Internal dynamics and guest binding of a sterically overcrowded host

Abstract

Substituent control in self-assembled host systems allows for a fine-tuning of structure, dynamics and guest preference. Flat banana-shaped ligands L¹ assemble with Pd(II) cations into the interpenetrated coordination cage dimer [3BF₄@Pd₄L¹₈], capable of sequential guest uptake. In contrast, the introduction of bulky adamantyl groups in ligand L² prevents dimerization and results in the clean formation of monomeric cage species [Pd₂L²₄]. Owing to steric crowding, the adamantyl substituent is considerably bent sideways with respect to the ligand backbone, and is rapidly flipping between both faces of the free ligand giving rise to two energetically degenerate conformers. Surprisingly, the flipping is preserved in the cage, albeit at a lower rate due to entropic reasons. Despite the very dense packing within the self-assembled structure, the cage is able to encapsulate a series of bis-anionic guests in an induced-fit fashion. Electronic structure calculations revealed a substantial contribution from dispersion interactions between the guest and the surrounding adamantyl groups that stabilize the host–guest complex. Guest exchange kinetics were quantified and the influence that encapsulated guests imparted on the ligand flipping dynamics was examined by a series of 2D NMR experiments. Four synchrotron X-ray structures of the cage and its host–guest complexes are presented, allowing for unprecedented insight into the host–guest interactions of a sterically overcrowded host and its guest-induced distortion.