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1. N‐Heterocyclic Carbene‐Based Copolymers for Templated Synthesis and Stabilization of Gold Nanoparticles

   https://doi.org/10.1002/chem.202500596  

   Nguyen, Suong T; Brown, Christopher M; Zhang, Wenxu; Kilgallon, Landon J; Johnson, Jeremiah A (April 2025, Chemistry – A European Journal)

   Abstract Surface functionalization and colloidal stability are pivotal for numerous applications of gold nanoparticles (Au‐NPs). Over the past decade, N‐heterocyclic carbenes (NHCs) have emerged as promising ligands for stabilizing Au‐NPs owing to their ease of synthesis, structural diversity, and strong metal‐ligand bonds. Here, we introduce new Au(I)–NHCcopolymer scaffolds as precursors to multidentate NHC‐protected Au‐NPs. Ring‐opening metathesis copolymerization of a norbornene‐appended Au(I)−NHC complex with another functionalized norbornene comonomer provides NHC–Au(I) copolymers with modular compositions and structures. Upon reduction, these copolymers yield multidentate polyNHC‐coated Au‐NPs with varied properties and corona functionalities dictated by the secondary monomer. These nanoparticles exhibit excellent size homogeneity and stability against aggregation in various buffers, cell culture media, and under exposure to electrolytes, oxidants, and exogenous thiols over extended periods. Moreover, we demonstrate post‐synthetic surface functionalization reactions of polyNHC−Au‐NPs while maintaining colloidal stability, highlighting their robustness and potential for applications such as bioconjugation. Overall, these findings underscore the potential of ROMP‐derived NHC‐containing copolymers as highly tunable and versatile multidentate ligands that may be suitable for other inorganic colloids and flat surfaces. 

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2. N-Heterocyclic carbene-stabilized gold nanoparticles and luminescent quantum dots

   https://doi.org/10.1117/12.2610485  

   Arabzadeh Nosratabad, Neda; Jin, Zhicheng; Du, Liang; Mattoussi, Hedi (March 2022, Colloidal Nanoparticles for Biomedical Applications XVII)

   Osiński, Marek; Kanaras, Antonios G. (Ed.)

   N-heterocyclic carbenes (NHCs) have attracted tremendous attention over the past decade, as it is expected to form strong coordination to transition metal complexes and surfaces. Here, we investigate the interactions between colloidal gold nanoparticles (AuNPs), or luminescent quantum dots (QDs) and a multidentate NHC-based polymer ligand. The ligand design relies on the nucleophilic addition reaction between several NHC anchoring groups, short polyethylene glycol (PEG) blocks, and a polymer chain. We find that such NHC-decorated ligands rapidly coordinate onto both sets of nanocrystals, which is attributed to the inherent σ-donating nature (soft Lewis base) of NHC groups combined with the soft Lewis acidic character of nanocrystal surfaces. We combine NMR spectroscopy, fluorescence spectroscopy, high-resolution transmission electron microscopy and dynamic light scattering to characterize the NHCstabilized nanocrystals and gain insights into the nature of the binding interactions. In particular, we find that the newly coated nanocrystals exhibit long-term colloidal stability over a broad range of conditions with no sign of degradation or aggregation build up, while preserving their photophysical properties, for at least one year of storage. 

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3. Ad aurum: tunable transfer of N-heterocyclic carbene complexes to gold surfaces

   https://doi.org/10.1039/d2qi01941h  

   Dominique, Nathaniel L.; Chen, Ran; Santos, Alyssa V.; Strausser, Shelby L.; Rauch, Theodore; Kotseos, Chandler Q.; Boggess, William C.; Jensen, Lasse; Jenkins, David M.; Camden, Jon P. (November 2022, Inorganic Chemistry Frontiers)

