Photoreduction Mechanism of CO₂ to CO Catalyzed by a Rhenium(I)-Polyoxometalate Hybrid Compound

The photoreduction mechanism of carbon dioxide to carbon monoxide by the Re-organic hybrid polyoxometalates (POMs) {NaH[PW₁₂O₄₀]^3-Re^IL(CO)₃DMA}^na (L = 15-crown-5 phenanthroline, DMA = N,N-dimethylacetamide) has been investigated by means of DFT and TD-DFT calculations. The reaction mechanism can be divided into several steps, including (i) photoexcitation and charge transfer, (ii) DMA release, (iii) CO₂ addition, (iv) protonation, and (v) CO release and regeneration of the catalyst. The charge transfer (CT) states, POM to Re complex, are efficiently induced by metal-centered (MC) excitations occurring on the reduced POM. Once one electron is transferred to the organometallic unit from the excited POM, the Re is able to bind and activate the CO₂ substrate. Subsequent steps that involve protonation of CO₂ and CO release are favorable thermodynamically and are induced by a second electron transfer from the POM to the Re complex. In this reaction, the POM acts as photosensitizer, electron reservoir, and electron donor.