Ruthenium-mediated selective cleavage of nitrogen–carbon bond of the diimine function. Synthesis, spectroscopic and redox properties of the complexes [Ru(L)₂{^-OC₆H₄C(CH₃)=N---H}][ClO₄] (L=2,2'-bipyridine and 1,10-phenanthroline) and the crystal structure of the bipyridine derivative

Abstract

The reactions of Ru(bpy)₂(CO₃) (bpy=2,2'-bipyridine) and Ru(phen)₂(CO₃) (phen=1,10-phenanthroline) with the binucleating phenolato diimine function, OH---C₆H₄---C(CH₃)=N---CH₂---C₆H₄---CH₂---N=(CH₃)C---C₆H₄---OH, H₂L in ethanol solvent under dinitrogen atmosphere result in ruthenium bipyridine/phenanthroline heterochelates [Ru(bpy)₂/(phen)₂L'](ClO₄) 1 where L' corresponds to the ketonic imine function ^-O---C₆H₄C(CH₃)=N---H incorporating the rare C=N---H fragment. In the course of the reaction the N---C bond of the diimine function in H₂L has been selectively cleaved. The formation of 1 has been authenticated by single-crystal X-ray structure of the bipyridine derivative (1a). The RuN₅O coordination sphere is distorted octahedral. The diamagnetic complexes 1 exhibit 1:1 conductivity in acetonitrile solution. The complexes show strong Ru^II→π^∗(bpy)/(phen) MLCT transitions in the visible region and intra-ligand transitions in the UV region. The complexes exhibit moderately strong emissions near 700 nm from the lowest energy MLCT bands (φ=1.7–2.2×10^-2). The complexes (1a and 1b) display reversible ruthenium(III)–ruthenium(II) couple near 0.5 V, irreversible ruthenium(III)→ruthenium(IV) oxidation near 1.7 V and one ligand-based (L') oxidation near 2.0 V versus SCE. The reductions of the bpy and phen ligands have been observed at the negative side of SCE. The electrochemically oxidized ruthenium(III) complexes (1a⁺ and 1b⁺) are found to be unstable at 298 K and exhibit rhombic EPR spectra having three distinct g values corresponding to the trivalent ruthenium(III) under distorted octahedral arrangement. The oxidized complexes (1a⁺ and 1b⁺) exhibit LMCT transitions near 750 nm.