Solvent-Induced Self-Assembly of Pd₆L₃ and Pd₈L₄ Coordination Cages for Benzene-Cyclohexane Separation

We report the solvato-controlled synthesis of novel Pd₂L-based bowl-shaped cages, specifically tetrafacial (n = 4) and trifacial (n = 3), constructed using cis-blocked 90° Pd(II) acceptors and a planar tetratopic ligand. The reaction of the ligand with (tmeda)Pd(ONO₂)₂ in water produced a trifacial cage (C1), which quickly converted into a tetrafacial cage (C2) in dimethyl sulfoxide (DMSO). Alternatively, the self-assembly of the ligand with (en)Pd(ONO₂)₂ in water yielded an equilibrium mixture of tetrafacial (C3) and trifacial (C4) cages that exhibited reversible structural transformations in response to solvent changes. Notably, the C3/C4 mixture effectively encapsulated and stabilized protonated merocyanine (MCH⁺), preventing its hydrolysis in aqueous media and enabling reversible photoisomerization over multiple cycles. Additionally, both C1 and the C3/C4 mixture displayed selective binding and extraction of benzene over cyclohexane in water, achieving over 99% purity in the separation process with excellent recyclability. This study presents the first instance of a water-soluble molecular host capable of selectively separating benzene from cyclohexane through simple aqueous extraction at ambient conditions, highlighting the potential of these cages for guest encapsulation, molecular stabilization, and selective chemical separations.