Interaction of [Ru(η⁶-p-cym)(H₂O)₃]²⁺ with citrate and tricarballate ions in aqueous solution; X-ray crystal structure of novel half-sandwich Ru(II)-citrato complexes.

The interaction between [Ru(η(6)-p-cym)(H(2)O)(3)](2+) and an important low molecular weight serum component, citric acid (citrH(3)), was studied with the aid of combined pH-potentiometric, (1)H NMR, (13)C NMR and electrospray ionization mass spectrometry (ESI-MS) methods in aqueous solution. For comparative purposes propane-1,2,3-tricarboxylic acid (tricarballylic acid, tricH(3)) having no alcoholic-OH group in position 2 was also investigated. Stoichiometries, stability constants and the most plausible solution structures of the complexes formed in the systems were determined. Depending upon the pH, citrate was found to coordinate to the metal ion via [COO(-), COO(-), OH] or [COO(-), COO(-), O(-)] fashion yielding mononuclear complexes with high stability. As a consequence at physiological pH the hydrolysis of the metal ion is completely hindered even at 1:1 metal to ligand ratio. Crucial role of the alcoholic/alcoholate function of the citric acid in [Ru(η(6)-p-cym)(H(2)O)(3)](2+) binding is reflected in the low stability of the species formed with tricarballylic acid. The X-ray crystal structures of [Ru(η(6)-p-cym)(citrH)]·H(2)O·CH(3)OH and 2[Ru(η(6)-p-cym)(citrH)]·3H(2)O being the first published structures of an organometallic Ru(II)-citrate and both featuring a [COO(-), COO(-), OH] coordinated ligand, are also reported.