Controlled assembly of hetero-binuclear 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 toward Ce-III or Coll precursors. The procedure avoids the customary use of a strong base. The controlled assembly of the TiOCr system exploits the selective redox reactivity of one metal toward another (Ti-III precursor reacting with anchored Cr-VI centers). The observed selectivity for linking a metal precursor to an already anchored partner versus formation of isolated centers ranges from a factor of 6 (TiOCe) to complete (TiOCr, TiOCo). Evidence for oxo bridges and determination of the coordination environment of each metal center is based on K-edge extended X-ray absorption fine structure spectroscopy (TiOCr), L-edge absorption spectroscopy (Ce), and X-ray absorption near edge structure measurements (Co, Cr). Electron paramagnetic resonance, optical, Fourier transform Raman, and Fourier transform infrared 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 degrees C) 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.