A molecular shuttle that operates inside a metal-organic framework

A 'molecular shuttle' is an interlocked molecular assembly in which a macrocyclic ring is able to move back and forth between two recognition sites. This large-amplitude translational motion was first characterized in solution in 1991. Since that report, many mechanically interlocked molecules (MIMs) have been designed, synthesized and shown to mimic the complex functions of macroscopic switches and machines. Here, we show that this fundamental concept-the translational motion of a molecular shuttle-can be organized, initiated and made to operate inside a crystalline, solid-state material. A metal-organic framework (MOF) designated UWDM-4 was prepared that contains a rigid linker that is a molecular shuttle. It was demonstrated by variable-temperature H-1-C-13 cross-polarization/ magic-angle spinning (CP/MAS) and C-13 2D exchange correlation spectroscopy (EXSY) solid-state NMR at 21.1 T on a C-13-enriched sample that the macrocyclic ring undergoes rapid shuttling along the rigid axle built between struts of the framework.