# N-Heterocyclic Carbene (NHC) Ligand Exchange of Bis(NHC)-Pd(II) Complexes

Qiaoqiao Teng, Wei Liu, Yaru Zhao, Lina Zhang, Hao Jin, Qi Meng, Gilles Frison

► To cite this version:

Qiaoqiao Teng, Wei Liu, Yaru Zhao, Lina Zhang, Hao Jin, et al. N-Heterocyclic Carbene (NHC) Ligand Exchange of Bis(NHC)-Pd(II) Complexes. Inorganic Chemistry, 2024, 63, pp.21806-21809.
10.1021/acs.inorgchem.4c03652 . hal-04785473

HAL Id: hal-04785473

https://hal.science/hal-04785473v1

Submitted on 15 Nov 2024

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# N-Heterocyclic Carbene (NHC) Ligand Exchange of Bis(NHC)-Pd(II) Complexes

Qiaoqiao Teng,\* Wei Liu, Yaru Zhao, Lina Zhang, Hao Jin, Qi Meng\*, Gilles Frison\*

## ABSTRACT

N-heterocyclic carbene (NHC)-Pd(II) bonds are recognized as rather robust bonds to equip palladium(II) centers with decent stability and stereoelectronic tunability. In this report, the authentic carbene-Pd(II) bonds in N-ammonium- and sulfonate-functionalized NHC complexes were activated, and NHC ligand exchange was triggered to afford four well-defined, sulfobetaine-type, zwitterionic heterobiscarbene complexes. Combined experimental studies and density functional theory calculations indicated that electrostatic interactions play a minimal role in the energy of this reaction; instead, the exchange appears to be driven primarily by the precipitation of the resulting zwitterionic complexes.

N-heterocyclic carbene (NHC) is a prestigious ligand for transition metals due to its strong σ donation to form robust carbene-M bonds.¹ In earlier applications, NHC-M bonds were considered essentially inert, providing metal complexes with enhanced stability and stereoelectronic tunability. However, recent reports have shown that the carbene ligand may be labile under certain conditions.

Öfele and co-workers first reported that irradiation of a [Cr(NHC)(CO)₅] complex led to the formation of a biscarbene chromium complex [Cr(NHC)₂(CO)₄] in boiling THF.² Later, analogous Mo and W complexes were prepared by similar reactions (Figure 1, eq. 1).³ More recently, group
11 metal-NHC complexes have been more intensively investigated for their carbene ligand redistribution (Figure 1, eq. 2), as represented by the works of Lin,⁴ Bertrand,⁵ Albrecht,⁶ Huynh,⁷ Gust,⁸ and others.⁹ Nowadays, Ag-NHC transfer has become the most widely used method to prepare NHC-metal complexes.¹⁰

**Öfele, Liu and others:**

$$ \mathrm{NHC-M-(CO)_5} \rightarrow \mathrm{cis-(NHC)_2M-(CO)_4} + \mathrm{M(CO)_6} \quad \mathrm{M} = \mathrm{Cr, Mo, W} $$

**Lin, Bertrand, Huynh, Albrecht, Gust and others:**

$$ \mathrm{NHC-M-X} \rightarrow \underset{\mathrm{NHC-M}^{\mathrm{+}}\mathrm{NHC}}{\overset{\mathrm{MX}_2^{\mathrm{\ominus}}}{\mathrm{I}}} \quad \mathrm{M} = \mathrm{Ag, Au, Cu} $$

**Herrmann et al:**

$$ \mathrm{NHC-Rh-(CO)_2} \rightarrow \underset{\mathrm{CO}}{\overset{\mathrm{Cl}}{\mathrm{NHC-Rh-NHC}}} + [\mathrm{RhCl(CO)_2}]_2 $$

**Caddick et al:**

$$ \chemfig{**6(=-=-(-N-[90]N)-=-(-Pd-[270]N)-=-(-N-[90]N)-)} + \chemfig{**6(=-=-(-N-[90]N)-=-(-Pd-[270]N)-=-(-N-[90]Ar)-)} \xrightarrow{C_6D_6} 2 \chemfig{**6(=-=-(-N-[90]Ar)-=-(-Pd-[270]N)-=-(-N-[90]Ar)-)} $$

