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 1
J(13C-195Pt) couplings, 2D NMR spectra
were acquired, revealing two types of Pt sites. For each of them, 1
J(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.