A butterfly-shaped B ← N open box for gas-phase benzene/cyclohexane separation.

A butterfly-shaped B ← N open molecular box, (TPX)(BMCT)2(PhCl)4, is constructed via a [4 + 2] coordination mode, and the activated form (TPX)(BMCT)2 is obtained after removal of chlorobenzene guests. The two methyl groups on the central benzene ring of the tetradentate pyridine ligand create steric hindrance, forcing the dative B ← N bonds to become nearly parallel and achieving perfect geometric complementarity with the bidentate boronate ester. After activation, the characteristic infrared bands of the open molecular box remain unchanged, confirming its structural integrity. Static gravimetric adsorption experiments show much higher benzene uptake than cyclohexane, and mixed-vapor adsorption followed by 1H NMR further confirms the high selectivity toward benzene. Dynamic breakthrough experiments demonstrate effective separation of benzene/cyclohexane mixtures with good cycling stability. The mechanism originates from the charge polarization effect induced by the dative B ← N bond, creating a strong positive electrostatic potential at the box center complementary to the negative potential of benzene. This is the first B ← N small-molecule structure capable of gas-phase dynamic breakthrough separation.