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

Abstract

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.