Iridium(III) Mediated Reductive Transformation of Closed-Shell Azo-Oxime to Open-Shell Azo-Imine Radical Anion: Molecular and Electronic Structure, Electron Transfer, and Optoelectronic Properties

The hydrogen bonded bis azo-oximato [IrCl2(L(NOH))(L(NO))] 2 and its deprotonated form (Et3NH)[IrCl2(L(NO))2] (Et3NH)(+)3(-) have been isolated in the crystalline state by a facile synthetic method. The azo-oxime frameworks in 3(-) have been conveniently transformed to the azo-imine by reduction with NaBH4 or ascorbic acid. Notably, the coordinated azo-imines accept an extra electron thereby furnishing the azo-imine radical anion complex 4. The underlying reductive transformation can be best described by proton-coupled electron transfer (PCET) process. Both the coordinated ligands (azo-oxime) in 3(-) are typically closed-shell monoanion (L(NO-)), but their reduced form (azo-imine) can behave as open-shell monoanion (L(NH•-)) owing to the presence of highly stabilized virtual orbitals. Remarkable enhancement of the π-acidity in azo-imine relative to the precursor azo-oxime has also been reflected from the electrochemical study. The irido complexes display rich optoelectronic properties, and the origin of the transitions has been scrutinized by the TD-DFT method. The molecular geometries of the complexes 2 and 3(-) reveal that the syn orientation of the azo-oximes frameworks is favored because of strong noncovalent H-bonding and π-π stacking interactions. In the course of the reduction of 3(-), the sterically encumbered disposition of the azo-oximes is converted to the relaxed anti form in the transformed azo-imines. Diffraction study reveals the electronic structure of 4 as [Ir(III)Cl2{(L(NH))2(•-)}]. The superior stabilization of the unpaired spin on the ligand array rather than metal has also been substantiated from EPR and DFT studies. Theoretical analysis reveals that the odd electron delocalizes primarily over both the azo-imine moieties ([IrCl2(L(NH•-))(L(NH))] ↔ ([IrCl2(L(NH))(L(NH•-))]) with no apparent contribution from metal, and this type of ligand-centered mixed valency (LCMV) can be best expressed as Robin-Day class III (fully delocalized) in nature.