**this work:**

$$ \chemfig{**6(=-=-(-SO_3^-K)^2-)} + \chemfig{**6(=-=-(-Br-Pd-[270]N)-=-(-Br-Pd-[270]N)-)} \xrightarrow{\text{2-ethynyl-1,3-butadiene}} \chemfig{**6(=-=-(-Et_3N)^2-)} + \chemfig{**6(=-=-(-Br)Pd-[270]N-)} $$

<caption>**Figure 1.** Representative carbene ligand redistribution.</caption>
Scattered cases of NHC ligand redistribution were noted for other groups of transition metals. One notable example is the slow redistribution of [RhCl(NHC)(CO)₂] into a Vaska-type *trans*-[RhCl(NHC)₂(CO)] complex, as found by Herrmann et al. (Figure 1, eq. 3).¹¹ For Group 10 metal-NHC complexes, which are known for their extensive catalytic applications, only one example of NHC ligand exchange has been reported, specifically for palladium (Figure 1, eq. 4). This example involved d¹⁰ Pd(0) centers, which are isoelectronic with Au(I), Ag(I) and Cu(I).¹²

Herein, we will unveil the first example of NHC-Pd(II) labilization involving only molecular Pd(II) species (Figure 1, eq. 5).¹³ This was realized by simply mixing *N*-ammonium- and sulfonate-functionalized homobiscarbene complexes in water, leading to the formation of four well-defined, sulfobetaine-type, zwitterionic heterobiscarbene complexes via NHC ligand exchange under mild conditions.

The sulfonate- and ammonium-functionalized biscarbene complexes were synthesized using pre- and post-functionalization methodology, respectively (Scheme S1).¹⁴ Depending on the solubility, the oppositely charged complexes were mixed in various combinations in H₂O or mixed H₂O/DMSO solutions. A rapid precipitation was commonly observed, and the isolated precipitate was identified as the unprecedented sulfobetaine-type, zwitterionic heterobiscarbene complex (Table 1). For instance, since both **1a** and **2a** are highly soluble in water, pure H₂O was used to prepare complex **3**, yielding a satisfactory 85%. In other reactions, DMSO was added to ensure solubility of both substrates, which significantly lowered the yields of complexes **4–6** due to the substantial recovery of the starting materials. The solvent effect was further confirmed by adding DMSO to the reaction of **1a** and **2a**. Indeed, a lower yield of complex **3** was obtained using the H₂O/DMSO mixture, and no conversion was observed at all in pure DMSO. The good solubilities of **3-6** in DMSO were assumed to hamper the ligand-exchange reactions.
<figure><img src="image_2.png" /><figcaption>**Table 1.** NHC ligand exchange to form zwitterionic, heterobiscarbene-Pd(II) complexes.<sup>*a*</sup></figcaption></figure>

<sup>*a*</sup>1 (0.08 mmol), 2 (0.08 mmol). <sup>*b*</sup>H<sub>2</sub>O (10 mL). <sup>*c*</sup>H<sub>2</sub>O (5 mL) + DMSO (5 mL). <sup>*d*</sup>DMSO (10 mL). <sup>*e*</sup>50 °C.

The heterobiscarbene complexes are hydrophobic and more soluble in organic solvents. Molar conductance measurements revealed a significant decrease in the ionic strength (Table S1). In the <sup>1</sup>H NMR spectra of the homobiscarbene complexes, two sets of signals with varying ratios corresponding to *trans*-anti and *trans*-syn rotamers were observed.<sup>15</sup> Notably, the ammonium-tethered complexes favored *anti* rotation, whereas the dianionic complexes displayed a preference
for the *syn* rotamer. In sharp contrast, only one set of signals, assignable to the *trans*-syn rotamers, was observed for the hetero-biscarbene complexes. The distinct differences between the resonances of α-CH₂ moieties in the NEt₃⁺ and SO₃⁻ functional groups allowed for an unambiguous identification of both ligands in a 1:1 integration ratio. Additionally, two carbene resonances were recorded at ~168 or (and) ~182 ppm, confirming the presence of both types of ligands.

