Synthesis and structure of tris(1,2-benzoquinone diimine)osmium(II) perchlorate, and its redox and spectroelectrochemical series

Abstract

Reaction of [NH₄]₂[Os^IVBr₆] with 1,2-diaminobenzene (H₂opda) in aerobic conditions afforded [Os(bqdi)₃][ClO₄]₂ (bqdi = 1,2-benzoquinone diimine), a diamagnetic osmium(II) complex containing three redox non-innocent α,α′-diimine ligands. In the crystal structure the relatively long CN distances together with short Os–N distances indicate the existence of considerable metal(dπ)–ligand(pπ) interactions. Cyclic voltammetry of [Os(bqdi)₃][ClO₄]₂ shows one electron redox processes at −0.24, −0.67 and −1.78 V versus Fc–Fc⁺ (Fc = ferrocene) which are assigned as successive reductions of the coordinated bqdi, and an irreversible Os^II–Os^III couple at +0.99 V. The solution electronic spectra of the mixed-valent (ligand) compounds, generated by electrolysis of [Os(bqdi)₃]²⁺ in an optically transparent thin-layer electrode cell at potentials past the first and the second cathodic responses, show near-IR transitions at 1405 and 1164 nm, respectively, assignable to inter-valence charge transfer between the reduced 1,2-diiminosemiquinone monoanion (dsq) and non-reduced (bqdi) ligands. The spectrum of the triply reduced form is more consistent with the formulation [Os^III(dsq)₂(opda)]⁻ than with the expected formulation [Os(dsq)₃]⁻, suggesting that an internal redox isomerism has occurred.