# Formation Of Gold-Astatine Bonds in N-Heterocyclic Carbene Complexes

Mathilde Ligeour, Sébastien G Guillet, Clémence Maingueneau, Mikaël Croyal, Aurélien Planchat, Jean-François Gestin, Nicolas Galland, François Guérard

► To cite this version:

Mathilde Ligeour, Sébastien G Guillet, Clémence Maingueneau, Mikaël Croyal, Aurélien Planchat, et al.. Formation Of Gold-Astatine Bonds in N-Heterocyclic Carbene Complexes. Chemistry - A European Journal, 2025, Online ahead of print. 10.1002/chem.202500826 . inserm-05063252

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

Distributed under a Creative Commons Attribution 4.0 International License
# FORMATION OF GOLD-ASTATINE BONDS IN N-HETEROCYCLIC CARBENE COMPLEXES

Mathilde Ligeour,[a] Sébastien G. Guillet,[a] Clémence Maingueneau,[a] Mikaël Croyal,[b] Aurélien Planchat,[c] Jean-François Gestin,[a] Nicolas Galland*[c] and François Guérard*[a]

[a] Dr. Mathilde Ligeour, Dr. Sébastien G. Guillet, Dr. Clémence Maingueneau, Dr. Jean-François Gestin, and Dr. François Guérard
CRCI2NA
Nantes Université, Inserm, CNRS, Université d'Angers
8 quai Moncousu, 44007 Nantes Cedex 1- FRANCE
E-mail: Francois.Guerard@univ-nantes.fr

[b] Dr. Mikaël Croyal

Nantes Université, CHU Nantes, Inserm, CNRS, SFR Santé,
Inserm UMS 016, CNRS UMS 3556, F-44000 Nantes, France

[c] Aurélien Planchat, Dr. Nicolas Galland
CEISAM
CNRS, Nantes Université, UMR 6230,
44000 Nantes, France
E-mail: nicolas.galland@univ-nantes.fr

Supporting information for this article is given via a link at the end of the document.

**Abstract:** With the purpose of exploring alternatives to the poorly stable astatoaryl bond found in most ²¹¹At-labeled radiopharmaceuticals, the coordination between astatine and metal-NHC (NHC = N-heterocyclic carbene) complexes was investigated. To orient this work, relativistic DFT calculations were performed, highlighting a superiority of gold(I) over rhodium(I) and iridium(I), both regarding the strength of the metal-astatine bond and the metal's preference for astatine over other halogens. The DFT calculations also revealed a more stable metal-astatine bond with electron withdrawing NHCs. Accordingly, two Au(I) complexes, [AuCl(IPr)] (IPr = N,N'-bis[2,6-(di-isopropyl)phenyl]imidazol-2-ylidene) and [AuCl(IAd)] (IAd = N,N'-bis[adamantyl]imidazol-2-ylidene) were radiolabelled by halide exchange in excellent radiochemical yields with iodine-125 and astatine-211 in a water/acetonitrile mixture with a reducing agent. While the adamantyl substituted complex was not investigated further due to its lack of solubility in aqueous or semi aqueous media, the stability of [Au²¹¹At(IPr)] in presence of phosphate buffered saline was evaluated and proved excellent (96% after two half-lives) encouraging further studies with relevant cancer targeting vectors.

## Introduction

Over the past decade, the study of astatine chemistry has seen an increasing attention due to the high potential of one of its isotopes, ²¹¹At, for targeted α therapy (TAT) of cancers.¹,² The high energy of the α particles emitted upon decay (5.9-7.4 MeV), their short track (80-100 µm) and the intermediate half-life of the radionuclide (t₁/₂ = 7.2 h) make ²¹¹At very appropriate for the treatment of small cell clusters or isolated cells (micro

metastases, residual disease and haematological cancers, while β emitters such as ¹³¹I and ¹⁷⁷Lu with their longer track are more appropriate to larger cell clusters and tumours with the advantage of distant irradiation of non-targeted cells).³ However, one of the main challenges for this radionuclide success in nuclear medicine resides in the production of stable ²¹¹At-labelled radiopharmaceuticals. Inspired by lighter halogens chemistry, the most common labelling approach currently relies on astatoaryl compounds (Fig. 1A).⁴ The latter have however often shown a lack of *in vivo* stability, leading to release of the radionuclide from its targeting vector and subsequent unwanted irradiation of non-targeted organs.⁵,⁶ Recent studies have reported the possibility to produce more stable scaffolds based on C-At bonding.⁷,⁸ Other strategies rely on exploring alternatives to the C-At chemistry. For instance, B-At bonds in boron clusters have shown improved stability,⁹ while the coordination of astatide as a ligand to a metal centre such as Rh(III) and Ir(III) in complexes also proved promising.¹⁰

<figure><img src="image_2.png" /><figcaption>**Figure 1.** A) Typical ²¹¹At-labelled compound based on C-At chemistry. B) General structure of compounds reported in this work.</figcaption></figure>

Despite a few proofs of concept, the idea of a metal-astatine bond as a radiolabelling strategy remains widely
unexplored, while the large number of metals and ligands available offers an important width of perspectives. Astatide being a large and highly polarizable anion, HSAB theory predicts an optimal binding with soft metallic metal centres. Therefore, a particular interest lies with heavy late transition metals with a known +I oxidation state, such as rhodium (which was already reported as a good candidate),¹¹ iridium or gold.

