Fast motion of molecular rotors in metal-organic framework struts at very low temperatures

The solid state is typically not well suited to sustaining fast molecular motion, but in recent years a variety of molecular machines, switches and rotors have been successfully engineered within porous crystals and on surfaces. Here we show a fast-rotating molecular rotor within the bicyclopentane-dicarboxylate struts of a zinc-based metal-organic framework-the carboxylate groups anchored to the metal clusters act as an axle while the bicyclic unit is free to rotate. The three-fold bipyramidal symmetry of the rotator conflicts with the four-fold symmetry of the struts within the cubic crystal cell of the zinc metal-organic framework. This frustrates the formation of stable conformations, allowing for the continuous, unidirectional, hyperfast rotation of the bicyclic units with an energy barrier of 6.2 cal mol^-1 and a high frequency persistent for several turns even at very low temperatures (10¹⁰ Hz below 2 K). Using zirconium instead of zinc led to a different metal cluster-carboxylate coordination arrangement in the resulting metal-organic framework, and much slower rotation of the bicyclic units.