Universidad Nacional Autónoma de México, Facultad de Química, 04510, Mexico City, Mexico.
Dalton Trans. 2020 Feb 25;49(8):2452-2467. doi: 10.1039/c9dt04733f.
Two tridentate and one tetradentate new ligands containing the terminal oxime group separated from secondary amino and pyridine groups as additional binding sites by two or three methylene groups have been prepared. Their acid-base properties, as well as the composition and stability of their complexes with Zn(ii) and Cd(ii) ions, were determined by potentiometric and spectrophotometric titrations. The X-ray structure of a Cd(ii) complex of a related tridentate oxime ligand previously studied in solution was determined. All oximate complexes show high reactivity in the cleavage of aryl acetates, paraoxon, parathion and 4-nitrophenyl diphenyl phosphate, with rate constants significantly surpassing the limiting rate constants observed for highly basic free oximate anions. The second-order rate constants for individual oximate complexes in solution are assigned to each ligand, metal cation and substrate. The results of the cleavage of 4-substituted phenyl acetates were analyzed in terms of Brønsted correlations with the leaving group pKa, which demonstrated a change in the rate determining step from the nucleophilic attack to the leaving group departure upon an increase in the leaving group basicity. The zero slope of the Brønsted correlation for the nucleophilic attack indicates transition state stabilization through electrophilic assistance by the metal ion. This interpretation is supported by metal selectivity in the relative efficiency of the cleavage of paraoxon and parathion. The existence of the alpha-effect in ester cleavage by coordinated oximates is confirmed by an analysis of the Brønsted correlations with the nucleophile basicity for metal bound oximate and alkoxo or hydroxo nucleophiles. The very high reactivity of the oximate complexes of the new ligands is attributed to transition state stabilization and to the removal of the solvational imbalance of oximate anions that impedes the expected increase in the reactivity of highly basic free anions.
已经制备了两个三齿和一个四齿新配体，它们含有末端肟基，通过两个或三个亚甲基与仲氨基和吡啶基隔开，作为额外的结合位点。通过电位滴定和分光光度滴定法测定了它们的酸碱性质以及与 Zn(ii)和 Cd(ii)离子形成的配合物的组成和稳定性。先前在溶液中研究过的相关三齿肟配体的 Cd(ii)配合物的 X 射线结构已被确定。所有肟配合物在芳基乙酸酯、对氧磷、对硫磷和 4-硝基二苯磷酸酯的裂解中表现出高反应性，其速率常数明显超过了高碱性游离肟阴离子观察到的限速常数。在溶液中，单个肟配合物的二级速率常数被分配给每个配体、金属阳离子和底物。用离去基团 pKa 对 4-取代基苯乙酸酯的裂解结果进行了 Brønsted 相关分析，结果表明，随着离去基团碱性的增加，速率决定步骤从亲核进攻变为离去基团离去。亲核进攻的 Brønsted 相关的零斜率表明，通过金属离子的亲电辅助稳定了过渡态。这种解释得到了金属对 paraoxon 和 parathion 裂解相对效率的选择性的支持。通过分析与金属结合的肟和烷氧基或羟氧基亲核试剂的亲核性的 Brønsted 相关关系，证实了配位肟酯裂解中的 alpha 效应的存在。新配体的肟配合物具有非常高的反应性，这归因于过渡态稳定和消除肟阴离子的溶剂化不平衡，这阻碍了高碱性游离阴离子预期的反应性的增加。
