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，可得到两种截然不同的寡聚体结构。在氘代氯仿(CDCl3)配制的1的氢核磁共振(1H NMR)谱图中，可观测到至少两组对应吡啶环α位与β位氢原子的信号峰。电喷雾电离质谱(ESI-MS)、蒸气压渗透法(VPO)及薄层色谱(TLC)分析结果显示，1在溶液中主要通过分子间硼-氮配位键自组装，形成环状五聚体与环状六聚体的混合物。采用气相扩散法，以乙醇(EtOH)为扩散溶剂在四氢呋喃(THF)中结晶1，或直接以三氯甲烷(CHCl3)为溶剂结晶1，分别得到了包合1个四氢呋喃分子的环状六聚体(16·THF)，以及包合1.5摩尔当量三氯甲烷分子的环状六聚体(16·CHCl3)。同理，将1的苯-正己烷混合溶液进行结晶，可得到包合2个苯分子的环状五聚体(15·C6H6)。研究表明，溶剂差异会影响1的两种环状寡聚体的结晶行为，且两种寡聚体均主要与溶剂分子发生共结晶。对所得晶体的热重分析及溶液态核磁共振研究表明，主体寡聚体与客体溶剂分子间的非共价相互作用强度不足以将客体分子稳定束缚于空腔中。