Inner-sphere electron transfer at the ruthenium-azo interface

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 51; no. 6; pp. 2547 - 2559
• Main Authors: Panda, Sanjib, Singh, Aditi, Dey, Sanchaita, Huang, Kuo-Wei, Lahiri, Goutam Kumar
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 08.02.2022
• Subjects: Coordination compounds; Electron transfer; Electrons; Ruthenium
• ISSN: 1477-9226; 1477-9234; 1477-9234
• DOI: 10.1039/d1dt03934b

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-abim 2− )] ( 2 )/[{Ru III (acac) 2 } 2 (μ-abim&z.rad; − )]ClO 4 ([ 2 ]ClO 4 ) frameworks. Structural, spectroscopic, electrochemical, and theoretical analysis of the complexes revealed the varying redox states of the azo functionality of abim, i.e. , [-N&z.dbd;N-] 0 , [-NN-]&z.rad; − , and [-N-N-] 2− in 1 , [ 2 ]ClO 4 , 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-]&z.rad; − ) 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 I 2 (μ-abim) 3 ](BF 4 ) 2 ([ 3 ](BF 4 ) 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). Intramolecular metal-to-ligand electron transfer induced multi-step reduction of azobis(1-methylbenzimidazole) and its varying binding scenarios have been illustrated on selective {Ru II (acac) 2 } (acac=acetylacetonate) and Cu I -platforms.