Kinetic Studies on the Oxidation of η5-Cyclopentadienyl Methyl Tricarbonyl Molybdenum(II) and the Use of Its Oxidation Products as Olefin Epoxidation Catalysts
journal contributionposted on 2009-01-26, 00:00 authored by Ahmad M. Al-Ajlouni, Draganco Veljanovski, Alejandro Capapé, Jin Zhao, Eberhardt Herdtweck, Maria José Calhorda, Fritz E. Kühn
The oxidation of η5-cyclopentadienyl(methyl)(tricarbonyl)molybdenum(II) (1) with excess tert-butylhydroperoxide (TBHP) initially yields η5-cyclopentadienyl(methyl)(dioxo)molybdenum(VI) (2), which further reacts with TBHP, forming η5-cyclopentadienyl(methyl)(oxo)(peroxo)molybdenum(VI) (3). The solid-state structure of 3 has been determined by single-crystal X-ray crystallography. Detailed kinetic studies have been carried out on the oxidation of 1 with TBHP as an oxidizing agent and on the catalytic activities of the resulting oxidation products, 2 and 3, in olefin epoxidation. In the absence of oxidant, neither of the molybdenum species is able to transfer an O-atom to an olefin. However, both Mo(VI) species act as catalysts for the epoxidation of olefins with TBHP through the formation of active intermediates. It has been found that compound 3 reacts with excess TBHP to give an active intermediate, which exists in equilibrium with the catalyst precursor 3 with a Keq close to 1. This intermediate is slowly formed in a reversible initial step. It reacts rapidly with an olefin, while it decomposes in the absence of olefin. Furthermore, the kinetic results indicate the formation of another active intermediate, originating from 2, that is 3−5 times more active in epoxidation catalysis than the active intermediate formed from 3. A mechanistic scheme is proposed, based on the kinetic results.