Role of Mediator and Effects of Temperature on ortho-C-N Bond Fusion Reactions of Aniline Using Ruthenium Templates: Isolation and Characterization of New Ruthenium Complexes of the in-Situ-Generated Ligands

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-III(terpy)Cl-3] (terpy = 2,2':6',2 ''-terpyridine) resulted in a trans bis-aniline ruthenium(II) complex [1](+) which upon oxidation with H2O2 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 degrees C produced a bis-chelated ruthenium complex (3). A previously reported complex [Ru-II(N-phenyl-1,2-benzoquinonediimine)(aniline)(2)(Cl)(2)] (5) undergoes similar oxidation by air at 185 degrees C to produce complex [3]. A separate chemical reaction between aniline and strongly oxidizing tetra-n-propylammonium perruthenate [(n-pr)(4)N](+)[RuO4](-) in air produced a ruthenium complex [4] of a N-4-tetraamidophenylmacrocycle ligand via multiple ortho-C-N bond fusion reaction. Notably, the yield of this product is low (5%) at 100 degrees 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-II(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-4-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(av)=2.06. The reduced complex [2], on the other hand, shows a single-line EPR signal at g(av)=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](2+) 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-III(terpy)Cl-3] and [(n-pr)(4)N](+)[RuO4)](-), 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-4-tetraamidophenylmacrocyclic ligand. Valence ambiguity in the complexes of the resultant redox-active ligands is scrutinized.