Kinetic studies on closure of a second condensed ring in [bis(2-aminoethyl) methylamine hydrochloride]dichloro- and dichloro[tris(2-aminoethyl)amine bis(hydrochloride)]-platinum(II) complexes

Abstract

The title complexes [PtCl₂(bama·HCl)] and [PtCl₂(taa·2HCl)] with the tri- and quadri-dentate ligands acting as bidentate have been prepared and the kinetics of closure of the second condensed ring have been studied in aqueous solution. From stopped-flow measurements in basic media at 25 °C, the rate constants for the reactions [PtCl₂-(bama)]→[PtCl(bama)]⁺+ Cl^– and [PtCl₂(taa)]→[PtCl(taa)]⁺+ Cl^– are k₁= 2.1 ± 0.1 and k₁″= 4.3 ± 0.2 s^–1. This implies that only a probability factor of two can satisfactorily account for the difference in the observed rates. Kinetic measurements at pH < 7 have show that under these conditions the reactive species are unprotonated [PtCl₂(bama)] and monoprotonated [PtCl₂(taa·HCl)]; the ring-closure process is faster for the tris(2-aminoethyl)amine (taa) complex than for the bis(2-aminoethyl)methylamine (bama) complex, and anchimeric assistance of the protonated end of taa on entry of the unprotonated end (by ion pairing with the leaving chloride) is proposed.