Kinetic Studies on the Oxidation of η⁵-Cyclopentadienyl Methyl Tricarbonyl Molybdenum(II) and the Use of Its Oxidation Products as Olefin Epoxidation Catalysts

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 K_eq 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.