Amongst all the reported ligands, N-heterocyclic carbenes (NHC) were selected for this study for multiple reasons. Metal-NHC complexes are straightforward to access *via* user friendly routes,¹² and these ligands are now well known in organometallic chemistry for their easy steric¹³ and electronic¹⁴ tunability, with known methods of characterization, which would allow for a fine study and optimization of the metal-astatine bond. In addition, NHC metal complexes are increasingly studied as anti-infective and anti-cancerous agents.¹⁵ Eventually, the chloride ligand present in many of the common metal-NHC complexes can act as a leaving group for introduction of ²¹¹At by halide exchange.

Herein are presented the investigations on the preparation of ²¹¹At-metal-NHC complexes (Figure 1B) and the impact of the metal and NHC N-substituents. Radiolabelling and stability results obtained both experimentally and computationally are discussed in view of developing novel approaches for the production of ²¹¹At-radiopharmaceuticals.

## Results and Discussion

### DFT calculations

With radioelements such as atstatine, for which the usual spectroscopic tools are not applicable, the scarce experimental data can be advantageously supplemented by the results of quantum mechanical calculations.¹⁶ The latter are often necessary for interpreting observations,¹⁷,¹⁸ or even anticipating the outcome of experiments.¹⁹ Since At is also a

heavy element (Z = 85), and following previous successful works,¹⁶⁻¹⁹ the density-functional theory (DFT) in a two-component relativistic formalism was used. According to a former benchmark study focusing on At-species,²⁰ the PBE0 hybrid functionals clearly appeared to be one of the very best among 36 DFT methods tested. For this study, it was used with triple zeta quality basis sets (TZVP) to calculate accurate energies. As some of the chloro and iodo NHC-complexes (1a-b and 7a-b in Fig. 2) have been crystallized and characterized by X-ray diffraction (Table S1),²¹ the precision of our computational methodology can be assessed on key geometrical parameters. The mean absolute error (MAE) was determined to be 1.2% on metal-halogen distances and 1.7% on carbon-metal-halogen valence angles (Table S2). It is worth noting that on precisely computed structures of At-species,²⁰ the PBE0 functional has led to a MAE of 1.1%.

Hence, DFT calculations were used to help the design of a metal complex that would efficiently bind atstatine. The influence of both the chosen metal and NHC were probed, combining three metals (rhodium, iridium and gold), with five different NHCs differing by their electron density and steric hindrance (Fig. 2). As a first characterization of stability, the intrinsic strength of the M-At bond was quantified. Two-component relativistic DFT calculations have notably been shown to be accurate for evaluating the dissociation energies of bonds formed by atstatine.²² Fig. 3A displays the bond dissociation energies (BDE) calculated for the heterolytic dissociation of the M-At bond in various NHC complexes (cf. reaction S1 and related details in SI). First of all, the evolution of the BDE as a function of the different metals follows a very similar trend, whatever the NHCs, and the BDE is notably higher with *p*-nitrophenyl substituents (poorly electron donating). But the most noticeable fact is that the Au-At bond appears systematically stronger than the ones with group IX metals: BDE values are between 16 and 21% higher compared to Ir-At bonds, and between 23 and 30% higher compared to Rh-At bonds.

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

**1a-d:** M = Au, L = none
X = Cl, I, ¹²⁵I, ²¹¹At

**3a-b:** M = Ir, L = cod
X = Cl, I

**4a-b:** M = Rh, L = cod
X = Cl, I

<figure><img src="image_4.png" /></figure>

**2a:** X = Cl

**2b:** X = I

**2c:** X = ¹²⁵I

**2d:** X = ²¹¹At

<figure><img src="image_5.png" /></figure>

**5a-b:** M = Au, L = none
X = Cl, I

**6a-b:** M = Ir, L = cod
X = Cl, I

**7a-b:** M = Rh, L = cod
X = Cl, I

<figure><img src="image_6.png" /></figure>

**8:** M = Au, L = none

**9:** M = Ir, L = cod

**10:** M = Rh, L = cod

<figure><img src="image_7.png" /></figure>

**11:** M = Au, L = none

**12:** M = Ir, L = cod

**13:** M = Rh, L = cod

<caption>**Figure 2.** Structures of the studied metal-NHC complexes. cod = cyclooctadiene.</caption>
<figure><img src="image_8.png" /><figcaption>**Figure 3.** A) Computed BDE of M-At bonds as a function of the different metals and NHCs. B) Reaction free energy of the halide exchange when chloride substitutes for astatide.</figcaption></figure>

The trends shown by the BDE values remain relevant for the experimental study in solution, according to the calculations of the Gibbs free energies of reaction (see Fig. S4). In aqueous phase, the heterolytic dissociation of the Au-At bond is about 107 and 120 kJ/mol more difficult than those of the Ir-At and Rh-At bonds, respectively.

However, the design of a metal complex that efficiently binds astatine cannot rely solely on maximizing the M-At bond strength. The metal's selectivity towards competitive ligands must also be taken into account. Indeed, the high concentration of chloride encountered *in vivo* can lead to astatine displacement, releasing [²¹¹At]astatide with subsequent irradiation of healthy tissues. Using the implicit CPCM solvent model, the halide exchange in aqueous phase was studied, where chloride replaces astatide. Fig. 3B shows the free energy of reaction ΔG₂₉₈' calculated for the different metals and NHCs. Similar to what was observed with BDE, and independently of the NHC, a clear superiority of gold versus rhodium and iridium appeared, the latest being the least promising of all three. The highest and systematically positive values of ΔG₂₉₈' for Au(I) complexes unambiguously demonstrate an unfavourable exchange towards chloride, and thus, a greater selectivity for astatide. Au(I) also seems to limit the impact of the NHC *N*-substituents, compared to group IX metals. Moreover, while IPr proved to be the worst of all five NHCs when combined with rhodium and iridium, [AuAt(IPr)] **1d** appeared to be a near optimal choice regarding halogen exchange stability.

