Comparative solution and structural studies of half-sandwich rhodium and ruthenium complexes bearing curcumin and acetylacetone

Abstract Half-sandwich organometallic complexes of curcumin are extensively investigated as anticancer compounds. Speciation studies were performed to explore the solution stability of curcumin complexes formed with [Rh(η 5 -C 5 Me 5 )(H 2 O) 3 ] 2+ . Acetylacetone (Hacac), as the simplest β-diketone ligand bearing (O,O) donor set, was involved for comparison and its Ru(η 6 ‑ p ‑cymene), Ru(η 6 ‑toluene) complexes were also studied. 1 H NMR, UV–visible and pH-potentiometric titrations revealed a clear trend of stability constants of the acac complexes: Ru(η 6 ‑ p ‑cymene) > Ru(η 6 ‑toluene) > Rh(η 5 -C 5 Me 5 ). Despite this order, the highest extent of complex formation is seen for the Rh(η 5 -C 5 Me 5 ) complexes at pH 7.4. Formation constant of [Rh(η 5 -C 5 Me 5 )(H 2 curcumin)(H 2 O)] + reveals similar solution stability to that of the acac complex. Additionally, structures of two complexes were determined by X-ray crystallography. The in vitro cytotoxicity of curcumin was not improved by the complexation with these organometallic cations.