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.