Inner-sphere electron transfer at the ruthenium-azo interface

Abstract

Metal complexes exhibiting multiple reversible redox states have drawn continuing research interest due to their electron reservoir features. In this context, the present article describes ruthenium-acac complexes (acac = acetylacetonate) incorporating redox-active azo-derived abim (azobis(1-methylbenzimidazole)) in mononuclear [Ru^II(acac)₂(abim)] (1) and dinuclear [{Ru^III(acac)₂}₂(μ-abim²⁻)] (2)/[{Ru^III(acac)₂}₂(μ-abim˙⁻)]ClO₄ ([2]ClO₄) frameworks. Structural, spectroscopic, electrochemical, and theoretical analysis of the complexes revealed the varying redox states of the azo functionality of abim, i.e., [–NN–]⁰, [–NN–]˙⁻, and [–N–N–]²⁻ in 1, [2]ClO₄, and 2, respectively. Comparison between the calculated azo bond distances of analogous {Ru(acac)₂}-coordinated azoheteroaromatics, i.e., abim and previously reported abbt (azobis(benzothiazole)) and abpy (azobis(pyridine)) examples, revealed the impact of varying amounts of intramolecular metal-to-azo electron transfer (i.e., the case of back-bonding) on stabilising radical anionic ([–NN–]˙⁻) and hydrazido ([–N–N–]²⁻) bridging modes in the complexes. An evaluation of the electronic forms of the complexes in accessible redox states via combined experimental and theoretical studies suggested a preferred resonance configuration rather than a precise description, primarily due to the severe mixing of metal-abim frontier orbitals. Moreover, the newly developed corresponding Cu-abim complex [Cu^I₂(μ-abim)₃](BF₄)₂ ([3](BF₄)₂) demonstrated the unique scenario of varying bridging modes of abim within the same molecular unit, involving both coordinated and non-coordinated azo functionalities. This also reemphasised the concept of the coordination-induced lengthening of the azo bond of abim (∼1.30 Å), via dπ(Cu^I) → π*(azo, abim) back-bonding, with reference to its non-coordinating counterpart (1.265(6) Å).