C(sp)-H bond activation by the carboxylate-adduct of osmium tetroxide (OsO).

The reaction of osmium tetroxide (OsO) and carboxylate anions (acetate: X = AcO and benzoate: X = BzO) gave 1 : 1 adducts, [OsO(X)] (1X), the structures of which were determined by X-ray crystallographic analysis. In both cases, the carboxylate anion X coordinates to the osmium centre to generate a distorted trigonal bipyramidal osmium(VIII) complex. The carboxylate adducts show a negative shift of the redox potentials () and a red shift of the stretches as compared to those of tetrahedral OsO itself. Despite the negative shift of , the reactivity of these adduct complexes 1X was enhanced compared to that of OsO in benzylic C(sp)-H bond oxidation. The reaction obeyed the first-order kinetics on both 1X and the substrates, giving the second-order rate constant (), which exhibits a linear correlation with the C-H bond dissociation energy (BDE) of the substrates (xanthene, 9,10-dihydroanthracene, fluorene and 1,2,3,4-tetrahydronaphthalene) and a kinetic deuterium isotope effect (KIE) of 9.7 ((xanthene-)/(xanthene-)). On the basis of these kinetic data together with the DFT calculation results, we propose a stepwise reaction mechanism involving rate-limiting benzylic hydrogen atom abstraction and subsequent rebound of the generated organic radical intermediate to a remaining oxido group on the osmium centre.