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 ‐Bu 2 pzH) and 3,5‐di‐ tert ‐butyl‐ 4‐bromopyrazole ( t ‐Bu 2 ‐4‐BrpzH). The products [MoO 2 Cl(η 2 ‐ t ‐Bu 2 pz)] ( 1 ), [MoO 2 (η 2 ‐ t ‐Bu 2 pz) 2 ] ( 2 ), [MoO 2 (η 2 ‐ t ‐Bu 2 ‐4‐Brpz) 2 ] ( 3 ), [WO 2 (η 2 ‐ t ‐Bu 2 pz) 2 ] ( 4 ) and [WO 2 (η 2 ‐ t ‐Bu 2 ‐4‐Brpz) 2 ] ( 5 ) were characterised by spectroscopic techniques. The X‐ray structure of complex 3 reveals a distorted trigonal prismatic geometry with two η 2 ‐co‐ordinated pyrazolate ligands. These high‐valent compounds participate in oxygen‐transfer reactions and catalyse the oxidation of PPh 3 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 PEt 3 or treatment of [MoOCl 2 (PMe 3 ) 2 ] with two equivalents of t ‐Bu 2 pzK mononuclear mono‐oxo compounds of the type [MoO( t ‐Bu 2 pz) 2 (PR 3 ) 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.