The hydrolysis products of cis-diamminedichloroplatinum(II) 5. The anation kinetics of cis-Pt(X)(NH 3) 2(OH 2) + (XCl, OH) with glycine, monohydrogen malonate and chloride

Abstract

The spectrophotometrically determined rate constants, k obs (s −1), for the anation of cis-PtCl(NH 3) 2(OH 2) + with glycine (Nu=gly at pH=7.4), or sodium hydrogen malonate (Nu=Hmal − at pH=4.3) can be represented by the equation k obs= k o+ k Nu[Nu], where k o is an hydrolysis rate constant independent of the nucleophile concentration but dependent on pH, and k Nu is a nucleophile dependent path. With Nu=Hmal −, k o∼0 and the first step is monodentate coordination, followed by a second ( k cy), nucleophile independent path, due to ring closure. With Nu=gly, the ring closure reaction is sufficiently fast as to appear synchronous with coordination and k o is the base hydrolysis rate constant for cis- PtCl(OH)(NH 3) 2 at pH=7.4. Kinetic parameters associated with the anation are k Hmal (25.0 °C, pH=4.3, I=1.0 M)=9.90×10 −4 M −1 s −1, Δ H ≠=74.8 kJ mol −1, Δ S ≠=−52 J K −1 mol −1; k gly (25.0 °C, pH=7.4, I=1.0 M) 2.75×10 −4 M −1 s −1, Δ H ≠=62.7 kJ mol −1, Δ S≠=−103 J K −1 mol −1; k cy (25.0 °C, pH=4.3, I=1.0 M)=1.03×10 −5 s −1, Δ H ≠=88.9 kJ mol −1, Δ S ≠=−42 J K −1 mol −1. The rate ( k 3, s −1) of the chloride release from cis-PtCl(OH)(NH 3) 2to give cis-Pt(OH)NH 3) 2(OH 2) + and rate of chloride uptake (k −3, M −1 s −1) have been measured at pH=7.4. Kinetic parameters are k 3=2.39×10 −5 s −1 (25.0 °C, pH=7.4, I=1.0 M) Δ H ≠=82.4 kJ mol −1, Δ S ≠=−57 J K −1 mol −1 and k −3=9.37×10 −4 M −1 s −1 (25.0 °C, pH=7.4, I=1.0 M), Δ H ≠=61.7 kJ mol −1 Δ S ≠=−96 J K −1 mol −1. From these rate constants, K 3=( k 3[ k −3] −1) is calculated to be 2.55×10 −2 at 25.0 °C. These data have been used to comment on the nucleophilicity of cis-PtCl(NH 3) 2(OH 2) +.