Inner-sphere electron transfer at the ruthenium-azo interface.

Sanjib Panda, Aditi Singh, Sanchaita Dey, Kuo-Wei Huang, Goutam Kumar Lahiri

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8 Scopus citations


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 [RuII(acac)2(abim)] (1) and dinuclear [{RuIII(acac)2}2(μ-abim2−)] (2)/[{RuIII(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., [–N[double bond, length as m-dash]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(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(CuI) → π*(azo, abim) back-bonding, with reference to its non-coordinating counterpart (1.265(6) Å).
Original languageEnglish (US)
JournalDalton transactions (Cambridge, England : 2003)
StatePublished - Jan 25 2022

Bibliographical note

KAUST Repository Item: Exported on 2022-01-28
Acknowledgements: Financial support received from SERB (J. C. Bose Fellowship, G. K. L.), UGC (fellowship to A. S. and S. D.), New Delhi, India, and KAUST (S. P. and K. W. H.) is gratefully acknowledged.

ASJC Scopus subject areas

  • Inorganic Chemistry


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