<figure><img src="image_3.png" /></figure>

**Figure 2.** Molecular structure of **1a**, **2a** and **6** showing 50% probability ellipsoids. Hydrogen atoms and most of the counterions are omitted for clarity. Selected bond lengths (Å) and bond angles (deg) are as follows: **1a**: Pd1–C4 2.022(7), Pd1–C26 2.020(7), Pd1–Br1 2.4359(9), Pd1–Br2 2.4361(9); C4–Pd1–Br1 91.0(2), C4–Pd1–Br2 89.2(2), C26–Pd1–Br1 89.7(2), C26–Pd1–Br2 90.1(2). PdC₂Br₂/NHC (left and right) dihedral angle: 79.0° and 75.9°. **2a**: Pd1–C10 2.016(5), Pd1–Br2 2.4193(6); C10–Pd1–Br2 90.56(15), C10–Pd1–Br2A 89.44(15). PdC₂Br₂/NHC dihedral angle: 72.9°. **6**: Pd1–C28 2.020(9), Pd1–C7 2.034(9), Pd1–Br1 2.4341(14), Pd1–Br2 2.4335(14); C28–Pd1–Br1 89.1(3), C28–Pd1–Br2 90.3(3), C7–Pd1–Br1 87.2(3), C7–Pd1–Br2 93.4(3). PdC₂Br₂/NHC (left and right) dihedral angle: 77.1° and 75.2°.
Single crystals of complexes **1a**, **2a** and **6** were analyzed using X-ray diffraction (Figure 2). The rotameric preferences in the solid states are in line with those found in solution. The *anti* preference of the two ammonium moieties can be attributed to the static electron repulsion. This preference is, however, compensated for by the chelation of a K⁺ counterion between the two sulfonate groups in the dianionic complex. To hamper such chelation, 18-crown-6 ether was added to an NMR solution of **1a**. As a result, the *trans-anti* rotamer became the dominant conformation (Table S1). In the heterobiscarbene complex **6**, the two NHC ligands are favorably *trans-syn* oriented, which is attributed to the electrostatic attraction between the oppositely charged functional groups.

To gain better insight into the driving force behind this ligand-exchange reaction, computational studies were carried out at the density functional theory (DFT) level (Table 2). For a simplified model without benzyl substituents and counterions, the reaction showed a zero free energy change (entry 1). Modeling the complete system with benzyl groups (entry 4) resulted in a slight endergonicity (+2.2 kJ/mol), which increased further when counterion effects were included. The first potassium cation (K⁺) is strongly bound to **A1** (31.9 kJ/mol, entries 2 vs 1) and **A3** (38.9 kJ/mol, entries 5 vs 4), leading respectively to the formation of **A2** and **A4**, which feature a *syn* conformation where the two anionic arms enclose the cation, consistent with the crystallographic structure. Conversely, the second K⁺ cation and the two Br⁻ counteranions are weakly bound to their respective charged NHC₂Br₂Pd complexes, with their dissociation being only slightly exergonic by 2 kJ/mol (entries 5 vs 6). This result is consistent with the crystallographic structures, where these counterions appear to be weakly bound. The formation of KBr is exergonic by 9.3 kJ/mol (entries 6 vs 7). Overall, the dissociation of the counterions and the formation of KBr are
unfavorable by +22.3 kJ/mol, leading to an endergonic reaction of +24.6 kJ/mol (entry 7). In contrast, the ligand-exchange reaction with a nonionic biscarbene complex (entry 8) is slightly exergonic. Thus, electrostatic interactions do not appear to be the main driving force of the reaction.¹⁶

<figure><img src="image_4.png" /><figcaption>**Table 2.** DFT evaluation of NHC ligand-exchange reactions.</figcaption></figure>

