The complexation reaction between Cu(II) and a series of cyclam analogue macrocycles with dangling primary amine groups was investigated in aqueous solution. The pH dependence of the reaction was measured between pH 0 and 5.5 covering a range of different protonation stages of the ligands. The kinetics was observed by stopped-flow measurements followed at many wavelengths. An initial second order reaction results in an intermediate which isomerises to a stable complex. The observed rate constants for the initial step varied between 1.5 × 10−2 M−1 s−1 at pH 0 and 6.3 × 105 M−1 s−1 at pH 5.3; for the secondary step the rates varied between 2 × 10−4 s−1 at pH 1 and 24 s−1 at pH 5.3. The pH dependence was successfully analysed in terms of the different reactivities for differently protonated ligands for the first step and differently protonated complexes for the second step. Ligands with unprotonated dangling primary amines react with rate constants of 106–107 M−1 s−1, the rates drop dramatically upon protonation of the arms to ca. 10–102 M−1 s−1 which are very similar to the rates of the parent cyclam ligand of equivalent protonation. The pH dependence of the secondary isomerisation reaction is explained in similar ways: protonated, dangling arms inhibit the isomerisation reaction completely.
研究了
水溶液中 Cu(II) 与一系列带有悬浮
伯胺基团的环
胺类似大环之间的络合反应。在 pH 值 0 至 5.5 之间测量了反应的 pH 依赖性,涵盖了
配体的不同质子化阶段。通过多种波长的停流测量观察了反应动力学。最初的二阶反应会产生一个中间产物,该中间产物会异构为一个稳定的复合物。观察到的初始步骤的速率常数在 pH 值为 0 时为 1.5 × 10-2 M-1 s-1 和 pH 值为 5.3 时为 6.3 × 105 M-1 s-1 之间变化;次级步骤的速率在 pH 值为 1 时为 2 × 10-4 s-1 和 pH 值为 5.3 时为 24 s-1 之间变化。根据第一步中不同质子化
配体和第二步中不同质子化配合物的不同反应活性,成功地分析了 pH 依赖性。未质子化悬空
伯胺配体的反应速率常数为 106-107 M-1 s-1,当臂质子化时,反应速率急剧下降至约 10-102 M-1 s-1,与质子化程度相当的母环胺
配体的反应速率非常相似。二级异构化反应的 pH 值依赖性也有类似的解释:质子化的悬臂完全抑制了异构化反应。