Macrocycles and Coordination Polymers Derived from Self-Complementary Tectons Based on N‑Containing Boronic Acids

A series of N-containing boronic esters have been prepared by combination of 3-pyridineboronic acid (3-pyba), 4-pyridineboronic acid (4-pyba), and 5-isoquinolineboronic acid (5-iqba) with ethanol, 1,2-ethanediol, and 1,3-propanediol. The resulting self-complementary tectons (SCTs) assembled further through N → B bond formation to give one tetranuclear macrocyclic and four one-dimensional polymeric boron complexes: [(3-py)­B­(OEt)2]4 (2), [(3-py)­B­(OCH2CH2O)]n (3), [(4-py)­B­(OEt)2]n (4), [(5-iq)­B­(OCH2CH2O)]n (6), and [(5-iq)­B­(OCH2CH2CH2O)]n (7). In conjunction with the previously reported pentadecanuclear boroxine cage (1) and high-dimensional assemblies derived from pentaerythritol, [(4-py)­B­(OCH2)2C­(CH2O)2B­(4-py)]n·4nEtOH·nH2O·nC7H8 (5), and [(5-iq)­B­(OCH2)2C­(CH2O)2B­(5-iq)]n·nEtOH (8), it was shown that a single class of self-complementary boron-based tectons can give a varied series of finite and infinite supramolecular aggregates. A comparative structural characterization of boron complexes 2–4 and 6–7 by single-crystal X-ray diffraction analysis and quantum-chemical calculations revealed that the corresponding SCTs could have been organized in all cases in the form of tetrameric macrocycles, as it occurred for 2, indicating that intermolecular noncovalent interactions during crystallization probably play a significant role in the output of the assembly process and influence the formation of supramolecular isomers. This observation was supported by a comparison of the molecular geometries of a series of tetranuclear macrocyclic and related linear oligomeric boronic esters derived from 3-pyba, 4-pyba, and 5-iqba, whose structures have been optimized by quantum-chemical calculations at the (B3LYP/cc-pVDZ) level of theory. The calculated structure of [(3-py)­B­(OEt)2]4 was in good agreement with the structure obtained experimentally by single-crystal X-ray diffraction analysis. For the cyclo-tetramers derived from 3-pyba and 5-iqba, all common conformers known from calixarene chemistry have been analyzed (cone, 1,3-alternate, 1,2-alternate, and partial cone), showing that the boronic ester group has a significant influence on their relative stability.