Tuning the Hydrolytic Properties of Half-Sandwich-Type Organometallic Cations in Aqueous Solution

Hydrolysis of half-sandwich-type platinum metal cations with the general formula [M(6-arene)(H2O)3]2+ {M = Ru, Os; 6-arene = benzene, toluene, 1-methyl-4-isopropylbenzene (p-cym), or 1,3,5-triisopropylbenzene (tri-iPr)} or [Ir(5-Cp*)(H2O)3]2+ (Cp* = pentamethylcyclopentadienyl anion) was studied in aqueous solution in the presence of 0.20 M KNO3 or KCl as a background electrolyte to explore the effects of the type of metal ion, the moderately coordinating monodentate chloride ion, and the electron-donating ability of the arene ligand. Replacement of Ru by Os enhances the formation of the biologically less-active, triple hydroxido-bridged dimer, [{M(6-arene)}2(2-OH)3]+, whereas in the presence of Cl- complete hydrolysis is suppressed owing to the formation of various chlorido and mixed chlorido/hydroxido species as intermediates. A linear relationship was found between the stability constants of the [{Ru(6-arene)}2(2-OH)3]+ complexes in the presence of arenes with different electron-donating abilities and various atomic and molecular parameters of the corresponding [Ru(6-arene)(H2O)3]2+ cations; the triisopropylbenzene derivative was the most resistant to hydrolysis. The results of this study may help in rationalizing the bioactivity of anticancer half-sandwich metal complexes and can contribute to the rational design of metal compounds with increased biological activity.