Kinetics of anation in platinum(II) and palladium(II) complexes of N-alkylated diethylenetriamine

Abstract

The kinetics of replacement of H₂O in the pseudo-octahedral complexes [Pt(L–L–L)(H₂O)][ClO₄]₂(L–L–L = Me₂N[CH₂]₂NMe[CH₂]₂NMe₂) and [Pd(L–L–L)(H₂O)][ClO₄]₂(L–L–L = R₂N[CH₂]₂NMe[CH₂]₂NR₂, R = Me or Et] by the nucleophiles Cl^–, Br^–, I^–, (NH₂)₂CS, SCN^–, and S₂O₃^2– have been studied in water at 25 °C. All the reactions exhibit a first-order dependence on both the substrate and the nucleophile concentration. The kinetic behaviour of the complexes depends markedly on the size of the substituents at the nitrogen atoms in the ligand L–L–L. When L–L–L = Me₂N[CH₂]₂NMe[CH₂]₂NMe₂ the second-order rate constants follow the usual nucleophilicity scale and in the case of the platinum complex the kinetic behaviour can be related to that of the unhindered complex [Pt(dien)(H₂O)]²⁺(dien = H₂N[CH₂]₂NH[CH₂]₂NH₂). In the case of the palladium substrate where the ligand L–L–L bears four bulky ethyl groups the reactivity is largely reduced with respect to that where L–L–L = Me₂N[CH₂]₂NMe[CH₂]₂NMe₂ and the usual scale of nucleophilicity is no longer valid. The experimental findings are interpreted in terms of the usual bimolecular attack at the metal on the basis of the electrophilicity and the steric hindrance at the reaction centre caused by the substituents of the tridentate ligand.