7 th International Conference on Chemical Kinetics, MIT, 2011 A Laser Flash Photolysis Study of CO 2...
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7th International Conference on Chemical Kinetics, MIT, 2011
A Laser Flash Photolysis Study of CO2 Reduction: Kinetics Leading to the Design of a Renewable
Reducing Agent
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Outline of the Talk
• Computational and experimental study of photochemical reduction of CO2 by Et3N.
• Use of the lessons learned in the design of a renewable amine.
• Future directions: Is an all-organic, renewable, visible- light photoreductant for CO2 possible?
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Photochemical CO2 Reduction
h
H2O (l) + CO2 (g) 1/2 O2 (g) + HCO2H (l)
H° = +60.8 kcal/mol
< 470 nm
H2OPC–H• + HO–
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The Key Idea
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Photochemical CO2 Reduction
Matsuoka, S.; Kohzuki, T.; Pac, C.; Ishida, A.; Takamuku, S.; Kusaba, M.; Nakashima, N.; Yanagida, S., J. Phys. Chem. 1992, 96, 4437
PC =
h
HCO2H
PTP
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•–
Fujiwara, H.; Kitamura, T.; Wada, Y.; Yanagida, S.; Kamat, P. V. J. Phys. Chem. 1999, 103, 4874.
PTP•–
Photochemical CO2 Reduction
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Effect of Ionization on C–H Reactivity
Figures are H° in kcal/mol (exptl. + CBS–QB3)
H• lossH+ loss
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Computational Results
PCM model for CH3CN
These results from empiricallycorrected UB3LYP, calibrated against UMP2 and UCCSD
for smaller systems
Later results use UCAM-B3LYP
Hazardous system for common DFT functionals such as B3LYP, because of self-interaction error in radical ions and long-range exchange error in CT states.
J. Phys. Chem. A, 2007, 111, 3719
Self-Interaction Error in DFT:Bally, T.; Sastry, G. N. J. Phys. Chem. A, 1997, 101, 7923Braieda, B.; Hiberty, P. C.; Savin, A. J. Phys. Chem. A, 1998, 102, 7872Graefenstein, J.; Kraka, E.; Cremer, D. J. Chem. Phys. 2004, 120, 524
CAM-B3LYP:Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004, 393, 51.
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Reality Bites
[1] : 0.35
[1] : 2.0
0.3 M CO2, 0.25M amine in CH3CN
D2C
NCD2
CH3
CD2
CH3
H3C + CO2
hOPP-3
H2C
NCH2
CD3
CH2
CD3
D3C + CO2
hOPP-3
H–CO2– + D–CO2
–
H–CO2– + D–CO2
–
PTP
PTP
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Kanoufi, F.; Zu, Y.; Bard, A. J. J. Phys. Chem. B 2001, 105, 210.
Dimers of this radical detected inphotochemical CO2 reduction
A Radical New Mechanism
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XTransient stability, at best.
Radical cation would presumably be worse.
Blocking C–H Reactivity
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Proton transfer
CBS-QB3 Isodesmic Reactions
H-atom transfer
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XTransient stability, at best.
Radical cation would presumably be worse.
Stable to prolongedphotolysis; affords no CO2 reduction.
Blocking C–H Reactivity
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+ PTP
Generation of “PTP•–” with the New Amine
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440 nm
470 nm
285 nm
0 1 2 3 4 5 time / s
+ PTP
• Appearance quite different from that with Et3N• Amine radical cation should have no band from 400 – 500nm• Decay of “PTP•–” is much faster than with Et3N• Everything returns to baseline, whereas with Et3N it does not
Decay of “PTP•–” from the New Amine
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Ion pair(s)
Ion pair(s)
The Ion-Pair Hypothesis
Deprotonation blocks BET
“Long-lived” PTP •–
The dilemma: This radical seems tobe necessary for CO2 reduction, but:
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Spectra taken after 500 ps.10-4 M PTP, 1M NEt3
PTP•–
CO2•–
Picosecond Infrared Studies
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Picosecond Infrared Studies
12CO2•–
13CO2•–
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Spectra taken after 500 ps.10-4 M PTP, 1M NEt3
PTP•–
CO2•–
Picosecond Infrared Studies
Prompt CO2•– formed
by direct Et3N photo-ionization with 266 nm pump
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k1 k2
e–solv +
Picosecond Infrared Studies
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Nanosecond Infrared Studies
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Re-evaluation of the First Steps
•–•– –10 kcal/mol
[0] kcal/mol
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Re-evaluation of the First Steps
•–
•+ CO2
PTP + Et3N + CO2 PTP + Et3N•+ + CO2•–
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Formate Production as f (PTP, )
254 nm, no PTP
254 nm, sat. PTP
>290 nm, sat. PTP
>290 nm, no PTP
1 M Et3N in CH3CN
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What Have we Learned?
