3-[4’-(Diethylboryl)phenyl]pyridine: Exclusive Crystallization of the Cyclic Tetramer Shigeharu Wakabayashi,* [a] Natsu Sugiyama, [a] Yasuhiro Ohki, [b] Takahito Itoh, [c] and Toshikazu Kitagawa [c] Abstract: The crystallization of 3-[4’-(diethylboryl)phenyl]pyr- idine (1), which formed a mixture of oligomers in solution with the cyclic trimer as a major component, in acetone at 0 8C afforded a cyclic tetramer that co-crystallized with sol- vent molecules. Similarly, solutions of compound 1 in tolu- ene at 10 8C and in benzene at 8 8C furnished the cyclic tetramer with the incorporation of toluene and benzene molecules, respectively, thus suggesting that the cyclic tetra- mer was the minor component. 13 C CP/MAS NMR spectros- copy of precipitates of compound 1 suggested that precipi- tation from acetone and toluene each afforded mixtures of the cyclic trimer and the cyclic tetramer, whereas precipita- tion from benzene exclusively furnished the cyclic tetramer. Therefore, it appeared that crystallization readily shifted the equilibrium towards the cyclic tetramer in benzene. The thermodynamic parameters for the equilibrium between these two oligomers in [D 6 ]benzene, as determined from a van’t Hoff plot, were DH8 = 8.8 kcal mol 1 and DS8 = 23.7 cal mol 1 K 1 , which were coincident with previously reported calculations and observations. 1. Introduction The self-assembly of well-designed molecular components is a powerful tool for the development of functional supramolec- ular nanostructures, as well as an appropriate model for gain- ing deeper understanding of biological systems. [1] Some orga- noboranes can act as versatile building blocks in molecular self-assembly. [2] Recently, we extended our long-standing inter- est in borylpyridines, [3] which is based on the formation of macrocycles by using pyridine–boron coordination, to new sys- tems that also offer the possibility of host–guest complexation. A recent report on the self-assembly of 4-(diethylboryl)pyridine (2 ; Scheme 1) revealed the formation of a cyclic pentamer and a cyclic hexamer, either of which could be selectively crystal- lized through the judicious choice of solvent. [3e] In contrast, compound 1 afforded an equilibrium mixture of oligomers in solution, including a cyclic trimer as the major component. [3b] Although such self-organization has been re- ported previously, [4, 2d] little is known about the crystallization of such macrocyclic assemblies in equilibrium. [2d] Therefore, it would be of great interest to compare the crystallization pro- file of compound 1 with that of compound 2. Herein, we report the results of a crystallization study of compound 1. Analysis of the precipitates by using 13 C CP/MAS NMR spectros- copy, elemental analysis, and thermogravimetric analysis (TGA) suggested that the precipitates from benzene only contained the cyclic tetramer (Scheme 2). Thermodynamic studies of the equilibrium are also reported. 2. Results and Discussion 2.1. Mass Spectrometry Compound 1 was prepared according to a literature proce- dure. [3b] Reinvestigation of compound 1 by using ESI-MS in THF in the presence of LiCl in positive-ion mode revealed peaks at m/z = 893.64, 676.49, 670.48, 453.33, and 447.32, which were as- signed to [4 M+H] + , [3 M+Li] + , [3 M+H] + , [2 M+Li] + , and [2 M+H] + , respectively (Figure 1). [3b] The observed isotopic distri- butions of each fragment were in close agreement with the the- oretical values. Even though this result suggested the presence of a mixture of oligomers in solution and that the tetramer should be the minor component, we cannot use it to discuss Scheme 1. Structures of borylpyridines 1 and 2. [a] Prof. Dr. S. Wakabayashi, N. Sugiyama Department of Clinical Nutrition Faculty of Health Science Suzuka University of Medical Science Suzuka, Mie 510-0293 (Japan) E-mail : s-waka@suzuka-u.ac.jp [b] Prof. Dr. Y. Ohki Department of Chemistry Graduate School of Science Nagoya University Chikusa-ku, Nagoya 464-8602 (Japan) [c] Prof. Dr. T. Itoh, Prof. Dr. T. Kitagawa Department of Chemistry for Materials Graduate School of Engineering Mie University Tsu, Mie 514-8507 (Japan) Supporting information and the ORCID identification number(s) for the au- thor(s) of this article can be found under: https://doi.org/10.1002/asia.201801698. Chem. Asian J. 2019, 00,0–0 # 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 && These are not the final page numbers! ÞÞ Full Paper DOI: 10.1002/asia.201801698