Crystal Engineering of Multiple Supramolecular Heterosynthons in Cocrystals of Boronic Acids: Structures, Photoreactivities, and Catalysis

Cocrystallizations of four para-substituted arylboronic acids, namely, 4-carboxyphenylboronic acid (4-cpba), 4-aminophenylboronic acid (4-apba), 4-aminocarbonylphenylboronic acid (4-acpba), and 4-nitrophenylboronic acid (4-npba), with trans-1,2-bis(4-pyridyl)ethylene (bpe) afforded seven multicomponent crystals, i.e., [(4-cpba)₂(bpe)₃], [(4-apba)₂(bpe)₃], [(4-acpba)₂(bpe)], [(4-apba)(bpe)₂(H₂O)₂], [(4-acpba)(bpe)₂(H₂O)₂], [(4-cpbhme)(bpe)], and [(4-npbhme)₂(bpe)], with the last two containing the hemimethyl ester (hme) of 4-cpba and 4-npba, respectively. SCXRD analyses reveal (B)O–H···N_pyr hydrogen bonds among the boronic acid (or hme) and bpe molecules. With the presence of an additional hydrogen-bonding site in the para-position of the boronic acids, the formation of 1D strands is induced, which generally supports interstrand π-interactions to yield 2D layer structures. In most cases, adjacent bpe molecules in the crystal structures fulfill the topological criteria for a [2 + 2] photodimerization reaction and react to yield rctt-tetrakis(4-pyridyl)cyclobutane (tpcb). UV photoreaction experiments on single crystals for [(4-cpba)₂(bpe)₃] resulted in a single-crystal-to-single-crystal (SCSC) transformation. The boronic acid 4-cpba was also used to direct the photoreaction catalytically.