Interaction between [Ru(η6-p-cym)(H2O)3]2+ and dl-serine or dl-isoserine: The role of the side chain alcoholic OH group in metal ion binding

To explore the role of the side chain alcoholic OH group in metal ion binding the interaction between [Ru(η6-p-cym)(H2O)3]2+ (p-cym = 1-isopropyl-4-methylbenzene) and dl-serine (serH) or dl-isoserine (iseH) was studied by means of pH-potentiometry, 1H NMR and ESI-MS techniques in aqueous solution. The results indicate that, unlike simple amino acids with no donor atom in the side chain, ser and especially ise are potent ruthenium ion binders and are capable of preventing the metal ion from significant hydrolysis at pH = 7.4 due to the metal ion assisted deprotonation and coordination of the alcoholic OH function and forming stable joined chelates with [NH2, COO−, O−] donor set. Comparison of the speciations obtained for the two systems indicates that ise with (6 + 5) membered joined chelates of the [NH2, COO−, O−] donor set forms more stable complexes than ser which is suitable for (5 + 6) [NH2, COO−, O−] coordination. Analysis of the effect of the replacement of the amino group in the [NH2, COO−, O−] donor set of ise by a COO− (citrate) reveals that the latter is more effective in the acidic pH range; the two donor sets show comparable metal ion strengths under physiologically relevant conditions and ise is found to be a better metal ion binder in the basic pH range than citrate due to the basicity difference of the –NH2 and –COO− groups in ise and citrate ligands, respectively. [Ru(η6-p-cym)ise](CF3SO3)·0.5H2O (1) with [NH2, COO−, OH] coordination of the ligand was also synthesized and characterized in the solid state by various analytical techniques.