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 k^f_Cl= 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: K^a_Clk^f_Cl=(7.8 ± 0.4)× 10^–7(Me₄en) and (3.4 ± 0.3)× 10^–10(Me₄pn), and K^a_H2Ok^f_H2O=(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 K^a_Cl= 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.