Monomeric Mo(V) and Mo(VI) complexes with sterically constrained metal centers

Abstract

Bray has proposed a chemical mechanism for the reduction of xanthine oxidase by xanthine, which involves a monomeric molybdenum active center having fac stereochemistry [1]. This proposal has simulated our interest in preparation and characterization of monomeric Mo(V) and Mo(VI) complexes constrained fac configuration by polydentate ligands such as hydrotris(3,5-dimethylpyrazolyl) borate hereafter designated as HB(Me 2pz) − 3. The HB(Me 2pz) − 3 ligand has been extensively used to stabilize a variety of low valent molybdenum compounds [2]. Moreover, the same ligand has been found to stabilize the Mo(V) center in MoOCl 2{HB(Me 2pz) 3}( I) [3]. The relative stability of these compounds is attributed partly to the steric bulk of 3-methyl group on the ligand. Mo(V) complexes of the type MoOXY{HB(Me 2pz) 3} (where X = Y = NCS; X = Cl, Y = OR or SPh; X = Y = SPh) have been prepared by the substitution reactions of I and spectroscopically characterized. Esr spectra of Mo(V) centers are sensitive to X and Y. Substitutions by thiolate ligands give smaller A o(Mo) and larger g o values. These substitutions also shift the MoO stretching vibration significantly to lower wave numbers. A preliminary kinetic study has revealed that the rates of ligand substitution are very slow in these complexes, compared to those of known MoOCl 3L 2 complexes (where L is a monodentate ligand) [4]. Mo(VI) complexes of the type MoO 2X{HB(Me 2pz)3} (X = Cl, Br, NCS) have been synthesized for the first time by the reaction of MoO 2X 2 (X = Cl, Br) or MoO 2(NCS) 2− 4 with the ligand, and characterized by spectroscopic methods including 95Mo NMR. Electrochemical studies and structural studies on these Mo(V) and Mo(VI) complexes will also be described.