Abstract:
Structural equivalence is a general tool applied in crystal engineering for the predictable construction of cocrystals. We studied 17 molecular complexes of 4-halophenylboronic acids (Cl, Br, and I) with different N-donor compounds in the context of the structural equivalence of halogen bonded Br and 1, as proposed by Jones and co-workers (Chem. Mater. 2008, 20, 6623-6626; Chem.-Eur. J. 2008, 14, 747-753). The 4-Cl- and 4-Br-phenylboronic acids make isostructural complexes with 4,7-phenanthroline (phen) and 1,2-bis(4-pyridyl)ethene (bpye); in contrast, with phenazine (phenz) and acridine (acr) I and Br derivatives are analogous. Thus, the structural equivalence of Cl/Br or Br/I derivatives is equally probable and is largely determined by the interplay between hydrogen and halogen bonds, besides the size, electron charge shifting ability, and steric crowding adjacent to the binding sites of coformers. Thanks to its bigger size and higher polarizability of the electron cloud, I is capable of making I center dot center dot center dot I, I center dot center dot center dot C((sp2)), and I center dot center dot center dot N interactions, while smaller Cl is consistent in making C-H center dot center dot center dot Cl interactions. The intermediate Br shifts its grouping depending on the crystallization conditions and coformers. In combination with the experimental observation and a CSD analysis, we have also analyzed the crystal motifs involving boronic acid hydrates to derive the probable structural landscape and further to ascertain their recurrence.
