Radical versus Nonradical States of Azobis(benzothiazole) as a Function of Ancillary Ligands on Selective Ruthenium Platforms

The paper deals with the electronic impact of ancillary ligands on the varying redox features of azobis(benzothiazole) (abbt) in the newly introduced mononuclear ruthenium complexes [Ru(pap)₂(abbt)] (1) and [Ru(bpy)₂(abbt)] (2), where pap = 2-phenylazopyridine and bpy = 2,2'-bipyridine. In this regard, the complexes [Ru^II(pap)₂(abbt^•-)]ClO₄ ([1]ClO₄), [Ru^II(pap)₂(abbt⁰)](ClO₄)₂ ([1](ClO₄)₂), [Ru^II(bpy)₂(abbt⁰)](ClO₄)₂ ([2](ClO₄)₂), and [Ru^II(bpy)₂(abbt^•-)]ClO₄ ([2]ClO₄) were structurally and spectroscopically characterized. Unambiguous assignments of the aforestated radical and nonradical forms of abbt in 1/2 and 1/2, respectively, were made primarily based on their redox-sensitive azo (N═N) bond distances as well as by their characteristic electron paramagnetic resonance (EPR)/NMR signatures. Although the radical form of abbt^•- was isolated as an exclusive product in the case of strongly π-acidic pap-derived 1, the corresponding moderately π-acidic bpy ancillary ligand primarily delivered an oxidized form of abbt⁰ in 2, along with the radical form in 2 as a minor (<10%) component. The oxidized abbt⁰-derived [1](ClO₄)₂ was, however, obtained via the chemical oxidation of [1]ClO₄. Both 1 and 2 displayed multiple closed by reversible redox processes (one oxidation O1 and four successive reductions R1-R4) within the potential window of ±2.0 V versus saturated calomel electrode. The involvement of metal-, ligand-, or metal/ligand-based frontier molecular orbitals along the redox chain was assigned based on the combined experimental (structure, EPR, and spectroelectrochemisry) and theoretical [density functional theory (DFT): molecular orbitals, Mulliken spin densities/time-dependent DFT] investigations. It revealed primarily ligand (abbt/pap or bpy)-based redox activities, keeping the metal ion as a simple spectator. Moreover, frontier molecular orbital analysis corroborated the initial isolation of the radical and nonradical species for the pap-derived 1 and bpy-derived 2 as well as facile reduction of pap and abbt in 1 and 2, respectively.