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 A1 assembled through dative B←N bonds
between 2-(naphthalen-1-yl)naphtho[2,3-d][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 A1·2Cy and A1·2Bz 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 A1·2Cy that gradually converts to the more stable A1·2Bz. Recyclability of the sorbent materials was accomplished without
heating through the reversible transformation of A1·2Bz toward the unsolvated form of A1 or A1·MeCN. Receptor A1 is prone to crystallize with MeCN in various
stoichiometric ratios with noncentrosymmetric conformations displaying syn orientation of the naphthylboronic fragments, whereas
crystals of A1 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 A1·MeCN, A1·2MeCN, and A1·2Bz confirmed a favorable solid phase
transformation toward the Bz solvate due to two factors: (i) the conformational
adaptability of receptor A1 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.