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N-Heterocyclic carbenes (NHCs) are widely used ligands in transition metal catalysis. Notably, they are increasingly encountered in heterogeneous systems. While a detailed knowledge of the possibly multiple metal environments would be essential to understand the activity of metal-NHC-based heterogeneous catalysts, only a few techniques currently have the ability to describe with atomic-resolution structures dispersed on a solid support. Here, we introduce a new dynamic nuclear polarization (DNP) surface-enhanced solid-state nuclear magnetic resonance (NMR) approach that, in combination with advanced density functional theory (DFT) calculations, allows the structure characterization of isolated silica-supported Pt-NHC sites. Notably, we demonstrate that the signal amplification provided by DNP in combination with fast magic angle spinning enables the implementation of sensitive 13C-195Pt correlation experiments. By exploiting 1J(13C-195Pt) couplings, 2D NMR spectra were acquired, revealing two types of Pt sites. For each of them, 1J(13C-195Pt) value was determined as well as 195Pt chemical shift tensor parameters. To interpret the NMR data, DFT calculations were performed on an extensive library of molecular Pt-NHC complexes. While one surface site was identified as a bis-NHC compound, the second site most likely contains a bidentate 1,5-cyclooctadiene ligand, pointing to various parallel grafting mechanisms. The methodology described here represents a new step forward in the atomic-level description of catalytically relevant surface metal-NHC complexes. In particular, it opens up innovative avenues for exploiting the spectral signature of platinum, one of the most widely used transition metals in catalysis, but whose use for solid-state NMR remains difficult. Our results also highlight the sensitivity of 195Pt NMR parameters to slight structural changes. 

Weakly Coordinating Anions (WCAs) containing electron deficient delocalized anionic fragments that are reasonably inert allow for the isolation of strong electrophiles. Perfluorinated borates, perfluorinated aluminum alkoxides, and halogenated carborane anions are a few families of WCAs that are commonly used in synthesis. Application of similar design strategies to oxide surfaces is challenging. This paper describes the reaction of Al(OR F ) 3 *PhF (R F = C(CF 3 ) 3 ) with silica partially dehydroxylated at 700 °C (SiO 2-700 ) to form the bridging silanol Si–OH⋯Al(OR F ) 3 ( 1 ). DFT calculations using small clusters to model 1 show that the gas phase acidity (GPA) of the bridging silanol is 43.2 kcal mol −1 lower than the GPA of H 2 SO 4 , but higher than the strongest carborane acids, suggesting that deprotonated 1 would be a WCA. Reactions of 1 with NOct 3 show that 1 forms weaker ion-pairs than classical WCAs, but stronger ion-pairs than carborane or borate anions. Though 1 forms stronger ion-pairs than these state-of-the-art WCAs, 1 reacts with alkylsilanes to form silylium type surface species. To the best of our knowledge, this is the first example of a silylium supported on derivatized silica. 

We demonstrate that natural isotopic abundance 2D heteronuclear correlation (HETCOR) solid-state NMR spectra can be used to significantly reduce or eliminate the broadening of 1 H and 13 C solid-state NMR spectra of organic solids due to anisotropic bulk magnetic susceptibility (ABMS). ABMS often manifests in solids with aromatic groups, such as active pharmaceutical ingredients (APIs), and inhomogeneously broadens the NMR peaks of all nuclei in the sample. Inhomogeneous peaks with full widths at half maximum (FWHM) of ∼1 ppm typically result from ABMS broadening and the low spectral resolution impedes the analysis of solid-state NMR spectra. ABMS broadening of solid-state NMR spectra has previously been eliminated using 2D multiple-quantum correlation experiments, or by performing NMR experiments on diluted materials or single crystals. However, these experiments are often infeasible due to their poor sensitivity and/or provide limited gains in resolution. 2D 1 H– 13 C HETCOR experiments have previously been applied to reduce susceptibility broadening in paramagnetic solids and we show that this strategy can significantly reduce ABMS broadening in diamagnetic organic solids. Comparisons of 1D solid-state NMR spectra and 1 H and 13 C solid-state NMR spectra obtained from 2D 1 H– 13 C HETCOR NMR spectra show that the HETCOR spectrum directly increases resolution by a factor of 1.5 to 8. The direct gain in resolution is determined by the ratio of the inhomogeneous 13 C/ 1 H linewidth to the homogeneous 1 H linewidth, with the former depending on the magnitude of the ABMS broadening and the strength of the applied field and the latter on the efficiency of homonuclear decoupling. The direct gains in resolution obtained using the 2D HETCOR experiments are better than that obtained by dilution. For solids with long proton longitudinal relaxation times, dynamic nuclear polarization (DNP) was applied to enhance sensitivity and enable the acquisition of 2D 1 H– 13 C HETCOR NMR spectra. 2D 1 H– 13 C HETCOR experiments were applied to resolve and partially assign the NMR signals of the form I and form II polymorphs of aspirin in a sample containing both forms. These findings have important implications for ultra-high field NMR experiments, optimization of decoupling schemes and assessment of the fundamental limits on the resolution of solid-state NMR spectra.