NMR kinetic studies of the interactions between [Ru(terpy)(bipy)(H2O)]2+ and some sulfur-donor ligands

Substitution reactions of monofunctional [Ru(terpy)(bipy)(H2O)]2+ complex, where terpy = 2,2′:6′,2″-terpyridine and bipy = 2,2′-bipyridine, with biologically relevant sulfur-donor nucleophiles, such as thiourea, l-methionine, l-cysteine and glutathione, are studied in aqueous solutions by 1H and 13C NMR spectroscopy techniques. All reactions are studied at pH 7.4, in the presence of phosphate buffer, to mimic the physiological environment during the substitution process. The reactions are carried out at 295 K under the second-order conditions. Observed results show that the rate of substitution strongly depends on the type and the structure of the entering nucleophile. The obtained order of reactivity for the entering sulfur-donor ligands is: thiourea > l-methionine > l-cysteine > glutathione. The pKa value of coordinated water ligand was determined in D2O by the 1H NMR technique. These results confirm the strong influence of inert polypyridyl ligands on the chemical behavior of Ru(II) complex as well as their influence on the lability of coordinated water molecule.

Graphical abstract1H NMR spectra and Guggenheim plot for the reaction between [Ru(terpy)(bipy)(H2O)]2+complex (4 mM) and l-Met (4 mM) in D2O at 295 K, pD = 7.4.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► NMR kinetic study of the interactions between [Ru(terpy)(bipy)(H2O)]2+ complex with sulfur-donor ligands. ► The acid dissociation constant (pKa) of the aqua complex were determined by 1H NMR spectroscopy. ► The second order rate constants were calculated from time dependent NMR spectra.