Maximized axial helicity in a Pd2L4 cage: inverse guest size-dependent compression and mesocate isomerism

• Published in: Chemical science (Cambridge) 2023-02, Vol.14 (6), p.1524-1531
• Main Authors: Bloch, Witold M, Horiuchi, Shinnosuke, Holstein, Julian J, Drechsler, Christoph, Wuttke, Axel, Hiller, Wolf, Mata, Ricardo A, Clever, Guido H
• Format: Article
• Language: English
• Subjects: Cages; Cavity expansion; Chemistry; Electronic structure; Helicity; Ionic mobility; Mass spectrometry; NMR spectroscopy; Separation; Single crystals; Titration calorimetry
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d2sc06629g

Helicity is an archetypal structural motif of many biological systems and provides a basis for molecular recognition in DNA. Whilst artificial supramolecular hosts are often helical, the relationship between helicity and guest encapsulation is not well understood. We report a detailed study on a significantly coiled-up Pd2L4 metallohelicate with an unusually wide azimuthal angle (∼176°). Through a combination of NMR spectroscopy, single-crystal X-ray diffraction, trapped ion mobility mass spectrometry and isothermal titration calorimetry we show that the coiled-up cage exhibits extremely tight anion binding (K of up to 106 M−1) by virtue of a pronounced oblate/prolate cavity expansion, whereby the Pd–Pd separation decreases for mono-anionic guests of increasing size. Electronic structure calculations point toward strong dispersion forces contributing to these host–guest interactions. In the absence of a suitable guest, the helical cage exists in equilibrium with a well-defined mesocate isomer that possesses a distinct cavity environment afforded by a doubled Pd–Pd separation distance.