Anchored Metal-to-Metal Charge-Transfer Chromophores in a Mesoporous Silicate Sieve for Visible-Light Activation of Titanium Centers

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Abstract
Binuclear metal-to-metal charge-transfer (MMCT) moieties consisting of a Ti and a CuI or a Ti and a SnII center were obtained in a MCM-41 silicate sieve along with isolated metal centers when exposing Ti-grafted MCM-41 to CuI or SnII precursors featuring highly labile CH3CN ligands. Fourier transform infrared (FT-IR) spectroscopy revealed complete removal of the labile CH3CN ligands of the metal precursor and the formation of CuI−O−Ti, CuI−O−Si, and corresponding SnII linkages on the pore surface. Optical and FT-IR difference spectroscopy upon oxidation of CuI (SnII) allowed assignment of the CuI−O (642 cm-1) and SnII−O (610 cm-1) bond modes of the MMCT moiety. The visible-light-absorbing TiIV−O−CuI MMCT chromophore extends from the UV to 600 nm, the corresponding TiIV−O−SnII absorption to 470 nm. Electron paramagnetic resonance monitoring of the TiSnII−MCM-41 sieve following photoexcitation of the MMCT transitions at cryogenic temperature confirmed that Ti is reduced to TiIII under visible light. Assembly of inorganic MMCT sites inside high-surface-area mesoporous silicates with each metal in a preselected oxidation state opens up activation of catalytically important metal centers under visible light.

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