Ruthenium(ii) complexes containing RuN4O2 spheres assembled viapyridine-imine-amide coordination. Syntheses, structures, properties and protonation behaviour of coordinated amide

Abstract

A series of ruthenium(II) complexes of general formula [RuL₂] with the pyridine-N, the imine-N and the amide-O donor N-(aroyl)-N′-(picolinylidene)hydrazines (HL) has been synthesized. The ligands differ on the substituent at the para position of the aroyl fragment. The complexes have been characterized by analytical, ¹H NMR, electronic absorption spectroscopy and cyclic voltammetry. X-Ray structures of representative complexes have been determined. The lowest energy MLCT (Ru(dπ) → L(π*)) transitions for these complexes are observed at essentially identical wavelength (544 ± 1 nm). The complexes display a metal centred oxidation and a ligand centred reduction in the potential ranges 0.44 to 0.59 V and −1.49 to −1.35 V (vs. Ag/AgCl), respectively. The differences in the metal and ligand redox potentials (ΔE_1/2) are practically same (1.94 ± 0.01 V) for all the complexes. The identical MLCT band positions and the same ΔE_1/2 values suggest that in this series of complexes, the energy gap between the metal-dπ and the ligand-π* levels is constant. The effective pK_a values of the species obtained by protonation of the coordinated amide functionalities in one of the complexes have been evaluated by spectrophotometric titration. The corresponding diprotonated species has been characterized by X-ray crystallography.