Knight Shift in ¹³C NMR Resonances Confirms the Coordination of N‐Heterocyclic Carbene Ligands to Water‐Soluble Palladium Nanoparticles

Abstract

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 C 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 complex. NMR studies after CO adsorption established that the KS shifts the C 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 C-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.

Synthesis of Water‐Soluble Palladium Nanoparticles Stabilized by Sulfonated N‐Heterocyclic Carbenes

“…The crystallite size calculated from the XRD data using the Scherrer equation (2.2 nm) was much smaller than the mean size obtained by TEM. A similar divergence between TEM and XRD radii was found in PdNPs 2 a , and was ascribed to the polycrystallinity of the nanoparticles observed in the HR‐TEM images (Figure b)…”

“…Concerning PdNPs, Glorius and co‐workers have reported the preparation of water‐stable Pd nanoparticles stabilized by charged NHC ligands . In a recent work, we have also communicated the synthesis and characterization of water‐soluble NHC‐stabilized PdNPs prepared by decomposition of a dimethyl NHC metal complex . The coordination of the NHC ligand to the metal surface of these nanoparticles was confirmed by the observation of a Knight‐shifted signal for the carbenic carbon in solid state 13 C NMR spectra.…”

“…We have recently described that the thermal decomposition of complex 1 a in water affords dark‐brown solutions containing PdNPs ( 2 a ) that remain dispersed in the aqueous solution for indefinite periods of time (Scheme ) . The size of these PdNPs was notably influenced by the heating ramp used to decompose the organometallic precursor, whereas other conditions (temperature, time of heating, or concentration of the metal precursor) had a minor effect.…”

Synthesis of water-soluble palladium(ii) complexes with N-heterocyclic carbene chelate ligands and their use in the aerobic oxidation of 1-phenylethanol

“…We have recently shown that the thermal decomposition of [Pd(NHC) 2 Me 2 ] complexes produces Pd nanoparticles that are solubilized and stabilized in water by sulfonated NHC ligands coordinated to their surface (Scheme 6). 8 These nanoparticles (mean diameter of 3.8 ± 0.8 nm) are reminiscent of the type of Pd aggregates that might form in the above catalytic solutions. Our guess was supported by the observation that these nanoparticles were also able to afford full conversions in the oxidation of 1-phenylethanol under the same conditions that complex 4 S , indicating that they are at least as active as this complex in this reaction(Scheme 6).…”

“…5 We are interested in the synthesis, reactivity, and applications of transition-metal complexes with water-soluble NHC ligands. 6,7,8 Herein, we report the synthesis and characterization of new palladium complexes with Nheterocyclic chelate ligands functionalized with sulfonate, carboxylate or ammonium groups. An account of the aqueousphase reactivity of these complexes, and their behavior as catalysts in the aerobic oxidation of alcohols 9,10 in water, is reported.…”