Site-specific oxygen-18 labelling of silica-supported vanadium(V) complexes: Implications for oxidation catalysis

The incorporation of 18 O isotope labels into specific positions of oxide-supported vanadium complexes has been achieved by grafting VOCl 3 onto silica. The gas phase reaction of VOCl 3 with the hydroxyl groups of the silica surface yields exclusively SiOV(O)Cl 2 . When the hydroxyl groups are first exchanged with H 2 , 18 O, the subsequent reaction with V 16 OCl 3 gives uniquely Si 18 OV 16 OCl 2 . Reaction of V 18 OCl 3 with unlabelled silica gives only Si 16 OV 18 OCl 2 . Exchange of the chloride ligands with alcohols gives SiOV(O)Cl(OR) and SiOV(O)(OR) 2 sequentially, without displacing or diluting the isotope labels. The thermal and photochemical reactions of SiOV(O)Cl 2 with the substrates ArNCO and CO, respectively, proceed by oxygen transfer uniquely from the terminal oxo position, as shown by the isotope distribution of the CO 2 product formed in each reaction. When the reactions are run under catalytic conditions, below 70°C, no incorporation of the lattice oxygen of silica into CO 2 was observed.