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 TiSnII-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.