Photoreduction Mechanism of CO₂to CO Catalyzed by a Three-Component Hybrid Construct with a Bimetallic Rhenium Catalyst

The mechanism of the photochemical reduction of carbon dioxide to carbon monoxide coupled to hydrocarbon dehydrogenation catalyzed by a three-component hybrid construct has been studied by combining density functional theory and complete active space self-consistent field/CASPT2 calculations. The electron and proton coupled transfer mechanism is summarized as follows. Graphitic carbon nitride (g-CN), as the photosensitive semiconductor, absorbs visible blue light and transfers electrons to polyoxometalate (POM), a process that is driven by the relative energy of the lowest unoccupied molecular orbital levels of the two species. Then, the hydrocarbon substrate (cyclohexene or cyclohexadiene) is dehydrogenated by the effect of holes formed during the photoexcitation of g-CN, which leads to the transfer of electrons to the oxidized g-CN and protons to the reduced POM. Under red light irradiation, the reduced POM transfers two electrons to the bimetallic rhenium [Re(I)] CO2 reduction catalyst {[Re1(bipyNNH2)(CO)3Cl-Re2(bipy)(CO)3Cl], bipy = 2,2′-bipyridine}. The activated catalyst reduces CO2 to CO, while the proton-rich POM transfers two protons to generate the reduction co-product, H2O. Acting as an electron/proton switch, POM promotes hydrocarbon dehydrogenation by increasing the electron/hole separation upon photoactivation of g-CN and facilitates the transfer of electrons and protons to the Re catalyst for CO2 reduction. The first electron transfer to the Re complex is to the bipyNNH2 ligand. The unpaired electron is then transferred to the Re 5dz2 orbital, activating the coordination of CO2. The combination of a strong interaction between the nearly degenerate d-I orbitals of Re and the bipyNNH2 ligand, the low reduction potential of-0.85 V, and an extremely low free energy barrier of +2.4 kcal mol-1 for CO2 ligation makes the bimetallic Re compound one of the best catalysts for transforming CO2 to CO.