Mass spectrometry

Starting with IPr as a model ligand for the experimental study, [AuCl(IPr)] **1a**, [IrCl(cod)(IPr)] **3a** and [RhCl(cod)(IPr)] **4a** were prepared using the weak base route,¹² and their iodinated analogues **1b**, **3b** and **4b** were synthesized by treatment with potassium iodide, to be used as standard for comparison with retention indexes of astatinated complexes in HPLC and TLC in the absence of stable At isotopes. Interestingly, when running routine electrospray ionization mass spectrometry (ESI-MS) analyses of the synthesized compounds, a different behaviour for the Rh(I) and Ir(I) complexes was noticed, compared to the Au(I) ones. While the Au(I) chloro and iodo complexes **1a** and **1b** were mainly observed as [M+Na]⁺ adducts (Fig. 4A), it is the fragment issued by the halide abstraction ([M-Cl]⁺ or [M-I]⁺) that was observed with Ir(I) (**3a**, **3b**) and Rh(I) (**4a**, **4b**) compounds, and this, even when decreasing voltage from 60 to 10 V (Fig. 4B and C).

This highlights the possibility, after the addition of Na⁺ to the metal complex during the ionization, for the gas phase adduct to dissociate and release a neutral NaX fragment. It depends on how much energy of [M+Na]⁺ was lost in a radiative or non-radiative way, and on the free energy of the M-X bond dissociation (cf. reaction S2). The corresponding values obtained by DFT calculations (Table S3) indicate that such dissociation is unfavorable only for M = Au (ΔG₂₉₈° is positive). Au(I) complexes show here again their superior stability.
<figure><img src="image_9.png" /><figcaption>**Figure 4.** ESI-MS analyses of complexes **1b** (A, ΔG<sub>298</sub>° = 31.1 kJ/mol), **3b** (B, ΔG<sub>298</sub>° = -111.0 kJ/mol) and **4b** (C, ΔG<sub>298</sub>° = -139.1 kJ/mol). While the [M+Na]<sup>+</sup> fragment is clearly visible in A), only the [M-I]<sup>+</sup> ones can be observed in B) and C), despite the lower voltage. Analyses for complexes **1a**, **3a** and **4a** can be found in the supporting information (Fig. S1-S3).</figcaption></figure>

Radiolabelling

Radiolabelling experiments were first performed with ¹²⁵I, which is more convenient for setting up the radioanalytical procedure due to its long t₁/₂ (59.9 days). Iodide is expected to react and behave similarly to astatide in nucleophilic processes. The halide exchange was performed using a similar procedure reported previously by Rajerison et al. for the radiolabelling of a chloro Rh(I) -NHC complex.¹¹ Complex **1a** was chosen as a first model, and led to 97 ± 3 % radiochemical yield (RCY) of **1c** after heating with [¹²⁵I]NaI at 80°C for 30 min in acetonitrile (Scheme 1). The formation of **1c** was identified by the nearly identical retention time in HPLC than the non-radioactive iodinated reference **1b** (Fig. S5 and S9).

These conditions were then adapted to <sup>211</sup>At-labelling. While <sup>125</sup>I is available commercially as sodium iodide in aqueous NaOH solution, <sup>211</sup>At was delivered by the Arronax cyclotron in chloroform with a suspected, but not clearly identified, zero oxidation state that requires a further reduction prior to use.<sup>23</sup> Sodium sulfite, a reducing agent frequently used for generating astatide in nucleophilic reactions,<sup>24,25</sup> unexpectedly provided a low RCY (28±5%). Using NaOH as reducing agent produced some high RCYs at room temperature (rt), however, reproducibility was low with RCY varying from 36 to 91%. The varying quality of distilled At solutions may explain this lack of reproducibility. Heating during reaction appeared to be a more robust approach to produce the required nucleophilic astatine species in NaOH, with more consistent RCY (84±21%) (Fig. S10). PPh<sub>3</sub>, that was recently reported as efficient in iodonium ylide astatination,<sup>26</sup> was also evaluated and provided a RCY of 70%.

Improved results were obtained with sulfhydryl based
reducing agents. L-cysteine, previously reported for labelling a
Rh(I) complex with good RCY,<sup>10</sup> yielded the desired
astatinated complex **1d** with excellent RCY (96±1%) (Fig.
S11).<sup>10</sup> A degradation of the precursor was observed, which
may be explained by the affinity of sulfhydryl for gold, leading
to a side reaction of exchange with the chloride ligand.

<figure><img src="image_10.png" /><figcaption>**Scheme 1.** Preparation of complexes **1c** and **1d** from precursor **1a**.
Labelling condition: **1a** (0.86 µmol) in 40 µL CH<sub>3</sub>CN and 18 µL of [¹²⁵I]NaI (0.2-2MBq) or ²¹¹At (0.5-2 MBq) in NaOH (0.1 M) or cysteine (5 g/L) solution.
RCY determined by HPLC/TLC analysis of crude product (average of n≥3 replicates).</figcaption></figure>

It is noteworthy that the same reaction conditions applied
to the iodinated precursor **1b** led to a significantly lower RCY
of 26±6% at 80 °C when using NaOH, which increased to
66±8% when increasing temperature to 100°C (Fig. S12), and
67±15% when using cysteine as reducing agent (Fig. S13).
This is in line with DFT calculations. The halide exchange from
chlorinated precursor **1a** to **1d** is rather favourable, with a value
of ΔG<sub>298</sub>* = -9.2 kJ/mol (Fig. 3B), while the one obtained for the
labelling of iodinated precursor **1b** to **1d** (-1.7 kJ/mol) highlights
a much less favourable halide exchange.

