A controlled tandem synthetic route to azines from various alcohols and hydrazine hydrate by the use of a Ni(II) complex of 2,6-bis(phenylazo)pyridine as a catalyst is reported. In marked contrast to the previous report, the reaction is operative using an earth-abundant metal catalyst, milder reaction conditions, and aerobic conditions, which though are desirable but unprecedented in the literature. The catalytic reaction has a vast substrate scope including a single-step synthesis of phthalazine from 1,2-benzenedimethanol and hydrazine hydrate via intramolecular coupling. Mechanistic investigation suggests that the coordinated ligand redox controls the reaction by the use of a reversible azo (N═N)/ hydrazo (NH-NH) redox couple where the metal center is used primarily as a template.

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In this work, ortho-C-N bond fusion reactions of aniline are followed by the use of two different ruthenium mediators. Reaction of aniline with [Ru(terpy)Cl] (terpy = 2,2':6',2″-terpyridine) resulted in a trans bis-aniline ruthenium(II) complex [1] which upon oxidation with HO produced compound [2] of a bidentate ligand, N-phenyl-1,2-benzoquinonediimine, due to an oxidative ortho-C-N bond fusion reaction. Complex [1] and aniline (neat) at 185 °C produced a bis-chelated ruthenium complex (3). A previously reported complex [Ru(N-phenyl-1,2-benzoquinonediimine)(aniline)(Cl)] (5) undergoes similar oxidation by air at 185 °C to produce complex [3]. A separate chemical reaction between aniline and strongly oxidizing tetra-n-propylammonium perruthenate [(n-pr)N][RuO] in air produced a ruthenium complex [4] of a N-tetraamidophenylmacrocycle ligand via multiple ortho-C-N bond fusion reaction. Notably, the yield of this product is low (5%) at 100 °C but increases to 25% in refluxing aniline. All these complexes are characterized fully by their physicochemical characterizations and X-ray structure determination. From their structural parameters and other spectroscopic studies, complex [2] is assigned as [Ru(terpy)(N-phenyl-1,2-benzoquinonediimine)(Cl)] whereas complex [4] is described as a ruthenium(VI) complex comprised of a reduced deprotonated N-phenyl-1,2-diamidobenzene and N-tetraamidophenylmacrocyclic ligand. Complex [2] exhibits one reversible oxidation at 1.32 V and one reversible reduction at -0.75 V vs Ag/AgCl reference electrode. EPR of the electrogenerated complexes has revealed that the oxidized complex is a ruthenium(III) complex with an axial EPR spectrum at g= 2.06. The reduced complex [2], on the other hand, shows a single-line EPR signal at g= 1.998. In contrast, complex [4] shows two successive one-electron oxidation waves at 0.5 and 0.8 V and an irreversible reduction wave at -0.9 V. EPR studies of the oxidized complexes [4] and [4] reveal that oxidations are ligand centered. DFT calculations were employed to elucidate the electronic structures as well as the redox processes associated with the above complexes. Aerial ortho-C-N bond fusion reactions of aniline using two different mediators, viz. [Ru(terpy)Cl] and [(n-pr)N][RuO], have been followed. It is found that in the case of oxidizable Ru(III) mediator complex, C-N bond fusion is limited only to dimerization reaction whereas the high-valent Ru(VII) salt mediates multiple C-N bond fusion reactions leading to the formation of a novel tetradentate N-tetraamidophenylmacrocyclic ligand. Valence ambiguity in the complexes of the resultant redox-active ligands is scrutinized.

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In this work, a new Ru(II) complex with the redox‐active pincer 2,6‐bis(phenylazo)pyridine ligand (L) is reported which acts as a metal‐ligand bifunctional catalyst for transfer hydrogenation reactions. The isolated complex [(L)Ru(PMe2Ph)2(CH3CN)](ClO4)2; [1](ClO4)2 is characterized by a host of spectroscopic measurements and X‐ray structure determination. It is diamagnetic and single‐crystal X‐ray structure analysis reveals that [1]2+ adopts a distorted octahedral geometry where L binds Ru center in meridional fashion. The observed elongation in the coordinated azo bond length (1.29 Å) is attributed to the extensive π‐back bonding, dπ(RuII)→π*(azo)L. The complex [1](ClO4)2 acts as an efficient catalyst, which brings about catalytic transfer hydrogenation reactions of a broad array of aldehydes and ketones in isopropanol and in inert conditions. The selectivity of the catalyst for aldehyde reduction over the other reducible functional groups such as nitro, nitrile, ester etc was also investigated. Mechanistic studies, examined by suitable control reactions and isotope labelling experiments, indicate synergistic participation of both ligand and metal centres via the formation of a fleeting Ru−H intermediate and hydrogen walking to the coordinated azo function of L.