• Electron addition to CO2 is difficult, and probably doesn’t occur from PTP•–
except by “inner-sphere” carboxylation mechanism.
• BET to Et3N•+ can occur from both PTP•– and carboxylated PTP•– in ion pairs
• Deprotonation of Et3N•+ blocks BET and generates –amino radical
• –Amino radical seems to be necessary for CO2 reduction, but...
• –Amino radical is also responsible for several of the byproducts
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NHR
R
NHR
R
NHR
R
+ e– + e–
NR
R
+ H+ + e–N
RR
+ H
IP (amine)
~PA (amine)
H°trans
–IP (H)
–BDE (C–H)
ΔH°trans = 414.6 – IP(amine) – PA(amine) (in kcal/mol)
An Idea for the New Amine
. J. Am. Chem. Soc. 2008, 130, 3169
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Aliphatic amines
ArNH2
ArNMe2
NH3
Sweet spot
An Idea for the New Amine
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An Idea for the New Amine
Janovsky, I.; Knolle, W.; Naumov, S.; Williams, F. Chem. Eur. J. 2004, 10, 5524.
e– Beam
Freon
•+
+•
‡
••
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An Idea for the New Amine
Adamantane-like TS for H transfer H transfer blocks
BET hole
Bridgehead blocks –amino radical
formation
Replaces –H of –amino radical
Simple alkeneshould be easily
hydrogenated
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Synthesis and Testing
H
H
hPTP
~ 2x Et3N
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A Lot More Synthesis
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Nature Chem. 2011, 3, 301.
250–300 nm
PTP
PTP
How it Works in Practice
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c.f. Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O. J. Am. Chem. Soc. 2008, 130, 2023–2031.
> 400 nm
Re(Bipy)(CO)3
(EtO)3PRe(Bipy)(CO)3+
It Also Works with Visible Light
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One Long Term Plan...
N. Itoh, W. C. Xu, S. Hara, K. Sakaki, Catal. Today 2000, 56, 307
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Outline of the Talk
• Computational and experimental study of photochemical reduction of CO2 by Et3N.
• Use of the lessons learned in the design of a renewable amine.
• Future directions: Is an all-organic, renewable, visible- light photoreductant for CO2 possible?
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Computational Results
J. Phys. Chem. A, 2007, 111, 3719
< 390 nm
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Some Useful Information
Reichardt, R.; Vogt, R. A.; Crespo-Hernández, C. E. J. Chem. Phys. 2009, 224518.
Görner, H.; Döpp, D. J. Chem. Soc., Perkin Trans. 2, 2002, 120.
Predicted pH-dependent rotational profile about red C-C bond
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PErel
(kcal/mol)
Dihedral Angle
+
B3LYP/6-31+G(d,p) PE Profile
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Putting the Pieces Together
~73 kcal/mol~63 kcal/mol
[0] kcal/mol
Barrier ~4 kcal/mol
56 kcal/mol
43 kcal/mol
33 kcal/mol
Barrier 12kcal/mol
CAM-B3LYP/6-31+G(d,p)G° (298 K, 1 M standard state)
PCM model for CH3CN
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An Unexpected Outcome…
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Acknowledgments
Rob RichardsonEd Holland
Chris StanleyClaire Minton
Mike GeorgeSun Xue-ZhongJames Calladine
Charlotte Clark
The Leverhulme Trust Royal Society/Wolfson Foundation