Chirality Induction in a Hydrophilic Metallohelicate

The title compound is a water-soluble triply stranded metallohelicate 1(Fe) composed of TEG-chain-installed calix[4]arene-based bis-bidentate ligands and Fe3+ cations. This compound can incorporate up to two molecules of small chiral cations (G1 and G2). Interestingly, G1 shows positive cooperativity for host-guest complexation, whereas G2 exhibits noncooperativity, despite having a greater affinity for 1(Fe) than G1. Density functional theory (DFT) calculations indicate the metallohelicate cavity expands upon encapsulation of the first guest. This conformational change facilitates the entrapment of the second guest and explains why the interaction parameters of G1 and G2 are larger than 1, despite electrostatic repulsion between the guests. The DFT calculation predicts an intermolecular interaction between the phenyl groups of G1 in the cavity, whereas no such interaction is found for G2. This difference can explain the distinct molecular recognition of G1 and G2. CD spectroscopy shows that both (R)-G1 and (R)-G2 induce (M)-helicity in 1(Fe) and vice versa. DFT calculations suggest that the chirality of the guests is transmitted to 1(Fe) via CH/O hydrogen bonds between the guest and the C=O bond on the calix[4]arene strand. The CD intensity of 1(Fe) is nonlinear in the ee% of (R)- and (S)-G1, indicating that the presence of the first guest increases the affinity of 1(Fe) toward a second guest with the same chirality due to preorganization by the first guest.

Automated summary

A water-soluble triply stranded metallohelicate capable of incorporating small molecular cations was studied. It was found that some cations exhibit positive cooperativity in host-guest complexation, while others show noncooperativity. Density functional theory calculations revealed that the conformation of the helicate cavity changes upon encapsulation of the first guest, which in turn affects the binding of the second guest. Circular dichroism spectroscopy showed that the chirality of the guests can be transmitted to the helicate, and the CD intensity is nonlinear with the enantiomeric excess of the guests.