<table><thead><tr><th>Entry</th><th>A</th><th>B</th><th>C</th><th>R<sub>1</sub></th><th>R</th><th>m</th><th>n</th><th>o</th><th>p</th><th>q</th><th>ΔG<sub>r</sub><sup>a</sup></th></tr></thead><tbody><tr><td>1</td><td>A1</td><td>B1</td><td>C1</td><td>SO<sub>3</sub><sup>-</sup></td><td>H</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>-0.0</td></tr><tr><td>2</td><td>A2</td><td>B1</td><td>C1</td><td>SO<sub>3</sub><sup>-</sup></td><td>H</td><td>1</td><td>0</td><td>0</td><td>1</td><td>0</td><td>+31.9</td></tr><tr><td>3</td><td>A2</td><td>B2</td><td>C1</td><td>SO<sub>3</sub><sup>-</sup></td><td>H</td><td>1</td><td>1</td><td>1</td><td>0</td><td>0</td><td>+15.5</td></tr><tr><td>4</td><td>A3</td><td>B3</td><td>C2</td><td>SO<sub>3</sub><sup>-</sup></td><td>Ph</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>+2.2</td></tr><tr><td>5</td><td>A4</td><td>B3</td><td>C2</td><td>SO<sub>3</sub><sup>-</sup></td><td>Ph</td><td>1</td><td>0</td><td>0</td><td>1</td><td>0</td><td>+41.1</td></tr><tr><td>6</td><td>A5</td><td>B4</td><td>C2</td><td>SO<sub>3</sub><sup>-</sup></td><td>Ph</td><td>2</td><td>2</td><td>0</td><td>2</td><td>2</td><td>+43.1</td></tr><tr><td>7</td><td>A5</td><td>B4</td><td>C2</td><td>SO<sub>3</sub><sup>-</sup></td><td>Ph</td><td>2</td><td>2</td><td>2</td><td>0</td><td>0</td><td>+24.6</td></tr><tr><td>8</td><td>A6</td><td>B1</td><td>C3</td><td>CH<sub>3</sub></td><td>H</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>-5.7</td></tr></tbody></table>

<footnote><sup>a</sup>Gibbs free energy of reaction, ΔG<sub>r</sub>, in kJ/mol was computed at the IEFPCM(H<sub>2</sub>O)-M06/def2-TZVPP//IEFPCM(H<sub>2</sub>O)-M06/def2-SVP level.</footnote>
In conclusion, we have disclosed the first example of carbene ligand exchange between NHC-Pd(II) complexes at the molecular level, resulting in the formation of four well-defined sulfobetaine-type, zwitterionic heterobiscarbene-Pd(II) complexes. DFT calculations indicated that electrostatic interactions play a minimal role in the energy of this reaction; instead, the exchange appears to be driven primarily by the prompt precipitation of the resulting zwitterionic complexes, as disclosed by the significant solvent effects on the ligand-exchange experiments. Further studies are needed to fully elucidate the mechanism of this ligand-exchange reaction. Moreover, investigations of heterometallic carbene ligand exchange and applications of such ligand exchange in catalysis and medicine are underway in our laboratory.

## ASSOCIATED CONTENT

### Supporting Information

Preparation scheme for the homobiscarbene complexes, Table of key characterization data of prepared complexes, Experimental section, DFT calculations data, NMR spectra and X-ray crystallographic data (PDF), Cartesian coordinates (xyz)

## AUTHOR INFORMATION

### Corresponding Author

Qiaoqiao Teng – Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, 213164, China; Email: tqq@cczu.edu.cn
Qi Meng – Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, 213164, China; Email: mengqi@cczu.edu.cn

Gilles Frison – Sorbonne Université, CNRS, Laboratoire de Chimie Théorique, LCT, F-75005 Paris, France; Email: gilles.frison@cnrs.fr

## Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## Notes

The authors declare no competing interests.

## ACKNOWLEDGMENT

The authors thank the National Natural Science Foundation of China (22001022) for financial support and Analysis and Testing Centre, NERC Biomass of Changzhou University for technical help.