To examine an alternative to the aryl substituted NHC
above, the possibility to radiolabel adamantyl substituted **2a**
was investigated. Unfortunately, the ¹²⁵I-labelling of adamantyl
precursor **2a** failed due to its low solubility in CH₃CN, resulting
in 7 ± 7% RCY of **2c**. Consequently, its good solubility in
CH₂Cl₂ was capitalized on by using a biphasic system. With
tetrabutylammonium hydroxide (TBAOH) as phase transfer
agent, a satisfying RCY of 86 ± 7% was obtained in 30 min, and increased to 98 ± 2% after 1h at room temperature (Scheme 2 and Fig. S14). Similar to the ²¹¹At-labelling of **1a**, a comparison of reducing agents for the labelling of **2a** with ²¹¹At was performed and highlighted a superiority of sodium metabisulfite in the production of **2d**, for a RCY of 80 ± 5% after 1h at room temperature (Table S4 and Fig. S15).

<figure><img src="image_11.png" /><figcaption>**Scheme 2.** Radiolabelling of complex **2a** to form compounds **2c** and **2d**. Labelling condition: **2a** (0.86 µmol) in 40 µL CH₂Cl₂, TBAOH (1.0M in MeOH) 8 µL and 18 µL of [¹²⁵I]Nal solution (0.2-2MBq) or ²¹¹At in Na₂S₂O₅ (10 mg/mL) solution (0.5-2 MBq). RCY were determined by HPLC/TLC of crude product (n≥3).</figcaption></figure>

Stability assays

Having obtained Au(I) complexes radiolabelled with ¹²⁵I and ²¹¹At with high efficiencies, a stability assay was performed in presence of phosphate-buffered saline (PBS). This solution mimics pH and chloride concentration of blood, Cl ions being potential competitors that could release the astatide ligand from the complexes by halide exchange. The IPr compounds **1c** and **1d** were selected, since adamantyl analogues **2c** and **2d** lacked solubility even at trace concentration for these assays performed in semi aqueous media. Both ¹²⁵I- and ²¹¹At-labelling complexes showed persistence in this solution for at least two ²¹¹At half-lives with a radiochemical purity superior to 99% for **1c** and 96% for **1d** after 14h (Fig. 5). Early time points showed a higher ratio of released astatine, the reason of which was not elucidated, but if slightly degraded, the reformation of the ²¹¹At-complex over time in these physiological conditions highlight the high affinity of Au(I) for astatide over chloride and the potential of this approach for further investigation for in vivo use.

<figure><img src="image_12.png" /><figcaption>**Figure 5.** Stability of **1c** and **1d** in presence of PBS over time (n= 3). % of intact complex determined by analysis of an aliquot of the medium by radio-HPLC.</figcaption></figure>

Conclusion

Labelling of metal-NHC complexes by halogen exchange appears as a new potential strategy to access stable ²¹¹At-labelling structures. DFT calculations and experimental results suggest a superiority of Au(I) to reach the most stable metal-astatine bonds. Chemisorption of astatine at the surface of gold nanoparticles was previously investigated,²⁷ evidencing a high affinity between both elements. To our knowledge this work represents the first example of preparation of monogold astatine compounds. The low release of At from complex **1d** in presence of PBS, highlighting a resistance against competitive chloride ion and physiological pH, warrants further investigations of this approach. Accordingly, the evaluation of the nature of the NHC ligands with improved aqueous solubility, and additional transition metals to optimize stability as well as the development of functionalized analogues for bioconjugation to biological vectors will be reported in due time.

Methods and instruments

Commercial Reagents and solvents were purchased from SigmaAldrich, Fisher Scientific, TCI, Strem or Fluorochem. Deuterated solvents were provided from Sigma Aldrich. [AuCl(IAd)] **2a** was obtained commercially from Strem and [Aul(IPr)] **1b** was obtained following the procedure by T. J. Robilotto *et al.*²⁸ Precursor 1-benzyl-3-(4-nitrophenyl)imidazolium bromide for **5a** synthesis was obtained following the procedure by H. Rajerison *et al.*¹¹

[¹²⁵I]Nal was obtained commercially from Perkin Elmer in 10⁻⁵ M NaOH solution with a volumic activity of 100 µCi/µL. Before use, the solution was diluted in trace select grade water from Fisher Scientifics. ²¹¹At was produced at the Arronax cyclotron facility using the ²⁰⁹Bi⁻ (α,2n)²¹¹At reaction and recovered in chloroform solution using a dry distillation protocol adapted from the procedure described by Lindegren *et al*,²⁹ and was dried under a gentle stream of nitrogen before reduction with the appropriate agent. ²¹¹At-labelling experiments were
performed within the 7 hours following distillation. Activities involved for ¹²⁵I and ²¹¹At radiolabelling were respectively in the 0.2-4 MBq and 0.5-2 MBq range. All reactions were monitored both by radio-HPLC and radio-TLC.

A Puriflash 600 (Interchim) with 30 µm silica pre-packed columns was used to purify the compounds.

