Antitumor pentamethylcyclopentadienyl rhodium complexes of maltol and allomaltol: Synthesis, solution speciation and bioactivity

• RhIII(Cp*)-hydroxypyrone complexes prepared under standard and microwave conditions.
• Antiproliferative activity of RhIII(Cp*) complexes of hydroxypyrones.
• Solution equilibrium studies of RhIII(Cp*) complexes of hydroxypyrones.
• Exclusive formation of mono-ligand complexes with moderate stability.

The reaction of the dimer [RhIII(pentamethylcyclopentadienyl)(μ-Cl)Cl]2 ([RhIII(Cp*)(μ-Cl)Cl]2) with the hydroxypyrone ligands maltol and allomaltol affords complexes of the general formula [RhIII(Cp*)(L)Cl] under standard and microwave conditions. The organometallic compounds were characterized by standard analytical methods and in the case of the allomaltol derivative in the solid state by single-crystal X-ray diffraction analysis. The complexes showed similar cytotoxicity profiles and were proved to be moderately active against various human cancer cell lines. The stoichiometry and stability of these complexes were determined in aqueous solution by pH-potentiometry, 1H NMR spectroscopy and UV–visible spectrophotometry. Speciation was studied in the presence and in the absence of chloride ions. Hydrolysis of [RhIII(Cp*)(H2O)3]2 + gave dimeric mixed hydroxido species [(RhIII(Cp*))2(μ-OH)3]+ and [(RhIII(Cp*))2(μ-OH)2Z2] (Z = H2O/Cl−). Formation of the mononuclear complexes [RhIII(Cp*)(L)Z] of maltol and allomaltol with similar and moderate stability was found. These species predominate at physiological pH and decompose only partially at micromolar concentrations. In addition, hydrolysis of the aqua complex or a chlorido/hydroxido co-ligand exchange resulted in the formation of the mixed-hydroxido species [RhIII(Cp*)(L)(OH)] in the basic pH range. Replacement of the chlorido by an aqua ligand in the complex [RhIII(Cp*)(L)Cl] was monitored and with the help of the equilibrium constants the extent of aquation at various chloride concentrations of the extra- and intracellular milieu can be predicted. Complexation of these RhIII complexes was compared to analogous [RuII(η6-p-cymene)] species and higher conditional stabilities were found in the case of the RhIII compounds at pH 7.4.

RhIII(Cp*)(L)Cl complexes of hydroxypyrones were prepared under standard and microwave conditions and their antiproliferative activity was tested against cancer cell lines. Stoichiometry and stability of the complexes were characterized by combination of different methods in aqueous solution and formation of mono complexes with moderate stability was found.Figure optionsDownload as PowerPoint slide