Molecular Complexes of 4‑Halophenylboronic Acids: A Systematic Exploration of Isostructurality and Structural Landscape

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 I, 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···I, I···C(sp2), and I···N interactions, while smaller Cl is consistent
in making C–H···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.