NMR analysis were performed with a Bruker AC spectrometer at 400 MHz (¹H) or 100 MHz (¹³C). Chemical shifts (δ) are expressed in part per million (ppm) in accordance to deuterated solvents CDCl₃ (7.26 (¹H), 77.16 (¹³C)) or CD₃CN (1.94 (¹H), 1.32(¹³C)). The multiplicity is reported with the following symbols: s (singlet), d (doublet), t (triplet), q (quadruplet) and m (multiplet), br (broad signal).

Mass spectrometry analysis were operated on a Synapt G2 HRMS Q-TOF mass spectrometer (Waters) equipped with an electrospray ionization (ESI) interface operating in the positive mode.

Reactions were followed by thin-layer chromatography (TLC) using 60 F254 silica gel plates on aluminium support (Merck) and revealed by UV lamp (254 nm). TLC were eluted in a (3:2) heptane/acetone eluant for compounds **1, 2, 5**, with a (1:1) hexane/methylene chloride eluant for compounds **3, 4**, and in a 1% MeOH in DCM solution for **1d** (with pre-impregnation of the plate in a saturated solution of cysteine in acetone). Radio-TLC were analysed using a Cyclone phosphorimager scanner (Perkin Elmer) or a Sapphire FL Biomolecular Imager (Azure Biosystem).

HPLC analyses were performed on an HPLC system Alliance e2695 system equipped with a Berthold radioFlowStar LB 513 Radio Flow Detector (column Symmetry Shield RP18 5µm 4,66x150 mm), or a Waters ARC Premier system equipped with a Elysia Raytest Ramona star 2" radiodetector and either a Waters 2998 PDA or a 2498 UV/Vis detector for UV (column Waters XBridge™ Premier BEH C18 2.5 µm 4.6 x 50 mm). UV chromatograms were measured at 254 nm. Analytical condition (for both systems): Elution solvent was A: water with 0.05% trifluoroacetic acid (TFA) and B: acetonitrile with 0.05% TFA. Flow: 1.5 mL/min; gradient: t = 0: 90% A, 10% B; t = 10 min: 1% A, 99% B; t = 15 min: 1% A; 99% B. Sample temperature: 20 °C, Column temperature: 30 °C.

Since free astatine-211 exhibits an affinity for HPLC materials and column, a washing run consisting of injecting 50 µL sodium sulfite solution (10 mg/mL) in the HPLC system was performed after each run to collect non-released activity during analysis ("washing" chromatogram incrusted in every HPLC result). RCY and radio-purity of analysed samples were corrected by subtracting the activity released during this washing run determined by the area of the peak observed.

## Synthesis of metal NHC complexes

### [AuCl(IPr)] (1a)

N,N'--(2,6-diisopropyl)phenyl imidazolium chloride (0.1 g, 0.24 mmol) and chloro(dimethylsulfide)gold(I) were dissolved in acetone (10 mL). The solution was heated at 60°C for 10 minutes. Sodium acetate (0.031 g, 0.36 mmol) was then added and the mixture stirred at 60°C for 2h. The solvent was removed under reduced pressure and methylene chloride was

added then the solution was filtered through a pad of celite®. The solvent was removed under reduced pressure and the residue was purified by flash chromatography on a silica column (methylene chloride/hexane 4:1). A white solid was obtained (0.44 g, 64%). The solid was dissolved in methylene chloride and after hexane addition the solution was stored in the fridge overnight. Translucent crystals were obtained.

¹H NMR (400 MHz, CD₃CN): δ 7.58 (t, 2H), 7.50 (s, 2H), 7.40 (d, 4H), 2.55 (m, 4H), 1.31 (d, 12H), 1.23 (d, 12H); ¹³C NMR (100 MHz, CD₃CN): δ 175.18, 147.04, 135.26, 131.75, 125.29, 124.98, 29.68, 24.65, 24.11; HPLC (RP18 Symmetryshield): t<sub>R</sub> = 9.834 min, Rf (heptane/acetone: 6/4) = 0.38; HRMS [M+Na]<sup>+</sup>: theory: 643.2125 m/z; [M+Na]<sup>+</sup>: exp: 643.2097 m/z

### [Aul(IAd)] (2b)

[AuCl(IAd)] **2a** (0.1 g, 0.24 mmol) and sodium iodide (0.18 g, 1.2 mmol) were suspended in ethanol (15 mL) under argon. The mixture was stirred for 24h. The solvent was removed under reduced pressure. Methylene chloride was added and the solution was filtered through a pad of celite®. The solvent was removed under reduced pressure. A light-yellow solid was obtained (60 mg, 71%).

¹H NMR (400 MHz, CDCl₃): δ 7.07 (s, 2H), 2.57 (s, 12H), 2.27 (s, 6H), 1.76 (t, 12H); ¹³C NMR (100 MHz, CDCl₃): δ 178.48, 115.33, 59.59, 44.36, 36.04, 30.21; HPLC (RP18 Symmetryshield): t<sub>R</sub> = 11.619 min; Rf (heptane/acetone: 6/4) = 0.45.

