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

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• 作者：Marjaana Mikola ; Karel D. Klika ; Anu Hakala ; and Jorma Arpalahti
• 刊名：Inorganic Chemistry
• 出版年：1999
• 出版时间：February 8, 1999
• 年：1999
• 卷：38
• 期：3
• 页码：571 - 578
• 全文大小：144K
• 年卷期：v.38,no.3(February 8, 1999)
• ISSN：1520-510X

文摘

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 thepH range 1.4-8.3 at different temperatures. Reactions of both complexes with I^- follow the usual associativetwo-path mechanism throughout the pH range studied, as do reactions with thiourea under neutral conditions (pH6.5). With both nucleophiles (Y), the observed rate constant linearly increases with increasing [Y] up to 1000-fold excess of Y. Plots of k_1,obs vs [Y] were employed to calculate the rate parameters k_S for the solvent path andk_Y for the nucleophile-dependent path by the equation k_1,obs = k_S + k_Y[Y] at different temperatures. The followingactivation parameters were obtained at 298.2 K for the reaction of thiourea with 1, H^d.gif"> = (72 ± 1) kJ mol^-1 andS^d.gif"> = (-79 ± 4) J K^-1 mol^-1, and with 2, H^d.gif"> = (72.8 ± 0.3) kJ mol^-1 and S^d.gif"> = (-86 ± 1) J K^-1 mol^-1.The corresponding data for I^- with 1 are H^d.gif"> = (83 ± 3) kJ mol^-1 and S^d.gif"> = (-49 ± 8) J K^-1 mol^-1, and with2, H^d.gif"> = (78 ± 3) kJ mol^-1 and S^d.gif"> = (-67 ± 10) J K^-1 mol^-1. Activation parameters for the solvent path areH^d.gif"> = (85 ± 1) kJ mol^-1 and S^d.gif"> = (-105 ± 2) J K^-1 mol^-1 for 1, and H^d.gif"> = (87 ± 7) kJ mol^-1 and S^d.gif"> =(-98 ± 22) J K^-1 mol^-1 for 2, on the basis of the data found for reactions with thiourea. Rate parameters for theformation and solvolytic decomposition gave log K values of 7.5 ± 0.1 and 6.1 ± 0.2 for the equilibrium constantsof 1 and 2, respectively, in aqueous 0.1 M NaClO₄ solution at 298.2 K. Ring opening of the tridentate dien groupin acidic solution provides a competing route for the overall substitution by thiourea. All experimental data foundare consistent with an associative mechanism involving pseudorotation of the five-coordinate intermediate formedby the attack of thiourea, including activation parameters (298.2 K) H^d.gif"> = (69.2 ± 0.3) kJ mol^-1 and S^d.gif"> =(-81 ± 1) J K^-1 mol^-1 for 1, and H^d.gif"> = (70.9 ± 0.7) kJ mol^-1 and S^d.gif"> = (-79 ± 2) J K^-1 mol^-1 for 2.According to kinetic analysis, about 60% of 1 and 70% of 2 yield free nucleoside via the ring-opening step,whereas the remainder give free nucleoside by direct replacement with thiourea. The ratio of these routes ispractically independent of thiourea concentration and temperature. The ¹H, ¹³C, and ¹⁹⁵Pt NMR spectroscopicdata for the isolated ring-opened species 3(1) and 3(2) (from 1 and 2, respectively) are consistent with a four-coordinate species [Pt(dienH)(L-N7)(tu)]³⁺, in which the dien group acts a bidentate ligand and one of the dienamino groups is trapped by protonation. Although both 3(1) and 3(2) are stable in cold acidic solution, theydecompose predominantly back to the starting material when the pH of the solution is increased. According toHPLC analysis, the former gives 1 and guanosine in a 12:1 ratio, and the latter yields 2 and adenosine in a 10:1ratio. The ability of the dien-NH₂ group to displace coordinated thiourea from Pt(II) contradicts the trans effectS > N and exemplifies the nucleophilic power of the NH₂ group of a partially chelated amine.