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)}{mu-(NH2(CH2)(6)H2N)}{PtCl(biPY)}]Cl(ClO4) (bipy is 2,2'-bipyridine), has been prepared and characterized by elemental microanalysis, IR, H-1 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 NaCI. 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)}{mu-(NH2(CH2)(6)H2N)}{PtCl(biPY)}]Cl(ClO4) complex with all selected ligands were confirmed by H-1 NMR spectroscopy at 295 K. Additionally, the two pK(a) values of studied diaqua complex, [(Pd(H2O)(bipy)}(mu-(NH2(CH2)(6)H2N)}(Pt(H2O)(biPY)}](4+), were determined by spectrophotometric pH titration. The large negative values for the entropy of activation, Delta S-not equal, support an associative substitution mechanism. (C) 2015 Elsevier Ltd. All rights reserved.