Maximized axial helicity in a Pd2L4 cage: inverse guest size-dependent compression and mesocate isomerism. Issue 6 (19th January 2023)

• Record Type: Journal Article
• Authors: Bloch, Witold M.; Horiuchi, Shinnosuke; Holstein, Julian J.; Drechsler, Christoph; Wuttke, Axel; Hiller, Wolf; Mata, Ricardo A.; Clever, Guido H.
• Is Part Of: Chemical science. Volume 14:Issue 6(2023)
• Page Start: 1524
• Page End: 1531
• Publication Date: 2023-01-19
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d2sc06629g
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : The extreme axial twist of a Pd2 L 4 cage facilitates tight encapsulation of mono-anions through prolate-oblate cavity adaptation. In the absence of a suitable guest, the helical cage exists in equilibrium with an 'unwound' mesocate isomer. Abstract : 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 Pd2 L 4 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 10 6 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.