STRUCTURAL PROPERTIES OF THE F4_MIL-140A(CE) MOF BY SOLID-STATE NMR SPECTROSCOPY

Metal-Organic Frameworks (MOFs) are a class of crystalline compounds whose scaffolding derives from metal clusters or ions that are interconnected by organic linkers. The high number of possible combinations of metals and ligands leads to high tunability of macroscopic properties and thus it is possible to employ MOFs in many fields of applications, including gas storage [1], gas separation [2], catalysis [3] and others [4,5]. During the design and development of a new MOF, it is extremely important to completely understand every macroscopic property of the compound and to relate it to its microscopic origin. Many techniques can be exploited and combined to characterize the structural and dynamic molecular properties of a MOF. Among them, Solid State Nuclear Magnetic Resonance (SSNMR) spectroscopy is certainly one of the most important because it can shed light on many aspects of the compound at a molecular level, such as 3D structure [6], porosity [7], local dynamics [8], and host-guest interactions [9]. In this work, 1H, 13C, and 19F SSNMR spectroscopy has been employed to gain an in-depth knowledge of a MOF belonging to the MIL class, precisely F4_MIL-140A(Ce), in which chain-like inorganic building units of CeriumIV are interconnected by tetrafluoroterephtalates. This MOF is extremely promising for possible applications, in particular as a sorbent for gas separation, because of its water-based synthesis and its step-shaped CO2 adsorption isotherm [10]. High-resolution SSNMR techniques and 2D correlation spectra have been used to obtain, also by comparison with powder X-ray diffraction results, a detailed characterization of the framework structure both in the presence and after removal of crystallization water, highlighting the presence of different molecular environments with different symmetry. Particular attention has been put into the investigation of dynamic processes involving the fluorinated aromatic rings through the variable-temperature analysis of 19F spin-lattice relaxation times and 13C chemical shift anisotropy. References [1] M. Eddaoudi et al., Science 2002, 295, 469. [2] Q. Qian et al., Chem. Rev. 2020, 120, 8161. [3] J. Lee et al., Chem. Soc. Rev. 2009, 38, 1450. [4] B. Chen et al., Angew. Chemie - Int. Ed. 2006, 45, 1390. [5] X. L. and H. T. Jian Cao, Curr. Med. Chem. 2020, 27, 5949. [6] T. Loiseau et al., Chem. - A Eur. J. 2004, 10, 1373. [7] N. Klein et al., Phys. Chem. Chem. Phys. 2010, 12, 11778. [8] X. Kong et al., J. Am. Chem. Soc. 2012, 134, 14341. [9] A. E. Khudozhitkov et al., J. Phys. Chem. C 2016, 120, 21704. [10] R. D'Amato et al., ACS Sustain. Chem. Eng. 2019, 7, 394.