Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55D-70569 Stuttgart, Germany.
Chemphyschem. 2010 Dec 3;11(17):3752-65. doi: 10.1002/cphc.201000444.
A series of N-4-(4'-alkoxybiphenyl)-N',N',N",N"-tetramethylguanidinium salts was synthesized with varying alkoxy chain lengths and additional N-alkyl substituents, each with a number of different counterions. X-ray crystal-structure analyses of 1b I, 1b PF(6), 2a I, and 4a I reveal bilayer structures in the solid state and, for the 1b and 1b PF(6) salts, a hydrogen-bond-type connectivity between the guanidinium N-H group and the anion is found. For the N-alkyl homologues 2a I and 4a I the anion is still oriented close to the head group, although at a larger distance. Ion pairs are present also in solution, as demonstrated by (1)H NMR: the N-H chemical shift shows a good linear correlation with the radius, and hence the hardness, of the anion. The intramolecular conformational flexibility of 1b I, 2b I, 3b I, and 4b I was studied by temperature-dependent (1)H NMR spectroscopy and discrete activation barriers were determined for rotations about each of the three C-N partial double bonds of the guanidinium core. The relative heights of the individual barriers change between the N-H and the N-alkylguanidinium salts. A fourth barrier is observed for the rotation about the N-biphenyl bond. DFT calculations of charge densities show that the positive charge resides primarily on the central carbon atom. Rotational barriers were calculated for N'-substituted 2-amino-1,3-dimethylimidazolidinium cations as models, and are in qualitatively good agreement with the NMR data. Mesomorphic properties were studied by differential-scanning calorimetry, polarizing optical microscopy, and X-ray diffraction (WAXS/SAXS). All liquid-crystalline guanidinium salts exhibit smectic A mesophases. Clearing temperatures show a linear correlation with the anionic radius. Substitution of the N-H group with methyl, ethyl, or propyl results in decreasing mesophase widths and a concomitant shrinkage of the layer spacings.
合成了一系列具有不同烷氧基链长和额外的 N-烷基取代基的 N-4-(4'-烷氧基联苯)-N',N',N",N"-四甲基胍盐，每个盐都有许多不同的抗衡离子。1b I、1b PF(6)、2a I 和 4a I 的 X 射线晶体结构分析表明，在固态中存在双层结构，并且对于 1b 和 1b PF(6)盐，发现胍鎓 N-H 基团与阴离子之间存在氢键型连接。对于 N-烷基同系物 2a I 和 4a I，阴离子仍然靠近头基，但距离较大。正如（1）H NMR 所示，离子对也存在于溶液中：N-H 化学位移与阴离子的半径和硬度呈良好的线性关系。1b I、2b I、3b I 和 4b I 的分子内构象灵活性通过温度依赖的（1）H NMR 光谱研究，并确定了胍核心中三个 C-N 部分双键的每个旋转的离散活化能垒。各个势垒的相对高度在 N-H 和 N-烷基胍盐之间发生变化。还观察到关于 N-联苯键的旋转的第四个势垒。对 N'-取代的 2-氨基-1,3-二甲基咪唑啉鎓阳离子进行密度泛函理论（DFT）计算，得到的电荷密度表明正电荷主要位于中心碳原子上。计算了作为模型的 N'-取代 2-氨基-1,3-二甲基咪唑啉鎓阳离子的旋转势垒，与 NMR 数据在定性上非常吻合。通过差示扫描量热法、偏光显微镜和 X 射线衍射（WAXS/SAXS）研究了介晶性质。所有液晶胍盐都表现出近晶 A 型介晶相。清亮点温度与阴离子半径呈线性相关。用甲基、乙基或丙基取代 N-H 基团会导致介晶相宽度减小，层间距收缩。
