Kinetics and mechanism of substitution reactions of the new bimetallic [{PdCl(bipy)}{μ-(NH2(CH2)6H2N)}{PtCl(bipy)}]Cl(ClO4) complex with important bio-molecules

The new dinuclear bimetallic complex, [{PdCl(bipy)}{μ-(NH2(CH2)6H2N)}{PtCl(bipy)}]Cl(ClO4) (bipy is 2,2′-bipyridine), has been prepared and characterized by elemental microanalysis, IR, 1H NMR spectroscopy and MALDI-TOF mass spectrometry. Substitution reactions of the studied complex with selected biologically important ligands such as: thiourea (Tu), l-methionine (l-Met), l-cysteine (l-Cys), l-histidine (l-His) and guanosine-5′-monophosphate (5′-GMP), were studied under the pseudo-first order conditions as a function of concentration and temperature using stopped-flow and UV–Vis spectrophotometry. The reactions were monitored in 0.1 M NaClO4 at pH 5.0, in the presence of 40 mM NaCl. All fast reactions were monitored by stopped-flow at three temperatures (288 K, 298 K, 308 K) to determine the activation parameters, while the reactions studied by UV–Vis spectrophotometry were tested only at 298 K. Observed order of reactivity of the used ligands is: Tu > l-Met > l-Cys > l-His > 5′-GMP. Substitution reactions of the investigated bimetallic complex with Tu, l-Cys and l-His were followed by degradation to the corresponding substituted mononuclear complexes of palladium (II) and platinum (II), [Pd(bipy)(Nu)2] and [Pt(bipy)(Nu)2] (where Nu = Tu, l-Cys, l-His), by releasing of the bridge ligand,1,6-diaminohexane. In contrast, during the substitution reactions with l-Met and 5′-GMP, the structure of starting bimetallic complex was preserved and the process of degradation can be halted. The proposed pathways of the substitution reactions of [{PdCl(bipy)}{μ-(NH2(CH2)6H2N)}{PtCl(bipy)}]Cl(ClO4) complex with all selected ligands were confirmed by 1H NMR spectroscopy at 295 K. Additionally, the two pKa values of studied diaqua complex, [{Pd(H2O)(bipy)}{μ-(NH2(CH2)6H2N)}{Pt(H2O)(bipy)}]4+, were determined by spectrophotometric pH titration. The large negative values for the entropy of activation, ΔS≠, support an associative substitution mechanism.

Graphical abstractSubstitution reactions of the new bimetallic complex, [{PdCl(bipy)}{μ-(NH2(CH2)6H2N)}{PtCl(bipy)}]Cl(ClO4), with thiourea. l-cysteine, l-methionine, l-histidine and guanosine-5′-monophosphate were studied by stopped-flow and UV-VIS spectrophotometry.Figure optionsDownload full-size imageDownload as PowerPoint slide