Ortho-Carom–N bond fusion in aniline associated with electrophilic chlorination reactions at ruthenium(III) coordinated acetylacetonates

Abstract

In an unusual reaction of [RuIII(acac)2(CH3CN)2](ClO4) ([1], acac=acetylacetonate) and aniline (Ph-NH2), resulted in the formation of ortho-semidine due to dimerisation of aniline via oxidative ortho-Carom–N bond formation reaction. This oxidation reaction is associated with stepwise chlorination of coordinated acac ligands at the γ-carbon atom resulting in the formation of [RuIII(acac)2L−] [2a], [RuIII(Cl-acac)(acac)L−] [2b], [RuIII(acac)(Cl-acac)L−] [2c] and [RuIII(Cl-acac)2L−] [2d] (L−=N-phenyl-ortho-semiquinonediimine) complexes, respectively. These have been characterized by 1H NMR, UV–Vis–NIR, ESI-MS and cyclic voltammetry studies. Single crystal X-ray structures of 2c and 2d are reported. Crystallographic structural bond parameters of 2c and 2d revealed bond length equalization of C–C, C–O and M–O bonds. It has been shown that perchlorate (ClO4-) counter anion, present in the starting ruthenium complex, acts as the oxidizing agent in bringing about oxidation of Ph-NH2 to ortho-semidine. The chloronium ions, produced in situ, chlorinate the coordinated acac ligands at the γ-carbon atom. Such electrophilic substitution of coordinated acac ligands indicates that the Ru-acac metallacycles in the reference compounds are aromatic. The complexes showed an intense and featureless band centered near 520nm, and a structured band near 275nm. These displayed one reversible cathodic response in the range, −1.1 to −0.8V and one reversible anodic response between 0.4 and 0.6V versus the Saturated Calomel reference Electrode, SCE. The response at the anodic potential is due to oxidation of the coordinated ligand L, while the reversible response at cathodic potential is due to reduction of the metal center.