# Insights Into the Exchange of N -Heterocyclic Carbene Ligands on Gold Nanoparticles: Necessity of Oxygen

Salem Ba Sowid, Dimitri Mercier, Yves Gimbert, François Ribot, Louis Fensterbank

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Salem Ba Sowid, Dimitri Mercier, Yves Gimbert, François Ribot, Louis Fensterbank. Insights Into the Exchange of N -Heterocyclic Carbene Ligands on Gold Nanoparticles: Necessity of Oxygen. ChemistryEurope, 2026, 4 (2), pp.e202500385. 10.1002/ceur.202500385. hal-05506037

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# Insights Into the Exchange of N-Heterocyclic Carbene Ligands on Gold Nanoparticles: Necessity of Oxygen

Salem Ba Sowid<fnref n="1" />,²,³ | Dimitri Mercier<fnref n="4" /> ID | Yves Gimbert⁵ | François Ribot<fnref n="1" /> ID | Louis Fensterbank²,³ ID

**Correspondence:** François Ribot (francois.ribot@sorbonne-universite.fr) | Louis Fensterbank (louis.fensterbank@college-de-france.fr)

**Received:** 16 September 2025 | **Revised:** 28 November 2025 | **Accepted:** 18 December 2025

**Keywords:** gold nanoparticles | ligand exchange | N-heterocyclic carbenes | NMR | X-ray photoelectron spectroscopy

## ABSTRACT

Gold nanoparticles (AuNPs) stabilized by N-heterocyclic carbenes (NHCs) represent robust alternatives to thiol-protected counterparts owing to the NHCs' strong Au–C bonds that provide enhanced stability. While NHC-capped AuNPs with a single type of carbene have been reported many times, strategies to introduce multiple NHC ligands on the same nanoparticle remain unexplored. Here, we investigate NHC-for-NHC ligand exchange on AuNPs stabilized either by benzimidazol-2-ylidene or by mesoionic triazolylidene scaffolds, employing an in situ generated free carbene route. Prior to ligand exchange, the formation and stability of the studied NHCs from the corresponding azolium salts were examined and characterized by NMR spectroscopy. In situ NMR studies of the exchange reaction showed no evidence of ligand substitution under argon, the conventional atmosphere for free NHC intermediates. Although no exchange was detected by NMR, X-ray photoelectron spectroscopy (XPS) analysis of the purified AuNPs revealed that ligand substitution had in fact occurred. Further investigation indicated that oxygen plays a key role in promoting the exchange, and that its involvement is also associated with oxidation of the gold core and the formation of NHC-Au(I) complexes. Taken together, our results highlight the complexity of NHC-for-NHC exchange on AuNPs by the free carbene route and point to the need for alternative strategies to achieve controlled ligand substitution.

## 1 | Introduction

Gold nanoparticles (AuNPs) have been one of the key elements in the field of nanoscience, thanks to their unique optical and electronic properties [1, 2]. Without surface ligands, AuNPs are prone to aggregation and degradation; hence, they are typically stabilized by organic ligands that preserve their colloidal stability. The nature of stabilizing ligands not only maintains the stability of AuNPs but also affects their surface chemistry, thereby defining their interfacial properties and functionality [3–6]. Mixed-ligands monolayer stabilized AuNPs (MLAuNPs), where two or more different ligands are attached to the

nanoparticle surface, have proven as a valuable tool to expand the utility of nanoparticles and fine-tune their properties [7–9]. A mixed-ligand system is particularly appealing when a complex ligand environment is needed, rendering it useful in applications such as catalysis [10], biomedicine [11], and sensing [12]. Thiol-based ligands, with their soft Lewis base character and well-established chemistry with gold nanomaterials, are widely used for surface functionalization owing to their comparatively strong Au–S bonds [13]. In particular, thiol-for-thiol ligand exchange, one of the most studied exchange reactions, is commonly employed in the preparation of MLAuNPs [14, 15]. This approach

<footnote n="1">¹Sorbonne Université, CNRS, Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP), Paris, France | ²Sorbonne Université, CNRS, Institut Parisien de Chimie Moléculaire (IPMC), Paris, France | ³Collège de France, CNRS, Laboratoire d'Activation en Chimie Moléculaire, Paris, France |</footnote>

<footnote n="4">⁴PSL Research University, CNRS, Chimie Paris Tech, Institut de Recherche de Chimie Paris (IRCP) - Physical Chemistry of Surfaces Research Group, Paris, France | ⁵Université Grenoble Alpes, Département de Chimie Moléculaire (UMR CNRS 5250), Grenoble, France</footnote>

<footnote>Dedicated to Jieping Zhu on this 60th birthday, with all our esteem.</footnote>

<footnote>This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
is favored because it usually results in incomplete ligand replacement, allowing for the fabrication of tailored ligand compositions [16, 17]. However, despite their broad applicability, thiols can be unstable under certain conditions, which has driven the exploration of alternative ligands for more robust surface functionalization [18, 19].

N-heterocyclic carbenes (NHCs), classically based on imidazol-2-ylidene and benzimidazol-2-ylidene scaffolds, have rapidly gartered significant recognition as promising alternatives to thiols for surface functionalization, owing to their high affinity toward gold centers and their synthetic modularity that enables diverse ligand structural designs [20–22]. Their ability to fabricate highly stable AuNPs, even under harsh conditions, highlights their potential as future ligands of choice for AuNPs stabilization [23–25]. Recently, other nonclassical NHCs such as nitrogen acyclic carbenes [26], mesoionic carbenes (MICs) [27–29], and (alkyl)(amino)carbene [30] have also found their place in AuNPs stabilization, expanding the diversity of carbene-based ligands. Nevertheless, despite these advancements, the current synthesis protocols of NHC-AuNPs have focused mainly on installing single-type carbene ligands onto the AuNP surface, either via reduction of NHC-Au(I) complexes [24, 25, 31] or gold–azolium salts [32, 33] (bottom-up), or through full ligand exchange of free NHCs, in situ generated or preisolated, with labile ligands such as thioethers (top-down) [23, 25, 34, 35]. Actually, the ligand exchange approach was among the first strategies reported for anchoring NHCs onto AuNPs [34]. Recently, NHC-Au(I) complexes have been used to append NHCs onto citrate-stabilized AuNPs [36], while NHC-Ag(I) and NHC-Cu(I) complexes have also been investigated as alternative NHC transfer reagents for the same system [37, 38].

