Electrochemical Reduction of CO2 Catalyzed by Re(pyridine-oxazoline)(CO)3Cl Complexes

• Published in: Inorganic chemistry Vol. 56; no. 6; pp. 3214 - 3226
• Main Authors: Nganga, John K, Samanamu, Christian R, Tanski, Joseph M, Pacheco, Carlos, Saucedo, Cesar, Batista, Victor S, Grice, Kyle A, Ertem, Mehmed Z, Angeles-Boza, Alfredo M
• Format: Journal Article
• Language: English
• Published: American Chemical Society 20.03.2017
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.6b02384

A series of rhenium tricarbonyl complexes coordinated by asymmetric diimine ligands containing a pyridine moiety bound to an oxazoline ring were synthesized, structurally and electrochemically characterized, and screened for CO2 reduction ability. The reported complexes are of the type Re­(N-N)­(CO)3Cl, with N-N = 2-(pyridin-2-yl)-4,5-dihydrooxazole (1), 5-methyl-2-(pyridin-2-yl)-4,5-dihydrooxazole (2), and 5-phenyl-2-(pyridin-2-yl)-4,5-dihydrooxazole (3). The electrocatalytic reduction of CO2 by these complexes was observed in a variety of solvents and proceeds more quickly in acetonitrile than in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The analysis of the catalytic cycle for electrochemical CO2 reduction by 1 in acetonitrile using density functional theory (DFT) supports the C–O bond cleavage step being the rate-determining step (RDS) (ΔG ⧧ = 27.2 kcal mol–1). The dependency of the turnover frequencies (TOFs) on the donor number (DN) of the solvent also supports that C–O bond cleavage is the rate-determining step. Moreover, the calculations using explicit solvent molecules indicate that the solvent dependence likely arises from a protonation-first mechanism. Unlike other complexes derived from fac-Re­(bpy)­(CO)3Cl (I; bpy = 2,2′-bipyridine), in which one of the pyridyl moieties in the bpy ligand is replaced by another imine, no catalytic enhancement occurs during the first reduction potential. Remarkably, catalysts 1 and 2 display relative turnover frequencies, (i cat/i p)2, up to 7 times larger than that of I.