Binuclear ZrOCo Metal-to-Metal Charge-Transfer Unit in Mesoporous Silica for Light-Driven CO2 Reduction to CO and Formate

Oxo-bridged heterobinuclear units of the type Zr(IV)OCo(II) covalently anchored on the pore surface of mesoporous silica SBA-15 have been synthesized with high selectivity. The unit exhibits a visible light absorbing metal-to-metal charge-transfer absorption (MMCT) extending to about 550 nm. The oxo-bridged structure of the binuclear moiety is manifested by spectral blue-shifts of the optical Co(II) spin-orbit bands due to reduced pi-electron donating ability of the bridging oxygen caused by the electron-withdrawing Zr center. EXAFS measurements of the Zr and Co K-edges and curve fitting analysis revealed a Zr to Co distance of 3.4 angstrom. The coordination geometry of the Zr and Co metal centers in monometallic Zr and Co-SBA-15 samples is closely preserved in the ZrOCo unit. Illumination of the MMCT absorption at 420 nm and shorter wavelengths resulted in the reduction of CO2 to gas phase CO and HCO2-, the latter adsorbed on the silica pore surface. The branching between carbon monoxide and formate was found to be determined by the fate of the sacrificial donor (triethyl- or diethylamine), namely proton transfer versus H atom transfer to CO2 interacting with the transient Zr(III) center. The ZrOCo(II) unit on a silica surface constitutes the first example of an all-inorganic heterobinuclear unit for the photoinduced splitting of CO2 to free CO. Moreover, transient Co(III) formed upon M.MCT excitation should possess sufficient oxidation potential for driving a catalyst for water oxidation, thereby opening up opportunities for replacing the sacrificial donor by water as electron source.