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.

鉴于硼酸具备独特的动态共价反应活性，可形成四硼酸酯衍生物，其芳基衍生物在材料科学与超分子化学中的应用研究热度逐渐攀升。然而，这类衍生物形成氢键配合物的能力却鲜有深入探究。本文首次针对一系列采用顺-顺（syn–syn）构象的芳香族硼酸，与互补氢键受体配对物形成的双氢键供体-受体（DD·AA）型配合物，开展溶液与固态结合行为研究。本研究首次测定了邻位取代硼酸在溶液中的结合常数（Ka），结果显示1:1结合模式下的Ka值介于300至6900 M⁻¹之间。通过对二聚1:1、三聚1:2与2:1型配合物的结晶培养，实现了其固态结构的深入表征，证实了−B(OH)₂基团通过与对应AA受体单元形成一对正面氢键，从而选择顺-顺构象。无邻位取代的硼酸会形成“平面型”配合物；而带有邻位取代基的位阻型类似物则通过ArB(OH)₂基团的旋转维持其识别性能，进而形成“T型”配合物。针对二硼酸与四氮杂并四苯的固态研究，首次观测到了超分子氢键聚合带状结构的形成。基于−B(OH)₂官能团的构象动态性，本研究成果有望为无金属催化或有机晶体工程领域开辟新方向：双氢键供体型硼酸可作为适宜的有机催化剂或模板，用于开发具有定制化组织性能的功能材料。