Organometallics

Organometallic complexes have long been potent homogeneous CO₂ reduction catalysts. One such complex that has garnered much interest lately, ReCl(bpy)(CO)₃, requires the rare and expensive metal rhenium. However, recent studies have found that manganese, two periods above rhenium, also functions as a CO-generating catalyst with bipyridine and CO ligands^1a-e
Recently, in collaboration with Dr. Jay Agarwal at the University of Georgia, new variations on these manganese catalysts have been prepared. Among them are two which have the bipyridine units replaced with pyridine-imidazole ligands, which bind through the nitrogen on the pyridine but the C2 carbon on the imidazole (forming a carbene).² These N-heterocyclic carbene-containing complexes demonstrate excellent selectivity for CO₂ reduction to CO; no H₂ production is observed at any point of the electrolysis.

A major drawback of these manganese tricarbonyl catalysts, both with bipyridine and with pyridine-imidazole ligands, is their high sensitivity to UV and visible light, necessitating the use of a darkroom for stability. The culprit is the readily excited metal-ligand charge transfer (MLCT) band, which leads to dissociation of one of the ligands. Exchanging the axial ligand X, which is typically a bromine, for a cyanide shifts the MLCT band to lower wavelengths and improves the stability of the catalyst, especially when exposed to visible light.³

We also proposed the use of a phenol-substituted bipyridine manganese complex, MnBr(6-(2-hydroxyphenol)-2,2-bipyridine)(CO)₃ for CO₂ electroreduction; this complex contains a phenolic proton close to the CO₂ binding site and therefore is useful for proton-assisted C−O bond cleavage. As a result, compared with the parent complex, MnBr(bpy)(CO)₃, this compound produces an electrocatalytic current that is enhanced by a factor of seven. Moreover, theoretical studies suggest that this increased activity is a result of a lower activation barrier for the intramolecular proton-assisted C-O cleavage mechanism.⁴

In addition to mononuclear complexes, our group is also interested in dinuclear metal complexes due to their cooperative effects which are often observed in biological systems. Our preliminary studies using in situ observations have shown strong evidence that di-manganese complexes have promising catalytic activities toward CO₂ reduction.

1. (a) Bourrez, M.; Molton, F.; Chardon-Noblat, S.; Deronzier, A. Angew. Chem., Int. Ed. 2011, 50, 9903−9906. (b) Smieja, J. M.; Sampson, M. D.; Grice, K. A.; Benson, E. E.;Froehlich, J. D.; Kubiak, C. P. Inorg. Chem. 2013, 52, 2484−2491. (c) Bourrez, M.; Orio, M.; Molton, F.; Vezin, H.; Duboc, C.; Deronzier, A.; Chardon-Noblat, S. Angew. Chem., Int. Ed. 2013, 53, 240−243. (d) Compain, J.-D.; Bourrez, M.; Haukka, M.; Deronzier, A.; Chardon-Noblat, S. Chem. Commun. 2014, 50, 2539−2542. (e) Sampson, M. D.; Nguyen, A. D.; Grice, K. A.; Moore, C. E.; Rheingold, A. L.; Kubiak, C. P. J. Am. Chem. Soc. 2014, 136, 5460−5471. 
2. Agarwal, J.; Shaw, T. W.; Stanton III, C. J.; Majetich, G. F.; Bocarsly, A. B.; Schaefer III, H. F. NHC-Containing Manganese(I) Electrocatalysts for the Two-Electron Reduction of CO2. Angew. Chem. Int. Ed.2014, 53(20), 5152-5155. 
3. Agarwal, J.; Stanton III, C. J. ; Shaw, T. W.; Vandezande, J. E.; Majetic, G. F.; Bocarsly, A. B.; Schaefer III, H. F. Exploring the effect of axial ligand substitution (X = Br, NCS, CN) on the photodecomposition and electrochemical activity of [MnX(NC)(CO)3] complexes. Dalton Trans. 2015, 44, 2122-2131. 
4. Agarwal, J.; Shaw, T. W.; Schaefer III, H. F.; Bocarsly, A. B. Design of a Catalytic Active Site for Electrochemical CO2 Reduction with Mn(I)-Tricarbonyl Species. Inorg. Chem. 2015, 54, 5285-5294.