Self-Assembly of 3-[4 '-(diethylboryl)phenyl]pyridine and 3-[3 '-(diethylboryl)phenyl]pyridine: Synthesis, structural features, and stability in solution

The self-assembly of 3-[4'-(diethylboryl)phenyl]pyridine (3) and 3-[3'-(die thylboryl)phenyl]pyridine (4) was investigated by H-1 and B-11 NMR spectros copies and vapor pressure osmometry. It can be seen that in solution 3 affo rds an equilibrium mixture of oligomers including a cyclic trimer as a majo r component via intermolecular boron-nitrogen coordination bonds, which is supported by temperature-, concentration-, and solvent-dependent behaviors of 3. In contrast, 4 seems to afford a mixture of oligomers including a cyc lic dimer in solution. Judging from the low coalescence temperature in vari able-temperature NMR in toluene-d(8) and easy scrambling with the correspon ding 3-[(diethylboryl)pheny]-5-methoxypyridines at ambient temperature, it is concluded that the intermolecular boron-nitrogen coordination bonds made by 3 or 4 are weaker than those formed by 3-(diethylboryl)pyridine (1) or 2-(diethylboryl)-5-methylpyridine (2). Poor thermal stability was supported by electrospray mass spectrometry and structural analyses utilizing a semi -empirical molecular orbital calculation AM1. It is noted that in the assem bly of 4 there is a tendency to maintain high tetrahedral character at the boron atom in the complex at the expense of strain energy. Thus, these feat ures of 3 and 4 in solution are in marked contrast to those of 1 and 2.