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

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.