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)­Cl₃] (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 °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 °C to produce complex [3]. A separate chemical reaction between aniline and strongly oxidizing tetra-n-propylammonium perruthenate [(n-pr)₄N]⁺[RuO₄]⁻ in air produced a ruthenium complex [4] of a N⁴-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^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 N⁴-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)­Cl₃] and [(n-pr)₄N]⁺[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 N⁴-tetraamidophenylmacrocyclic ligand. Valence ambiguity in the complexes of the resultant redox-active ligands is scrutinized.