Effect of Extra-Framework Anion Substitution on the Properties of a Chiral Crystalline Sponge

Chiral metal-organic materials, CMOMs, are of interest as they can offer selective binding sites for chiral guests. Such binding sites can enable CMOMs to serve as chiral crystalline sponges (CCSs) to determine molecular structure and/or purify enantiomers. We recently reported on the chiral recognition properties of a homochiral cationic diamondoid, dia, network {[Ni(S-IDEC)(bipy)(H₂O)][NO₃]}_n (S-IDEC = S-indoline-2-carboxylicate, bipy = 4,4′-bipyridine), CMOM-5[NO₃]. The modularity of CMOM-5[NO₃] means there are five feasible approaches to fine-tune structures and properties via substitution of one or more of the following components: metal cation (Ni²⁺); bridging ligand (S-IDEC); linker (bipy); extra-framework anion (NO₃^-); and terminal ligand (H₂O). Herein, we report the effect of anion substitution on the CCS properties of CMOM-5[NO₃] by preparing and characterizing {[Ni(S-IDEC)(bipy)(H₂O)][BF₄]}_n, CMOM-5[BF₄]. The chiral channels in CMOM-5[BF₄] enabled it to function as a CCS for determination of the absolute crystal structures of both enantiomers of three chiral compounds: 1-phenyl-1-butanol (1P1B); methyl mandelate (MM); ethyl mandelate (EM). Chiral resolution experiments revealed CMOM-5[BF₄] to be highly selective toward the S-isomers of MM and EM with enantiomeric excess, ee, values of 82.6 and 78.4%, respectively. The ee measured for S-EM surpasses the 64.3% exhibited by [DyNaL(H₂O)₄] 6H₂O and far exceeds that of CMOM-5[NO₃] (6.0%). Structural studies of the binding sites in CMOM-5[BF₄] provide insight into their high enantioselectivity.