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Ruthenium(II) complexes [RuL(CO)Cl(PPh3)2], 1a–4a, have been synthesized using ligands PhN＝NC(Ar)NOH, HL 1–4, respectively, by varying the pendant aryl (Ar) groups. The single crystal X-ray diffraction studies of complexes reveal that there are certain changes in Ru–Nazo and Ru–Noxime bond lengths that may be explained primarily on the basis of electronic effects of pendant aryl groups. In 4a, all Ru–N bond lengths are longer and this is attributed to oximato–O⋯O (hydroxyphenyl) interactions. Furthermore, there are weak intramolecular F⋯π interactions in 3a. The complexes display multiple reductive responses ascribable to electron acceptance within the azo-oxime framework of the coordinated ligand and the corresponding one-electron reduced metastable anion radical complexes of type [Ru(L˙−)(CO)Cl(PPh3)2]−[1]˙− have been generated. This property of the complexes has been exploited in electron transfer catalysis via trapping of electrons in the azo-oxime skeleton in two types of reactions: (1) α-alkylation of ketones with primary alcohols and (ii) synthesis of 2-substituted quinoline derivatives from 2-aminobenzyl alcohols and substituted acetophenones/alkyl methyl ketones. The scope of catalysis has been studied and the probable catalytic pathway has been established from experimental results. The catalytic pathway is ligand-centric and redox-driven for the dehydrogenation process and the initial step involves formation of a coordinated anion radical. This leads to conversion of the starting 1° alcohol to the corresponding carbonyl via the HAT pathway, with the ruthenium(II) centre practically behaving as a template and remaining redox inactive. The catalytic reactions have been proven to be affected by the nature of pendant aryl (Ar) groups within the coordinated ligand. [ABSTRACT FROM AUTHOR]

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes. [ABSTRACT FROM AUTHOR]