EXPLORING THE INTERACTION OF WATER WITH OPEN METAL SITES IN MIL-101(CR): A 1H NMR RELAXOMETRY AND MOLECULAR DYNAMICS STUDY

The presence of Lewis acid sites in MOFs plays an important role in selective adsorption of substrates[1] and in heterogeneous catalysis[2]. These sites may be coordinatively unsaturated metal sites (CUSs), also referred to as open metal sites. MIL-101(Cr) has been successfully employed in these application fields exploiting its CUSs. However, the mobility of molecules bound to the open sites, and in particular their lifetime within the metal coordination shell, has been rarely investigated even though it is expected to affect the reactivity of the metal ions. In the presented work, we explored the mobility of water molecules bound to CUSs of MIL-101(Cr), and their exchange between the first and the second coordination shell of a Cr ion, whose presence was clearly highlighted by Molecular Dynamics simulations. To this aim, we recorded the 1H NMR relaxometric behaviour as a function of the magnetic field strength of an aqueous suspension of MIL-101(Cr) in the 35-70 °C temperature interval using Field Cycling NMR relaxometry[3]. The data, interpreted within the Solomon-Bloembergen-Morgan model[4], provided residence times of tens of nanoseconds for water molecules on the Cr open metal sites. Field Cycling NMR measurements were complemented by 1H fast Magic Angle Spinning NMR spectroscopy and 1H T1 and T2 relaxometry at fixed frequency on samples at different water loadings. Evidence on the presence of structural order of water molecules confined in the pores of fully loaded MIL-101(Cr) was provided by the observation of 1H-1H residual dipolar interaction. References [1] N. Abedin Khan, Z. Hasan, S. Hwa Jhung J. Hazard. Mater. 2013, 244- 245, 444-456. [2] A. Bavykina, et al. Chem. Rev. 2020, 120, 8468-8535. [3] R. Kimmich, E. Anoardo Prog. Nucl. Magn. Reson. Spectrosc. 2004, 44, 257-320. [4] N. Bloembergen, L. O. Morgan J. Chem. Phys. 1961, 34, 842-850.