Theoretical studies of molybdenum peroxo complexes [MoO(n)(O₂)(3-n)(OPH₃)] as catalysts for olefin epoxidation

The equilibrium geometries of the molybdenum oxo/peroxo compounds MoO(n)(O₂)(3-n) and the related complexes [MoO(n)(O₂)(3-n)(OPH₃)] and [MoO(n)(O₂)(3-n)(OPH₃)(H₂O)] (n = 0-3) have been calculated using gradient-corrected density-functional theory at the B3LYP level. The structures of the peroxo complexes with ethylene ligands [MoO(n)(O₂)(3-n)(C₂H₄)] and [MoO(n)(O₂)(3-n)(OPH₃)(C₂H₄)] (n = 1, 2) where ethylene is directly bonded to the metal have also been optimized. Calculations of the metal - ligand bond-dissociation energies show that the OPH₃ ligand in [MoO(n)(O₂)(3-n)(OPH₃)] is much more strongly bound than the ethylene ligand in [MoO(n)(O₂)(3-n)(C₂H₄)]. This makes the substitution of phosphane oxide by olefins in the epoxidation reaction unlikely. An energy-minimum structure is found for [MoO(O₂)₂(OPH₃)(C₂H₄)], for which the dissociation of C₂H₄ is exothermic with D₀ = -5.2 kcal/mol. The reaction energies for the perhydrolysis of the oxo complexes with H₂O₂ and the epoxidation of ethylene by the peroxo complexes have also been calculated. The peculiar stability of the diperoxo complex [MoO(O₂)₂(OPH₃)(H₂O)] can be explained with the reaction energies for the perhydrolysis of [MoO(n)(O₂)(3-n)(OPH₃)(H₂O)]. The first perhydrolysis step yielding the monoperoxo complex is less exothermic than the second perhydrolysis reaction, but the further reaction with H₂O₂ yielding the unknown triperoxo complex is clearly endothermic. CDA analysis of the metal - ethylene bond shows that the binding interactions are mainly caused by charge donation from the ligand to the metal.