Oxidatively Induced Isomerization of Square-Planar [Ni(1,4,8,11-tetraazacyclotetradecane)](ClO(4))(2).
Oxidatively Induced Isomerization of Square-Planar [Ni(1,4,8,11-tetraazacyclotetradecane)](ClO(4))(2).
复制标题
方形平面 [Ni(1,4,8,11-四氮杂环十四烷)](ClO(4))(2) 的氧化诱导异构化。
作者：
D. Pierce;T. L. Hatfield;E. J. Billo;Yao Ping
先前的质子核磁共振和电子光谱研究表明，Ni（II）与1，4，8，11-四氮杂环十四烷（cyclam）的方形平面配合物存在两种稳定的异构体构象，即R、S、R、S形式或反式-I异构体和R、R、S、S形式或反式-III异构体。Ni（II）-cyclam的反式-I异构体的电化学分析已经证明在氧化成Ni（III）之后快速转化成反式-III cyclam构象。采用循环伏安法（CV）技术，通过有限差分计算模拟CV迹线，研究了这种氧化诱导异构化的机理和动力学。最关键的机械指标被认为是瞬态的trans-I-Ni（III）物种。在pH 2-4范围内通过CV检测到该中间体，并发现其表观半衰期约为1.5。室温下400 ms。值得注意的是，这种寿命比相应的trans-I-Ni（II）物种短约十亿倍。在不同的溶液温度和pH值的测量表明，氧化诱导的异构化遵循一个明显的平方计划，其中的反式-I/III异构化过程的Ni（III）是独立的pH值。这一发现排除了碱催化的异构化过程，已被确定为Ni（II）系统。阿克里尼乌斯图的正向异构化速率常数允许提取的Ni（III）过程的活化参数。这些参数进行了讨论相对于可能的速率确定步骤。
Previous proton NMR and electronic spectroscopy studies have demonstrated that the square-planar complex of Ni(II) and 1,4,8,11-tetraazacyclotetradecane (cyclam) exists in two stable isomeric conformations, namely the R,S,R,S form or trans-I isomer and the R,R,S,S form or trans-III isomer. Electrochemical analysis of the trans-I isomer of Ni(II)-cyclam has demonstrated rapid conversion to the trans-III cyclam conformation following oxidation to Ni(III). The mechanism and kinetics for this oxidatively-induced isomerization were studied by the technique of cyclic voltammetry (CV) with simulation of CV traces by finite-difference computations. The most crucial mechanistic indicator was found to be the transient trans-I-Ni(III) species. This intermediate was detected by CV over a pH range 2-4 and was found to have an apparent half-life of ca. 400 ms at room temperature. Remarkably, this life-time was approximately a billionfold shorter than the corresponding trans-I-Ni(II) species. Measurements made at varied solution temperature and pH demonstrated that the oxidatively induced isomerization followed an apparent square-scheme, where the trans-I/III isomerization process of Ni(III) was independent of pH. This finding precluded a base-catalyzed isomerization process that has been previously identified for the Ni(II) system. Arrhenius plots of the forward isomerization rate constant allowed the extraction of activation parameters for the Ni(III) process. These parameters are discussed with respect to possible rate-determining steps.