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

Abstract

Eleven mixed-ligand ruthenium(II) complexes of the general formula [Ru(bpy)₂ (L)]CIO, [bpy = 2,2′-bipyridine, L == salicylaldiminate or 2-(arylazo) phenolate anion] have been synthesized and characterized. The complexes are diamagnetic (low-spin d⁶, 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 ceric solution and isolated as perchlorate salts. These oxidized complexes are paramagnetic (low-spin d⁵, S = ½)) 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-l-oxime, [Ru^II(bpy)₂Cl₂] and [Fe^II(C₅H₅)₂] to produce N,N,N′,N′-tetramethylbenzidine, 1,2-napthoquinone-l-iminoxy radical, [Ru^III(bpy)₂Cl₂]⁺ and [Fe^III(C₅H₅(₂]⁺, respectively, which have been characterized by spectroscopic and electrochemical techniques.