Visible Light Absorption of Binuclear TiOCo Charge-Transfer Unit Assembled in Mesoporous Silica

Grafting of Co(NCCH3)2Cl2 onto mesoporous Ti-MCM-41 silica in acetonitrile solution affords binuclear Ti-O-Co sites on the pore surface under complete replacement of the precursor ligands by interactions with anchored Ti centers and the silica surface. The Co ligand field spectrum signals that the Co centers are anchored on the pore surface in tetrahedral coordination. FT-infrared action spectroscopy using ammonia gas adsorption reveals Co-O-Si bond modes at 831 and 762 cm. No Co oxide clusters are observed in the as-synthesized material. The bimetallic moieties feature an absorption extending from the UV into the visible to about 600 nm which is attributed to the Ti-O-Co Ti-OCo metal-to-metal charge-transfer (MMCT) transition. The chromophore is absent in MCM-41 containing Ti and Co centers isolated from each other; this material was synthesized by grafting Co onto a Ti-MCM-41 sample with the Ti centers protected by a cyclopentadienyl ligand. The result indicates that the appearance of the charge-transfer absorption requires that the metal centers are linked by an oxo bridge, which is additionally supported by XANES spectroscopy. The MMCT chromophore of Ti-O-Co units has sufficient oxidation power to serve as visible light electron pump for driving multi-electron transfer catalysts of demanding uphill reactions such as water oxidation.