### [IrCl(cod)(IPr)] (3a)

The protocol was adapted from the procedure by R. Savka *et al.*³⁰

1,3-Bis-(2,6-diisopropylphenyl)imidazolium chloride (0.27 g, 0.64 mmol) and chloro(1,5-cyclooctadiene)iridium(I) dimer (0.22 g, 0.33 mmol) were dissolved in acetone (20 mL). Potassium carbonate (0.29 g, 1.9 mmol) was added and the solution was stirred at 60°C for 20h. The solvent was removed under reduced pressure. The residue was dissolved in methylene chloride and the solution was filtered through a pad of celite®. The solvent was removed under reduced pressure and the residue was purified by flash chromatography on a silica column (hexane/methylene chloride 4:1). The solvent was removed under reduced pressure. A yellow solid was obtained (183 mg, 80%).

NMR ¹H (400 MHz, CDCl₃): δ 7.47 (t, 2H), 7.34 (br, 4H), 7.2 (s, 2H), 4.19 (t, 2H), 3.43 (s, 2H), 2.89 (t,2H), 2.70 (s,2H), 1.70-1.66 (m, 2H), 1.58-1.15 (m,16H) 1.10 (d, 12H), 0.89 (d, 2H); NMR ¹³C (400 MHz, CDCl₃): δ 181.69, 136.39, 129.95, 124.55, 83.07, 51.61, 33.69, 29.09, 28.90, 26.62, 23.46, 22.70; Rf (heptane/acetone: 6/4) = 0.33; HRMS [M+H]<sup>+</sup>: theory: 725.3213 m/z; [M+Cl]<sup>+</sup>: exp: 689.3491 m/z.

### [IrCl(cod)(IPr)] (3b)

[IrCl(cod)(IPr)] (3a) (0.05 g, 0.07 mmol) and potassium iodide (0.1 g, 0.7 mmol) were added in ethanol (10 mL) and the mixture was stirred for 24h. The solvent was removed under reduced pressure. Methylene chloride was added and the solution was filtered through a pad of celite®. The residue was purified by flash chromatography on a silica column (hexane/methylene chloride 7:3). The solvent was removed
under reduced pressure. A yellow solid was obtained (17 mg, 33%).

NMR ¹H (400 MHz, CDCl₃): δ 7.46 (t, 2H), 7.34 (d, 2H), 7.26 (d, 2H), 7.06 (s, 2H), 4.44 (t, 2H), 3.64 (m, 2H), 3.05 (t, 2H), 2.78 (m, 2H), 1.67-1.59 (m, 2H), 1.55-1.25 (m, 16H), 1.11 (m, 12H), 0.95-0.86 (m, 2H); NMR ¹³C (100 MHz, CDCl₃): δ 182.37, 147.05, 155.30, 136.31, 129.31, 124.71 (d), 122.73, 80.63, 54.60, 32.62, 29.42, 28.94 (d), 26.32, 23.82, 22.09; Rf (heptane/acetone: 6/4) = 0.46; HRMS [M+H]⁺<sup>theory</sup>: 817.2570 m/z; [M-I]⁺<sup>exp</sup>: 689.3488 m/z.

[RhCl(cod)(IPr)] (4a)

The protocol was adapted from the procedure by R. Savka *et al.*³⁰

1,3-Bis-(2,6-diisopropylphenyl)imidazolium chloride (0.17 g, 0.4 mmol) and chloro(1,5-cyclooctadiene)rhodium(I) dimer (0.1 g, 0.2 mmol) were dissolved in acetone (3 mL). Potassium carbonate (0.2 g, 1.2 mmol) was added and the solution was stirred at 60°C for 20h. The solvent was removed under reduced pressure. The residue was dissolved in methylene chloride and the solution was filtered through a pad of celite®. The solvent was removed under reduced pressure. Hexane was added and the mixture was kept in the fridge overnight. The solution was filtered and the filtrate was evaporated to dryness affording a yellow solid (100 mg, 38%).

NMR ¹H (400MHz, CDCl₃): δ 7.52 (t, 2H), 7.35 (br, 4H), 7.02 (s, 2H), 4.57 (m, 2H), 3.59 (br, 2H), 3.24 (m, 2H), 2.51 (br, 2H), 1.86-1.04 (m, 32H); NMR ¹³C (100 MHz, CDCl₃): δ 186.52(d), 147.98, 145.56, 136.63, 130.03, 125.05, 124.83, 123.21, 96.27(d), 68.14 (d), 32.89, 29.07, 28.49, 26.78, 24.25, 22.94; Rf (heptane/acetone: 6/4) = 0.49; HRMS [M+H]⁺<sup>theory</sup>: 635.2639 m/z; [M-Cl]⁺<sup>exp</sup>: 599.2921 m/z.

[RhI(cod)(IPr)] (4b)

[RhCl(cod)(IPr)] (4a) (0.06 g, 0.09 mmol) and sodium iodide (0.08 g, 0.5 mmol) were added in methylene chloride (5 mL). The mixture was stirred for 20h. The mixture was filtered through a pad of celite® and the solvent was removed under reduced pressure. The residue was purified by flash chromatography on a silica column (hexane/methylene chloride 4:1). A yellow solid was obtained (34.5 mg, 50%).

NMR ¹H (400MHz, CDCl₃): δ 7.51-7.47 (t, 2H), 7.40 (d, 2H), 7.27 (d, 2H), 7.07 (s, 2H), 4.83 (t, 2H), 3.76-3.70 (m, 2H), 3.53 (t, 2H), 2.59-2.52 (m, 2H), 1.81-1.25 (m, 20H), 1.11-1.05 (m, 12H); NMR ¹³C (100 MHz, CDCl₃): δ 147.47, 146.57, 136.55, 129.91, 125.14, 124.94, 122.90, 93.63-93.56 (d), 71.28-71.74 (d), 32.02, 29.13-29.02 (d), 26.69-26.60 (d), 24.08, 22.19; Rf (heptane/acetone: 6/4) = 0.52; HRMS [M+H]⁺<sup>theory</sup>: 727.1995 m/z; [M-I]⁺<sup>exp</sup>: 599.2921 m/z.

