Hydrogen Bond Networks between Amines and Spiroborates for Crystal and Organogel Engineering

Utilizing small molecules as building blocks to construct morphologically well-defined nano- or microcrystals is of great implications for both fundamental research and practical applications. However, versatile and general molecular crystal engineering platforms remain scarce because the self-assembly driving forces and crystal growth processes are susceptible to molecular structures. Herein, we show that anionic spiroborates can interact with various protonated amines (ranging from primary, secondary, to tertiary amines, also including both aromatic and alkyl amines) through electrostatic-force-reinforced hydrogen bonding, affording uniform amine+spiroborate– crystals ranging from zero-dimensional (0D) to three-dimensional (3D) morphologies. In particular, ultrafine one-dimensional (1D) amine+spiroborate– nanowires form organogels in conventional organic solvents even under a solid content as low as 0.3 wt %. Noteworthy, our crystal and organogel engineering strategy of amine+spiroborate– does not start with spiroborates. We unexpectedly found that amines can trigger the homolysis of diborates into spiroborates, providing a one-step route to the growth of amine+spiroborate– crystals. We clarify the homolysis mechanism of diborates through a set of spectroscopic analyses and establish the interactions between amines and spiroborates by single-crystal characterization. Our findings may open a new avenue for supramolecular chemistry because of the big family of amines and the unexplored properties of spiroborates.