The role of diaminocyclohexane and diaminobenzene linking bridges on the aqua substitution of chelated dinuclear Pt(II) complexes by nitrogen donor heterocycles. A kinetic and mechanistic study
The substitution of the aqua ligands from six Pt(II) complexes, viz., [Pt(H2O)(N,N-bis(2-pyridylmethyl)cyclohexylamine](ClO4)2 (Pt1); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-trans-1,4-cyclohexyldiamine)](ClO4)4 (Pt2); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-methylenedicyclohexyldiamine)](ClO4)4 (Pt3); [Pt(H2O)N,N-bis(2-pyridylmethyl)phenylamine)](ClO4)2 (Pt4); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,4-phenyldiamine](ClO4)4 (Pt5); and [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-methylenediphenyldiamine)](ClO4)4 (Pt6), by nitrogen heterocyclic ligands{viz., pyrazole (Pz); 3-methylpyrazole (mPz); 1,2,4-triazole (Tz) and pyrazine (Pzn)} were studied in an aqueous 0.01 M perchloric acid medium. The substitutions were investigated under pseudo-first-order conditions as a function of the concentration of nucleophiles and reaction temperature using UV–visible spectrophotometry. The substitution of the aqua ligands by all the nitrogen donor heterocycles proceeded via a single step whose rate decreased in the respective orders: Pt1 > Pt3 > Pt2 and Pt4 > Pt6 > Pt5 in the two sets of complexes. Of the nucleophiles used in this study, pyrazine was the most reactive and the complete order of the rate of aqua substitution was Pzn >> Pz > Tz > mPz. The large and negative activation entropies and low but positive enthalpies of activation values support a significant contribution from bond making in the transition state of the substitution process.
