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

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?

Photochemical CO2 Reduction

h

H2O (l) + CO2 (g) 1/2 O2 (g) + HCO2H (l)

H° = +60.8 kcal/mol

< 470 nm

H2OPC–H• + HO–

The Key Idea

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

•–

Fujiwara, H.; Kitamura, T.; Wada, Y.; Yanagida, S.; Kamat, P. V. J. Phys. Chem. 1999, 103, 4874.

PTP•–

Photochemical CO2 Reduction

Effect of Ionization on C–H Reactivity

Figures are H° in kcal/mol (exptl. + CBS–QB3)

H• lossH+ loss

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.

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

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

XTransient stability, at best.

Radical cation would presumably be worse.

Blocking C–H Reactivity

Proton transfer

CBS-QB3 Isodesmic Reactions

H-atom transfer

XTransient stability, at best.

Radical cation would presumably be worse.

Stable to prolongedphotolysis; affords no CO2 reduction.

Blocking C–H Reactivity

+ PTP

Generation of “PTP•–” with the New Amine

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

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:

Spectra taken after 500 ps.10-4 M PTP, 1M NEt3

PTP•–

CO2•–

Picosecond Infrared Studies

Picosecond Infrared Studies

12CO2•–

13CO2•–

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

k1 k2

e–solv +

Picosecond Infrared Studies

Nanosecond Infrared Studies

Re-evaluation of the First Steps

•–•– –10 kcal/mol

[0] kcal/mol

Re-evaluation of the First Steps

•–

•+ CO2

PTP + Et3N + CO2 PTP + Et3N•+ + CO2•–

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

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

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

Aliphatic amines

ArNH2

ArNMe2

NH3

Sweet spot

An Idea for the New Amine

An Idea for the New Amine

Janovsky, I.; Knolle, W.; Naumov, S.; Williams, F. Chem. Eur. J. 2004, 10, 5524.

e– Beam

Freon

•+

+•

‡

••

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

Synthesis and Testing

H

H

hPTP

~ 2x Et3N

A Lot More Synthesis

Nature Chem. 2011, 3, 301.

250–300 nm

PTP

PTP

How it Works in Practice

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

One Long Term Plan...

N. Itoh, W. C. Xu, S. Hara, K. Sakaki, Catal. Today 2000, 56, 307

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?

Computational Results

J. Phys. Chem. A, 2007, 111, 3719

< 390 nm

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

PErel

(kcal/mol)

Dihedral Angle

+

B3LYP/6-31+G(d,p) PE Profile

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

An Unexpected Outcome…

Acknowledgments

Rob RichardsonEd Holland

Chris StanleyClaire Minton

Mike GeorgeSun Xue-ZhongJames Calladine

Charlotte Clark

The Leverhulme Trust Royal Society/Wolfson Foundation