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

Reaction of MoO2Cl2(THF)(2) with mono- (L-1) and bi-dentate (L-2) pyrazole type ligands leads to octahedral complexes of formula MoO2Cl2(L-1)(2) and MoO2Cl2(L-2) (1, L-1 = 3,5-Me(2)pz; 2, L-2 = Me2C(pz)(2)). The structure of 2 has been determined crystallographically. Reaction of MoO2X2(THF)(2) (X = Cl or Br) with tridentate ligands (L-3), 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(L-3)]X (3, X = Cl, L-3 = HC(pz)(3); 4, X = Br, L-3 = HC(pz)(3); 4, X = Cl, L-3 = HC(3,5-Me(2)pz)(3)) and MoO2Cl{(pz)(3)BH} 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 Mo-95 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.