While the abovementioned methods have enabled the robust functionalization of gold surfaces with a single type of carbene, strategies for introducing multiple NHC ligands on the same nanoparticle remain underexplored. In particular, the partial exchange of surface-bound NHCs by other NHCs to generate mixed-ligand AuNPs has not yet been documented. Such NHC-for-NHC exchange strategies could offer important advantages, especially when using NHCs incompatible with nanoparticle synthesis [36], when decoupling core size and surface chemistry is desired, or when varying the NHC ligand–shell composition is necessary. Moreover, the coassembly of different NHCs on a single AuNP surface could ultimately yield heteroself-assembled monolayers with tunable stability, reactivity, and new molecular geometry [39]. In a conceptually relevant context, Du, Glorius, and coworkers demonstrated that the coassembly of NHC/thiols on a flat Ag surface enhanced the surface anti-corrosion properties, conductivity, and catalytic activity for $C_2H_2$ hydrogenation [40]. Ligand exchange is also an elegant way to probe the relative affinity of different ligands for a surface [41]. Indeed, it is widely accepted that the strong σ-donation of NHCs is a key factor in the retention of their bonds on AuNP surfaces [20–22, 42]. We therefore investigated NHC-for-NHC ligand exchange with two different types of NHCs: the classical benzimidazole-2-ylidene, which is derived from benzimidazolium, and the mesoionic 1,2,3-triazol-5-ylidene, which is derived from 1,2,3-triazolium. Such an approach should provide insights into the expected higher relative affinity of 1,2,3-triazol-5-ylidene mesoionic carbens for AuNPs, in line with its stronger sigma donation ability [43]. Understanding the feasibility and dynamics

of such NHC-for-NHC ligand exchange appears essential for advancing the controlled surface functionalization of AuNPs with multiple NHC ligands. Given this significance, we are herein describing our attempts toward NHC-for-NHC ligand exchange on AuNPs using the supposedly simple in situ generated free carbene route.

## 2 | Results and Discussion

To this end, we studied exchange reactions on AuNPs stabilized by benzimidazol-2-ylidene ($1^F$NHC) and 1,2,3-triazol-5-ylidene (1-MIC) based NHCs (Scheme 1). First, we synthesized the azolium salts, $1^F$NHC·HBr and 1-MIC·HBr, both bearing a $C_{12}H_{25}$ chain at positions adjacent to the (pro)carbene center, which is known to promote the stabilization of AuNPs [25, 28, 29, 44]. The azolium salt $1^F$NHC·HBr was tagged with fluorine atoms to allow monitoring of the ligand exchange reaction by $^{19}$F nuclear magnetic resonance (NMR) and to serve as a reporter in X-ray photoelectron spectroscopy (XPS) analysis. It should be noted that the incorporation of fluorine into the $1^F$NHC backbone leads to only a minor decrease in its σ-donating ability compared to its nonfluorinated analog, as supported by Huynh's electronic parameter (HEP) [45] measurements (Figure S2, HEP = 178.68 ppm for $1^F$NHC vs. 179.09 ppm for its nonfluorinated analog 1-NHC). Using the same method, we further confirmed that 1-MIC is a stronger σ-donor, with a HEP value of 181.39 ppm, exceeding both $1^F$NHC and 1-NHC. These results were further corroborated by DFT calculations on simplified ligand models (Table S1), in which the long $C_{12}H_{25}$ chains adjacent to the carbenic center were replaced by $C_2H_5$ groups. The calculations show that 1-MIC has the highest HOMO energy (–5.60 eV), while the HOMO energies of 1-NHC (–6.23 eV) and $1^F$NHC (–6.47 eV) differ only slightly, further confirming the HEP measurements. Moreover, the calculations show that introducing fluorine into the benzimidazole backbone increases the π-acceptor character of $1^F$NHC compared to its proton analog 1-NHC, as reflected by its more stabilized LUMO (–0.89 eV for $1^F$NHC vs. –0.53 eV for 1-NHC). Nevertheless, since σ-donation is often regarded as the primary contributor to the Au–C bond on AuNP surfaces [46], any influence from π-backdonation is likely modest. These azolium salts were further used to prepare $1^F$NHC-AuNPs and 1-MIC-AuNPs, respectively, using our previously reported protocol [28, 33]. These optimized protocols slightly

<figure><img src="image_2.png" /><figcaption>**SCHEME 1** | Proposed $NHC^1$-for-$NHC^2$ ligand exchange reaction on AuNPs.</figcaption></figure>
differ because the starting azoliums exhibit different reactivities, but both yield purely Au(0) nanoparticles of comparable size, solely stabilized by NHC ligands. Briefly, 1-$^F$NHC-AuNPs were synthesized directly from 1-$^F$NHC·HBr salt and AuCl in a biphasic toluene/H$_2$O mixture using excess NaBH$_4$ as a reducing agent. Similarly, 1-MIC-AuNPs were prepared from 1-MIC·HBr salt, NaH, and HAuCl$_4$, followed by reduction with excess NaBH$_4$ in a biphasic CH$_2$Cl$_2$/H$_2$O mixture. The resulting nanoparticles were characterized by transmission electron microscopy (TEM), UV-Vis spectroscopy, thermogravimetric analysis (TGA), and solution-state NMR spectroscopy to confirm their size, ligand density, and to verify the absence of free molecular species prior to the ligand exchange reaction (Figures S3–S7).

