Inner-sphere electron transfer at the ruthenium-azo interface

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)(mu-abim(2-))] (2)/[{Ru-III(acac)(2)}(2)(mu-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., [-N=N-](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 [CuI2(mu-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 (similar to 1.30 angstrom), via d pi(Cu-I) -> pi*(azo, abim) back-bonding, with reference to its non-coordinating counterpart (1.265(6) angstrom).