Kinetic studies on the closure of a chelate ring in [(2-dimethylaminoethyl)dimethylammonium]- and [(3-dimethylaminopropyl)dimethylammonium]-trichloroplatinum(II) complexes

Abstract

The title complexes, [PtCl3(Me4-Hen)] and [PtCl3(Me4-Hpn)][Me4-Hen =(2-dimethylaminoethyl)dimethylammonium, Me4-Hpn =(3-dimethylaminopropyl)dimethylammonium], with the diamine ligand acting as unidentate, have been prepared and the kinetics of closure of the chelate ring studied in aqueous solution. The rate constants of the reaction [PtCl3(N–N)]–→[PtCl2(N–N)]+ Cl–(N–N = Me4en or Me4pn), measured in basic medium at 25 °C, were kfCl= 183 ± 6 (Me4en) and 1.00 ± 0.03 s–1(Me4pn); temperature-variation studies gave ΔH‡= 43 ± 3 (Me4en) and 57 ± 2 kJ mol–1(Me4pn), and ΔS‡=–49 ± 4 (Me4en) and –46 ± 3 J K–1 mol–1(Me4pn). These data are interpreted as a measure of the greater conformational stability of the five-versus six-membered ring. The presence of two methyl substituents on the N atoms causes a 50–100-fold rate enhancement which is a measure of the so called ‘Thorpe–Ingold’ or ‘gem-dimethyl’ effect. Kinetic measurements at pH < 7 are interpreted according to a reaction mechanism in which reversible solvolysis of a cis chloride from the ring-opened species leads to two parallel paths for ring closure. The kinetic constants for acid hydrolysis [k1=(9.5 ± 0.5)× 10–4(Me4en) and (5.7 ± 0.2)× 10–4 s–1(Me4pn)] and chloride anation of the solvento-species [k–1= 0.36 ± 0.01 (Me4en) and 0.14 ± 0.01 dm3 mol–1 s–1(Me4pn)] are in the usual range; the products of the acid-dissociation constant of semidetached (i.e. monoco-ordinated bidentate ligand) diamine and the kinetic constant of ring closing, in the chloro- and in the solvento-species are: KaClkfCl=(7.8 ± 0.4)× 10–7(Me4en) and (3.4 ± 0.3)× 10–10(Me4pn), and KaH2OkfH2O=(1.03 ± 0.05)× 10–5(Me4en) and (1.4 ± 0.1)× 10–9 mol dm–3s–1(Me4pn). From data in acidic and basic media, the acid-dissociation constant was calculated as KaCl= 4.3 × 10–9(Me4en) and 3.4 × 10–10 mol dm–3(Me4pn), intermediate between those of the di- and mono-protonated free diamine.