Homochiral BINAPO-Based Metal-Organic Frameworks for Luminescence Sensing of Phenylglycinol Enantiomers

Chiral metal-organic frameworks (CMOFs) with luminescent properties are promising enantioselective sensors for important chiral molecules. Herein, we report the synthesis and characterization of four CMOFs, Cu-L, Zn-L, Cd-L, and Mn-L, based on the BINAP oxide (BINAPO) ligand R-2,2′-Bis(diphenylphosphoryl)-1,(1′-binaphthyl)-4,4′-bis(4-carboxyphenyl) (H₂L). In the isostructural Cu-L and Zn-L MOFs, the metal cations coordinate with carboxylate groups and solvent molecules to form paddle-wheel shaped secondary building units (SBUs). These SBUs are further connected by the ligands as 4-connected nodes, resulting in square lattice (sql) networks. The metal centers in Cd-L and Mn-L coordinate with phosphine oxides and carboxylates, with each Mn cation additionally coordinating to one aqua molecule. The resulting frameworks adopt 2D and 3D networks of fes and qtz-h topologies, respectively. At a ligand concentration of 28 μM, the fluorescence of an H₂L solution in acetonitrile was quenched by R-phenylglycinol (R-PG) and S-PG, with Stern-Völmer constants (K_sv) of 69.6 ± 1.0 and 34.2 ± 0.5 M^-1, respectively. At the same ligand concentration, a Zn-L suspension in acetonitrile exhibited slightly lower quenching efficiencies, with K_sv values of 16.2 ± 0.3 M^-1 for R-PG and 27.5 ± 0.4 M^-1 for S-PG. In tetrahydrofuran, Zn-L was quenched by R-PG and S-PG with K_sv values of 44.5 ± 0.7 and 21.7 ± 0.4 M^-1, respectively, and H₂L exhibited lower quenching efficiencies. The reversed enantioselectivity observed in Zn-L in acetonitrile, compared to H₂L, suggests the presence of distinct chiral recognition sites, potentially influenced by solvent effects on the coordinated BINAPO ligand within the MOF.