Photocatalytic conversion of CO₂ to CO by Ru(II) and Os(II) octahedral complexes: a DFT/TDDFT study

The reaction mechanisms of the photocatalytic reduction of CO₂ to CO catalyzed by [(en)M(CO)₃Cl] complexes (M = Ru, Os, en = ethylenediamine) in the presence of triethanolamine (TEOA), R₃N (R = -CH₂CH₂OH), in DCM and DMF solvents, were studied by means of DFT/TDDFT electronic structure calculations. The geometric and free energy reaction profiles for two possible reaction pathways were calculated. Both reaction pathways studied, start with the 17e^-, catalytically active intermediate, [(en)M(CO)₃]˙⁺ generated from the first triplet excited state, T₁ upon reductive quenching by TEOA which acts as a sacrificial electron donor. In the first possible pathway, TEOA^- anion binds to the metal center of the catalytically active intermediate, [(en)M(CO)₃]˙⁺ followed by CO₂ insertion into the M-OCH₂CH₂NR₂ bond. The latter upon successive protonations releases a metal 'free' [R₂NCH₂CH₂OC(O)(OH)] intermediate which starts a new and final catalytic cycle, leading to the formation of CO and H₂O while regenarating TEOA. In the second possible pathway, the 17e^-, catalytically active intermediate, [(en)M(CO)₃]˙⁺ captures CO₂ molecule, forming an η¹-CO₂ complex. Upon 2H⁺/2e^- successive protonations and reductions, CO product is obtained along with regenarating the catalytically active intermediate [(en)M(CO)₃]˙⁺. The nature of the proton donor affects the reaction profiles of both mechanisms. The nature of the solvent does not affect significantly the reaction mechanisms under study. Finally, since photoexcitation and T₁ reductive quenching are common to both pathways, we have srutinized the photophysical properties of the [(en)M(CO)₃Cl] complexes along with their T₁ excited states reduction potentials, . The [(en)M(CO)₃Cl] complexes absorb mainly in the UV region while the absolute are in the range 6.4-0.9 eV.