Versatile self-adapting boronic acids for H-bond recognition: from discrete to polymeric supramolecules

I. Georgiou, S. Kervyn, A. Rossignon, F. De Leo, J. Wouters, G. Bruylants, D. Bonifazi
J. Am. Chem. Soc. 2017, 139, 2710 − 2727.
DOI: 10.1021/jacs.6b11362


By taking advantage of the peculiar dynamic covalent reactivity of boronic acids to form tetraboronate derivatives, interest has risen to use the aryl derivatives in materials science and supramolecular chemistry, 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 first double H-bonded DD•AA-type complexes of a series of aromatic boronic acids that adopt a synsyn conformation with suitable complementary H-bonding acceptor partners. The first determination of the Ka in solution of ortho substituted boronic acids showed that 1:1 association is in the range between 300 and 6900 M-1. Crystallization of dimeric 1:1, trimeric 1:2 and 2:1 complexes enabled 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 rotating 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. Given 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 developing functional materials with tailored organizational properties.

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