Structures of paramagnetic V^IV amido complexes grafted onto metal oxide surfaces: Model systems for heterogeneous vanadium catalysts

Well defined vanadium(IV) surface complexes were prepared via reaction of tetrakis(dimethylamido)-vanadium(IV), V[N(CH₃)₂]₄, with protons of surface hydroxyl groups of three different, mainly non-porous oxides SiO₂, Al₂O₃, and TiO₂. The grafting reaction was performed either from pentane solution or from the gaseous phase under inert conditions. The density and number of hydroxyl groups were adjusted by thermal pre-treatment of the oxides in vacuo. Chemical analysis (V, N) provided information about the chemical composition of the vanadium surface species after grafting onto the oxides. The coordination sphere and redox properties of the mononuclear paramagnetic vanadium(IV) surface complexes were examined by electron paramagnetic resonance (EPR). After exposure to air vanadium(V) species could be identified by ⁵¹V-solid-state-NMR. Ligand exchange reactions were investigated by in-situ infrared spectroscopy after the immobilization reaction.