Serendipity and Four Polymorphic Structures of Benzidine, C₁₂H₁₂N₂

The search for a co-crystal of benzidine as a bifunctional hydrogen-bond donor with potential hydrogen-bond acceptors has instead revealed four polymorphs of the source material benzidine for which, somewhat surprisingly, no structure has been reported as of the November 2005 update of the CSD. All four structures are characterized by a rather unusual number of molecules in the asymmetric unit (Z‘ = 1.5, 3, and 4.5), which are found in only 0.25%, 0.4%, and 0.002% of structures in the CSD. Forms I and IV (Z‘ = 4.5) exhibit very similar crystal habits and are not distinguishable visually. In all forms except Form II (Z‘ = 3), one of the molecules lies on a crystallographic inversion center, requiring the molecule to be planar; other molecules are nonplanar. Spectroscopic and thermodynamic characterizations of the system, including at least two possible additional forms of benzidine obtained by HT polymorph screening are reported.本研究原本旨在探索联苯胺（benzidine）作为双功能氢键给体与潜在氢键受体形成共晶的可能性，却颇为意外地获得了原料联苯胺的四种多晶型体。颇为意外的是，截至2005年11月剑桥晶体结构数据库（Cambridge Structural Database, CSD）的更新版本，尚未有关于这四种多晶型体的晶体结构被报道。四种结构的不对称单元内均拥有非同寻常的分子计数（Z'=1.5、3和4.5），这类结构在CSD收录的全部晶体结构中占比仅为0.25%、0.4%和0.002%，属于极为罕见的晶型类型。其中晶型I与晶型IV（Z'=4.5）的晶体形貌高度相似，无法通过目视手段加以区分。除晶型II（Z'=3）外，其余所有晶型中均有一个分子位于晶体学倒反中心上，该分子因此必须呈平面构型，其余分子则为非平面构型。本研究还报道了该体系的光谱学与热力学表征结果，其中包含通过高温（High Temperature, HT）多晶型筛选得到的至少两种潜在联苯胺额外晶型。