Synthesis, Structure, and Electrochemical Properties of a Family of 2-(Arylazo)phenolate Complexes of Ruthenium with Unusual C−C Coupling and NN Cleavage

Reaction of 2-(4‘-R-phenylazo)-4-methylphenols (R = OCH3, CH3, H, Cl, and NO2) with [Ru(dmso)4Cl2] affords a
family of five ruthenium(III) complexes, containing a 2-(arylazo)phenolate ligand forming a six-membered chelate
ring and a tetradentate ligand formed from two 2-(arylazo)phenols via an unusual C−C coupling linking the two
ortho carbons of the phenyl rings in the arylazo fragment. A similar reaction with 2-(2‘-methylphenylazo)-4-methylphenol
with [Ru(dmso)4Cl2] has afforded a similar complex, in which one 2-(2‘-methylphenylazo)-4-methylphenolate ligand
is coordinated forming a six-membered chelate ring, and the other two ligands have undergone the C−C coupling
reaction, and the coupled species is coordinated as a tetradentate ligand forming a five-membered N,O-chelate
ring, a nine-membered N,N-chelate ring, and another five-membered chelate ring. Reaction of 2-(2‘,6‘-dimethylphenylazo)-4-methylphenol with [Ru(dmso)4Cl2] has afforded a complex in which two 2-(2‘,6‘-dimethylphenylazo)-4-methylphenols are coordinated as bidentate N,O-donors forming five- and six-membered chelate rings,
while the third one has undergone cleavage across the NN bond, and the phenolate fragment, thus generated,
remains coordinated to the metal center in the iminosemiquinonate form. Structures of four selected complexes
have been determined by X-ray crystallography. The first six complexes are one-electron paramagnetic and show
rhombic ESR spectra. The last complex is diamagnetic and shows characteristic 1H NMR signals. All the complexes
show intense charge-transfer transitions in the visible region and a Ru(III)−Ru(IV) oxidation on the positive side of
SCE and a Ru(III)−Ru(II) reduction on the negative side.