Aizawa Sen-Ichi, Takizawa Kohei, Aitani Momoko
Graduate School of Science and Engineering, University of Toyama 3190 Gofuku Toyama 930-8555 Japan
RSC Adv. 2019 Aug 13;9(43):25177-25183. doi: 10.1039/c9ra05507j. eCollection 2019 Aug 8.
The reactions of a dichloro(-cymene)ruthenium(ii) dimer, [RuCl(-cymene)], with citric acid and sulfur-containing amino acids gave only [Ru(L)(-cymene)]-type complexes (L = citrate (Cit), l-penicillaminate (l-Pen), -methyl-l-cysteinate (-Me-l-Cys) and l-methioninate (l-Met)) in aqueous solutions at various pHs and molar ratios of the reactants, where Cit and the amino acids act as a tridentate ligand. These sulfur-containing amino acid complexes with bound nitrogen, oxygen and sulfur atoms and η--cymene take absolute configuration around Ru(ii) selectively, having the α-proton oriented in the opposite direction from the Ru(ii) center. The concentration dependences of the observed pseudo-first-order rate constants were provided for the substitution reactions of the citrato complex, [Ru(Cit)(-cymene)], with a large excess of the sulfur-containing amino acids at various temperatures at pH 7.3, where solvolysis path was observed for -Me-l-Cys and l-Met as an intercept but not for l-Pen. The activation parameters for the substitution reactions by the direct attack of the amino acids were changed significantly, indicating that the reaction mechanism varies sensitively with the amino acids from an associative mechanism to an interchange one. The pH dependences of the rate constants of the substitution reactions suggest that the carboxylate group is an attacking group for -Me-l-Cys and l-Met under neutral conditions and the thiol group of l-Pen acts as an entering group constantly at any pH showing a considerably smaller activation energy compared with -Me-l-Cys and l-Met. Differences in stabilities of the amino acid complexes were obtained from the equilibrium constants for the substitution reactions between the amino acids. These results indicate that the activation energies for the substitution reactions of the citrato complex with the amino acids are moderately correlated with the stabilities of the formed amino acid complexes.
二氯（对异丙基苯）钌（II）二聚体[RuCl（对异丙基苯）]与柠檬酸和含硫氨基酸在不同pH值和反应物摩尔比的水溶液中反应，仅生成[Ru（L）（对异丙基苯）]型配合物（L =柠檬酸盐（Cit）、L-青霉胺（L-Pen）、α-甲基-L-半胱氨酸盐（α-Me-L-Cys）和L-蛋氨酸盐（L-Met）），其中Cit和氨基酸作为三齿配体。这些含有与氮、氧和硫原子结合的含硫氨基酸配合物以及η-对异丙基苯在Ru（II）周围选择性地采取绝对构型，α-质子的取向与Ru（II）中心相反。在pH 7.3的不同温度下，给出了柠檬酸盐配合物[Ru（Cit）（对异丙基苯）]与大量过量含硫氨基酸的取代反应中观察到的伪一级速率常数的浓度依赖性，其中对于α-Me-L-Cys和L-Met观察到溶剂解途径作为截距，但对于L-Pen未观察到。氨基酸直接进攻引起的取代反应的活化参数发生了显著变化，表明反应机理随氨基酸从缔合机理到交换机理而敏感变化。取代反应速率常数的pH依赖性表明，在中性条件下，羧基是α-Me-L-Cys和L-Met的进攻基团，L-Pen的硫醇基在任何pH下都始终作为进入基团，与α-Me-L-Cys和L-Met相比，活化能相当小。从氨基酸之间取代反应的平衡常数获得了氨基酸配合物稳定性的差异。这些结果表明，柠檬酸盐配合物与氨基酸取代反应的活化能与形成的氨基酸配合物的稳定性适度相关。
