Structure and dynamics of a perfluorinated Ce(IV)-based metal organic framework by solid-state NMR

In the last decades, metal-organic frameworks (MOFs) have emerged as inorganic/organic hybrid porous materials with exceptionally large surface area. In particular, Ce(IV)-based MOFs have been devised in the last decade to exploit the redox chemistry of this metal in catalysis and photocatalysis [1]. Furthermore, substitution of hydrogen with fluorine atoms in organic linkers has been applied to change the hydrophilic/hydrophobic properties of MOFs. In benzene dicarboxylate ligands (BDC2- ) such a substitution modifies the dihedral angle between the carboxylate groups and the phenylene ring, as well as the energy barrier for the phenylene ring rotation [2], thus affecting the MOF properties related to gas capture and storage. Solid State Nuclear Magnetic Resonance (SSNMR) has established as one of the most powerful techniques for the characterization of structural and dynamic properties at the molecular scale in MOFs [3,4]. In particular, chemical shift values in SSNMR spectra provide local structural information unavailable from other techniques, while 2D homonuclear and heteronuclear correlation experiments can be used to explore spatial proximities between nuclei and, therefore, molecular fragments. Morever, many nuclear observables (anisotropic line shapes, nuclear relaxation times, residual interactions) accessible by SSNMR give unique possibilities for the study of linker motions. In the present work, multinuclear SSNMR is applied to investigate structural and dynamic properties of F4_MIL140A(Ce), a recently synthesized perfluorinated Ce(IV)- based MOF with very good hydrothermal and mechanical stability. The framework is formed by cerium oxide chains units linked by tetrafluorobenzenedicarboxylate (TFBDC2-) linkers giving rise to small pores, which render these materials particularly suitable to CO2 adsorption. High-resolution 19F-13C cross-polarization (CP) and heteronuclear correlation (HETCOR) SSNMR experiments and 19F direct excitation experiments were exploited to unravel the local environment of crystallographically independent TFBDC2- linkers. Moreover, the interactions of the TFBDC2- ligands with water molecules present in as synthesized F4_MIL140A(Ce) were highlighted by 1H-13C CP experiments. The dynamics of TFBDC2- ligands was instead studied by analyzing the spin-lattice relaxation times (T1) and chemical shift anisotropy of 13C and 19F nuclei as a function of temperature and supporting the SSNMR measurements with DFT calculations. [1] J. Jacobsen et al., Dalton Trans. 49 (2020) 16551. [2] A. Gonzalez-Nelson et al., J. Phys. Chem. C 143 (2021) 12053. [3] Y. Fu et al., Coord. Chem. Rev. 427 (2021) 213563. [4] C. He et al., Solid State Nucl. Magn. Reson. 117 (2022) 101772