Aggregation tendency and reactivity toward AgX of cationic half-sandwich ruthenium(ii) complexes bearing neutral N,O-ligands

Abstract

The aggregation tendency of complexes [Ru(η⁶-cymene)(N,O)Cl]X [N,O = 2-benzoylpyridine (2-bzpy), 1, and 2-acetylpyridine (2-acpy), 2, X⁻ = BPh₄⁻ or PF₆⁻] has been studied by means of PGSE NMR experiments. It was found that complexes with PF₆⁻ as counterion are mainly present in CD₂Cl₂ as ion pairs at low concentration, as a mixture of ion triples and free anions at medium concentration and as ion quadruples at elevated concentration. ¹⁹F, ¹H-HOESY NMR experiments revealed that in ion triples and ion quadruples two cationic Ru-units pair up. Consistently, in the solid-state structure of 1PF₆, determined through X-ray single-crystal investigation, two cationic Ru-units are held together by an intermolecular π–π stacking interaction between the pyridyl rings. Complexes having BPh₄⁻ as counterion are only present in solution as even aggregates, namely ion pairs at low concentration and ion quadruples at elevated concentration. In such a case a counteranion bridges two cationic Ru-units as observed in the solid-state structure of 1BPh₄. The reactivity of complexes 1–2 toward AgX salts has been investigated in different solvents. Bicationic [Ru(η⁶-cymene)(N,O)(MeCN)]X₂ (N,O = 2-bzpy, 3, and 2-acpy, 4) and [Ru(MeCN)₄(N,O)]X₂ (N,O = 2-bzpy, 5, and 2-acpy, 6) complexes were obtained by the reaction of 1 and 2 with AgX in the presence of three equivalents of acetonitrile or in acetonitrile, respectively. The reaction of 1 with AgPF₆ in acetone afforded complex [Ru(η⁶-cymene)(N,O,O)]PF₆ (7, where N,O,O = 4-alcoxide-4-phenyl-4-(pyridin-2-yl)butan-2-one) from the C–C coupling of a deprotonated methyl group of the coordinated acetone and the CO moiety of 2-bzpy ligand.