Self-Assembly of 4‑(Diethylboryl)pyridine: Crystal Structures of the Cyclic Pentamer and Hexamer and Their Solvent-Dependent Selective Crystallization

Two
distinct oligomeric structures were obtained by the self-assembly
of 4-(diethylboryl)­pyridine (1). In the 1H
NMR spectrum of 1 in CDCl3, at least two sets
of signals were observed for the pyridyl α- and β-hydrogen
atoms. ESI-MS, VPO, and TLC analysis revealed that 1 assembles
mainly into a mixture of cyclic pentamers and hexamers in solution
via intermolecular boron–nitrogen coordination bonds. Crystallization
of 1 in THF by vapor diffusion of EtOH or in CHCl3 afforded the cyclic hexamer incorporating one THF molecule
(16·THF) or 1.5 mol equiv of chloroform
molecule (16·CHCl3), respectively.
Similarly, a solution of 1 in a mixture of benzene and
hexane furnished the cyclic pentamer bearing two benzene molecules
(15·C6H6). It seems
that the solvent differences affected the crystallization of the two
distinct cyclic oligomers of 1, either of which was cocrystallized
predominantly with the solvent molecule. Thermogravimetric analysis
of the crystals and NMR studies of the solution revealed that the
noncovalent interactions between the host and guest are not strong
enough to hold the guest molecule in the cavity.

通过4-(二乙基硼基)吡啶（1）的自组装，获得了两种独特的寡聚结构。在CDCl3中的1H NMR光谱中，观察到至少两组信号对应于吡啶α-和β-氢原子。电喷雾质谱（ESI-MS）、VPO和薄层色谱（TLC）分析显示，1主要在溶液中通过分子间硼-氮配位键组装成环状五聚体和六聚体的混合物。通过乙醇蒸汽扩散法在THF中结晶1或在CHCl3中结晶，分别得到了包含一个THF分子（16·THF）或1.5摩尔当量的氯仿分子（16·CHCl3）的环状六聚体。同样，将1溶解于苯和己烷的混合物中，得到了带有两个苯分子的环状五聚体（15·C6H6）。似乎溶剂的不同影响了两种独特的环状寡聚物1的结晶，其中任一均主要与溶剂分子共结晶。晶体热重分析及溶液的核磁共振（NMR）研究表明，主客体之间的非共价相互作用不足以将客体分子保持在空腔中。