Conformational variability exhibited by mellitic acid anions (MA−n) with organic amine cations: Structures with an abundance of NH ⋅s OC hydrogen bonds and a paucity of hydrogen bonds
作者:Isabella L. Karle、Y. B. R. D. Rajesh、Subramania Ranganathan
DOI:10.1007/s10870-005-4554-9
日期:2005.10
Benzenehexacarboxylic acid (mellitic acid) is an ideal substrate for the study of radially arranged ionic interactions with organic bases. Earlier work has shown that the net charge in MA (−4, −3, or −2) influences its assembly, respectively, either as ribbons or sheets. In the former case the cations intersperse the ribbons forming a tight network. In the MA sheets from the latter, the cations are arranged in orthogonal stacks. In the MA-phenanthroline complex such stacks are arranged exactly parallel resulting in significant π–π interactions. The present work relates to the modification of the phenanthroline motif to flexible structures. The reported ability of 2,3-bis-(2′-pyridyl) pyrazine (P2), prepared from 2,2′-pyridyl by reaction with ethylenediamine (EDA) followed by dehydrogenation of the resulting 2,3-bis-(2′-pyridyl)-5,6-dihydropyrazine, to form helical structures made it an obvious choice for complexation studies with MA. These resulted not only in the formation of the desired 7 MAP2 complex but also, surprisingly, with the dihydroprecursor complex with EDA 3 (MAEDA), a compound that eluded direct preparation with MA and EDA. The crystal structure of MAEDA reports the first complex of MA where MA−5 ions are encountered. The 15 hydrogen bonds, three for each N, result in hard packing. The complex with P2 7 (MAP2) showed the common features of MA−2 sheets. Unlike phenanthroline, the nonplanar disposition of the pyridine rings made it possible for both pyridine protons to be available for salt formation, resulting in the arrangement of the cations in layers interposed between the sheets of MA−2 ions. The pyrazine ring does not participate in hydrogen bonding or stacking in the crystal.
苯己羧酸(麦饭石酸)是研究径向排列的离子与有机碱相互作用的理想底物。早先的研究表明,MA 中的净电荷(-4、-3 或 -2)会分别影响其组装成带状或片状。在前一种情况下,阳离子穿插在带状结构中,形成一个紧密的网络。在后者的 MA 片材中,阳离子呈正交堆叠排列。在甲基丙烯酸甲酯-菲罗啉复合物中,这些叠层完全平行排列,从而产生显著的 π-π 相互作用。目前的工作涉及将菲罗啉基团修饰成柔性结构。据报道,2,3-双(2′-吡啶基)吡嗪(P2)是由 2,2′-吡啶与乙二胺(EDA)反应制备而成,然后对生成的 2,3-双(2′-吡啶基)-5,6-二氢吡嗪进行脱氢反应,从而形成螺旋结构。这些研究不仅形成了所需的 7 MAP2 复合物,还令人惊讶地形成了与 EDA 3 的二氢前体复合物(MAEDA),这是一种无法直接用 MA 和 EDA 制备的化合物。MAEDA 的晶体结构报告了第一个出现 MA-5 离子的 MA 复合物。15 个氢键(每个 N 有 3 个)形成了硬堆积。与 P2 7(MAP2)的复合物显示出 MA-2 片状的共同特征。与菲罗啉不同的是,吡啶环的非平面布局使得两个吡啶质子都可以形成盐,从而使阳离子排列在 MA-2 离子片之间的层中。吡嗪环没有参与晶体中的氢键或堆积。