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Herein, the reactivity of an aromatic diamine, 2,2′-diaminoazobenzene 1 [H2NLNH2], with rhodium salts was investigated. Diverse coordinations, specifically single and double NH deprotonated forms, of the title ligand are obtained during metallation, affording mononuclear 2 and dinuclear 3 complexes, respectively. An unprecedented edge-shared bioctahedral geometry with a syn configuration (sterically encumbered) about the RhIII2N2 core is obtained with pincer-type NNN ligation. Both complexes are redox-active and provide well-defined oxidative responses, and the reversibility of the redox-couples is enhanced with the decreasing temperature. The spectroscopic study indicates the formation of open-shell species derived from the precursor amido complexes, and these species are believed to be ligand-centered π radicals. The incorporation of both electron-poor azo and electron-rich amido moieties imparts the possibility of low-energy electronic transitions within the tridentate ligand (the push–pull effect). Furthermore, the occurrence of ILCT has been substantiated theoretically (TDDFT and NTO).

Luminescent nickel(II) complexes of type [NiII(L−I)2], 2, have been synthesized using photosensitizer redox-active oxime ligands HL, 1, incorporating the π-acidic azo and pyridyl functions. The redox non-innocent behaviour of the ligands has been exploited to isolate the open shell Ni(II)-bound azo-oxime anion radical complexes of type Et4N[NiII{(L−I)2}•−], 3, via reduction with NaBH4. The superior stabilization of the unpaired spin over the ligand framework has been established by EPR and DFT studies. The anti-bacterial activity of 2 has been scrutinized and it has been found to exhibit potential radical scavenging activity.

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.

Chelating behavior of a potential flexidentate bisazoaromatic molecule 2-(phenylazo)azobenzene (PAAB) toward palladium have been examined. Activation of one instead of the two C(Ph)–H bonds of PAAB (HL) in the course of the metallation furnishes an exclusive unsymmetrical mode of ligation. Consequently, another member of the important class of CNN pincer palladacycles is achieved in almost quantitative yields. Crystallographic analysis authenticates the monoanionic terdentate ligation of the bisazoaromatic molecule and neutral [PdII(L)X] (X = acetate and halides) complexes with planar palladacycle and out-of-plane pendant phenyl ring. Unlike the N-donor pincers incorporating imine moieties, the introduction of two azo chromophores not only makes the PAAB ligand strongly π-acidic but also significantly modulates the FMOs in the palladacycles. An appreciable lowering of ligand-centered (LC) π* orbitals is apparent upon complexation which is in accordance with the electrochemical responses where large anodic shifts of LC LUMO and LUMO + 1 by 0.8–0.9 V have been noted. The significant modification of FMOs leads to interesting optoelectronic properties in the palladacycles. The cyclopalladated compounds are luminescent in solution at room temperature. This property is plausibly a result of the LC emissive states (azo π*). The optoelectronic features are interpreted by the Time-Dependant (TD) density functional theory (DFT) and natural transition orbital (NTO) analyses. In addition, scrutiny of Suzuki–Miyaura and Heck-type reactions indicates that the pincer palladacycles may be employed in the development of useful catalysts for C–C coupling reactions.

The reaction between a potential flexidentate pyridyl-azo-oxime HL1 and Co(ClO4)2 yields novel homoleptic complexes of types [Co(III)(L(-I))3], 2 and [Co(III)(L(-I))2]ClO4, 3⁺ClO₄ in N6 and N4O2 coordination environments respectively. The FMOs of these complexes vary appreciably and are strongly modified by the coordination environment. This has striking influences on the spectral and redox properties of the metallo conjugates of ligand HL. The synthesized bis 2 and tris chelates 3⁺ possess well-defined optoelectronic and redox properties and these are scrutinized by the density functional theory (DFT) and time dependent density functional theory (TD-DFT) analyses. The visible excitations are primarily mixed singlet-manifold (1)ILCT and (1)LLCT transitions, with different amounts of ligand π-π* character while in the UV region, the excitations are essentially π-π* ILCT/LLCT transitions for the 3⁺ and ILCT/LLCT transitions along with the LMCT component for 2. The luminescent cobalt(III) species are rarely cited albeit these are found to be moderately blue emissive with slight quenching of the emission quantum yield (Φ) as compared to that of a free ligand. Computation reveals that the cobalt d orbital is involved in the triplet emissive excited states and this phenomenon is plausibly responsible for the quenching of the emission quantum yield in the complexes. Both types of complexes are electro-active in solution and the first reductive response, associated with the redox orbital comprising delocalized π orbital of a ligand, is shifted in the more positive potential (0.6 V) in 3⁺ relative to 2 and this observation is corroborated with the appreciable stabilization (~0.5 eV) of LUMO of 3⁺ (coordination mode A) as compared to that in 2 (coordination mode B). This provides us an opportunity to explore the cobalt-bound azo-oxime anion radical compound by reduction of the diamagnetic precursor 3⁺. The best description of the one-electron paramagnetic 3 can be ascertained as [Co(III){(L(-I))2}˙(-)] from the EPR and DFT studies where the unpaired spin is delocalized essentially over π* orbital comprising both the coordinated ligands (97%) with little participation of cobalt d(yz) (3%).