Cocrystallization of Trimesic Acid and Pyromellitic Acid with Bent Dipyridines

Bent dipyridyl bases, 2,5-bis(3-pyridyl)-1,3,4-oxadiazole (3-bpo) and its 4-N-donor analogue 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-bpo), were applied to crystallize with trimesic acid (benzene-1,3,5-tricarboxylic acid,
H3TMA) and pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid, H4BTA), affording new binary molecular
cocrystals [(H3TMA)·(3-bpo)]·DMF (1), [(H3TMA)·(4-bpo)] (2), [(H4BTA)·(3-bpo)]·2H2O (3), and a partly charge-transfer
salt [(H2BTA)1/2·(H-4-bpo)·(H4BTA)1/2]·H2O (4) under general conditions. An unexpected compound 5 (pyromellitic
dihydrazide) was obtained during the attempt to attain a cocrystal of 4-bpo and H4BTA by hydrothermal synthesis.
X-ray single-crystal diffraction studies reveal that supramolecular synthon I [

(7)] containing classical O−H···N
and weak C−H···O interactions is again testified to be involved in constructing the binary hydrogen-bonding networks
1−4, in which the heteromeric intermolecular interactions are often utilized as effective synthetic tools to provide
a significant portion of the stabilization energy of molecular crystals. In compound 1, 1-D acid/base zigzag chains
formed by synthon I are extended to a 2-D layered architecture via additional C−H···O interactions, the cavities of
which are occupied by guest solvents (N,N-dimethylformamide, DMF). Compound 2 displays fascinating 1-D triple-helical motifs via synthon I (formation of the single strand helix) and C−H···O interactions, which are further
expanded to a 3-D supramolecular array by multiple hydrogen bonds. Self-recognition via hydrogen bonding has
been evidently embodied in this unique supramolecular system. In compounds 3 and 4, water moieties are present
in both resultant assemblies and play different roles in the formation of 3-D supramolecular networks. For 3, each
water molecule acts as a 3-connected node, linking three 1-D acid/base double chains resulting from the combination
of synthon I and C−H···N bonds, to afford a 3-D sandwich network. For 4, water molecules only behave as guests
to further stabilize the robust 3-D hydrogen-bonding host framework. Compound 5 has a 2-D sheet structure via a
pair of strong head-to-tail N−H···O interactions. The thermal stability of cocrystals 1−4 has been investigated by
thermogravimetric analysis of mass loss.

将弯曲联吡啶碱类——2,5-双(3-吡啶基)-1,3,4-噁二唑（3-bpo）及其4-N供体类似物2,5-双(4-吡啶基)-1,3,4-噁二唑（4-bpo），分别与偏苯三甲酸（benzene-1,3,5-tricarboxylic acid, H3TMA）和均苯四甲酸（benzene-1,2,4,5-tetracarboxylic acid, H4BTA）进行结晶组装，于常规条件下得到新型二元分子共晶[(H3TMA)·(3-bpo)]·DMF（1）、[(H3TMA)·(4-bpo)]（2）、[(H4BTA)·(3-bpo)]·2H2O（3），以及部分电荷转移盐[(H2BTA)1/2·(H-4-bpo)·(H4BTA)1/2]·H2O（4）。在尝试通过水热合成法制备4-bpo与H4BTA的共晶时，意外获得了化合物5（均苯四甲酸二酰肼）。X射线单晶衍射研究表明，包含经典O−H···N与弱C−H···O相互作用的超分子合成子I [ (7) ] 被证实参与构建了二元氢键网络1~4，其中异质分子间相互作用常作为高效合成工具，为分子晶体提供了主要的稳定化能。在化合物1中，由合成子I形成的一维酸/碱锯齿链通过额外的C−H···O相互作用拓展为二维层状结构，层间空腔由客体溶剂N,N-二甲基甲酰胺（DMF）占据。化合物2通过合成子I（单链螺旋的形成）与C−H···O相互作用展现出引人入胜的一维三重螺旋基元，该基元进一步通过多重氢键拓展为三维超分子骨架。此独特超分子体系中，氢键介导的自识别效应得到了显著体现。在化合物3与4中，水分子均存在于最终组装体中，并在三维超分子网络的形成过程中发挥不同作用。对于化合物3而言，每个水分子作为3连接节点，将由合成子I与C−H···N键结合形成的一维酸/碱双链连接起来，从而构筑出三维三明状网络。对于化合物4，水分子仅作为客体组分进一步稳定坚固的三维氢键主体骨架。化合物5通过一对强头对尾N−H···O相互作用形成二维片层结构。已通过热重失重分析法对共晶1~4的热稳定性进行了表征。