## 2.1 | NHCs Formation and Stability

In order to conduct the ligand exchange reaction, we first assessed the free NHCs formation. Azolium salts could be converted into the corresponding NHCs by treating them with a strong base under an inert atmosphere [47, 48]. Accordingly, 1-$^F$NHC·HBr and 1-MIC·HBr were reacted with sublimed NaOtBu in degassed and dried THF-d$_8$ at room temperature under inert conditions. The NMR spectra of both reactions were recorded and compared side-by-side with those collected from the azolium salts (Figure 1a,b).

The $^1$H NMR spectrum (Figure 1a, bottom) of the benzimidazolium salt (1-$^F$NHC·HBr) showed peaks at 11.8 and 8.5 ppm attributed respectively to the C2 and C4 protons of the ring system. Additionally, two signals at 4.8 and 2.1 ppm correspond to the aliphatic protons of C6 and C7, respectively. The NMR spectrum collected from the crude 1-$^F$NHC solution (Figure 1a, top), recorded *ca.* 10 min after base addition, shows the disappearance of the benzimidazolium C2 proton signal at 11.8 ppm, indicating the successful conversion of the benzimidazolium salt to the free carbene, while the aromatic and aliphatic proton signals, particularly those close to the heterocyclic ring (e.g. C4 and C6 protons), are shifted upfield yet remaining identifiable. Further evidence for the formation of the free 1-$^F$NHC was obtained from $^{13}$C NMR and $^1$H-$^{13}$C heteronuclear multiple bond correlation (HMBC) spectroscopy (Figures S10 and S11). These experiments showed that the C2 carbon undergoes a downfield shift from 146.9 ppm in the benzimidazolium form to 230.3 ppm in the carbene form. Such a shift is consistent with a benzimidazol-2-ylidene carbene formation [49]. Over time, the $^1$H NMR spectrum of 1-$^F$NHC solution gradually showed a decrease of the free carbene signals, together with the growth of signals attributable to a dimeric 1-$^F$NHC species [50] (Figure 1c). This transformation proceeded slowly, as the free carbene was still clearly detectable after 7 days. Interestingly, deprotonation of benzimidazolium salts bearing methyl groups at the nitrogen positions in THF was reported to give exclusively the

<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | $^1$H NMR spectra of (a) 1-$^F$NHC·HBr salt before deprotonation (bottom) and after deprotonation and (b) 1-MIC·HBr salt before deprotonation (bottom) and after deprotonation (top). The green diamond marks the tBu-OH signals, and the red diamond marks residual THF solvent. Stacked $^1$H NMR spectra showing the evolution of (c) 1-$^F$NHC to 1-$^F$NHC dimer and (d) 1-MIO to 1,4-dodecyltriazole over time.</figcaption></figure>
dimer [51], showing that the 1<sup>F</sup>NHC used here, with its more hindered C<sub>12</sub>H<sub>25</sub> substituted, is more kinetically persistent toward dimerization.

In the case of triazolium salt (1-MIC·HBr), the ¹H NMR spectrum of the crude mixture of 1-MIC·HBr and base revealed deprotonation at the benzylic CH₂ of the N-benzyl substituent rather than at C5 proton, leading to the formation of mesoionic N-heterocyclic olefin (1-MIO) (Figure 1b). The ¹H NMR spectrum of 1-MIO showed a signal at 5.5 ppm, attributable to the benzylic C10 proton in the olefin form. This signal appears upfield relative to the benzylic CH₂ resonance observed at 6.3 ppm in the triazolium salt. Additionally, the C5 proton signal shifted upfield from 10.6 ppm in the triazolium salt to 7.2 ppm in the 1-MIO spectrum. Additional evidence for the formation of 1-MIO was provided by the ¹H-¹³C HMBC and ¹H-¹³C heteronuclear single quantum correlation (HSQC) spectra (Figure S16 and S17). In the HMBC spectrum of 1-MIO, the C5 carbon (121.9 ppm) clearly showed cross-peaks with the C6 proton (3.9 ppm) and C8 proton (2.4 ppm), and was shifted upfield compared to the corresponding C5 signal in the triazolium salt (131.7 ppm). The C10 carbon (86.3 ppm), corresponding to the benzylic olefinic position, showed a significant downfield shift compared to the same carbon in the triazolium salt (55.2 ppm). Moreover, its attached proton exhibited cross-peaks with the aromatic C11 and C12 carbons, further supporting the proposed structure. Additionally, the ¹H-¹³C HSQC spectrum of 1-MIO showed that both C5 and C10 each bear one proton, while in the triazolium salt, C5 bears one proton and C10 bears two. All these observations confirm that deprotonation occurred at the benzylic C10 position rather than at C5. Tautomerization between MIOs and their corresponding MIC forms has been reported in related systems, and is believed to proceed via a stepwise intermolecular proton transfer [52]. Notably, in such systems, the presence of Lewis acids (LAs) has been shown to favor the formation of MIC-LA adducts at room temperature by shifting the equilibrium toward the MIC tautomer. Guided by these observations, we investigated the reactivity of our 1-MIO species toward gold(I). Interestingly, upon addition of 0.5 equivalents of AuSMe<sub>2</sub>Cl to the 1-MIO solution, an immediate color change from orange to pale purple was observed. The ¹H NMR spectrum of the crude mixture after Au(I) addition revealed the expected NMR signals of the [(1-MIC)<sub>2</sub>Au][X] complex, where the characteristic triazolium C5 proton signal disappeared and the benzylic CH<sub>2</sub> signal was restored, consistent with carbene coordination to Au through the C5 carbon (Figure S18). Moreover, the ¹³C NMR spectrum showed a downfield shift of the C5 carbon, from 121.9 ppm in the 1-MIO to 172.6 ppm upon coordination (Figures S19 and S20). The latter value falls within the typical range reported for (1-MIC)<sub>2</sub>Au(I) complexes [53], confirming the assignment of the observed complex. These combined results confirmed the presence of an equilibrium between the 1-MIO and the 1-MIC. In this equilibrium, coordination to Au(I) drives the conversion toward the MIC form, thereby enabling (1-MIC)<sub>2</sub>Au(I) complex formation. In addition, monitoring the 1-MIO solution over time revealed its gradual decomposition into 1,4-didodecyltriazole (Figure 1d). Further confirmation of triazole formation was obtained by matching the ¹H NMR signals of the crude 1-MIO solution and 1,4-didodecyltriazole (Figure S22). Such a dealkylation pathway has previously been reported

