MOF Containing Layers of Dipolar-Rotor with Very High Mobility at Extremely Low Temperatures

Metal-organic frameworks (MOFs) present a platform that allows the insertion of ligands in a wide range of structural and chemical environments, thus tuning the ligand’s interactions with neighbouring moieties within the MOF. Conventionally, fast rotary dynamics is achieved by isolating molecular rotors, used as struts in MOFs, from their neighbours to reduce intermolecular interactions. Inserting bicyclo[1.1.1]pentandioate (FTR) rotors into a cubic framework yielded isolated rotors with a rotational energy barrier of only 6 cal/mol (Fig 1 A).1 The outstanding synthetic versatility of MOFs allows us to explore more dens arrangements of rotors to study the effect of cooperativity between the rotors. The FTR rotors were inserted into a pillar-and-layer Zn-MOF, and a geminal fluorinated FTR rotor into an Al-MOF. For the Zn-MOF, these rotors are still dynamic at very low temperatures through co-rotating pairs of rotors in a geared-like fashion. These geared molecular rotors have extremely low energy barriers for rotation (24 cal/mol) owing to the synchroneity of their rotation.2 Furthermore, the dipolar FTR-F2 rotor in the Al-MOF forms layers of interacting rotors oriented in different configurations. Contrary to expectation, these rotors are extremely dynamic down to 4K through a cooperative reorientation cascade of the dipolar rotors with a barrier for reorientation of ca. 17 cal/mol (Fig 1 B).3 REFERENCES 1. Perego, J.; Bracco, S.; Negroni, M.; Bezuidenhout, C. X.; Prando, G.; Carretta, P.; Comotti, A.; Sozzani, P., Nature Chem. 2020, 12, 845. 2. Perego, J.; Bezuidenhout, C. X.; Bracco, S.; G. Prando; L. Marchiò; M. Negroni; P. Carretta; P. Sozzani; and A. Comotti, JACS 2021, 143 (33), 13082-13090. 3. Perego, J.; Bezuidenhout, C. X.; Bracco, S.; Piva, S.; Prando, G.; Aloisi, C.; Carretta, P.; Kaleta, J.; Le, T. P.; Sozzani, P.; Daolio, A.; Comotti, A., Angew. Chem. Int. Ed 2023, 62, e202215893.