Mechanochemical Mediated Coexistence of B←N Coordination and Hydrogen Bonding.

Mechanochemistry afforded a photoactive cocrystal via coexisting (B)O-H⋅⋅⋅N hydrogen bonds and B←N coordination. Specifically, solvent-free mechanochemical ball mill grinding and liquid-assisted grinding of a boronic acid and an alkene resulted in mixtures of hydrogen-bonded and coordinated complexes akin to mixtures of noncovalent complexes that can be obtained in solution in equilibria processes. The alkenes of the hydrogen-bonded assembly undergo an intermolecular [2+2] photodimerization in quantitative conversion, effectively reporting the outcome of the self-assembly processes. Our results suggest that interplay involving noncovalent bonds subjected to mechanochemical conditions can lead to functional solids where, in the current case, the structure composed of the weaker hydrogen bonding interactions predominates.

Structures and Reactivities of Cocrystals Involving Diboronic Acids and Bipyridines: In Situ Linker Reaction and 1D-to-2D Dimensionality Change via Crystal-to-Crystal Photodimerization.

Cocrystallizations of diboronic acids [1,3-benzenediboronic acid (1,3-bdba), 1,4-benzenediboronic acid (1,4-bdba) and 4,4'-biphenyldiboronic acid (4,4'-bphdba)] and bipyridines [1,2-bis(4-pyridyl)ethylene (bpe) and 1,2-bis(4-pyridyl)ethane (bpeta)] generated the hydrogen-bonded 1 : 2 cocrystals [(1,4-bdba)(bpe)2 ] (1), [(1,4-bdba)(bpeta)2 ] (2), [(1,3-bdba)(bpe)2 (H2 O)2 ] (3) and [(1,3-bdba)(bpeta)2 (H2 O)] (4), wherein 1,3-bdba involved hydrated assemblies. The linear extended 4,4'-bphdba exhibited the formation of 1 : 1 cocrystals [(4,4'-bphdba)(bpe)] (5) and [(4,4'-bphdba-me)(bpeta)] (6). For 6, a hemiester was generated by an in-situ linker transformation. Single-crystal X-ray diffraction revealed all structures to be sustained by B(O)-H⋅⋅⋅N, B(O)-H⋅⋅⋅O, Ow -H⋅⋅⋅O, Ow -H⋅⋅⋅N, C-H⋅⋅⋅O, C-H⋅⋅⋅N, π⋅⋅⋅π, and C-H⋅⋅⋅π interactions. The cocrystals comprise 1D, 2D, and 3D hydrogen-bonded frameworks with components that display reactivities upon cocrystal formation and within the solids. In 1 and 3, the C=C bonds of the bpe molecules undergo a [2+2] photodimerization. UV radiation of each compound resulted in quantitative conversion of bpe into cyclobutane tpcb. The reactivity involving 1 occurred via 1D-to-2D single-crystal-to-single-crystal (SCSC) transformation. Our work supports the feasibility of the diboronic acids as formidable structural and reactivity building blocks for cocrystal construction.