Radiolabelling of metal NHC complexes

[¹²⁵I][Aul(Pr)] (1c)

To 18 µL of aqueous solution of [¹²⁵I]Nal (0.2-2 MBq) were added 40 µL of **1a** in acetonitrile (21.5 µmol/mL). After a few seconds of vigorous stirring, the mixture was incubated at 80°C for 30 minutes. t<sub>R</sub> (RP18 Symmetryshield) = 10.687 min.

[²¹¹At][AuAt(IPr)] (1d)

To dry [²¹¹At]At dissolved in 18 µL of reducing agent (sodium hydroxide 0.1M or cysteine 5 g/L) were added 40 µL of **1a** in acetonitrile (21.5 µmol/mL). After a few seconds of vigorous stirring the mixture was incubated at 80°C for 30 minutes. t<sub>R</sub> (RP18 Symmetryshield) = 10.748 min. t<sub>R</sub> (BEH C18) = 8.64 min.

[¹²⁵I][Aul(Ad)] (2c)

To 18 µL of aqueous solution of [¹²⁵I]Nal were added 40 µL of **2a** in methylene chloride (21.5 µmol/mL) and 8 µL of tetrabutylammonium hydroxide 0.1 M in methanol (TBAOH). After a few seconds of vigorous stirring the mixture was incubated at room temperature for 1h. Once the reaction completed the mixture was diluted in acetonitrile to obtain a single layer. t<sub>R</sub> (RP18 Symmetryshield) = 11.502 min.

[²¹¹At][AuAt(Ad)] (2d)

To 18 µL of sodium metabisulfite (10 mg/mL) with [²¹¹At]At were added 40 µL of **2a** in methylene chloride (21.5 µmol/mL) and 8 µL of tetrabutylammonium hydroxide 0.1 M in methanol (TBAOH). After a few seconds of vigorous stirring the mixture was incubated at room temperature for 1h. Once the reaction completed the mixture was diluted in acetonitrile to obtain a single layer. t<sub>R</sub> (RP18 Symmetryshield) = 11.853 min.

Stability studies in presence of PBS

To 150 µL of PBS/acetonitrile (2:3) were added 30 µL of the radiolabelling solution containing compound **1c** (2-4 MBq, 0.86-1.72 MBq/µmol) or **1d** (1-2 MBq 0.43-0.86 MBq/µmol). After a few seconds of vigorous stirring the mixture was stirred at room temperature for 14h. At each time point (30 min, 1h, 2h, 4h, 7h, 14h) analyses were performed by HPLC (ARC system).

Quantum mechanical calculations

The two-component relativistic density functional theory, which was proved to be accurate for investigating At-containing systems,³¹,¹⁹ requires to replace the orbital representation by spinors that are complex vector functions of two components (2c). The Generalized Kohn-Sham (GKS) method, implemented in the Gaussian 16 rev. A.03 program,³² takes advantages of relativistic pseudo-potentials containing scalar and spin-dependent terms to treat the electron correlation and the relativistic effects on an equal footing. We opted in this work for the global hybrid PBE0 functional,³³ which was successfully used to study astatine adsorption on a gold surface.³⁴ The small core pseudo-potentials ECPnMDF were used for the Rh, I, Ir, Au and At atoms, respectively.³⁵⁻³⁶ Two sets of basis functions were used for describing the explicitly treated electrons. The SVP one gathers the def2-SVP basis sets for all atoms.³⁷ This set is used for the geometry optimizations and subsequent harmonic frequency calculations at the scalar-relativistic level of theory. The vibrational frequencies help to ascertain the nature of the structures (minimum), and were used to compute gas phase Gibbs free energies at T = 298.15 K. The TZVP set, used solely for single-point energy calculations at the 2c-relativistic level of theory on previously optimized geometries, combines the dhf-TZVP
basis sets supplemented with two-component extensions for the At and I atoms,³⁸ and the def2-TZVP basis sets for the remaining atoms.³⁷

Solvent effects were introduced through the conductor-like polarizable continuum model (CPCM),³⁹ used in conjunction with the UAHF cavity model,⁴⁰ as recommended in ref. 29 for investigating atstatine chemistry in solution. Indeed, accurate Gibbs free energies of solvation can be predicted using implicit solvent models, provided that specific parameters are used to build At cavities. An atstatine sphere radius consistent with the UAHF cavity model have been recently introduced.³⁹ The procedure used to compute free energies of aqueous solvation (T = 298.15 K, 1 mol/L standard state) was to perform, on top of previously optimized geometries, gas-phase single-point and CPCM-UAHF single-point energy calculations at the HF/SVP level of theory.

## Acknowledgements

This work was supported by the French National Agency for Research, grants ANR-19-CE07-0036 (ExPAT), ANR-11-LABX-18-01 (Labex IRON), ANR-16-IDEX-0007 (ISITE NExT) and INCa-DGOS-INSERM-ITMO Cancer_18011. This work was carried out using HPC resources from CCIPL ("Centre de Calcul Intensif des Pays de la Loire"). Co-funded by the European Union (ERC, SAt-Radio, 101087175). Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. A CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission, in accordance with the grant's open access conditions.

