Hydrogen Bond-Directed Frameworks Based on 1,2,4,5-Benzene-Tetracarboxylate

The deprotonated forms of 1,2,4,5-benzenetetracarboxylic acid H4bta can act not only as hydrogen-bond acceptors but also as hydrogen-bond donors, depending on the deprotonated carboxyl groups given different
supramolecular adducts. A search of the Cambridge Structural Database (CSD) of cocrystal structures of H4bta
with L = 4,4‘-bpy or bpe has found three different adducts:  [HL]2[H2bta], [H2L][H2bta], and [H2L][H3bta], where
cocrystal structures of [HL]2[H2bta] with L = 4,4‘-bpy or bpe form similar supramolecular arrangements. To
investigate this similarity, two new organic salts of the formula {[H2-4,4‘-bpy][H2bta]}·2H2O (1) and {[H2bpe]
[H3bta]2}·4H2O (2) have been characterized. In the 1:1 adduct 1, the molecular components are linked by hydrogen
bonds of the N−H···O type creating chains. These chains are further linked by C−H···O type interactions (C−H
from the cation and O from the anion) creating a two-dimensional motif. Two types of centrosymmetric rings are
evident in the sheets. The stacked sheets are held together via π···π interactions to create a three-dimensional
network. Two types of channels, one hydrophilic and the other hydrophobic, run parallel to the a-axis. Water molecules
are hydrogen bonded in the hydrophilic channels. The supramolecular structure in the 2:1 adduct 2 is due to hydrogen
bonds of the O−H···O and N−H···O types that afford a railroad network along the a-axis. Two water molecules are
held in the railroad cavities by hydrogen bonds with carboxylate oxygen atoms. Adjacent railroad chains are linked
through hydrogen bonds involving water molecules, building a two-dimensional structure.

1,2,4,5-苯四甲酸（1,2,4,5-benzenetetracarboxylic acid, H4bta）的去质子化形式既可作为氢键受体，亦可作为氢键供体，其具体角色取决于去质子化的羧基位点，进而形成不同的超分子加合物。对剑桥结构数据库（Cambridge Structural Database, CSD）中H4bta与配体L（即4,4'-联吡啶（4,4'-bpy）或1,2-二(4-吡啶基)乙烯（bpe））的共晶体结构进行检索，共发现三种不同的加合物：[HL]2[H2bta]、[H2L][H2bta]与[H2L][H3bta]，其中当L为4,4'-bpy或bpe时，[HL]2[H2bta]型共晶体具有相似的超分子排布。为探究该结构相似性，本研究对两种新型有机盐进行了表征，其分子式分别为{[H2-4,4'-联吡啶（H2-4,4'-bpy）][H2bta]}·2H2O（标记为化合物1）与{[H2bpe][H3bta]2}·4H2O（标记为化合物2）。在1:1型加合物1中，分子组分通过N−H···O型氢键相互连接形成链状结构；该链状结构进一步通过C−H···O型相互作用（其中C−H来自阳离子，O来自阴离子）连接，构筑得到二维超分子骨架。该二维片层中存在两种中心对称环结构，堆叠的片层通过π···π相互作用进一步结合，形成三维超分子网络。体系中存在两类平行于a轴的通道，分别为亲水性通道与疏水性通道；水分子通过氢键作用固定于亲水性通道内。在2:1型加合物2中，超分子结构由O−H···O与N−H···O型氢键构筑得到沿a轴延伸的“铁路状”网络结构；两个水分子通过与羧酸根氧原子形成氢键，固定于该铁路状网络的空腔内；相邻的铁路状链通过水分子介导的氢键相互连接，最终形成二维超分子结构。