The ruthenium-substituted 'sandwich' type polyoxometalate [WZnRum(III)2(XW9O34)2]11- where X = Zn(II) or Co(II) has been shown to be unique in its ability to catalyze the selective hydroxylation of adamantane at the tertiary carbon position with molecular oxygen as the oxygen donor. The hydroxylation features an adamantane:dioxygen stoichiometry of 2:1. Kinetic studies of the reaction show that the reaction is second order in the ruthenium-substituted polyoxometalate and zero order in adamantane. The reaction rate dependence on molecular oxygen is complicated. Highest reaction rates were observed at approximately 1 atm of dioxygen, but the rates decreased at lower (0.2 atm) and higher (3 atm) dioxygen pressures. Kinetic analysis as a function of temperature showed a low entropy of activation, indicating a highly ordered transition state. The reaction commenced only after an induction period related to formation of the 'active' species from the precursor polyoxometalate. Alternatively, the induction period could be eliminated by adding a reducing agent. The induction period was found to be a function of both the reaction temperature and oxidation pressure. Coordination studies carried out via UV-vis and IR spectroscopy indicate the formation of a ruthenium(IV) oxo or μ-peroxo ruthenium(III) dimeric species as the active oxygenation species. This formulation is supported by the ESR spectra observed upon addition of spin traps such as 2- methyl-2-nitrosopropane or 5,5-dimethyl-1-pyrroline N-oxide to the reaction mixtures. In the oxidation of alkenes, catalytic and highly selective epoxidation may be observed. Especially informative was the catalytic epoxidation of trans-cyclooctene, which yielded a trans:cis-cyclooctene oxide ratio of 20:1, providing strong evidence of a nonradical oxidation pathway. On the basis of the reaction stoichiometry, spectroscopic evidence, reaction probes, and kinetic studies, a mechanism is proposed calling for a dioxygenase type activation of molecular oxygen via complexation to a ruthenium(II) species followed by formation of a ruthenium(IV)oxo species via a ruthenium(III) μ-peroxo intermediate.
ASJC Scopus subject areas