for 1,2,3-triazol-5-ylidene-based MICs bearing alkyl substituents at the N3 position of the heterocycle [54]. It could also be ascribed to an oxidation process caused by adventitious oxygen [55].

## 2.2 | Ligand Exchange Reaction

Liquid-state NMR spectroscopy is a powerful technique for monitoring ligand exchange reactions on nanoparticles surfaces in real-time. Free ligands in solution typically exhibit sharp NMR signals; those grafted onto a nanoparticle's surface often show broadened signals [31, 35, 56, 57]. This broadening is directly related to two main reasons. First, the variation of the grafted ligand's local environments leads to the distribution of their chemical shift. Second, the restricted mobility of grafted ligands results in fast transverse relaxation (shorter T2), which also contributes to signal broadening. In some cases, this broadening can become so important that signals, particularly those from ligands at the closest sites to the surface, eventually disappear into the baseline [41, 58]. Therefore, we have used this feature to monitor the ligand exchange reaction in real-time by following the release of either 1<sup>F</sup>NHC or 1-MIC from the AuNPs surface upon ligand exchange.

In a typical ligand exchange experiment, 8 equivalents of azolium salt (1-MIC·HBr or 1<sup>F</sup>NHC·HBr), relative to the amount of ligand on the AuNP surface, as estimated by TGA, were dissolved in THF-d<sub>8</sub> and treated with 1.1 equivalents of NaOtBu inside a glove box. The resulting mixture was then added to AuNPs stabilized by the NHC ligands (either 1<sup>F</sup>NHC-AuNPs or 1-MIC-AuNPs) dispersed in THF-d<sub>8</sub>. The exchange reaction was allowed to proceed for 24 h at room temperature. Afterward, the mixture was transferred to a J-Young NMR tube, and the NMR was recorded. Although we anticipated the ligand exchange would occur, neither reaction provided evidence of ligand substitution. In the case of 1-MIO solution added to 1<sup>F</sup>NHC-AuNPs, the ¹⁹F NMR spectrum of the final crude mixture surprisingly showed no features of free 1<sup>F</sup>NHC or related 1<sup>F</sup>NHC species released into the solution. To ensure that this absence of signal was not related to ¹⁹F NMR detection limit, we verified that a solution of 1<sup>F</sup>NHC·HBr at a concentration corresponding to 5% of the ligands originally present on 1<sup>F</sup>NHC-AuNPs was indeed showing a clear signal (Figure S23). The completely silent ¹⁹F NMR spectrum indicates that ligand exchange did not occur under these conditions, as 1<sup>F</sup>NHC remained tightly bound to the AuNP surface. Similarly, for the reverse reaction between 1<sup>F</sup>NHC and 1-MIC-AuNPs, the ¹H NMR spectrum showed no sign of related 1-MIC species released (Figure S24).

Despite our initial in situ NMR characterization, which did not evidence any exchange, we opted to further investigate the system using post-synthesis characterization techniques. So far, ligand exchange reactions to form NHC-AuNPs, by replacing weakly bound ligands with free NHCs, have indeed only been confirmed through post-synthesis characterization, typically by identifying the coordinated NHC on the AuNP surface [23, 25, 34, 59–62]. Given the discrepancy between our in situ NMR results and the expected exchange behavior, we decided to complement our study with UV-Vis, TEM, XPS, and TGA analysis, aiming to assess the presence of incoming ligands on the AuNP surface. Therefore, the crude mixtures from both reactions were taken out of the glove box and dried under vacuum. The resulting solids were dispersed
in CH₂Cl₂, washed three times with water to remove salts, and purified by two ethanol precipitation/centrifugation cycles, yielding the purified AuNP-1 (from the exchange of 1-MIO with 1-<sup>F</sup>NHC-AuNPs) and AuNP-2 (from the exchange of 1-<sup>F</sup>NHC with 1-MIC-AuNPs).

The optical properties of the resulting AuNP-1 and AuNP-2 remained largely unchanged (Figure 2). AuNP-1 exhibited a localized surface plasmon resonance (LSPR) at 516 nm, similar to the starting 1-<sup>F</sup>NHC-AuNPs, while AuNP-2 showed a slight blue shift of 2 nm compared to 1-MIC-AuNPs, resulting in an LSPR at 520 nm. This suggested that no significant size and shape variation in the gold core had occurred. To confirm this, a TEM analysis was performed. TEM showed that the sizes of AuNP-1 (2.8 ± 0.5 nm) and AuNP-2 (4.3 ± 0.7 nm) were only marginally smaller than the starting 1-<sup>F</sup>NHC-AuNPs (3.0 ± 0.5 nm) and 1-MIC-AuNPs (4.7 ± 0.8 nm), respectively (Figure 2a,b). However, this change remains within the experimental uncertainty of the TEM measurements and is therefore not statistically significant.

