Synthesis, characterization, electron-transfer properties and reactivities of a group of ruthenium(ii) complexes with RuN₂P₂X₂(X = Cl, Br) coordination spheres

Abstract

Four dihaloruthenium(II) complexes of general formula Ru(PPh₃)₂(L)X₂ [L = 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen); X = Cl, Br] have been synthesized by the reaction of L with Ru(PPh₃)₃X₂ in dichloromethane. The complexes are diamagnetic and show intense absorptions in the visible region which are assigned to the MLCT transitions. In dichloromethane solution they show a reversible ruthenium(II)-ruthenium-(III) oxidation near 0.4 vs SCE. Two [Ru^III(PPh₃)₂(L)(Cl₂)]⁺ ions have been generated in solution by electrochemical oxidation of Ru^II(PPh₃)₂(L)Cl₂. Solutions of the oxidized complexes show LMCT transitions in the visible region and at 77 K show rhombic EPR spectra. The distortions from octahedral symmetry have been quantified, in these species, with the help of EPR data analysis. The axial distortion is much larger than the rhombic one. The two X^- ligands are easily displaced from Ru(PPh₃)₂(L)X₂ by its reaction with Ag⁺ in ethanol producing [Ru(PPh₃)₂(L)(EtOH)₂]²⁺, which upon reacting with L′ [L′ = bpy, phen, acetylacetonate ion (acac-), oxalate ion (ox^2-)] affords complexes of type [Ru)PPh₃)₂(L)(L′)ⁿ⁺ (n = 0, 1, 2). Five such complexes have been synthesized and characterized. They all are diamagnetic and show intense MLCT transitions in the visible region. The ruthenium(II)-ruthenium(III) oxidation is very sensitive to the nature of L′. The potential decreases in the order (of L′); bpy (or phen) gt; acac^-gt; ox^2-. Reductions of the coordinated bpy and phen are also observed.