## REFERENCES

1 (a) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. An overview of N-heterocyclic carbenes. *Nature* **2014**, *510*, 485–496. (b) Huynh, H. V. Electronic Properties of N-heterocyclic carbenes and their experimental determination. *Chem. Rev.* **2018**, *118*, 9457–9492.
2 Öfele, K.; Herberhold, M. Tetracarbonyl bis(1,3-dimethyl-4-imidazolin-2-ylidene) chromium(0). *Angew. Chem., Int. Ed. Engl.* **1970**, *9*, 739–740.

3 Ku, R.-Z.; Huang, J.-C.; Cho, J.-Y.; Kiang, F.-M.; Reddy, K. R.; Chen, Y.-C.; Lee, K.-J.; Lee, J.-H.; Lee, G.-H.; Peng, S.-M.; Liu, S.-T. Metal ion mediated transfer and cleavage of diaminocarbene ligands. *Organometallics* **1999**, *18*, 2145–2154.

4 Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Coinage metal-N-heterocyclic carbene complexes. *Chem. Rev.* **2009**, *109*, 3561–3598.

5 Frey, G. D.; Dewhurst, R. D.; Kousar, S.; Donnadieu, B.; Bertrand, G. Cyclic (alkyl)(amino)carbene gold(I) complexes: A synthetic and structural investigation. *J. Organomet. Chem.* **2008**, *693*, 1674–1682.

6 Canseco-Gonzalez, D.; Petronilho, A.; Mueller-Bunz, H.; Ohmatsu, K.; Ooi, T.; Albrecht, M. Carbene transfer from triazolylidene gold complexes as a potent strategy for inducing high catalytic activity. *J. Am. Chem. Soc.* **2013**, *135*, 13193–13203.

7 (a) Sivaram, H.; Jothibasu, R.; Huynh, H. V. Gold complexes of an alicyclic indazole-derived N-heterocyclic carbene: syntheses, characterizations, and ligand disproportionation. *Organometallics* **2012**, *31*, 1195–1203. (b) Guo, S.; Sivaram, H.; Yuan, D.; Huynh, H. V. Gold and palladium hetero-bis-NHC complexes: characterizations, correlations, and ligand redistributions. *Organometallics* **2013**, *32*, 3685–3696. (c) Guo, S.; Bernhammer, J. C.; Huynh, H. V. 1,2,4-Triazole-derived carbene complexes of gold: characterization, solid-state aggregation and ligand disproportionation. *Dalton Trans.* **2015**, *44*, 15157–15165.
8 (a) Goetzfried, S. K.; Gallati, C. M.; Cziferszky, M.; Talmazan, R. A.; Wurst, K.; Liedl, K. R.; Podewitz, M.; Gust, R. N-Heterocyclic carbene gold(I) complexes: mechanism of the ligand scrambling reaction and their oxidation to gold(III) in aqueous solutions. *Inorg. Chem.* **2020**, *59*, 15312–15323. (b) Goetzfried, S. K.; Koenig, S. M. C.; Gallati, C. M.; Gust, R. Internal and external influences on stability and ligand exchange reactions in bromido[3-ethyl-4-aryl-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) complexes. *Inorg. Chem.* **2021**, *60*, 8546–8553.

9 (a) Furst, M. R. L.; Cazin, C. S. J. Copper N-heterocyclic carbene (NHC) complexes as carbene transfer reagents. *Chem. Commun.* **2010**, *46*, 6924–6925. (b) Su, H.-L.; Pérez, L. M.; Lee, S.-J.; Reibenspies, J. H.; Bazzi, H. S.; Bergbreiter, D. E. Studies of ligand exchange in N-heterocyclic carbene silver(I) complexes. *Organometallics* **2012**, *31*, 4063–4071. (c) Dinda, J.; Adhikary, S. D.; Sethb, S. K.; Mahapatra, A. Carbazole functionalized luminescent silver(I), gold(I) and gold(III)-N-heterocyclic carbene complexes: a new synthetic disproportionation approach towards Au(I)-NHC to provide Au(III)-NHC. *New J. Chem.* **2013**, *37*, 431–438. (d) Nahra, F.; Gómez-Herrera, A.; Cazin, C. S. J. Copper(I)-NHC complexes as NHC transfer agents. *Dalton Trans.* **2017**, *46*, 628–631. (e) Chen, Q.; Liu, Q.; Xiao, J.; Leng, X.; Deng, L. Catalytic method for the synthesis of deuterium-labeled N-heterocyclic carbenes enabled by a coordinatively unsaturated Ruthenium N-heterocyclic carbene catalyst. *J. Am. Chem. Soc.* **2021**, *143*, 19956–19965.

