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)] <b>2</b> and its deprotonated form (Et₃NH)­[IrCl₂(L^NO)₂] <b>(Et</b>_<b>3</b><b>NH)</b>^<b>+</b><b>3</b>^<b>–</b> have been isolated in the crystalline state by a facile synthetic method. The azo-oxime frameworks in <b>3</b>^<b>–</b> 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 <b>4</b>. The underlying reductive transformation can be best described by proton-coupled electron transfer (PCET) process. Both the coordinated ligands (azo-oxime) in <b>3</b>^<b>–</b> 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 <b>2</b> and <b>3</b>^<b>–</b> reveal that the <i>syn</i> 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 <b>3</b>^<b>–</b>, the sterically encumbered disposition of the azo-oximes is converted to the relaxed <i>anti</i> form in the transformed azo-imines. Diffraction study reveals the electronic structure of <b>4</b> as [Ir^IIICl₂{(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

Ambient-Stable Bis-Azoaromatic-Centered Diradical [(L^•)M(L^•)] Complexes of Rh(III): Synthesis, Structure, Redox, and Spin–Spin Interaction

Bis-azoaromatic electron traps, viz. 2-(2-pyridylazo)­azoarene <b>1</b>, have been synthesized by colligating electron-deficient pyridine and azoarene moieties, and they act as apposite proradical templates for the formation of stable open-shell diradical complexes [(<b>1</b>^•–)­Rh^III(<b>1</b>^•–)]⁺ ([<b>2</b>]⁺), starting from the low-valent electron reservoir [Rh^I]. The less stable monoradical [Rh^III(<b>1</b>^•–)­Cl₂(PPh₃)₃] (<b>3</b>) has also been isolated as a minor product. These π-radical complexes are multiredox systems, and the electron transfer processes occur exclusively within the pincer-type NNN ligand backbone <b>1</b>. Molecular and electronic structures of the diradicals and monoradicals have been ascertained with the aid of X-ray diffraction, electrochemical, spectroelectrochemical, and spectral (electronic, IR, NMR, and EPR) studies. In the diradicals [<b>2</b>]⁺, the orthogonal disposition of two ligand π orbitals linked via a closed-shell metal center (t₂⁶) impedes significant coupling between the radicals. Indeed, the observed magnetic moment of [<b>2a</b>]^<b>+</b> lies near ∼2.3 μ_B over the temperature range 50–300 K. A very weak antiferromagnetic (AF) intramolecular spin–spin interaction between two ligand π arrays in [<b>(1</b>^•–)­Rh^III(<b>1</b>^•–)]⁺ have been found experimentally (<i>J</i> ≈ −5 cm^–1), and this is further substantiated by density functional theory (DFT) calculations at the (U)­B3LYP/6-31G­(d,p) level