   The exceptional stability of N-heterocyclic carbene (NHC) monolayers on gold surfaces and nanoparticles (AuNPs) is enabling new and diverse applications from catalysis to biomedicine. Our understanding of NHC reactivity at surfaces; however, is quite nascent when compared to the long and rich history of NHC ligands in organometallic chemistry. In this work, well-established transmetalation reactions, previously developed for NHC transfer in homogeneous organometallic systems, are explored to determine how they can be used to create carbene functionalized gold surfaces. Two classes of NHCs, based on imidazole and benzimidazole scaffolds, were tested. The resulting AuNP surfaces were analyzed using X-ray photoelectron spectroscopy (XPS), laser desorption ionization mass spectrometry (LDI-MS), and surface-enhanced Raman spectroscopy (SERS). Reaction of either a Au( i ) or Ag( i ) isopropyl benzimidazole NHC complex with citrate-capped AuNPs yields, in both cases, a chemisorbed NHC that is bound through a Au adatom. Theoretical calculations additionally illustrate that binding through the Au adatom is favored by more than 10 kcal mol −1 , in good agreement with experiments. Surprisingly, reaction of Au( i ), Ag( i ), and Cu( i ) diisopropylphenyl imidazole NHCs do not follow the same pattern. The Cu complex undergoes transmetalation with very little deposition of Cu; whereas, unexpectedly, the Ag complex foregoes transmetalation and instead adducts to the AuNP with retention of the Ag–C bond. Theoretical calculations illustrate that the imidazole ligand affords significant dispersion interactions with the gold surface, which may stabilize binding through the Ag adatom motif, despite its less favorable bonding energies. Taken together these results suggest a unique ability to tune the reactivity by changing the carbene structure and raise critical questions about how established transmetalation reactions in organometallic chemistry can be applied to form NHC functionalized surfaces. 

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4. Comparing experimental and computational approaches for studying the binding of N-heterocyclic carbenes

   Santos, A; Simpson, S (March 2024, ACS Spring 2024)

   N-heterocyclic carbenes (NHCs) have grown in popularity in recent years due to their superior surface stability on metal nanoparticles and surfaces. This stability is often characterized experimentally by studying the σ-donation and π-backbonding as measured through NHC-selenium adduct NMR and the Huynh Electronic Parameter (HEP), respectively. However, recent work with NHCs on metal clusters suggests that the ligands can adopt a variety of orientations on the surface. Thus, the surface may have a pronounced impact on the σ-donation and π-backbonding observed for these NHCs. In this work, we aim to determine how well these experimental characterizations compare to trends observed via bond decomposition analysis. 

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5. Do polymer ligands block the catalysis of metal nanoparticles? Unexpected importance of binding motifs in improving catalytic activity

   https://doi.org/10.1039/D0TA03906C  

   Zhang, Lei; Wei, Zichao; Meng, Michael; Ung, Gaël; He, Jie (August 2020, Journal of Materials Chemistry A)

   null (Ed.)

   Metal nanoparticles (NPs) tethered by synthetic polymers are of broad interest for self-assembly, nanomedicine and catalysis. The binding motifs in polymer ligands usually as the end functional groups of polymers are mostly limited to thiolates. Since the binding motif only represents a tiny fraction of many repeating units in polymers, its importance is often ignored. We herein report the uniqueness of polymeric N-heterocyclic carbene (NHC) ligands in providing oxidative stability and promoting the catalytic activity of noble metal NPs. Two “grafting to” methods were developed for polymer NHCs for pre-synthesized metal NPs in various solvents and with different sizes. Remarkably, imidazolium-terminated polystyrene can modify gold NPs (AuNPs) within 2 min while reaching a similar grafting density to polystyrene-thiol (SH) requiring 6 h modification. We demonstrate that polymer NHCs are extremely stable at high temperature in air. Interestingly, the binding motifs of polymer ligands dominate the catalytic activity of metal NPs. Polymer NHC modified metal NPs showed improved activity regardless of the surface crowdedness. In the case of AuNPs, AuNPs modified with polystyrene NHCs are approximately 5.2 times more active than citrate-capped ones and 22 times more active than those modified with polystyrene thiolates. In view of ligand-controlled catalytic properties of metal NPs, our results illustrate the importance of binding motifs that has been overlooked in the past. 

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