Olefin epoxidation by peroxo complexes of Cr, Mo, and W. A comparative density functional study

The epoxidation of olefins by peroxo complexes of Cr(VI), Mo(VI) and W(VI) was investigated using the B3LYP hybrid density functional method. For the mono- and bisperoxo model complexes with the structures (NH3)(L)M(O)(2-n)(e ta(2)-O-2)(1+n) (n = 0, 1; L = none, NH3; M = Cr, Mo, W) and ethylene as mo del olefin, two reaction mechanism were considered, direct oxygen transfer and a two-step insertion into the metal-peroxo bond. The calculations revea l that direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a transition state of spiro structure is preferred as sig nificantly higher activation barriers were calculated for the insertion mec hanism than for the direct mechanism. W complexes are the most active in th e series investigated with the calculated activation barriers of direct oxy gen transfer to ethylene decreasing in the order Cr > Mo > W. Barriers of b isperoxo species are lower than those of the corresponding monoperoxo speci es. Coordination of a second NH3 base ligand to the mono-coordinated specie s, (NH3)M(O)(2)(eta(2)-O-2) and (NH3)MO(eta(2)-O-2)(2), results in a signif icant increase of the activation barrier which deactivates the complex. Fin ally, based on a molecular orbital analysis, we discuss factors that govern the activity of the metal peroxo group M(eta(2)-O-2), in particular the ro le of metal center.