Complexes of 2,2′-biphenol with ruthenium(ii), molybdenum(v), tungsten(v) and tungsten(vi): structures, electrochemistry and spectroscopy

Abstract

Complexes of the potentially bidentate ligand 2,2′-biphenol (H₂biph) have been prepared and studied by electrochemical and UV/Vis/NIR spectroelectrochemical methods. The complexes studied are [(Tp*)M(O)(biph)] [M = Mo, W; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate], [W(biph)₃], [Cl₆W₂(biph)₃] and [Ru(bpy)₂(biph)], and all have been structurally characterised. [(Tp*)M(O)(biph)] (M = Mo, W) both undergo reversible M(IV)/M(V) couples at negative potentials, with the redox potential for the W(IV)/W(V) couple being 580 mV more negative than that of the Mo(IV)/Mo(V) couple; the redox potentials are similar to those which occur in the complexes [(Tp*)M(O)(OC₆H₅)₂] with two monodentate phenolate ligands. The structure of [W(biph)₃] is essentially octahedral but with a distortion towards trigonal prismatic; the complex undergoes two metal-centred redox processes, W(IV)/W(V) and W(V)/W(VI) which were characterised spectroelectrochemically. An unexpected new W(VI) complex [Cl₆W₂(biph)₃] has the structure [{WCl₃(biph)}₂(μ-biph)], in which each W(VI) centre has a terminal chelating biphenolate ligand, and other highly twisted biphenolate ligand acts as a bis-monodentate bridge spanning the two W(VI) centres. [Cl₆W₂(biph)₃] undergoes two successive W(V)/W(VI) redox processes at negative potentials, with a separation of 170 mV indicating a through-space Coulombic interaction between the metal centres; spectroelectrochemistry showed no evidence of an inter-valence charge-transfer band in the mixed-valence W(V)–W(VI) state. [Ru(bpy)₂(biph)] has a Ru(II)/Ru(III) couple at a potential very similar to related ruthenium complexes with a (pyridine)₄(phenolate)₂ donor set.