Versatile Self-Adapting Boronic Acids for H‑Bond Recognition: From Discrete to Polymeric Supramolecules

Because of the peculiar dynamic covalent
reactivity of boronic
acids to form tetraboronate derivatives, interest in using their aryl
derivatives in materials science and supramolecular chemistry has
risen. Nevertheless, their ability to form H-bonded complexes has
been only marginally touched. Herein we report the first solution
and solid-state binding studies of the first double-H-bonded DD·AA-type
complexes of a series of aromatic boronic acids that adopt a syn–syn conformation with suitable
complementary H-bonding acceptor partners. The first determination
of the association constant (Ka) of ortho-substituted boronic acids in solution showed that Ka for 1:1 association is in the range between
300 and 6900 M–1. Crystallization of dimeric 1:1
and trimeric 1:2 and 2:1 complexes enabled an in-depth examination
of these complexes in the solid state, proving the selection of the
−B­(OH)2 syn–syn conformer through a pair of frontal H-bonds with the relevant AA
partner. Non-ortho-substituted boronic acids result
in “flat” complexes. On the other hand, sterically demanding
analogues bearing ortho substituents strive to retain
their recognition properties by rotation of the ArB­(OH)2 moiety, forming “T-shaped” complexes. Solid-state
studies of a diboronic acid and a tetraazanaphthacene provided for
the first time the formation of a supramolecular H-bonded polymeric
ribbon. On the basis of the conformational dynamicity of the −B­(OH)2 functional group, it is expected that these findings will
also open new possibilities in metal-free catalysis or organic crystal
engineering, where double-H-bonding donor boronic acids could act
as suitable organocatalysts or templates for the development of functional
materials with tailored organizational properties.