Molybdenum(VI) cis-dioxo complexes bearing (poly)pyrazolyl-methane and -borate ligands: syntheses, characterization and catalytic applications

Abstract

Reaction of MoO₂Cl₂(THF)₂ with mono- (L₁) and bi-dentate (L₂) pyrazole type ligands leads to octahedral complexes of formula MoO₂Cl₂(L₁)₂ and MoO₂Cl₂(L₂) (1, L₁ = 3,5-Me₂pz; 2, L₂ = Me₂C(pz)₂). The structure of 2 has been determined crystallographically. Reaction of MoO₂X₂(THF)₂ (X = Cl or Br) with tridentate ligands (L₃), e.g. tris(pyrazolyl)methane or tris(pyrazolyl)borate, leads to the replacement of both co-ordinated solvent molecules and one of the chloride ligands to give [MoO₂X(L₃)]X (3, X = Cl, L₃ = HC(pz)₃; 4, X = Br, L₃ = HC(pz)₃; 5, X = Cl, L₃ = HC(3,5-Me₂pz)₃) and MoO₂Cl{(pz)₃BH} 6. Depending on the donor ability and the steric bulk of the ligand, rapid rotation is observed at room temperature in some cases. At lower temperatures the structures are less fluxional. Nevertheless, bidentate and tridentate pyrazolyl ligands produce distinctly different chemical shifts in the ⁹⁵Mo NMR spectra. The turnover frequencies of the described complexes in olefin epoxidation, with t-butyl hydroperoxide as oxidizing agent, are in the range of 150–460 [mol/(mol catalyst h)]. This activity is in the middle of the range observed for MoO₂X₂L₂ complexes with N–donor ligands.