**Keywords:** Astatine • Gold • N-heterocyclic Carbene • Radiochemistry • Radiolabelling

<footnote n="1">1 F. Guérard, C. Maingueneau, L. Liu, R. Eychenne, J.-F. Gestin, G. Montavon and N. Galland, *Acc. Chem. Res.*, 2021, **54**, 3264-3275.</footnote>

<footnote n="2">2 S. Lindegren, P. Albertsson, T. Bäck, H. Jensen, S. Palm and E. Aneheim, *Cancer Biother. Radiopharm.*, 2020, **35**, 425-436.</footnote>

<footnote n="3">3 L. Marcu, E. Bezak and B. J. Allen, *Critical Reviews in Oncology/Hematology*, 2018, **123**, 7-20.</footnote>

<footnote n="4">4 M. Vanermen, M. Ligeour, M.-C. Oliveira, J.-F. Gestin, F. Elvas, L. Navarro and F. Guérard, *EJNMMI radiopharm. chem.*, 2024, **9**, 69.</footnote>

<footnote n="5">5 D. S. Wilbur, *Curr. Radiopharm.*, 2008, **1**, 144-176.</footnote>

<footnote n="6">6 D. Teze, D.-C. Sergentu, V. Kalichuk, J. Barbet, D. Deniaud, N. Galland, R. Maurice and G. Montavon, *Sci. Rep.*, 2017, **7**, 2579.</footnote>

<footnote n="7">7 Y. Dekempeneer, T. Bäck, E. Aneheim, H. Jensen, J. Puttemans, C. Xavier, M. Keyaerts, S. Palm, P. Albertsson, T. Lahoutte, V. Caveliers, S. Lindegren and M. D'Huyvetter, *Mol. Pharmaceutics*, 2019, **16**, 3524-3533.</footnote>

<footnote n="8">8 H. Suzuki, Y. Kaizuka, M. Tatsuta, H. Tanaka, N. Washiya, Y. Shirakami, K. Ooe, A. Toyoshima, T. Watabe, T. Teramoto, I. Sasaki, S. Watanabe, N. S. Ishioka, J. Hatazawa, T. Uehara and Y. Arano, *J. Med. Chem.*, 2021, **64**, 15846-15857.</footnote>

<footnote n="9">9 D. S. Wilbur, M.-K. Chyan, D. K. Hamlin, R. L. Vessella, T. J. Wedge and M. F. Hawthorne, *Bioconjugate Chem.*, 2007, **18**, 1226-1240.</footnote>

<footnote n="10">10 M. Pruszyński, A. Bilewicz and M. R. Zalutsky, *Bioconjugate Chem.*, 2008, **19**, 958-965.</footnote>

<footnote n="11">11 H. Rajerison, F. Guérard, M. Mougin-Degraef, M. Bourgeois, I. Da Silva, M. Chérel, J. Barbet, A. Faivre-Chauvet and J.-F. Gestin, *Nucl. Med. Biol.*, 2014, **41**, e23-e29.</footnote>

<footnote n="12">12 E. A. Martynova, N. V. Tzouras, G. Pisanò, C. S. J. Cazin and S. P. Nolan, *Chem. Commun.*, 2021, **57**, 3836-3856.</footnote>

<footnote n="13">13 H. Clavier and S. P. Nolan, *Chemical Communications*, 2010, **46**, 841-861.</footnote>

<footnote n="14">14 D. G. Gusev, *Organometallics*, 2009, **28**, 6458-6461.</footnote>

<footnote n="15">15 M. Mora, M. C. Gimeno and R. Visbal, *Chem. Soc. Rev.*, 2019, **48**, 447-462.</footnote>

<footnote n="16">16 N. Guo, R. Maurice, D. Teze, J. Graton, J. Champion, G. Montavon and N. Galland, *Nature Chem.*, 2018, **10**, 428-434.</footnote>

<footnote n="17">17 D.-C. Sergentu, D. Teze, A. Sabatié-Gogova, C. Alliot, N. Guo, F. Bassal, I. D. Silva, D. Deniaud, R. Maurice, J. Champion, N. Galland and G. Montavon, *Chem. Eur. J.*, 2016, **22**, 2964-2971.</footnote>

<footnote n="18">18 F. Bassal, J. Champion, S. Pardoue, M. Seydou, A. Sabatié-Gogova, D. Deniaud, J.-Y. L. Questel, G. Montavon and N. Galland, *Inorg. Chem.*, 2020, **59**, 13923-13932.</footnote>

<footnote n="19">19 N. Guo, D.-C. Sergentu, D. Teze, J. Champion, G. Montavon, N. Galland and R. Maurice, *Angew. Chem. Int. Ed. Engl.*, 2016, **55**, 15369-15372.</footnote>

<footnote n="20">20 D.-C. Sergentu, G. David, G. Montavon, R. Maurice and N. Galland, *J. Comput. Chem.*, 2016, **37**, 1345-1354.</footnote>

<footnote n="21">21 Crystallographic structures of compounds **7b**, **7a**, **1a** and **1b** can be respectively found with CCDC numbers 2298774, 2298778, 2298779, 2299132.</footnote>

<footnote n="22">22 T. Ayed, J. Pilmé, D. Tézé, F. Bassal, J. Barbet, M. Chérel, J. Champion, R. Maurice, G. Montavon and N. Galland, *Eur. J. Med. Chem.*, 2016, **116**, 156-164.</footnote>

<footnote n="23">23 E. Aneheim, S. Palm, H. Jensen, C. Ekberg, P. Albertsson and S. Lindegren, *Sci. