A new series of molybdenum-(iv), -(v), and -(vi) dithiolate compounds as active site models of molybdoenzymes: preparation, crystal structures, spectroscopic/electrochemical properties and reactivity in oxygen atom transfer

Abstract

A new set of molybdenum-(IV), -(V), and -(VI) compounds containing 3,6-dichloro-1,2-benzenedithiolate (bdtCl₂) were isolated and characterised by crystallographic and other spectroscopic techniques as active site models of arsenite oxidase, one of the molybdoenzymes. MoO₂ compounds were prepared in high yields by reaction of MoO₂Cl₂ with bdtCl₂, related dithiolene and thiocatecholate in methanol at low temperature. The bdtCl₂ ligand particularly stabilised the MoO compounds with oxidation numbers of +4 and +5 as well as the MoO₂ compound with an oxidation number of +6. The compounds (Et₄N)₂[Mo^VIO₂(bdtCl₂)₂] (1), (Et₄N)₂[Mo^IVO(bdtCl₂)₂] (2) and (Et₄N)[Mo^VO(bdtCl₂)₂] (3) were successfully isolated, whereas (Et₃NH)₂[MoO₂(thiocatecholate)₂] (6) gradually decomposed in acetonitrile. A distorted octahedral structure similar to that of (1) was suggested for the structure of the active site of the oxidised form of arsenite oxidase on the basis of a comparison of their bond distances and angles. The bond distances and angles around the molybdenum(IV) atom in (2) were similar to those around the molybdenum(IV) centre in the reduced form of arsenite oxidase. The reversible (2)/(3) couple exhibited a more positive redox potential than common MoO dithiolene compounds. (1) underwent an irreversible proton-coupled reduction process to yield (2). An oxygen atom transfer reaction of (1) with triphenylphosphine afforded (2) and triphenylphosphine oxide, and proceeded in second order as v = −d/dt [MoO₂] = k[MoO₂][PPh₃]. The structures and properties of the oxo-bridged dinuclear compound (Et₄N)₂[Mo^VIO₂(bdtCl₂)]₂(μ-O) (4), a dimer of bdtCl₂ (5) and (6) were also characterised.