Olefin epoxidation by molybdenum and rhenium peroxo and hydroperoxo compounds: A density functional study of energetics and mechanisms

A density functional study on olefin epoxidation by rhenium and molybdenum peroxo complexes has been carried out. Various intermediates and transition structures of the systems CH3ReO3/H2O2, H3NMoO3/H2O2, and H3NOMoO3/H2O2 we re characterized, including Ligated and unligated mono- and bisperoxo inter mediates as well as hydroperoxo derivatives. For the rhenium system the bis peroxo complex CH3ReO(O-2)(2).H2O was found to be most stable and the one w ith the lowest transition state for epoxidation of ethylene (activation bar rier of 16.2 kcal/mol), in line with experimental findings. However, partic ipation of monoperoxo and hydroperoxo complexes in olefin epoxidation canno t be excluded. For both molybdenum systems, hydroperoxo species with an add itional ammonia model ligand in axial position were calculated to be most s table. Inspection of calculated activation barriers of ethylene epoxidation reveals that, in both molybdenum systems, hydroperoxo mechanisms are compe titive if not superior to peroxo mechanisms. The reaction barriers elf the various oxygen transfer processes can be rationalized by structural, orbita l, and charge characteristics, exploiting a model that:interprets the elect rophilic nature of the reactive oxygen center.