Combining Anisotropic ¹¹⁹Sn NMR and Ab Initio Calculations to Probe the Stereoactive Lone Pair in Sn(II)...

The coordination environments of several Sn-(II) metal-organic frameworks (MOFs) have been investigated using ¹¹⁹Sn solid-state NMR in combination with density functional theory (DFT) calculations. The Sn-(II) nucleus exhibits high chemical shift anisotropy (CSA) due to its lone pair, with spans of up to Ω = 1280 ppm. The CSA is very sensitive to the local coordination environment; however, large CSAs are challenging to measure, which is overcome using a combination of variable magic-angle spinning and static ultra-wideline BRAIN-CP-WCPMG experiments. In some cases, the spectra are further complicated by the presence of Sn-Sn J coupling. We measured the CSA of six Sn-(II) carboxylate MOFs, with between 1 and 3 crystallographically distinct Sn sites per structure. Comparing the experimentally measured ¹¹⁹Sn NMR tensors with those calculated from GIPAW DFT, we find good agreement with the isotropic shift, but DFT systematically underestimates the CSA. By accounting for the anisotropic effect of the Sn-(II) lone pair on the shielding tensor, the experimental CSAs can be accurately calculated. The combination of anisotropic NMR and DFT sheds light on the stereoactive Sn lone pair, which directs the material properties.