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

Abstract

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 π-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 Å. 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 CO 2 to gas phase CO and HCO 2 –, 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 CO 2 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 CO 2 to free CO. Moreover, transient Co(III) formed upon MMCT 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.