A Laser Flash Photolysis Study of CO Kinetics Leading to the Design of a Renewable Reducing Agent

2

Reduction:

7 th International Conference on Chemical Kinetics, MIT, 2011

Outline of the Talk • Computational and experimental study of photochemical reduction of CO 2 by Et 3 N.

• Use of the lessons learned in the design of a renewable amine.

• Future directions: Is an all-organic, renewable, visible light photoreductant for CO 2 possible?

h  < 470 nm Photochemical CO 2 Reduction H 2 O (

l

) + CO 2 (

g

) 1/2 O 2 (

g

) + HCO 2 H (

l

) 

H

° = +60.8 kcal/mol H 2 O PC –H • + HO –

The Key Idea

h  Photochemical CO 2 Reduction Matsuoka, S.; Kohzuki, T.; Pac, C.; Ishida, A.; Takamuku, S.; Kusaba, M.; Nakashima, N.; Yanagida, S.,

J. Phys. Chem.

1992

,

96

, 4437 PC = PTP HCO 2 H

PTP •– Photochemical CO 2 Reduction Fujiwara, H.; Kitamura, T.; Wada, Y.; Yanagida, S.; Kamat, P. V.

J. Phys. Chem.

1999

,

103

, 4874.

Effect of Ionization on  C–H Reactivity H • + Figures are 

H

° in kcal/mol (exptl. + CBS– QB3)

Computational Results 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.

PCM model for CH 3 CN These results from empirically corrected UB3LYP, calibrated against UMP2 and UCCSD for smaller systems

J. Phys. Chem. A

,

2007

,

111

, 3719 Later results use UCAM-B3LYP Self-Interaction Error in DFT: Bally, T.; Sastry, G. N.

J. Phys. Chem. A J. Phys. Chem. A

D.

J. Chem. Phys

. , ,

1997 1998 2004

, ,

101 , 102 120

, 7923 Braieda, B.; Hiberty, P. C.; Savin, A.

, 7872 Graefenstein, J.; Kraka, E.; Cremer, , 524 CAM-B3LYP: Yanai, T.; Tew, D. P.; Handy, N. C.

Chem. Phys. Lett.

2004

,

393

, 51.

Reality Bites D 3 C H 2 C C H 2 N CD 3 CH 2 CD 3 + CO 2 h  H –CO 2 – + D –CO 2 – [1] : 2.0

H 3 C D 2 C C D 2 N CH 3 CD 2 CH 3 0.3

M

CO 2

,

0.25

M

amine in CH 3 CN + CO 2 h  H –CO 2 – + D –CO 2 – [1] : 0.35

A Radical New Mechanism Kanoufi, F.; Zu, Y.; Bard, A. J.

J. Phys. Chem. B

2001

,

105

, 210.

Dimers of this radical detected in photochemical CO 2 reduction

Blocking  C–H Reactivity X Transient stability, at best.

Radical cation would presumably be worse.

CBS-QB3 Isodesmic Reactions Proton transfer H-atom transfer

Blocking  C–H Reactivity X Transient stability, at best.

Radical cation would presumably be worse.

Stable to prolonged photolysis; affords no CO 2 reduction.

Generation of “PTP •– ” with the New Amine + PTP

Decay of “PTP •– ” from the New Amine + PTP 440 nm • • • • Appearance quite different from that with Et 3 N Amine radical cation should have no band from 400 – 500nm Decay of “PTP•–” is much faster than with Et 3 N Everything returns to baseline, whereas with Et 3 N it does not 470 nm 285 nm 0 1 2 3 time /  s 4 5

The Ion-Pair Hypothesis Ion pair(s) Deprotonation blocks BET “Long-lived” PTP •– The dilemma: This radical seems to be necessary for CO 2 reduction, but: Ion pair(s)

Picosecond Infrared Studies PTP •– Spectra taken after 500 ps .

10 -4

M

PTP, 1

M

NEt 3 CO 2 •–

Picosecond Infrared Studies 12 CO 2 •– 13 CO 2 •–

Picosecond Infrared Studies Prompt CO 2 •– formed by direct Et 3 N photo ionization with 266 nm pump PTP •– Spectra taken after 500 ps .

10 -4

M

PTP, 1

M

NEt 3 CO 2 •–

e – solv

+ Picosecond Infrared Studies

k

1

k

2

Nanosecond Infrared Studies

Re-evaluation of the First Steps •– [0] kcal/mol –10 kcal/mol •–

•+ •– Re-evaluation of the First Steps CO 2 PTP + Et 3 N + CO 2 PTP + Et 3 N •+ + CO 2 •–

Formate Production as

f

(PTP,  ) 254 nm, no PTP 1

M

Et 3 N in CH 3 CN 254 nm, sat. PTP >290 nm, sat. PTP >290 nm, no PTP

What Have we Learned?

• Electron addition to CO 2 is difficult, and probably doesn’t occur from PTP •– except by “inner-sphere” carboxylation mechanism.

• BET to Et 3 N •+ can occur from both PTP •– and carboxylated PTP •– in ion pairs • Deprotonation of Et 3 N •+ blocks BET and generates  –amino radical •  –Amino radical seems to be necessary for CO 2 reduction, but...

•  –Amino radical is also responsible for several of the byproducts

An Idea for the New Amine

R R N H R

IP (amine)

R N H

+ e –

R



H

°

trans

R N H

+ e –

R R N

–BDE (C–H) +

H R

–IP (H)

R N

~PA (amine) +

H

+ + e – Δ

H

°

trans

= 414.6

– IP(amine) – PA(amine) (in kcal/mol) .

J. Am. Chem. Soc.

2008

,

130

, 3169

An Idea for the New Amine

Sweet spot

Aliphatic amines ArNMe 2 NH 3 ArNH 2

An Idea for the New Amine •• Janovsky, I.; Knolle, W.; Naumov, S.; Williams, F.

5524.

Chem. Eur. J. 2004 ,

10

, e – Beam Freon + • ‡

Adamantane-like TS for H transfer An Idea for the New Amine Replaces  –H of  –amino radical Bridgehead blocks  –amino radical formation H transfer blocks BET hole Simple alkene should be easily hydrogenated

Synthesis and Testing H H h  PTP ~ 2 x Et 3 N

A Lot More Synthesis

How it Works in Practice PTP  PTP 250 –300 nm

Nature Chem.

2011

,

3

, 301.

It Also Works with Visible Light Re(Bipy)(CO) 3 (EtO) 3 PRe(Bipy)(CO) 3 + > 400 nm c.f. Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O.

J. Am. Chem. Soc.

2008

,

130

, 2023–2031.

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 CO 2 by Et 3 N.

• Use of the lessons learned in the design of a renewable amine.

• Future directions: Is an all-organic, renewable, visible light photoreductant for CO 2 possible?

Computational Results  < 390 nm

J. Phys. Chem. A

,

2007

,

111

, 3719

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

PE rel (kcal/mol) B3LYP/6-31+G(d,p) PE Profile + Dihedral Angle

Putting the Pieces Together ~73 kcal/mol ~63 kcal/mol CAM-B3LYP/6-31+G(d,p)  G ° (298 K, 1

M

standard state) PCM model for CH 3 CN Barrier ~4 kcal/mol 56 kcal/mol 43 kcal/mol Barrier 12 kcal/mol [0] kcal/mol 33 kcal/mol

An Unexpected Outcome…

Acknowledgments Rob Richardson Ed Holland Chris Stanley Claire Minton The Leverhulme Trust Mike George Sun Xue-Zhong James Calladine Charlotte Clark Royal Society/Wolfson Foundation