Mechanochemical Synthesis of Photoactive Boronic Acid-Alkene Cocrystals: Interplay of Hydrogen Bonding and B←N Coordination


Mechanochemistry, an interdisciplinary field bridging physical chemistry and materials science, leverages mechanical force to prompt chemical reactions, providing an eco-friendly alternative to solvent-based approaches and diminishing the environmental impact of chemical synthesis. This method, which employs grinding or milling to impart energy directly to substrates, enables reactions under less severe conditions and without toxic solvents, particularly beneficial in organoboron chemistry for the exploration and manipulation of boronic acids and their derivatives through noncovalent interactions. Boronic acids are essential in supramolecular chemistry because of their amphoteric nature and dual reactivity in forming dynamic bonds, and therefore play a crucial role in constructing supramolecular architectures via mechanochemistry.  A new study published in Angewandte Chemie International edition conducted by Dr. María Guadalupe Vasquez-Ríos and Professor Leonard MacGillivray from the University of Iowa, USA and presently at the Université de Sherbrooke, Canada together with Professor Gonzalo Campillo-Alvarado from  Reed College, USA the authors investigated the synthesis and properties of a photoactive cocrystal formed through the mechanochemical interaction of boronic acid and an alkene. These experiments were designed to explore the coexistence of (B)O-H…N hydrogen bonding and B←N coordination in the self-assembly of these molecules and to examine the photochemical behavior of the resulting assemblies. The researchers created mixtures of hydrogen-bonded and coordinated complexes using solvent-free mechanochemical ball mill grinding and liquid-assisted grinding to mix boronic acid (ba=phenylboronic acid) and an alkene (bpe=trans-1,2-bi-(4-pyridyl)ethylene) in a 1:1 ratio. Upon exposure to UV irradiation, the alkenes in the hydrogen-bonded assembly underwent an intermolecular [2+2] photodimerization, resulting in quantitative conversion.  The team performed extensive advanced analysis of the assemblies using single-crystal X-ray diffraction  which determined the crystalline structure and the nature of interactions within the assemblies, powder X-ray diffraction method was used to confirm the formation and coexistence of the hydrogen-bonded and coordinated complexes in the ground solid and density functional theory calculations  were performed to understand the driving forces behind the formation of the hydrogen-bonded complex versus the B←N coordination.

The team found that the mechanochemical process led to the coexistence of hydrogen-bonded and B←N coordination complexes within the same solid. This coexistence mimics the mixtures of noncovalent complexes that can be obtained in solution. Moreover, they reported that continuous grinding resulted in the exclusive generation of a hydrogen-bonded structure. This photoactive hydrogen-bonded assembly structure was preorganized for an intermolecular [2+2] photodimerization in the crystalline state, which proceeded quantitatively upon exposure to UV irradiation. Furthermore, the researchers found that the mechanochemical conditions favored the formation of the weaker hydrogen-bonded complex as the final product. Additionally, the authors stressed the importance of molecular packing in directing mechanochemical self-assembly outcomes, as indicated by density functional theory calculations. These findings suggest that packing density is a key factor in the preferential formation of the hydrogen-bonded complex over the coordination complex, emphasizing molecular packing’s role in supramolecular chemistry. It is also noteworthy to mention the new method aligns with green chemistry principles by reducing hazardous waste and energy consumption, showcasing the formation of complexes that mirror dynamic equilibria found in solution but achieved in solid-state.

The coexistence of (B)O-H…N hydrogen bond and B←N coordination under mechanochemical conditions, as demonstrated by Professor Leonard MacGillivray and colleagues, opens new avenues for the design and synthesis of functional materials. The ability to control the self-assembly of molecular building blocks through mechanochemistry paves the way for the development of novel supramolecular architectures with tailored properties. It offers a more environmentally friendly and efficient alternative to traditional synthesis methods. Furthermore, the integration of photoactive components into these structures offers promising applications in areas such as photonic devices, sensors, and catalysis. The exploration of mechanochemistry in organoboron chemistry thus represents a frontier in the quest for sustainable, functional materials crafted through the precision of molecular self-assembly. Professor MacGillivray notes that “we were surprised with the co-existence of the bonds, and we are now attempting to expand the observation to functional materials.”


Guadalupe Vasquez-Ríos M, Campillo-Alvarado G, MacGillivray LR. Mechanochemical Mediated Coexistence of B←N Coordination and Hydrogen Bonding. Angew Chem Int Ed Engl. 2023;62(35):e202308350. doi: 10.1002/anie.202308350.

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