Surprisingly, XPS analysis of AuNP-1 and AuNP-2 indicated successful ligand exchange. For AuNP-1, obtained from the reaction between 1-MIO and 1-<sup>F</sup>NHC-AuNPs, the presence of 1-MIC coordinated to AuNPs surface was assessed by analyzing the F 1s and deconvoluted N 1s XPS spectra (Figure 3a). We have shown previously that MIC molecules show N 1s signal with two components with a 1:2 ratio due to the different chemical environments of central and outer nitrogen atoms in the heterocycle [28]. The N 1s spectrum analysis of AuNP-1 showed the presence of a

complex photopeak between 403 and 399 eV. A fine deconvolution of this N 1s photopeak revealed the presence of four components, which were attributed to nitrogen atoms coming from 1-MIC, 1-<sup>F</sup>NHC, and a cationic (benz)imidazolium species, presumably 1-<sup>F</sup>NHC.HBr. Specifically, the two components at 401.8 eV (one N<sub>central</sub>) and 400.7 eV (two N<sub>outer</sub>) correspond to covalently bound 1-MIC, in agreement with our previous observation. Moreover, the covalently bound 1-<sup>F</sup>NHC, which has two equivalent nitrogen atoms, presents only one symmetrical component at 400.7 eV. The last symmetrical component at higher binding energy (401.9 eV) was attributed to cationic benzimidazolium species. Previous studies have likewise reported that covalently bound benzimidazole based NHCs on metallic nanoparticles exhibit lower binding energies compared to benzimidazolium species [33, 63]. We surmise that the presence of cationic benzimidazolium species is promoted by the excess halides present in the reaction medium during the exchange process. Based on the deconvoluted N 1s components, the intensity of the signals attributed to 1-MIC remained relatively low compared to those of 1-<sup>F</sup>NHC species, indicating limited exchange reaction. Consistent with this, analysis of the fluorine content in the F 1s region (Figure 3a and Table 1, entry 1) revealed that the ligand shell is still predominantly composed of 1-<sup>F</sup>NHC, with only partial exchange by 1-MIC. From the experimental atomic ratio of N to F (1:24:1), the amount of 1-MIC was estimated to be approximately 10 mol% of the surface ligands.

Interestingly, in the case of AuNP-2, which was obtained from the reaction of 1-<sup>F</sup>NHC with 1-MIC-AuNPs, the XPS analysis

<figure><img src="image_4.png" /><figcaption>FIGURE 2 | TEM and UV-Vis characterization of purified (a) AuNP-1 and (b) AuNP-2.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | XPS characterization of purified (a) AuNP-1 and (b) AuNP-2. High-resolution spectra of the N 1s and F 1s regions are shown for both AuNPs from left to right. The N 1s photopeaks were deconvoluted to reveal the contributions from the different species.</figcaption></figure>

<table><caption>**TABLE 1** | N and F concentrations of AuNP-1 and AuNP-2 obtained by XPS.</caption><thead><tr><td>Entry</td><td>AuNP</td><td>N%</td><td>F%</td><td>(N : F)</td></tr></thead><tbody><tr><td>1</td><td>AuNP-1</td><td>15.9</td><td>12.8</td><td>1.24 : 1</td></tr><tr><td>2</td><td>AuNP-2</td><td>15</td><td>14.4</td><td>1.04 : 1</td></tr></tbody></table>

of the N 1s and F 1s spectra indicated the predominant presence of covalently bound 1-$^F$NHC. This was evidenced by a photopeak centered at 400.7 eV in the N 1s spectrum and a corresponding peak at 687.79 eV in the F 1s spectrum (Figure 3b). Once again, a small shoulder at higher binding energy (~401.9 eV) in the N 1s spectrum was observed, which can be attributed to residual physiosorbed cationic benzimidazolium species. Moreover, the observed N : F ratio of 1.04 : 1 was consistent with the expected contributions from 1-$^F$NHC species only, further excluding the presence of 1-MIC (Table 1, entry 2). Altogether, these findings indicate that while ligand exchange took place in both reactions, the degree of exchange differed significantly, with 1-$^F$NHC more effectively replacing 1-MIC than the reverse.

Regarding the gold oxidation state, the Au 4f XPS spectra of AuNP-1 and AuNP-2 demonstrated that both AuNPs were only composed of Au(0) (Figures S27 and S28). This was evidenced by the presence of two components at 84.0 eV (Au 4f$_{7/2}$) and 87.6 eV

(Au 4f$_{5/2}$) with a binding energy difference of 3.6 eV, characteristic of metallic gold Au(0) [33].

TGA experiments of AuNP-1 and AuNP-2 showed a slight change in the weight loss% of both nanoparticles after ligand exchange, compared to the starting 1-$^F$NHC-AuNPs and 1-MIC-AuNPs (Figure S29). Since both ligands have comparable molar masses (491.7 g/mol for 1-$^F$NHC and 495.75 g/mol for 1-MIC), any change in the weight loss% should be reflected in the surface density, assuming that both ligands adopt the same binding configuration. Notably, we did not observe a significant increase weight loss%, which would indicate a pronounced coadsorption process of the incoming ligands rather than an exchange. This supports the conclusion that the observed XPS results are due to a ligand exchange process.

