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
(acac)
(abim)] (1) and dinuclear [{Ru
(acac)
}
(μ-abim
)] (2)/[{Ru
(acac)
}
(μ-abim˙
)]ClO
([2]ClO
) frameworks. Structural, spectroscopic, electrochemical, and theoretical analysis of the complexes revealed the varying redox states of the azo functionality of abim,
, [-NN-]
, [-NN-]˙
, and [-N-N-]
in 1, [2]ClO
, and 2, respectively. Comparison between the calculated azo bond distances of analogous {Ru(acac)
}-coordinated azoheteroaromatics,
, 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 (
, the case of back-bonding) on stabilising radical anionic ([-NN-]˙
) and hydrazido ([-N-N-]
) bridging modes in the complexes. An evaluation of the electronic forms of the complexes in accessible redox states
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(μ-abim)
](BF
)
([3](BF
)
) 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),
dπ(Cu
) → π*(azo, abim) back-bonding, with reference to its non-coordinating counterpart (1.265(6) Å).