One Coordination Cage, Many Pathways: Multiple Stimuli Drive Reversible Transformations

A trigonal prismatic coordination cage endowed with a large cavity is built by self-association of a bis-(rhodium) complex and a planar triazatruxene-based ligand. The remarkable versatility of this molecular building in promoting reversible structural transformations is demonstrated through manipulating three orthogonal external stimuli, i.e., cage concentration, guest identity, and pH. Four distinct stable discrete structures can be formed on-demand: a monomeric cage, two distinct host-guest complexes, and a mechanically interlocked cage dimer, and all these structures are fully and readily interconvertible. The cage uniquely couples (i) selective double guest encapsulation, (ii) stimulus-controlled reversible catenation, and (iii) pH-triggered pairwise guest release-uptake. The resulting structures were comprehensively characterized using 1D and 2D NMR spectroscopy, high-resolution mass spectrometry, theoretical calculations, and, in most cases, single-crystal X-ray diffraction.