Oxygen-Transfer Reactions of Molybdenum- and Tungstendioxo Complexes Containing η2-Pyrazolate Ligands

Dioxomolybdenum and ‐tungsten compounds containing sterically demanding pyrazolate ligands have been synthesised by treatment of dioxometal halides with the potassium salts of 3,5‐di‐ tert‐butylpyrazole (t‐Bu2pzH) and 3,5‐di‐tert‐butyl‐ 4‐bromopyrazole (t‐Bu2‐4‐BrpzH). The products [MoO2Cl(η2‐t‐Bu2pz)] (1), [MoO2(η2‐t‐Bu2pz)2] (2), [MoO2(η2‐t‐Bu2‐4‐Brpz)2] (3), [WO2(η2‐t‐Bu2pz)2] (4) and [WO2(η2‐t‐Bu2‐4‐Brpz)2] (5) were characterised by spectroscopic techniques. The X‐ray structure of complex 3 reveals a distorted trigonal prismatic geometry with two η2‐co‐ordinated pyrazolate ligands. These high‐valent compounds participate in oxygen‐transfer reactions and catalyse the oxidation of PPh3 with dimethyl sulphoxide. UV/VIS measurements of the oxo‐transfer reactions and the kinetics of the catalytic process are described. By the reaction of 2 with three equivalents of PEt3 or treatment of [MoOCl2(PMe3)2] with two equivalents of t‐Bu2pzK mononuclear mono‐oxo compounds of the type [MoO(t‐Bu2pz)2(PR3)2] (R=Et 6, R=Me 7) were obtained and characterised by X‐ray diffraction analyses. This points to biologically relevant mononuclear Mo(IV) intermediates in the catalytic process with this type of complex.