Dioxomolybdenum(VI) Complexes of the Hydrotris(3,5-Dimethyi-1-Pyrazolyi)borate Ligand. Synthesis and Oxygen atom Transfer Reactions

The complexes {HB(Me₂pz)₃}MoO₂X (HB(Me₂pz)₃⁻= hydrotris(3,5-dimethyl-l-pyrazolyl)borate; X = F⁻Cl⁻, Br⁻, NCS⁻, OPh⁻, OMe⁻, SPh⁻) have been prepared and characterized, and the crystal structure of the X = NCS⁻complex has been determined. {HB(Me₂pz)₃}MoO₂(NCS) crystallizes in the space group Pl, with a = 8.049 (1) Å, b = 8.669 (1) Å, c = 17.397 (2) Å, α = 79.98 (1)°, β = 81.30 (1)°, γ = 78.39 (1)°, and Z = 2; refinement resulted in R_w= 0.056 (3136 reflections with F_°²> MF_°²), 273 variable parameters). Three representative complexes (X = Cl⁻, SPh⁻, OPh⁻) react with Ph₃P at room temperature to yield Ph₃PO and a molybdenum-containing product having a composition dependent upon the solvent used. In DMF, pyridine, or acetonitrile, the Mo(IV) complexes {HB(Me₂pz₃)}MoOX(solvent) are formed as a result of the oxygen atom transfer reaction and subsequent solvent coordination. In dichloromethane, the ultimate products are the Mo(V) complexes {HB(Me₂pz)₃}MoOClX. In toluene, binuclear Mo(V) complexes {HB(Me₂pz)₃}₂Mo₂O₃X₂form from reaction of the transient {HB(Me₂pz)₃}MoOX and the starting complex. The identities of two reaction products have been confirmed by crystal structure determinations. {HB(Me₂pz)₃}MoO(Cl)(NCS) crystallizes in space group Pl, with a = 8.093 (1) Å, b = 8.840 (1) Å, c = 17.432 (3) Å, α = 79.30 (1)°, β = 80.66 (1)°, γ = 78.40 (1)°, and Z = 2; refinement resulted in R_w= 0.053 (2864 reflections with F_°²> 3σ(F_°²), 277 variable parameters). The structure is isomorphous with that of {HB(Me₂pz)₃}MoO₂(NCS). {HB(Me₂pz)₃}MoO(Cl)(NC₅H₅) crystallizes in Pl, with a = 9.636 (1) Å,b = 14.319 (2) Å, c = 20.802 (3) Å, α = 79.90 (1)°, β = 84.47 (1)°, γ = 77.82 (1)°, and Z = 4; refinement resulted in R_w= 0.071 (6777 reflections with F_°²> 3σ(F_°²), 589 variable parameters). The oxidation of Ph₃P to Ph₃PO by Me₂SO is catalyzed by {HB(Me₂pz)₃}MoO₂X complexes with X = Cl⁻, SPh⁻, and OPh⁻.