de Namor Angela F Danil, Zegarra-Fernandez Katherine
Laboratory of Thermochemistry, Chemistry Division, School of Biomedical and Molecular Sciences, University of Surrey, Guildford, Surrey, UK.
J Phys Chem B. 2007 Jun 28;111(25):7321-30. doi: 10.1021/jp072128+. Epub 2007 Jun 1.
The solution behavior of ethyl p-tert-butylcalix[5]arene pentanoate, EtCalix(5), in various protic and aprotic solvents has been assessed from the standard Gibbs energies of transfer of this ligand from acetonitrile to other solvents. These data were derived from solubility measurements of EtCalix(5) in different media. It is concluded that in solvents in which metal cation salts are predominant as ionic species in solution (within a low concentration range), the solvation of the ligand will not contribute significantly to the thermodynamics of cation complexation as a result of the medium effect. Various analytical techniques were used to identify the sites of ligand-cation interaction (1H NMR) and the composition (conductance and calorimetric measurements) of the metal-ion complexes in the various solvents. Titration calorimetry (direct and competitive) was used to thermodynamically characterize the cation binding ability of EtCalix(5) and gain quantitative information on the complex stability and the factors controlling it. Given the large bulk of data reported in the literature wrongly placed under the thermodynamic umbrella, particular emphasis is made about the need of identifying the process taking place in solution prior to proceeding with its thermodynamic characterization. The results are compared with those for the ethyl ester derivative of the cyclic tetramer, EtCalix(4). It is concluded that in moving from EtCalix(4) to EtCalix(5), (i) the selectivity of the latter for cations is swallowed toward the larger cations and (ii) although the number of binding sites increases, these are not able to exert the efficient control upon the ligand-cation binding ability as that found for the tetramer ester and these cations in these solvents.
通过对叔丁基杯[5]芳烃戊酸乙酯（EtCalix(5)）从乙腈转移至其他溶剂的标准吉布斯自由能，评估了该配体在各种质子性和非质子性溶剂中的溶液行为。这些数据源自EtCalix(5)在不同介质中的溶解度测量。得出的结论是，在金属阳离子盐作为溶液中主要离子物种的溶剂中（在低浓度范围内），由于介质效应，配体的溶剂化对阳离子络合的热力学贡献不大。使用了各种分析技术来确定配体 - 阳离子相互作用的位点（1H NMR）以及各种溶剂中金属离子络合物的组成（电导和量热测量）。滴定量热法（直接和竞争）用于从热力学角度表征EtCalix(5)的阳离子结合能力，并获取有关络合物稳定性及其控制因素的定量信息。鉴于文献中大量错误地归入热力学范畴的数据，特别强调了在进行热力学表征之前识别溶液中发生的过程的必要性。将结果与环状四聚体的乙酯衍生物EtCalix(4)的结果进行了比较。得出的结论是，从EtCalix(4)转变为EtCalix(5)时，（i）后者对阳离子的选择性向较大阳离子方向减弱，并且（ii）尽管结合位点的数量增加，但这些位点无法像四聚体酯及其在这些溶剂中的阳离子那样对配体 - 阳离子结合能力进行有效控制。
