Methane Generation from CO2 with a Molecular Rhenium Catalyst

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.

1IL and 3MLCT excited states modulated by H+: the structure and photophysical properties of [(2-bromo-5-(1H-pyrazol-1-yl)pyrazine)Re(CO)3Br]

The photophysical characterization of pyrazolyl–pyrazine Re(i) complex, shows a ¹IL and ³MLCT excited states, being just the ³MLCT able to react with trifluoroacetic acid to yield the protonated and long-lived ³ILH⁺ species. These findings make the compound a potential sensor for protons in solution in the presence of light.

[RuII(tpy)(bpy)Cl]+-Catalyzed reduction of carbon dioxide. Mechanistic insights by carbon-13 kinetic isotope effects

¹³C kinetic isotope effect determinations combined with DFT calculations provide insight on the CO₂ reduction reaction catalyzed by a ruthenium complex.

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

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.

CO2 reduction with Re(i)–NHC compounds: driving selective catalysis with a silicon nanowire photoelectrode

Excellent selectivity was observed in CO₂ reduction using Re(i)–NHC catalysts on a silicon nanowire photoelectrode.