Areneruthenium(II) 4-acyl-5-pyrazolonate derivatives: coordination chemistry, redox properties, and reactivity

Areneruthenium(II) molecular complexes of the formula Ru(arene)(Q)Cl], containing diverse 4-acyl-5-pyrazolonate ligands Q with arene = cymene or benzene, have been synthesized by the interaction of HQ and Ru(arene)Cl(micro-Cl)]2 dimers in methanol in the presence of sodium methoxide. The dinuclear compound {Ru(cymene)Cl}2Q4Q] (H2Q4Q = bis(4-(1-phenyl-3-methyl-5-pyrazolone)dioxohexane), existing in the RRuSRu (meso form), has been prepared similarly. Ru(cymene)(Q)Cl] reacts with sodium azide in acetone, affording Ru(cymene)(Q)N3] derivatives, where Cl- has been replaced by N3-. The reactivity of Ru(cymene)(Q)Cl] has also been explored toward monodentate donor ligands L (L = triphenylphosphine, 1-methylimidazole, or 1-methyl-2-mercaptoimidazole) and exo-bidentate ditopic donor ligands L-L (L-L = 4,4'-bipyridine or bis(diphenylphosphino)propane) in the presence of silver salts AgX (X = SO3CF3 or ClO4), new ionic mononuclear complexes of the formula Ru(cymene)(Q)L]X, and ionic dinuclear complexes of the formula {Ru(cymene)(Q)}2L-L]X2 being obtained. The solid-state structures of a number of complexes were confirmed by X-ray crystallographic studies. Their redox properties have been investigated by cyclic voltammetry and controlled potential electrolysis, which, on the basis of their measured RuII/III reversible oxidation potentials, have allowed the ordering of the bidentate acylpyrazolonate ligands according to their electron-donor character and are indicative of a small dependence of the HOMO energy upon the change of the monodentate ligand. This is accounted for by DFT calculations, which show a relevant contribution of acylpyrazolonate ligand orbitals to the HOMOs, whereas that from the monodentate ligand is minor.