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

Abstract Grafting of C

Controlled Assembly of Heterobinuclear Sites on Mesoporous Silica: Visible Light Charge-Transfer Units with Selectable Redox Properties

Mild synthetic methods are demonstrated for the selective assembly of oxo-bridged heterobinuclear units of the type TiOCrIII, TiOCoII, and TiOCeIII on mesoporous silica support MCM-41. One method takes advantage of the higher acidity and, hence, higher reactivity of titanol compared to silanol OH groups towards CeIII or CoII precursor. The procedure avoids the customary use of strong base. The controlled assembly of the TiOCr system exploits the selective redox reactivity of one metal towards another (TiIII precursor reacting with anchored CrVI centers). The observed selectivity for linking a metal precursor to an already anchored partner versus formation of isolated centers ranges from a factor of six (TiOCe) to complete (TiOCr, TiOCo). Evidence for oxo bridges and determination of the coordination environment of each metal centers is based on K-edge EXAFS (TiOCr), L-edge absorption spectroscopy (Ce), and XANES measurements (Co, Cr). EPR, optical, FT-Raman and FT-IR spectroscopy furnish additional details on oxidation state and coordination environment of donor and acceptor metal centers. In the case of TiOCr, the integrity of the anchored group upon calcination (350 oC) and cycling of the Cr oxidation state is demonstrated. The binuclear units possess metal-to-metal charge-transfer transitions that absorb deep in the visible region. The flexible synthetic method for assembling the units opens up the use of visible light charge transfer pumps featuring donor or acceptor metals with selectable redox potential

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In-situ Spectroscopy of Water Oxidation at Ir Oxide Nanocluster Driven by Visible TiOCr Charge-Transfer Chromophore in Mesoporous Silica

An all-inorganic photocatalytic unit consisting of a binuclear TiOCr charge-transfer chromophore coupled to an Ir oxide nanocluster has been assembled on the pore surface of mesoporous silica AlMCM-41. In situ FT-Raman and EPR spectroscopy of an aqueous suspension of the resulting IrxOy-TiCr-AlMCM-41 powder reveal the formation of superoxide species when exciting the Ti(IV)OCr(III) --> Ti(III)OCr(IV) metal-to-metal charge-transfer chromophore with visible light. Use of H218O confirms that the superoxide species originates from oxidation of water. Photolysis in the absence of persulfate acceptor leads to accumulation of Ti(III) instead. The results are explained by photocatalytic oxidation of water at Ir oxide nanoclusters followed by trapping of the evolving O2 by transient Ti(III) centers to yield superoxide. Given the flexibility to select donor metals with appropriate redox potential, photocatalytic units consisting of a binuclear charge-transfer chromophore coupled to a water oxidation catalyst shown here constitute a step towards thermodynamically efficient visible light water oxidation units

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

Grafting of CoII(NCCH3)2Cl2 onto mesoporous Ti-MCM-41 silica in acetonitrile solution affords binuclear Ti-O-CoII sites on the pore surface under complete replacement of the precursor ligands by interactions with anchored Ti centers and the silica surface. The CoII 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-1. 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 TiIV-O-CoII?3 TiIII-O-CoIII 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 CoII 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-CoII 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