Preferential molecular recognition of heterochiral guests within a cyclophane receptor

Abstract

The discrimination of enantiomers by natural receptors is a well-established phenomenon. In contrast the number of synthetic receptors with the capability for enantioselective molecular recognition of chiral substrates is scarce and for chiral cyclophanes indicative for a preferential binding of homochiral guests. Here we introduce a cyclophane composed of two homochiral core-twisted perylene bisimide (PBI) units connected by p-xylylene spacers and demonstrate its preference for the complexation of [5]helicene of opposite helicity compared to the PBI units of the host. The pronounced enantio-differentiation of this molecular receptor for heterochiral guests can be utilized for the enrichment of the P-PBI-M-helicene-P-PBI epimeric bimolecular complex. Our experimental results are supported by DFT calculations, which reveal that the sterically demanding bay substituents attached to the PBI chromophores disturb the helical shape match of the perylene core and homochiral substrates and thereby enforce the formation of syndiotactic host-guest complex structures. Hence, the most efficient substrate binding is observed for those aromatic guests, e. g. perylene, [4]helicene, phenanthrene and biphenyl, that can easily adapt in non-planar axially chiral conformations due to their inherent conformational flexibility. In all cases the induced chirality for the guest is opposed to those of the embedding PBI units, leading to heterochiral host-guest structures.

Dictating Packing and Interactions of Perylene Bisimides within Cyclophane Structures in the Solid State

“…For optimal stacking behaviour, a slip‐stack arrangement is present along the a axis, as previously shown for monomeric PBI derivatives with sterically demanding bay substituents [15a,21] . Our flexible linker enables the π‐surfaces to be much closer together than in comparable molecules with rigid linkers [11j,17] . This results in one half of each chromophore interacting intramolecularly with another half, allowing the corresponding other half to form intermolecular π‐π interactions (see Figure 2e).…”

“…In a bay‐bridged PBI cyclophane, the PBI units are not heterochirally but homochirally arranged [14b] . A homochiral arrangement is also observed for a cyclophane with tert ‐butylphenyl bay substituents and p ‐xylylene spacers [17] . In contrast, a cyclophane with the same tert ‐butylphenoxy bay substituents as P2 but a rigid biphenylene linker shows a heterochiral arrangement of the chromophores within the crystal, but without inter‐ or intramolecular π‐π stacking due to the large distances between the PBI moieties [11j] .…”

“…[14b] A homochiral arrangement is also observed for a cyclophane with tert-butylphenyl bay substituents and p-xylylene spacers. [17] In contrast, a cyclophane with the same tert-butylphenoxy bay substituents as P2 but a rigid biphenylene linker shows a heterochiral arrangement of the chromophores within the crystal, but without inter-or intramolecular π-π stacking due to the large distances between the PBI moieties. [11j] In addition, solvent molecules are inside the cavity, [11j] which is not possible in our case due to the strong ππ interactions.…”

“…Furthermore, in most of the technological applications PBIs are finally incorporated in solid state and polycrystalline thin films, [4a,16] that is why it is compelling to investigate and control the properties in the solid state. So far, there are few crystal structures of macrocycles with multiple PBI building blocks, two cyclophanes with rigid linkers at the imide positions [11j,17] , two others where the chromophores are connected via the bay positions [14b,18] and three metalla[3]catenanes [19] . Hence, after the detailed investigation of our flexible PBI cyclophanes in solution [11k] , our aim with the present contribution is to elucidate the packing of the PBI chromophores in the solid state to get a better understanding how the PBI units are oriented to each other in different aggregate states.…”

Methylene-bridged naphthotubes: new macrocyclic arenes with great potential for supramolecular chemistry

“…[25][26][27] Designing macrocyclic hosts with unique structures and functions is the core research direction of supramolecular chemistry and also an important step to innovate supramolecular systems and promote functional applications. [28][29][30] Among various macrocyclic hosts, macrocyclic arenes, including calixarenes, pillararenes, biphenarenes, prismarenes, geminiarenes, hybridarenes, and calixnaphtharenes, have attracted tremendous attention due to their special structural features, ease of functionalization, and outstanding host-guest properties (Fig. 1).…”