Separation of Benzene–Cyclohexane at the Liquid–Solid and Vapor–Solid Interfaces Using Adaptive Molecular Crystals of Acyclic B←N Receptors with a 1,2,4,5-Tetrazine Core

Separation of mixtures containing low-molecular-weight hydrocarbons using sustainable and less energy-intensive processes is one of the grand challenges facing the chemical sciences. In this work, we report the peculiar ability of molecular crystals containing boron-based receptors <b>A1</b> assembled through dative B←N bonds between 2-(naphthalen-1-yl)naphtho[2,3-<i>d</i>][1,2,3]dioxaborolane and 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine (BiPyTz), to readily capture and separate small molecules of great industrial interest like benzene (Bz) and cyclohexane (Cy) via the formation of stoichiometric solvates <b>A1·2Cy</b> and <b>A1·2Bz</b> with high conversions. Sorption in mixtures Bz/Cy 1:1 (v/v) showed high selectivity (≥99%) for Bz over Cy in liquid-solid experiments and up to 96% in vapor-solid tests. The dynamics of the vapor-solid sorption process revealed a transient formation of <b>A1·2Cy</b> that gradually converts to the more stable <b>A1·2Bz</b>. Recyclability of the sorbent materials was accomplished without heating through the reversible transformation of <b>A1·2Bz</b> toward the unsolvated form of <b>A1</b> or <b>A1·MeCN</b>. Receptor <b>A1</b> is prone to crystallize with MeCN in various stoichiometric ratios with noncentrosymmetric conformations displaying <i>syn</i> orientation of the naphthylboronic fragments, whereas crystals of <b>A1</b> containing Bz or Cy exhibited a more stable conformation with antiparallel disposition of the aromatic substituents and crystallographic inversion symmetry. Computational studies of adduct-adduct and solvent-adduct interaction energies in the crystals of <b>A1·MeCN</b>, <b>A1·2MeCN</b>, and <b>A1·2Bz</b> confirmed a favorable solid phase transformation toward the Bz solvate due to two factors: (i) the conformational adaptability of receptor <b>A1</b> to optimize adduct-adduct aromatic interactions, accounting for the major contribution to the crystal stability, and (ii) a larger attachment energy of Bz over MeCN. Overall, our studies demonstrate the potential of receptor-based molecular crystalline materials containing flexible and adaptive acyclic B←N adducts to effectively separate a very challenging hydrocarbon solvent system under gentle conditions and with recycling capabilities.