Synthesis and catalytic application of octahedral lewis base adducts of dichloro and dialkyl dioxotungsten(VI)

Complexes of the composition W(O)₂(CI)₂L₂ and W(O)₂(R)₂L₂ (R = Me, Et; L₂ = bidentate Lewis base ligand) have been prepared and are fully characterized (including an exemplary X-ray crystal structure of W(O)₂(CI)₂(4,4′-ditert-butyl-2,2′ -bipyridine)). This latter compound crystallizes in the orthorhombic space group P2₁2₁2₁ with a = 8.3198(1) Å b = 13.3224(2) Å, c = 18.0415(2) Å, and Z = 4. The title complexes are applied as catalysts in olefin epoxidation catalysis with tert-butyl hydroperoxide (TBHP) as the oxidizing agent. The W(VI) complexes display only moderate turnover frequencies but can be reused several times without loss of catalytic activity. The highest activity can be achieved at reaction temperatures of ca. 90 °C. Chloro derivatives are somewhat more active than alkyl complexes, and sterically less crowded complexes show also higher activities than their congeners with bulky ligands L₂. Kinetic examinations show that the catalyst formation is the rate determining step and it is observed that tert-butyl alcohol, the byproduct of the epoxidation reaction, acts as a competitor for TBHP, thus lowering the reaction velocity during the course of the reaction but not irreversibly destroying the catalyst.