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

Abstract

Dioxomolybdenum and -tungsten compounds containing sterically demanding pyrazolate ligands have been synthesised by treatment of dioxometal halides with the potassium salts of 3,5-ditert-butylpyrazole (t-Bu 2 pzH) and 3,5-di-tert-butyl-4-bromopyrazole (t-Bu 2 -4-BrpzH). The products (5) were characterised by spectroscopic techniques. The X-ray structure of complex 3 reveals a distorted trigonal prismatic geometry with two h 2 -co-ordinated pyrazolate ligands. These high-valent compounds participate in oxygentransfer 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 ] (R¼ Et 6, R¼ 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.

Oxidomolybdenum(IV), ‐(V), ‐(VI) Complexes with Relevance to Molybdenum Enzymes: Oxygen Atom Transfer, Redox Chemistry and EPR Spectroscopy

“…These findings are commensurate with the results of Mösch-Zanetti et al, who employed β-ketiminato or η 2 -pyrazolato ancillary ligands, and with our results for the 1/ PMe 3 (n = 3) system. [14][15][16] With the larger triphenylphosphane (Tolman cone angle 145° [ 20] ) the reaction stops at the µ-oxido dimer 4, and no triphenylphosphane complex is observed. These reactions are summarized in Scheme 3.…”

“…[13] In some instances, excess of phosphane substrate has been shown to result in phosphane-coordinated molybdenum(IV) species, e.g., with ancillary β-ketiminato, η 2 -pyrazolato or iminopyrrolato ligands [Scheme 1, Equation (5)]. [14][15][16] Stable molybdenum(V) species with relevance to a biomimetic catalytic cycle have been prepared by reduction of dioxidomolybdenum(VI) complexes employing, e.g., the exceptionally versatile hydridotris(3,5-dimethylpyrazol-1-yl)borato co-ligands. [17,18] Double fixation of dioxidomolybdenum(VI) complexes to insoluble polymer supports by two iminopyrrolato ligands has been successfully used by our group to suppress comproportionation [Scheme 1, Equation (6)] and Scheme 1.…”

Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

“…Dioxido di(ˇ-ketiminato) molybdenum(VI) complexes Mo(L NO ) 2 (O) 2 investigated by Mösch-Zanetti transfer an oxygen atom to phosphanes [Scheme 4f; (L NO ) − = ˇ-ketiminato] [75,76]. In contrast to the trispyrazolylborato complexes the phosphane oxide is substituted by a phosphane ligand in OAT reactions with phosphane [75,76].…”

Molybdenum and Tungsten Oxidoreductase Models

“…In order to force a sixfold coordinated molybdenum centre into a trigonal prismatic coordination geometry, Most et al used nitrogendonor pyrazolate (pz) ligands instead of sulfur-donor dithiolene ligands, the former having significantly smaller bite angles (Figure 5a). [17] The resulting [MoO 2 (pz) 2 ] complexes were indeed of trigonal prismatic geometry, as evidenced by X-ray crystallography. For a comparison, octahedral Schiff base (SB) complexes, [MoO 2 (SB) 2 ] (Figure 5b), which coordinated to molybdenum via nitrogen and oxygen donors, were synthesised.…”