Ruthenium phenolates. Synthesis, characterization and electron-transfer properties of some salicylaldiminato and 2-(arylazo)phenolato complexes of ruthenium

Eleven mixed-ligand ruthenium(II) complexes of the general formula [Ru(bpy), (L)]ClO4 [bpy = 2,2'-bipyridine, L = salicylaldiminate or 2-(arylazo) phenolate anion] have been synthesized and characterized. The complexes are diamagnetic (low-spin d(6), S = 0) and in solution show intense MLCT transitions in the visible region. In acetonitrile solution they all show a reversible ruthenium(II)-ruthenium(III) oxidation in the range 0.5-0.9 V versus SCE and an irreversible ruthenium(III)-ruthenium (IV) oxidation near 1.6 V versus SCE. The potential of the ruthenium(II)-ruthenium(III) couple is sensitive to the nature of substituents on the ligand L. Two successive one-electron reductions of the coordinated bpy are also observed near -1.5 and -1.8 V versus SCE. Two representative [Ru-III(bpy)(2)(L)](2+) complexes have been synthesized by chemical oxidation of their ruthenium(II) precursors by aqueous eerie solution and isolated as perchlorate salts; These oxidized complexes are paramagnetic (low-spin d(5), S = 1/2) and show rhombic ESR spectra at 77 K. They show intense LMCT transitions in the visible region in acetonitrile solution together with weak ligand-field transitions at lower energies. Chemical reduction of these ruthenium(III) complexes by hydrazine gives back the parent ruthenium(II) complexes. In acetonitrile solution the [Ru-III(bpy)(2)(L)](2+) complexes oxidize N,N-dimethyl aniline, 1,2-napthoquinone-1-oxime, [Ru-II(bpy)(2)Cl-2] and [Fe-II(C5H5)(2)] to produce N,N,N',N'-Fe-III(C5H5)(2)](+), tetramethylbenzidine, 1,2-napthoquinone-1-iminoxy radical, [Ru-III(bpy)(2)Cl-2](+) and [Fe-III(C5H5)(2)](+), respectively, which have been characterized by spectroscopic aad electrochemical techniques.