Dichloro and dimethyl dioxomolybdenum(vi)–diazabutadiene complexes as catalysts for the epoxidation of olefins

Abstract

The dioxomolybdenum(VI) complex [MoO₂Cl₂{p-tolyl(CH₃DAB)}] has been prepared in good yield by reaction of the solvent adduct MoO₂Cl₂(THF)₂ with one equivalent of the bidentate ligand N,N-p-tolyl-2,3-dimethyl-1,4-diazabutadiene. Treatment of the dichloro complex with the Grignard reagent CH₃MgCl gives the dimethyl derivative [MoO₂(CH₃)₂{p-tolyl(CH₃DAB)}]. The complexes are highly active and selective catalysts for the homogeneous epoxidation of cyclooctene using tert-butyl hydroperoxide (TBHP) as the oxidant. In both cases, the initial activity is ca. 175 mol mol⁻¹_Mo h⁻¹ and cyclooctene oxide is obtained quantitatively within 4 h. It was possible to recover the dimethyl complex at the end of the reaction and reuse it in a second run with only a small decrease in activity. The complexes are also active and selective for the epoxidation of other olefins, such as 1-octene, 2-octene, cyclododecene and (R)-(+)-limonene, with TBHP. The catalytic production of cyclooctene oxide was investigated in detail, varying either the reaction temperature or the initial concentrations of substrate, oxidant and catalyst precursor. Kinetic studies show that the catalyst precursor–oxygen donor complex formation is first-order in TBHP and in the metal complex [MoO₂Cl₂{p-tolyl(CH₃DAB)}]. A specific rate of 3.2 mol⁻¹ dm³ s⁻¹ was found for catalyst formation at 25 °C. Activation parameters for this reaction have also been measured (ΔH^≠ = 48 ± 3 kJ mol⁻¹, ΔS^≠ = −112 ± 10 J mol⁻¹ K⁻¹).