To understand the electrocatalytic CO2 reduction of metal carbonyl complexes without "non-innocent" ligands, the electrochemical responses of group 6 M(CO)6 (M = Cr, Mo, or W) and group 7 M2(CO)10 (M = Mn or Re) complexes were examined under Ar and CO2 at a glassy carbon electrode. All of the complexes showed changes in their cyclic voltammograms under CO2. The group 6 hexacarbonyl species show a significant increase in current under CO2 during metal-based reduction, corresponding to catalytic reduction of CO2. Bulk electrolysis experiments with Mo(CO)6 showed that CO was the primary product. The group 7 dimers showed very little change during metal-based reduction, but return oxidation responses disappeared, indicative of a chemical reaction after exposure to CO2 without catalysis. Addition of H2O, a proton source, to the solutions under CO2 decreased the catalytic current of the group 6 carbonyls and had no effect on the responses of the group 7 carbonyls. The group 6 M(CO)6 species are notable in that that they are effective catalysts without the need for an added "non-innocent" ligand such as 2,2'-bipyridine.

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 CO reduction ability. The reported complexes are of the type Re(N-N)(CO)Cl, 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 CO 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 CO 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). 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)Cl (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/i), up to 7 times larger than that of I.

The atomic-level tunability of molecular structures is a compelling reason to develop homogeneous catalysts for challenging reactions such as the electrochemical reduction of carbon dioxide to valuable C1–C n products. Of particular interest is methane, the largest component of natural gas. Herein, we report a series of three isomeric rhenium tricarbonyl complexes coordinated by the asymmetric diimine ligands 2-(isoquinolin-1-yl)-4,5-dihydrooxazole (quin-1-oxa), 2-(quinolin-2-yl)-4,5-dihydrooxazole (quin-2-oxa), and 2-(isoquinolin-3-yl)-4,5-dihydrooxazole (quin-3-oxa) that catalyze the reduction of CO2 to carbon monoxide and methane, albeit the latter with a low efficiency. To our knowledge, these complexes are the first examples of rhenium­(I) catalysts capable of converting carbon dioxide into methane. Re­(quin-1-oxa)­(CO)3Cl (1), Re­(quin-2-oxa)­(CO)3Cl (2), and Re­(quin-3-oxa)­(CO)3Cl (3) were characterized and studied using a variety of electrochemical and spectroscopic techniques. In bulk electrolysis experiments, the three complexes reduce CO2 to CO and CH4. When the controlled-potential electrolysis experiments are performed at −2.5 V (vs Fc+/0) and in the presence of the Brønsted acid 2,2,2-trifluoroethanol, methane is produced with turnover numbers that range from 1.3 to 1.8. Isotope labeling experiments using 13CO2 atmosphere produce 13CH4 (m/z = 17) confirming that methane originates from CO2 reduction. Theoretical calculations are performed to investigate the mechanistic aspects of the 8e–/8H+ reduction of CO2 to CH4. A ligand-assisted pathway is proposed to be an efficient pathway in the formation of CH4. Delocalization of the electron density on the (iso)­quinoline moiety upon reduction stabilizes the key carbonyl intermediate leading to additional reactivity of this ligand. These results should aid the development of more robust catalytic systems that produce CH4 from CO2.

In order to understand the effect of intramolecular proton acidity on CO 2 reduction by Mn bipyridyl species, three fac-Mn(CO) 3 bipyridine complexes containing intramolecular phenol groups of varying acidities were synthesized and electrochemical, spectroscopic, and computational studies were performed. While the phenol group acidity has minimal influence on the metal center, the complex containing a fluoro-substituted (more acidic) phenol, MnBr(F-HOPh-bpy)(CO) 3 , exhibits a decreased catalytic to peak current ratio following the second reduction in comparison to the complexes with unsubstituted or methylsubstituted phenol groups (MnBr(HOPh-bpy)(CO) 3 and MnBr(Me-HOPh-bpy)(CO) 3, respectively). A second process is also present in the catalytic wave for MnBr(F-HOPh-bpy)(CO) 3 . Furthermore, MnBr(F-HOPh-bpy)(CO) 3 exhibits decreased CO production and increased H 2 production in comparison to MnBr(HOPh-bpy)(CO) 3 . Spectroelectrochemistry under an inert atmosphere in the presence of water shows that following the first reduction, for both MnBr(F-HOPh-bpy)(CO) 3 and MnBr(HOPh-bpy)(CO) 3 , the major product is a phenoxide-coordinated fac-(CO) 3 species formed from reductive deprotonation and the minor product is a six-coordinate Mn(I) hydride. For both species, the major species following the second reduction is a five-coordinate anion believed to be the active catalyst for CO 2 reduction, but the Mn(I) hydride persists as a minor species. The IR assignments are supported by theoretical calculations. These findings show that changes to the acidity of an intramolecular substituent can have significant effects on the catalytic performance and product selectivity of Mn(CO) 3 bipyridine catalysts despite having minimal effect on the metal center, with a more acidic intramolecular substituent increasing H 2 production at the expense of CO 2 reduction.