Disclosing Interfaces of ZnO Nanocrystals Using Dynamic Nuclear Polarization: Sol‐Gel versus Organometallic Approach

The unambiguous characterization of the coordination chemistry of nanocrystal surfaces produced by wet‐chemical synthesis presently remains highly challenging. Here, zinc oxide nanocrystals (ZnO NCs) coated by monoanionic diphenyl phosphate (DPP) ligands were derived by a sol‐gel process and a one‐pot self‐supporting organometallic (OSSOM) procedure. Atomic‐scale characterization through dynamic nuclear polarization (DNP‐)enhanced solid‐state NMR (ssNMR) spectroscopy has notably enabled resolving their vastly different surface‐ligand interfaces. For the OSSOM‐derived NCs, DPP moieties form stable and strongly‐anchored μ₂‐ and μ₃‐bridging‐ligand pairs that are resistant to competitive ligand exchange. The sol‐gel‐derived NCs contain a wide variety of coordination modes of DPP ligands and a ligand exchange process takes place between DPP and glycerol molecules. This highlights the power of DNP‐enhanced ssNMR for detailed NC surface analysis and of the OSSOM approach for the preparation of ZnO NCs.     

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 Pd^II 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.