As the XPS showed that the exchange took place, we hypothesized that the ligand exchange may have occurred when the nanoparticles were in contact with air during the purification step, with oxygen potentially playing a key role in the exchange reaction. Previous studies have shown that ligand exchange between AuNPs stabilized by weakly coordinated ligands and free NHC to produce NHC-AuNPs can also involve the formation of NHC-Au(I) complexes during the course of the exchange reaction. The formation of these complexes was attributed to the leaching of NHC-Au(I) species from the AuNP surface, where the strong affinity of NHCs for surface Au atoms disrupts
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**FIGURE 4** | (a) Staked ¹⁹F[{H} NMR spectra of AuNP-1 crude before and after oxygen bubbling at different time points. (b) High-resolution XPS Au 4f spectra analysis of pure 1-<sup>F</sup>NHC-AuNP, crude AuNP-1 under Argon, and crude AuNP-1 under oxygen.</figcaption></figure>

Au–Au bonds and promotes the dislodging of gold atoms as NHC-Au(I) complexes [59, 60]. Recently, Reithofer and coworkers proposed that NHC, generated from masked-NHC, may bind to Au(0) surface through an ‘oxidative’ coordination step, in which NHCs generate partially oxidized Au surface atoms or (ad)atoms [64]. However, the proposed rationalizations did not consider the need for an external oxidant, such as air or an oxidizing reagent, to generate Au(I) from the nanoparticles that are composed of Au(0). Therefore, to test the oxygen involvement in the exchange reaction presented in this study, the ligand exchange experiment between 1-MIO and 1-<sup>F</sup>NHC-AuNPs was repeated using the same protocol described, and the ¹⁹F NMR spectra were recorded before and after bubbling oxygen in the reaction medium, as shown in Figure 4a. Interestingly, the ¹⁹F NMR spectrum, recorded after 20 min (under an oxygen atmosphere), showed the appearance of a sharp peak at -147.5 ppm, which was not observed under argon (Figure 4a, middle). The sharpness of this peak indicates that this species is not attached to the AuNP surface but is free in solution. This peak was assigned to the 1-<sup>F</sup>NHC=O (azolone) species [65] (see Figure S30 for details of the assignment). It is worth noting that a similar NHC=O (azolone) byproduct, along with homoleptic (NHC)₂Au(I) complexes, was previously reported to form upon reaction of NHCs with citrate-stabilized AuNPs [66]. Furthermore, the NMR spectrum recorded after 18 h (under an oxygen atmosphere) displayed, in addition to the 1-<sup>F</sup>NHC=O signal, two new signals shifted

downfield (Figure 4a, top). These signals may correspond to 1-<sup>F</sup>NHC-Au(I) complexes, as their ¹⁹F NMR chemical shifts in the crude mixture (-139.6 and -139.8 ppm) are close to those of the independently prepared homoleptic [(1-<sup>F</sup>NHC)₂Au(I)][PF₆] and heteroleptic [(1-<sup>F</sup>NHC)(1-MIC)Au(I)][PF₆] complexes (-138.6 and -138.9 ppm, respectively). The slight differences may arise from the presence of different counter-anions in the crude mixture. To further support their identification, electrospray ionization high resolution mass spectrometry (ESI-HRMS) analysis of the AuNP-1 supernatant revealed the presence of multiple NHC-Au(I) complexes, including (1-MIC)₂Au(I), (1-<sup>F</sup>NHC)₂Au(I), and the heteroleptic (1-MIC)(1-<sup>F</sup>NHC)Au(I) species (Figure S31). The ¹H NMR spectrum of the crude mixture also showed 1,4-didodecyltriazole as the major product (Figure S32). A control experiment, where 1-MIO was reacted with oxygen in the absence of 1-<sup>F</sup>NHC-AuNPs, also led to the formation of 1,4-didodecyltriazole as the major product, indicating the reactivity of 1-MIO toward oxygen (Figure S33). The presence of this species, together with 1-<sup>F</sup>NHC=O in the crude mixture, shows that an oxidative pathway competes with the ligand-exchange process.

Moreover, since the starting 1-<sup>F</sup>NHC-AuNPs are composed exclusively of Au(0) as demonstrated earlier by XPS, we aimed to determine whether the introduction of oxygen facilitates gold oxidation and potentially the formation of the observed NHC-Au(I) complexes. Consequently, XPS was performed on the crude AuNP-1 sample before and after oxygen bubbling, and the Au 4f spectra were compared to the starting 1-<sup>F</sup>NHC-AuNPs (Figure 4b). Interestingly, after oxygen bubbling, the crude AuNP-1 suspension showed the appearance of Au(I) signals alongside Au(0), evidencing partial oxidation of the gold core. In the crude AuNP-1 sample maintained under argon, a very small amount of Au(I) was also detected, much less than in the oxygen-bubbled sample, which can presumably be attributed to slight oxidation during sample transfer for XPS analysis. These results outline that NHC-AuNPs are prone to oxidation in the presence of free NHCs and oxygen in the reaction medium. Indeed, it has been reported that (NHC)₂Au(I) complexes can be obtained by reacting metallic Au(0) with imidazolium salts, CuSO₄, and ammonia under air [67]. In this reaction, ammonia serves as both, a base (to deprotonate the imidazolium) and as a stabilizer for Cu(II), enabling Cu(II) to facilitate the oxidation of Au(0) to Au(I). Interestingly, the authors also demonstrated that (NHC)₂Au(I) complexes still form, though in lower yields, even without Cu(II). Therefore, the involvement of in situ generated free NHCs in gold oxidation in the latter case cannot be excluded. It is also consistent with the observation of the azolone and the 1,4-dodecyltriazole derivatives, which are markers of free NHCs oxidation [55, 65]. At the same time, NHC-AuNPs are generally stable in the presence of oxygen when stabilized by NHCs with functional groups that provide sufficient steric stabilization [68], similar to those used in this study. As a control experiment, we bubbled oxygen into a suspension of 1-<sup>F</sup>NHC-AuNPs, which did not lead to any detectable release of species by NMR (Figure S34). Therefore, it is unlikely that the oxidation observed for crude AuNP-1 arises from a direct reaction between oxygen and 1-<sup>F</sup>NHC-AuNPs.

