7 th International Conference on Chemical Kinetics, MIT, 2011

1. A Laser Flash Photolysis Study of CO2 Reduction: Kinetics Leading to the Design of a Renewable Reducing Agent 7th International Conference on Chemical Kinetics, MIT, 2011

2. 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?

3. H2O (l) + CO2 (g) 1/2 O2 (g) + HCO2H (l) PC–H• + HO– H2O < 470 nm H° = +60.8 kcal/mol Photochemical CO2 Reduction h

4. The Key Idea

5. PC = 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 PTP h HCO2H

6. • – Photochemical CO2 Reduction PTP•– Fujiwara, H.; Kitamura, T.; Wada, Y.; Yanagida, S.; Kamat, P. V. J. Phys. Chem.1999, 103, 4874.

7. H+ loss H• loss Effect of Ionization on  C–H Reactivity Figures are H° in kcal/mol (exptl. + CBS–QB3)

8. 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 CH3CN These results from empirically corrected UB3LYP, calibrated against UMP2 and UCCSD for smaller systems Later results use UCAM-B3LYP Self-Interaction Error in DFT: Bally, T.; Sastry, G. N. J. Phys. Chem. A, 1997, 101, 7923 Braieda, B.; Hiberty, P. C.; Savin, A. J. Phys. Chem. A, 1998, 102, 7872 Graefenstein, 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. J. Phys. Chem. A, 2007, 111, 3719

9. PTP PTP Reality Bites [1] : 2.0 0.3 M CO2, 0.25M amine in CH3CN [1] : 0.35

10. Dimers of this radical detected in photochemical CO2 reduction A Radical New Mechanism Kanoufi, F.; Zu, Y.; Bard, A. J. J. Phys. Chem. B2001, 105, 210.

11. Blocking  C–H Reactivity X Transient stability, at best. Radical cation would presumably be worse.

12. CBS-QB3 Isodesmic Reactions Proton transfer H-atom transfer

13. Blocking  C–H Reactivity X Transient stability, at best. Radical cation would presumably be worse. Stable to prolonged photolysis; affords no CO2 reduction.

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

15. Decay of “PTP•–” from the New Amine + PTP 440 nm • 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 470 nm 285 nm 0 1 2 3 4 5 time / s

16. Deprotonation blocks BET The dilemma: This radical seems to be necessary for CO2 reduction, but: “Long-lived” PTP •– Ion pair(s) The Ion-Pair Hypothesis Ion pair(s)

17. Picosecond Infrared Studies PTP•– Spectra taken after 500 ps. 10-4 M PTP, 1M NEt3 CO2•–

18. Picosecond Infrared Studies 12CO2•– 13CO2•–

19. Picosecond Infrared Studies Prompt CO2•– formed by direct Et3N photo-ionization with 266 nm pump PTP•– Spectra taken after 500 ps. 10-4 M PTP, 1M NEt3 CO2•–

20. Picosecond Infrared Studies k1 k2 e–solv +

21. Nanosecond Infrared Studies

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

23. PTP + Et3N + CO2 PTP + Et3N•+ + CO2•– Re-evaluation of the First Steps CO2 •+ •–

24. Formate Production as f (PTP, ) 254 nm, no PTP 1 M Et3N in CH3CN 254 nm, sat. PTP >290 nm, sat. PTP >290 nm, no PTP

25. 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

26. An Idea for the New Amine ΔH°trans = 414.6 – IP(amine) – PA(amine) (in kcal/mol) . J. Am. Chem. Soc.2008, 130, 3169

27. An Idea for the New Amine Aliphatic amines Sweet spot ArNMe2 NH3 ArNH2

28. ‡ + • An Idea for the New Amine Janovsky, I.; Knolle, W.; Naumov, S.; Williams, F. Chem. Eur. J.2004, 10, 5524. + • e– Beam Freon • •

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

30. h PTP H H Synthesis and Testing ~ 2x Et3N

31. A Lot More Synthesis

32. PTP PTP How it Works in Practice 250–300 nm Nature Chem.2011, 3, 301.

33. Re(Bipy)(CO)3 (EtO)3PRe(Bipy)(CO)3+ It Also Works with Visible Light > 400 nm c.f. Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O. J. Am. Chem. Soc. 2008, 130, 2023–2031.

34. One Long Term Plan... N. Itoh, W. C. Xu, S. Hara, K. Sakaki, Catal. Today 2000, 56, 307

35. 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?

36.  < 390 nm Computational Results J. Phys. Chem. A, 2007, 111, 3719

37. 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 Some Useful Information

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

39. Barrier ~4 kcal/mol ~73 kcal/mol ~63 kcal/mol 56 kcal/mol CAM-B3LYP/6-31+G(d,p) G° (298 K, 1 M standard state) PCM model for CH3CN 43 kcal/mol Barrier 12 kcal/mol 33 kcal/mol [0] kcal/mol Putting the Pieces Together

40. An Unexpected Outcome…

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