EFRC seminar 10/10/2013

10 Oct 2013

"Strategies for Two-Electron CO2 Reduction Catalysis"

 

James T. Muckerman

Chemistry Department

Brookhaven National Laboratory, Upton, NY

 

Abstract:

Tabulated standard reduction potentials at pH 7 in water clearly indicate that multi-electron reduction of CO2 is necessary to avoid having to produce aqueous CO2•-. This can be seen graphically in the form of a Latimer-Frost (free energy vs. oxidation state) diagram taken from the older pulse radiolysis literature1 (Fig. 1) that shows the three additional odd-electron intermediates in CO2 reduction beyond CO or formic acid that have very high energy. Such a diagram underscores the need for carrying out at least two-electron reductions to convert CO2 to fuels. We are exploring several successful strategies for carrying out two-electron reductions in this context with a combined experimental and theoretical approach, and have identified at least three distinct routes. One is to bind CO2 to certain transition-metal complexes that have three available adjacent oxidation states, e.g., Co(III), Co(II) and Co(I). If such a catalyst is designed with the middle state as the resting state, it can be reduced once, bind CO2 through a two-electron oxidative addition reaction, then reduced again to return to its resting state. The reduction chemistry can occur after the first or second reduction.

An example of such a catalyst is the Co(HMD)2+ (HMD = a 1,4,8,11-tetraaza macrocycle) complex2 shown in Fig. 2. Another approach is to exploit a disproportionation reaction involving two one-electron-reduced (OER) species. This strategy is exemplified by our recent work on the Ru(bpy)2(pbn)2+ (pbn = 2-(2-pyridyl)benzo[b]- 1,5-naphthyridine) photocatalyst that produces the NADPH-like Ru(bpy)2(pbnHH)2+ hydride donor via the disproportionation of two OER Ru(bpy)2(pbnH•-)2+ intermediates,3 and also by the Re(bpy)(CO)3(Cl)0 catalyst that forms a carboxylate-bridged dimer between two catalyst species through the reaction of CO2 with two OER Re(bpy)(CO)30 species. This dimer can react with another CO2 molecule resulting in the net disproportionation of two CO2•- species.4 A third approach is the sequential ionic hydrogenation reaction on CO2 or on CO ligands of a TM complex. Both protonation followed by hydride transfer and hydride transfer followed by protonation pathways are possible. The choice of TM complexes and hydride donors can be guided by consideration of thermodynamic hydricities. A variant of the hydride ion as the carrier of the two electrons involved in a reduction step is the insertion of CO2 into a metal-hydride bond, as has been demonstrated for the case of CO2 insertion into Ru(H)(bpy)(tpy)+.5

 

 

References

  1. Koppenol, W. H.; Rush, J. D. J. Phys. Chem. 1987, 91, 4429-4430. Reprinted with permission from ref. 1. Copyright (1987) American Chemical Society
  2. Schneider, J; Jia, H; Muckerman, J. T.; Fujita, E. Chem. Soc. Rev. 2012, 41, 2036-2051.
  3. Polyansky, D. E.; Cabelli, D.; Muckerman, J. T.; Fukushima, T.; Tanaka, K.; Fujita, E. Inorg. Chem. 2008, 47, 3958-3968.
  4. Agarwal, J.; Fujita, E.; Schaefer, H. F. III; Muckerman, J. T. J. Am. Chem. Soc. 2012, 134, 5180-5186.
  5. Creutz, C.; Chou, M. H.; Hou H.; Muckerman, J. T. Inorg. Chem. 2010, 49, 9809-9822.
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