Complex Formation between [(η6-p-cymene)Ru(H2O)3]2+ and Hydroxycarboxylates or their Sulfur Analogues - The Role of Thiolate Groups in Metal Ion Binding

The interactions between [(η6‐p‐cymene)Ru(H2O)3]2+ and L‐malic acid (H2mal), L‐tartaric acid (H2tart), and their sulfur‐containing analogues dl‐thiomalic acid (H3thiomal) and meso‐2,3‐dimercaptosuccinic acid (H4dmsa) were studied by pH potentiometry, NMR spectroscopy, and ESI‐MS. The hydroxycarboxylates are potent metal ion binders and prevent hydrolysis at pH 7.4 in aqueous solution. Although H2mal forms mononuclear complexes with different degrees of protonation by the involvement of the alcohol/alkoxide group in addition to the carboxylate groups, the presence of binding isomers is detected for the H2tart system. The replacement of the alcohol group by a thiolate group results in the formation of dinuclear [Ru2A2] complexes as the single species over a wide pH range. Complexes with identical binding modes with H3thiomal or polymeric species with H4dmsa are identified, and the X‐ray structure of [{(η6‐p‐cym)Ru}2(Hthiomal)2] is reported. The stereochemistry of the chiral‐at‐metal complexes was explored by NMR spectroscopy and is also discussed. Although the hydroxycarboxylates L‐malic acid and L‐tartaric acid form mostly mononuclear complexes with [(η6‐p‐cymene)Ru(H2O)3]2+ and can prevent hydrolysis of the metal ion, their sulfur analogues thiolactate and thiomalate prefer to bind through bridging thiolate groups to yield highly stable dinuclear [Ru2A2]‐type complexes.