Mapping Out Chemically Similar, Crystallographically Nonequivalent Hydrogen Sites in Metal–Organic Frameworks by ¹H Solid-State NMR Spectroscopy

Metal–organic frameworks (MOFs) are important materials with many actual and potential applications. Crystal structure of many MOFs is determined by single-crystal X-ray diffraction. However, due to the inability of XRD to accurately locate hydrogen atoms, the local structures around framework hydrogen are usually poorly characterized even if the overall framework has been accurately determined. ¹H solid-state NMR (SSNMR) spectroscopy should, in principle, be used as a complementary method to XRD for characterizing hydrogen local environments. However, the spectral resolution of ¹H SSNMR is severely limited by the strong ¹H–¹H homonuclear dipolar coupling. In this work, we demonstrate that high-resolution ¹H MAS spectra of MOF-based material can be obtained by ultrafast sample spinning at high magnetic field in combination with isotopic dilution. In particular, we examined an important MOF, microporous α-Mg₃(HCOO)₆ and α-Mg₃(HCOO)₆ in the presence of several guest species. All six chemically very similar, but crystallographically, nonequivalent H sites of these MOFs were resolved in a chemical shift range as small as 0.8 ppm. Although the assignment of ¹H peaks due to crystallographically nonequivalent hydrogens is difficult due to that they all have almost identical chemical environments, we are able to show that they can be assigned from ¹H–¹H proximity maps obtained from 2D ¹H–¹H double quantum (DQ) experiments in conjunction with theoretical calculations. ¹H MAS spectra of framework hydrogen are very sensitive to the guest molecules present inside the pores and they provide insight into host–guest interaction and dynamics of guest molecule. The ability of achieving very high resolution for ¹H MAS NMR in MOF-based materials and subsequent spectral assignment demonstrated in this work allows one to obtain new structural information complementary to that obtained from single-crystal XRD.