Metathesis activity of monomeric Mo-methylidene centres on (1 0 0) and (1 1 0)C surfaces of γ-Al2O3––a comparative DFT study

Pathways of propene addition to monomeric Mo-methylidene centres situated on the (1 0 0) and (1 1 0)C surfaces of γ-Al2O3 are studied with DFT, applying the cluster model approach. Trigonal bipyramidal molybdacyclobutanes, being intermediates of propene metathesis, as well as the square pyramidal ones, have been localised during investigations of both paths. According to the calculations, formation of the trigonal bipyramidal intermediate on the (1 0 0) alumina surface is endothermic and has a relatively high activation barrier. On the other hand, a nucleophilic propene attack on the Mo-methylidene centre situated on the (1 1 0)C alumina surface results in exothermic formation of a π-complex that rearranges to the stable trigonal bipyramidal molybdacyclobutane. The relative stabilities of the trigonal bipyramidal and square pyramidal molybdacyclobutanes also depend on their localisation on alumina.