10 Wang, H. M. J.; Lin, I. J. B. Facile synthesis of silver(I)-carbene complexes. Useful carbene transfer agents. *Organometallics* **1998**, *17*, 972–975.
11 Bittermann, A.; Herdtweck, E.; Härter, P.; Herrmann, W. A. Rhodium(I), a carbene-transfer transition-metal ion and a synthetic route to symmetrical and asymmetrical substituted trans-RhCl(CO)(NHC)(NHC) complexes. *Organometallics* **2009**, *28*, 6963–6968.

12 Titcomb, L. R.; Caddick, S.; Geoffrey, F.; Cloke, N.; Wilson, D. J.; McKerrecher, D. Unexpected reactivity of two-coordinate palladium-carbene complexes; synthetic and catalytic implications. *Chem. Commun.* **2001**, 1388–1389.

13 One might argue that NHC-Pd(II) is not novel because NHC-stabilized nanoparticles have often been identified as the actual active species in catalysis involving NHC-Pd(II) complex. It is noteworthy that their formation is typically associated with Pd(II)-Pd(0) reduction, a process that commonly requires elevated temperatures or the addition of extra reducing agents. Selected examples: (a) Canseco-Gonzalez, D.; Gniewek, A.; Szulmanowicz, M.; Müller-Bunz, H.; Trzeciak, A. M.; Albrecht, M. PEPPSI-type palladium complexes containing basic 1,2,3-triazolylidene ligands and their role in Suzuki–Miyaura catalysis. *Chem. Eur. J.* **2012**, *18*, 6055–6062. (b) Kostyukovich, A. Y.; Tsedilin, A. M.; Sushchenko, E. D.; Eremin, D. B.; Kashin, A. S.; Topchiy, M. A.; Asachenko, a. f.; Nechaev, M. S.; Ananikov, V. P. In situ transformations of Pd/NHC complexes with Nheterocyclic carbene ligands of different nature into colloidal Pd nanoparticles. *Inorg. Chem. Front.* **2019**, *6*, 482–492. (c) Ovezova, M.; Eroglu, Z.; Metin, O.; Çetinkaya, B.; Gülcemal, S. Unveiling the catalytic nature of palladium-N-heterocyclic carbene catalysts in the α-alkylation of ketones with primary alcohols. *Dalton Trans.* **2021**, *50*, 10896–10908. (d) Denisova, E. A.; Kostyukovich, A. Y.; Fakhrutdinov, A. N.; Korabelnikova, V. A.; Galushko, A. S.; Ananikov, V. P. Hidden” nanoscale catalysis in alkyne hydrogenation with well-defined molecular Pd/NHC complexes. *ACS Catal.* **2022**, *12*, 6980–6996.
14 Nguyen, V. H.; Ibrahim, M. B.; Mansour, W. W.; Ali, B. M. E.; Huynh, H. V. Postmodification approach to charge-tagged 1,2,4-triazole-derived NHC palladium(II) complexes and their applications. *Organometallics* **2017**, *36*, 2345–2353.

15 Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A. 13C NMR Spectroscopic Determination of Ligand Donor Strengths Using N-Heterocyclic Carbene Complexes of Palladium(II). *Organometallics* **2009**, *28*, 5395–5404.

16 To better understand the limited influence of electrostatic interactions, the interaction energy between the arms of complexes **A3**, **B3**, and **C2** were estimated in the supporting information.
<figure><img src="image_5.png" /></figure>

NHC ligands of *N*-ammonium and sulfonate functionalized homobiscarbene-Pd(II) complexes were exchanged to afford well-defined, sulfobetaine-type, zwitterionic heterobiscarbene-Pd(II) complexes.