### 3 | Conclusion

In conclusion, the ligand exchange between two different NHCs scaffolds, namely, 1-<sup>F</sup>NHC and 1-MIC on AuNPs, was
investigated using in situ generated free carbene route. Our
results showed that while the ligand exchange reaction is achiev-
able through this route, they also highlight that the exchange
mechanism is more complex than a simple replacement of the
existing surface-bound ligands by free NHCs present in the solu-
tion. Indeed, direct in situ NMR monitoring of the exchange reac-
tion under argon, an atmosphere typically used to preserve free
NHCs and commonly applied in such reactions, failed to provide
clear evidence of ligand displacement. However, XPS characteri-
zation of the purified AuNPs after the exchange reaction con-
firmed that ligand exchange had occurred, suggesting that the
exchange reaction likely proceeded during the work-up proce-
dure used to purify the AuNPs. These observations pointed
toward a potential role of oxygen in facilitating the exchange pro-
cess, which was subsequently confirmed. Further experiments
confirmed that oxygen is directly involved in the exchange pro-
cess, and that its presence also results in partial oxidation of the
gold core along with the formation of NHC-Au(I) complexes. We
infer that the formation of these complexes could play an impor-
tant role in driving the exchange reaction on AuNPs. Indeed, XPS
analysis showed that 1-MIC was more efficiently replaced by
1-<sup>F</sup>NHC than the reverse, which initially seems counterintuitive.
Since 1-MIC is the stronger σ-donor, one would have expected it
to form a stronger bond to the gold surface and be more resistant
to exchange. However, the formation of NHC-Au(I) complexes as
byproducts suggests that 1-MIC may preferentially leave the
Au(0) nanoparticle surface to form Au(I) complexes, which
are more stable than those of 1-<sup>F</sup>NHC. A recent study by
Nazemi, Frenette, and coworkers supports this view [46], show-
ing through theoretical calculations that NHCs bind more
strongly to Au(I) complexes than to Au(0) nanoparticles. This
could rationalize the more efficient replacement of 1-MIC by
1-<sup>F</sup>NHC observed by XPS, despite the stronger σ-donating ability
of 1-MIC. Nevertheless, differences in steric effects and surface
arrangement between the two scaffolds may also contribute to
this outcome. Further studies will aim at gaining deeper insight
into the mechanism of NHC-for-NHC exchange on AuNPs, with
the goal of developing robust routes that allow controlled ligand
substitution and fine-tuning of the ligand shell.

Acknowledgments

The authors would like to acknowledge Sorbonne Université, CNRS, and
Collège de France for funding, and the SM3 platform of Sorbonne
Université for high-resolution mass spectrometry.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

