Engineering Homochiral Dinuclear Ir(III)-Metallohelix-Based Porous Molecular Crystals for Atropisomer Enantioseparation

Abstract

Porous molecular crystal materials offer promising applications for enantioselective separation and asymmetric catalysis, while regulating the chiral channel environment remains challenging yet. Herein, we report the introduction of homochiral dinuclear Ir(III) triple-stranded helicates as molecular blocks to fabricate chiral porous crystals for the effective enantioseparation of atropisomers. The chiral cyclohexylamine-functionalized robust helicates Δ 2 R 6 -H and Λ 2 S 6 -H exhibit remarkable stereochemical stability (>6 months) and present superb enantioseparation abilities toward atropisomers, yielding >99% ee for BINOL in only one adsorption and separation cycle. Importantly, these molecular crystals can provide consistent ee values (>99%) even after 10 cycles of enantioseparation. With simple regulation of the helicands with achiral ethylenediamine, the resulting helicates Δ 2 -H and Λ 2 -H display similar adsorption ability toward BINOL but no enantioselectivity. This study highlights that introducing and fine-tuning the configuration of homochiral helicates to fabricate porous molecular crystals is anticipated for enantioselective separation.

Single‐Crystal‐to‐Single‐Crystal Transformation and Alcohols Enantioseparation of Homochiral Ir(III)‐Metallohelix‐Based Porous Molecular Crystal

“…[44][45] The Brunauer-Emmett-Teller surface area (BET) was estimated to be 812 m 2 • g À 1 and the pore volume was calculated to be 0.71 cm 3 • g À 1 . A pore-size distribution analysed by non-local density functional theory (NLDFT) revealed a main peak in the micropore region of 5.5 Å which is comparable with the diameters of the windows of individual helicate of approximately 5.16 Å × 6.17 Å.. [43] Considering the high ratio of imine groups (i. e., six -CH=N-groups per helicate) that were decorating the inner pore, both the polymorphs might offers a favorable environment to be the porous materials for CO 2 capture. In this regard, CO 2 gas adsorption experiments were performed.…”

“…Interestingly, this Synthetic route for the racemic and homochiral triple-stranded dinuclear Ir(III) helicates reported in our previous work. [43] Scheme 1. Packing of the chiral helicate in different crystallization conditions, showing the two polymorphs and the SCSC transformation approach (Δ 2 R 6 -H as an example).…”

“…Importantly, we found that isopropanol solvent could act as an effective trigger to motivate polymorph transformation from α-Δ 2 R 6 -H to β-Δ 2 R 6 -H. After immersing of α-Δ 2 R 6 -H in isopropanol and stirring at room temperature for 48 h, the PXRD revealed that crystallinity of transformed β-Δ 2 R 6 -H was in good agreement with the as-synthesized one (Figure 4). Interestingly, when the S-BINOL was wrapped in extrinsic 3D channels of α-Δ 2 R 6 -H, the SCSC transformation was unsuccessful, [43] which was due to interaction between the guest and channels inhibits reorientation or positional translation of Δ 2 R 6 -H modules. By contrast, the solvent induced structural conversion from β-Δ 2 R 6 -H to α-Δ 2 R 6 -H cannot be achieved in the solid state, except redissolving in CH 2 Cl 2 and recrystallizing with ether.…”

“…Encouragingly, the obtained porous molecular crystals with chiral cyclohexyl decorated extrinsic 3D channels presented superb and durable enantioseparation abilities toward BINOL atropisomers. [43] Interestingly, we showed herein that a new β-polymorph molecular crystalline material (β-Δ 2 R 6 -H/β-Λ 2 S 6 -H, with a needle-like geometry) based on the same Δ 2 R 6 -H/Λ 2 S 6 -H helicate modules can be effectively obtained based on a SCSC transformation approach from the α-polymorph in the solid state in response to a specific chemical trigger, or by simply altering the crystallization conditions of Δ 2 R 6 -H/Λ 2 S 6 -H (Scheme 1). Compared with the αpolymorph, the β-polymorph displayed higher CO 2 capacity and better selectivity for CO 2 over N 2 .…”

“…Figure 1. Synthetic route for the racemic and homochiral triple-stranded dinuclear Ir(III) helicates reported in our previous work [43]. …”