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

Binuclear metal-to-metal charge-transfer (MMCT) moieties consisting of a Ti and a Cu(I) or a Ti and a Sn(II) center were obtained in a MCM-41 silicate sieve along with isolated metal centers when exposing Ti-grafted MCM-41 to Cu(I) or Sn(II) 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 Cu(I)-O-Ti, Cu(I)-O-Si, and corresponding Sn(II) linkages on the pore surface. Optical and FT-IR difference spectroscopy upon oxidation of Cu(I) (Sn(II)) allowed assignment of the Cu(I)-O (642 cm(-1)) and Sn(II)-O (610 cm(-1)) bond modes of the MMCT moiety. The visible-light-absorbing Ti(IV)-O-Cu(I) MMCT chromophore extends from the UV to 600 nm, the corresponding Ti(IV)-O-Sn(II) absorption to 470 nm. Electron paramagnetic resonance monitoring of the TiSn(II)-MCM-41 sieve following photoexcitation of the MMCT transitions at cryogenic temperature confirmed that Ti is reduced to Ti(III) 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.