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

<div><p>The substitution of the aqua ligands from six Pt(II) complexes, viz., [Pt(H<sub>2</sub>O)(<i>N,N</i>-bis(2-pyridylmethyl)cyclohexylamine](ClO<sub>4</sub>)<sub>2</sub> (<b>Pt1</b>); [{Pt(H<sub>2</sub>O)}<sub>2</sub>(<i>N,N,N′,N′</i>-tetrakis(2-pyridylmethyl)-<i>trans</i>-1,4-cyclohexyldiamine)](ClO<sub>4</sub>)<sub>4</sub> (<b>Pt2</b>); [{Pt(H<sub>2</sub>O)}<sub>2</sub>(<i>N,N,N′,N′</i>-tetrakis(2-pyridylmethyl)-4,4′-methylenedicyclohexyldiamine)](ClO<sub>4</sub>)<sub>4</sub> (<b>Pt3</b>); [Pt(H<sub>2</sub>O)<i>N,N</i>-bis(2-pyridylmethyl)phenylamine)](ClO<sub>4</sub>)<sub>2</sub> (<b>Pt4</b>); [{Pt(H<sub>2</sub>O)}<sub>2</sub>(<i>N,N,N′,N′</i>-tetrakis(2-pyridylmethyl)-1,4-phenyldiamine](ClO<sub>4</sub>)<sub>4</sub> (<b>Pt5</b>); and [{Pt(H<sub>2</sub>O)}<sub>2</sub>(<i>N,N,N′,N′</i>-tetrakis(2-pyridylmethyl)-4,4<i>′</i>-methylenediphenyldiamine)](ClO<sub>4</sub>)<sub>4</sub> (<b>Pt6</b>), by nitrogen heterocyclic ligands{viz., pyrazole (<b>Pz</b>); 3-methylpyrazole (<b>mPz</b>); 1,2,4-triazole (<b>Tz</b>) and pyrazine (<b>Pzn</b>)} 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: <b>Pt1 </b>> <b>Pt3 </b>> <b>Pt2</b> and <b>Pt4 </b>> <b>Pt6 </b>> <b>Pt5</b> 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 <b>Pzn</b> >> <b>Pz </b>> <b>Tz </b>> <b>mPz</b>. 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.</p></div>