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## Supporting Information

Additional supporting information can be found online in the Supporting Information section. The authors have cited additional references within the Supporting Information [69–75]. **Supporting Fig. S1:** ¹³C{¹H} NMR spectra (carbene region) of the independently synthesized homoleptic complexes [(1-FNHC)₂Au][PF₆] and [(1-NHC)²Au][PF⁶] (top, reference spectra), compared with the reaction mixtures obtained during the attempted synthesis of the heteroleptic complexes [(1-NHC)(2-NHC) Au][PF₆] and [(1-FNHC)(2-NHC)Au][PF₆] (bottom). Light colored markers identify the characteristic carbene signals of each ligand in the homoleptic complexes: purple = ¹FNHC, green = ¹-NHC, brown = ²-NHC. [(2-NHC)₂Au][PF₆] carbene signal was assigned based on the literature [74]. Reference spectra were used to assign the signals of the homoleptic species present in the mixtures. The remaining carbene resonances (red and blue dots) correspond to the heteroleptic complexes, confirmed by ¹H-¹³C HMBC correlations. **Supporting Fig. S2:** a. Schematic representation of heteroleptic bis-NHCAu(I) complexes containing 2-NHC and table showing the 2-NHC carbon signals of different heteroleptic bis-NHCAu(I) complexes (the chemical shifts presented in the table are referenced to CDCl₃ at 77.7 ppm, while those reported in the synthesis part are referenced to CDCl₃ at 7.16 ppm. They are accordingly downfield shifted by 0.54 ppm). b. Scale representation showing the difference in sigma-donation nature of 1-MIC, 1-NHC, and 1-FNHC ligands. **Supporting Fig. S3:** UV-Vis spectrum in CH₂Cl₂ (top-left), TEM and NP size distribution (top-right), and TGA analysis (bottom) of ¹FNHC-AuNP. **Supporting Fig. S4:** UV-Vis spectrum in CH₂Cl₂ (top-left), TEM and NP size distribution (top-right), and TGA analysis (bottom) of 1-MIC-AuNP. **Supporting Fig. S5:** ¹H NMR spectrum of ¹FNHC-AuNP (300 MHz, 300 K) in CD₂Cl₂; normal spectrum (top) and increased intensity (bottom). **Supporting Fig. S6:** ¹⁹F{H} NMR spectrum of ¹FNHC-AuNP (282 MHz, 300 K) in CD₂Cl₂. **Supporting Fig. S7:** ¹H NMR spectrum of 1-MIC-AuNP (300 MHz, 300K) in CD₂Cl₂. **Supporting Figure S8:** ¹H NMR spectrum of the free ¹FNHC crude solution 10 mins after the base addition (400 MHz, 300K) in THF-d₈. **Supporting Fig. S9:** ¹⁹F{H} NMR spectrum of the free ¹FNHC crude 13 mins after the base addition (376 MHz, 300K) in THF-d₈. **Supporting Fig. S10:** ¹³C{H} NMR spectrum of free ¹FNHC 35 mins after the base addition (101 MHz, 300K) in THF-d₈. **Supporting Fig. S11:** ¹H¹³C{H} HMBC NMR spectrum of free ¹FNHC 1 h after the base addition (400/101 MHz, 300K) in THF-d₈. **Supporting Fig. S12:** Stacked ¹H NMR spectra showing the slow conversion of the free ¹FNHC to the ¹FNHC dimer over time (300 MHz, 300K) in THF-d₈. **Supporting Fig. S13:** ¹³C{H} NMR spectrum showing the co-existence of free ¹FNHC and ¹FNHC dimer after 20 h of aging the ¹FNHC solution in THF-d₈ (282 MHz, 300K). **Supporting Fig. S14:** ¹H NMR spectrum of the ¹MIO solution 10 mins after the base addition (400 MHz, 300K) in THF-d₈ (top). The ¹MIC·HBr spectrum in THF-d₈ is presented for comparison purposes (bottom). **Supporting Fig. S15:** ¹³C{H} spectrum of the ¹MIO solution 15 h after the base addition (126 MHz, 300K) in THF-d₈. **Supporting Fig. S16:** ¹H¹³C{H} HMBC NMR spectrum of ¹MIO solution 18 h after the base addition (500/126 MHz, 300K) in THF-d₈. **Supporting Fig. S17:** ¹H¹³C{H} HSQC NMR spectrum of ¹MIO (top) and ¹MIC·HBr (bottom) (500/126 MHz, 300K) in THF-d₈. **Supporting Fig. S18:** Stacked ¹H NMR spectra of freshly prepared ¹MIO solution (top) and of (1-MIC)₂Au(I) formed upon addition of (SMe₂)AuCl to the ¹MIO solution (bottom), in THF-d₈ (300 MHz, 300 K). **Supporting Fig. S19:** ¹³C{H} NMR spectrum of the (1-MIC)₂Au(I) after the addition of (SMe₂)AuCl to the ¹MIO solution in THF-d₈ (300 MHz, 300 K). **Supporting Fig. S20:** ¹H¹³C{H} HMBC NMR spectrum of the crude (1-MIC)₂Au(I) after the addition of (SMe₂)AuCl to the ¹MIO solution in THF-d₈ (300 MHz, 300 K). **Supporting Fig. S21:** Stacked ¹H NMR spectra showing the slow degradation of the ¹MIO to the ¹,4-dodecyltriazole over time (300 MHz, 300K) in THF-d₈. **Supporting Fig. S22:** Stacked ¹H NMR spectra of ¹MIO solution aged for 48 h (bottom) and authentic ¹,4-didodecyltriazole (top) in THF-d₈ (300 MHz and 300 K).
Blue dots highlight the matching signals consistent with the
formation of **1,4-didodecyltriazole** as a decomposition product of **1-MIO**. **Supporting Fig. S23:** ¹⁹F{H} NMR spectrum of 1-MIO + 1-¹F*NHC-AuNP* crude 24 h – argon atm, the expected signals of 1-¹F*NHC* highlighted in the dashed red box (top). Control experiment showing that a ¹⁹F{H} signal is detectable at a concentration corresponding to 5% of the ligands initially present on the 1-¹F*NHC-AuNPs*, using 1-¹F*NHC-HBr* as a model compound (bottom). **Supporting Fig. S24:** ¹H NMR spectrum of 1-¹F*NHC* + 1-MIC-AuNP crude 24 h – argon atm, the expected signals of 1-MIC are highlighted in the dashed red box. **Supporting Fig. S25:** UV-Vis spectrum in CH₂Cl₂ (top-left), TEM and NP size distribution (top-right), and TGA analysis (bottom) of AuNP-1. **Supporting Fig. S26:** UV-Vis spectrum in CH₂Cl₂ (top-left), TEM and NP size distribution (top-right), and TGA analysis (bottom) of AuNP-2. **Supporting Fig. S27:** Au 4f High-resolution XPS spectrum of purified AuNP-1. **Supporting Fig. S28:** Au 4f High-resolution XPS spectrum of purified AuNP-2. **Supporting Fig. S29:** TGA characterization of purified a. AuNP-1 compared to 1-¹F*NHC-AuNP* and b. AuNP-2 compared to 1-MIC-AuNP. **Supporting Fig. S30:** Stacked ¹⁹F{¹H} NMR spectra of AuNP-1 crude mixture after oxygen bubbling (bottom) and after addition of 1-¹F*NHC=O* (top), showing increased intensity of the same peak. **Supporting Fig. S31:** ESI-HRMS analysis of the supernatant from AuNP-1. **Supporting Fig. S32:** ¹H NMR spectrum of AuNP-1 crude mixture after oxygen bubbling. Blue dots highlight the signals of **1,4-didodecyltriazole**. **Supporting Fig. S33:** Stacked ¹H NMR spectra of freshly prepared 1-MIO solution under argon (top) and after oxygen bubbling (bottom), where the bottom spectrum shows 1,4-didodecyltriazole as the major product. **Supporting Fig. S34:** ¹H (top) and 1 F{H} (bottom) NMR spectra of 1-¹F*NHC-AuNP* after oxygen bubbling confirm the absence of any released species. **Supporting Table S1:** HOMO and LUMO energy level values of a simplified model of 1-MIC, 1-NHC, and 1-¹F*NHC* calculated using PBE0/def2-TZVP method.