Novel half-sandwich Ru(II)-hydroxamate complexes: synthesis, characterization and equilibrium study in aqueous solution

Novel half-sandwich Ru(II)-benzohydroxamate complexes were synthesized. [Ru(η⁶-p-cymene)(μ-bha)]₂Cl₂ (bhaH = benzohydroxamic acid) with (O,O) coordination of the ligand, was characterized by elemental analysis, spectroscopic (¹H-NMR, IR) and ESI-MS methods. Replacement of the chloride of the precursor by CF₃SO₃⁻ and reaction of the obtained [Ru(η⁶-p-cym)(acetone)₃](CF₃SO₃)₂ with bhaH affords [Ru(η⁶-p-cymene)(μ-bha)]₂(CF₃SO₃)₂, the crystal and molecular structure of which has been determined by X-ray crystallography. In this complex, the first reported structure of an organometallic Ru(II)-hydroxamate, the carbonyl oxygens of the bha ligands coordinate to one of the Ru units and the hydroxamate oxygens bridge the two Ru atoms. Hydrolytic behaviour of [Ru(η⁶-p-cym)Cl_x(H₂O)_3-x]^(2-x)+ (x = 0–3) present in aqueous solution and its complex forming capabilities with N-methyl-acetohydroxamic acid (meahaH) were studied by pH-potentiometric, ¹H-NMR, UV-Vis and ESI-MS methods. Formation constants of the various species present in solution are reported. The data are consistent with the formation of the stable, monomeric [Ru(η⁶-p-cym)(meaha)]⁺ as the major species at physiological pH.