η6-(2-phenoxyethanol) ruthenium(II)-complexes of 2,2′-bipyridine and its derivatives: Solution speciation and kinetic behaviour.

A novel family of Ru II -arene compounds with the general formula of [Ru II (η 6 -(2-phenoxyethanol))(L)Cl] + (L: 2,2′-bipyridine (bpy) ( 3 ), 4,4′-dimethyl-2,2′-bipyridine ( 4 ) and 4,4′-diyldimethanol-2,2′-bipyridine ( 5 )) was synthesized and characterized by standard spectroscopic and analytical methods. Complex 3 was further studied by single-crystal X-ray diffraction analysis, showing a pseudo octahedral geometry and strong π-π lateral stacking interactions in the crystal packing. Effect of the substituents on the electrochemical properties and on the aqueous solution stability was monitored by cyclic voltammetry, UV–Vis and 1 H NMR spectroscopy. Complexes 3 – 5 presented multiple irreversible redox processes according to their cyclic voltammograms recorded in acetonitrile, and their Ru II → Ru III oxidation peaks were found at ca. +1.6 V. Hydrolysis of the binuclear [Ru II (η 6 -(2-phenoxyethanol))(μ 2 -Cl)Cl] 2 precursor ( 1 ) resulted in binuclear hydroxido bridged species [(Ru II (η 6 -(2-phenoxyethanol))) 2 (μ-OH) 3 ] + and [(Ru II (η 6 -(2-phenoxyethanol))) 2 (μ-OH) 2 Z 2 ] (Z = H 2 O/Cl − ) in the presence of chloride ions in water. The hydrolytic behaviour of this Ru II precursor is similar to that of the analogous species [Ru II (η 6 - p -cymene)(μ 2 -Cl)Cl] 2 regarding the hydrolysis products and their stability constants. Formation of complexes 3 – 5 by reaction of the Ru II precursor with the (N,N) bidentate ligands was found to be relatively slow in aqueous solution. The complexation is complete already at pH 1 due to the formation of [Ru II (η 6 -(2-phenoxyethanol))(L)Z] complexes of significantly high stability in all cases, which are predominant species up to pH 6. However, besides the formation of the mixed hydroxido species [Ru II (η 6 -(2-phenoxyethanol))(L)(OH)] + at neutral and basic pH values, the slow oxidation of the Ru II centre takes place as well leading to the partial loss of the arene moiety. The rate of these processes depends on the pH and its maximum was found at pH 8–9. Additionally the chlorido/aqua co-ligand exchange processes of the [Ru II (η 6 -(2-phenoxyethanol))(L)Cl] + species were also monitored and only ∼5% of the chlorido ligand was found to be replaced by water in 0.1 M chloride ion containing aqueous solutions at pH 5. [ABSTRACT FROM AUTHOR]