 molecule

› O and S bridging atoms› Something else?

MoS

SPh

Ph

O

MoS

S Ph

Ph

O

R2

R1

Looking Forward

We would like to crystallize the product and identify the final product

Monoatomic Mo and W complexes have been known to dimerize Example: Jun’s W compound

Use labelled CO2 to track the oxygen atoms; start to deduce a mechanism

References

1. Crowley, T. J.; Berner, R. A. Science 2001, 292, 870-872.

2. Ha, S.; Dunbar, Z.; Masel, R.I. J. Power Sources 2006, 158, 129-136.

3. From a paper’s press notes: http://www.usu.edu/science/htm/one-step-closer-usu-biochemists-convert-greenhouse-gas-to-fuel/

4. Boyington, J. C.; Gladyshev, V. N.; Khangulov, S. V.; Stadtman, T. C.; Sun, P. D. Science 1997, 275, 1305-1308.

5. (a) Lim B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263-273. (b) Lim B. S.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1920-1930.

6. Paper in press.

7. (a) Tate, D. P.; Knipple, W. R.; Augi, J. M. Inorg. Chem. 1962, 1, 433-434. (b) Schrauzer, G. N.; Mayweg, V. P. J. Am. Chem. Soc. 1965, 87, 1483-1489. (c) Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263-273.

Acknowledgements

• Camly Tran• Dr. Eunsuk Kim• Jun Seo• The Kim lab• Brown University

› Undergraduate Teaching and Research Award for funding (summer 2012)