Dynamics of Fast-Rotating Rotors in a Metal-Organic Framework as Explored by Solid-State NMR

The design and creation of solid materials containing fast-rotating components, such as molecular rotors, is highly attractive for developing advanced responsive machines capable of converting external chemical or physical stimuli into actions.[1] We successfully synthesised a new flexible pillared Metal-Organic Framework (MOF) named FTR-P2, whose structure comprises layers of byciclopentane dicarboxylate molecular rotors (BCP) and Zn metal nodes pillared by azobypyridyl ligands to generate a 3D network.[2] The framework structure consists of two centred interpenetrated nets that undergo reciprocal sliding along the pillar direction in the presence of guests in the pores. VT 13C {1H} CP MAS spectra were collected down to 95 K (Fig. 1 Left) to study the structural changes. The BCP methylene groups always show a singlet indicating fast motional exchange, while more complex behaviour is observed for the pillars that have perfectly symmetric pyridyl rings. However, the carbons of the pyridyl rings only coalesce at temperatures above 150 K, attributed to the fast pedal-like motion of the azo group with kexch > 3600 Hz. After adsorption of iodine molecules, the new off-centred structure (FTR-P2-I2) presents two non-equivalent pyridyl rings: one (Ring A) is exposed to the channel and gains freedom, while the other (Ring B) sits in the plane of the 2D layers and forms hydrogen bonds with the oxygen atoms of the metal nodes that hinder its mobility. The coalesce peaks of the syn- and anti-conformations of C2 shift toward the central peak upon increasing temperature, according to the Boltzmann distribution of the two conformations. Finally, the dynamics of the BCP rotors were studied by VT 13C and 1H T1 NMR relaxation times measurements, demonstrating the BCP hyper-mobility even at extremely low temperatures (Fig. 1 Right). Surprisingly, the structural changes induced by iodine molecules prompt an increase in cooperativity that enables even faster mobility with lower energy barriers, shifting the maximum relaxation rate (33.8 MHz) from 85 K to as low as 44 K. [1] J. Perego et al. J. Am. Chem. Soc. 2021, 143, 13082-13090 [2] J. Perego et al. Angew. Chem. Int. ed. 2024, e202317094