Substitution Reactions of Platinum(II)-Nucleobase Complexes by Associative Mechanism Involving Pseudorotation of the Five-Coordinate Intermediate.

Substitution reactions of N7-platinated guanosine and adenosine complexes {[Pt(dien)(Guo-N7)]2+ (1), [Pt(dien)(Ado-N7)]2+ (2), dien = diethylenetriamine} by thiourea (tu) and I- have been studied in aqueous solution in the pH range 1.4−8.3 at different temperatures. Reactions of both complexes with I- follow the usual associative two-path mechanism throughout the pH range studied, as do reactions with thiourea under neutral conditions (pH 6.5). With both nucleophiles (Y), the observed rate constant linearly increases with increasing [Y] up to 1000-fold excess of Y. Plots of k1,obs vs [Y] were employed to calculate the rate parameters kS for the solvent path and kY for the nucleophile-dependent path by the equation k1,obs = kS + kY[Y] at different temperatures. The following activation parameters were obtained at 298.2 K for the reaction of thiourea with 1, ΔH⧧ = (72 ± 1) kJ mol-1 and ΔS⧧ = (−79 ± 4) J K-1 mol-1, and with 2, ΔH⧧ = (72.8 ± 0.3) kJ mol-1 and ΔS⧧ = (−86 ± 1) J K-1 mol-1. The corresponding dat...