Photochemical Properties of Re(CO)₃ Complexes with and without a Local Proton Source and Implications for CO₂ Reduction Catalysis

Herein, we present a detailed study on the photophysical properties and the excited state reactivity of two mononuclear Re­(CO)₃ complexes with imdazol-pyridine ligands equipped with and without a local proton source, [Re­(CO)₃LCl], where for 1: L = 2,4-ditert-butyl-6-(6-(1-methyl-1H-imidazol-2-yl)­pyridin-2-yl)­phenol and 2: L = 2-(3,5-ditert-butyl-2-methoxyphenyl)-6-(1-methyl-1H-imidazol-2-yl)­pyridine. Time-resolved IR and UV/vis spectroscopy revealed that excitation of 1 and 2 is followed by population of the triplet excited state within <100 fs, where structural and vibrational relaxation to the T₁ equilibrium structure is observed on the picosecond time scale. The T₁ state can be viewed as a MLCT state as all ν­(CO) features in the transient infrared (TRIR) spectra are shifted to higher wavenumbers upon excitation, which is indicative for a decreasing Re → CO π-backdonation. The T₁ states have considerably long lifetimes at room temperature of 160 ns for 1 and 430 ns for 2 in dmf and they can be successfully quenched by the sacrificial electron donors triethanolamine (TEOA) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole (BIH). The quenching rates are 2 orders of magnitude larger for BIH than for TEOA, as the latter reaction is endergonic. However, both species are not active in the photochemical CO₂-to-CO conversion. We rationalize this for 2 by the low steady-state concentration of the initial reduction product, [Re­(CO)₃LCl]⁻, which ejects chloride rather fast. Thus, the second, homogeneous electron transfer process between [Re­(CO)₃LCl]⁻ and [Re­(CO)₃L­(solvent)] forming the active species [Re­(CO)₃L]⁻, has a very low probability and decomposition pathways come to the fore. 1 decomposes under irradiation in the presence of BIH or TEOA forming the initial photoproduct 3. We tentatively assume that the ligand in 3 is deprotonated and switches from a N,N- to a N,O^–-coordination mode. This indicates that in the excited state the Re–N bond is cleaved quite easily, as this decomposition pathway has not been observed under electrochemical conditions.