Horvat Gordan, Frkanec Leo, Cindro Nikola, Tomišić Vladislav
Division of Physical Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia.
Phys Chem Chem Phys. 2017 Sep 13;19(35):24316-24329. doi: 10.1039/c7cp03920d.
The complexation of alkali metal cations by lower rim N,N-dihexylacetamide (L1) and newly synthesized N-hexyl-N-methylacetamide (L2) calix[4]arene tertiary-amide derivatives was thoroughly studied at 25 °C in acetonitrile (MeCN), benzonitrile (PhCN), and methanol (MeOH) by means of direct and competitive microcalorimetric titrations, and UV and H NMR spectroscopies. In addition, by measuring the ligands' solubilities, the solution (transfer) Gibbs energies of the ligands and their alkali metal complexes were obtained. The inclusion of solvent molecules in the free and complexed calixarene hydrophobic cavities was also investigated. Computational (classical molecular dynamics) investigations of the studied systems were also carried out. The obtained results were compared with those previously obtained by studying the complexation ability of an N-hexylacetamidecalix[4]arene secondary-amide derivative (L3). The stability constants of 1 : 1 complexes were determined in all solvents used (the values obtained by different methods being in excellent agreement), as were the corresponding complexation enthalpies and entropies. Almost all of the examined reactions were enthalpically controlled. The most striking exceptions were reactions of Li with both ligands in methanol, for which the entropic contribution to the reaction Gibbs energy was substantial due the entropically favourable desolvation of the smallest lithium cation. The thermodynamic stabilities of the complexes were quite solvent dependent (the stability decreased in the solvent order: MeCN > PhCN ≫ MeOH), which could be accounted for by considering the differences in the solvation of the ligand and free and complexed alkali metal cations in the solvents used. Comparison of the stability constants of the ligand L1 and L2 complexes clearly revealed that the higher electron-donating ability of the hexyl with respect to the methyl group is of considerable importance in determining the equilibria of the complexation reactions. Additionally, the quite strong influence of intramolecular hydrogen bond formation in compound L3 (not present in ligands L1 and L2) and that of the inclusion of solvent molecules in the calixarene hydrophobic cone were shown to be of great importance in determining the thermodynamic stability of the calixarene-cation complexes. The experimental results were fully supported by those obtained by MD simulations.
在25℃下，于乙腈（MeCN）、苯甲腈（PhCN）和甲醇（MeOH）中，通过直接和竞争微量热滴定以及紫外和核磁共振氢谱法，深入研究了下缘N,N - 二己基乙酰胺（L1）和新合成的N - 己基 - N - 甲基乙酰胺（L2）杯[4]芳烃叔酰胺衍生物与碱金属阳离子的络合作用。此外，通过测量配体的溶解度，获得了配体及其碱金属络合物的溶液（转移）吉布斯自由能。还研究了游离和络合的杯芳烃疏水腔内溶剂分子的包合情况。对所研究体系进行了计算（经典分子动力学）研究。将所得结果与先前通过研究N - 己基乙酰胺杯[4]芳烃仲酰胺衍生物（L3）的络合能力所获得的结果进行了比较。在所有使用的溶剂中测定了1:1络合物的稳定常数（通过不同方法获得的值高度一致），相应的络合焓和熵也是如此。几乎所有研究的反应都是由焓控制的。最显著的例外是锂与两种配体在甲醇中的反应，由于最小的锂阳离子在熵上有利的去溶剂化作用，该反应对反应吉布斯自由能的熵贡献很大。络合物的热力学稳定性强烈依赖于溶剂（稳定性按溶剂顺序降低：MeCN > PhCN ≫ MeOH），这可以通过考虑所用溶剂中配体、游离和络合的碱金属阳离子溶剂化的差异来解释。配体L1和L2络合物稳定常数的比较清楚地表明，己基相对于甲基更高的给电子能力在确定络合反应的平衡中具有相当重要的意义。此外，化合物L3中分子内氢键形成（配体L1和L2中不存在）以及杯芳烃疏水锥内溶剂分子包合的相当强的影响在确定杯芳烃 - 阳离子络合物的热力学稳定性方面显示出非常重要的作用。实验结果得到了分子动力学模拟结果的充分支持。
