Knight Shift in 13C NMR Resonances Confirms the Coordination of N-Heterocyclic Carbene Ligands to Water-Soluble Palladium Nanoparticles
The coordination of N-heterocyclic carbene (NHC) ligands to the surface of 3.7 nm palladium nanoparticles (PdNPs) can be unambiguously established by observation of Knight shift (KS) in the 13C resonance of the carbenic carbon. In order to validate this coordination, PdNPs with sizes ranging from 1.3 to 4.8 nm were prepared by thermal decomposition or reduction with CO of a dimethyl NHC PdII complex. NMR studies after 13CO adsorption established that the KS shifts the 13C resonances of the chemisorbed molecules several hundreds of ppm to high frequencies only when the particle exceeds a critical size of around 2 nm. Finally, the resonance of a carbenic carbon is reported to be Knight-shifted to 600 ppm for 13C-labelled NHCs bound to PdNPs of 3.7 nm. The observation of these very broad KS resonances was facilitated by using Car–Purcell–Meiboom–Gill (CPMG) echo train acquisition NMR experiments.On the (K)nightshift: The coordination of N-heterocyclic carbene (NHC) ligands to the surface of palladium nanoparticles (PdNPs) can be unambiguously established by observation of Knight shift (KS) in the 13C resonance of the carbenic carbon. The effect is only observed for particles exceeding a critical size of around 2 nm.
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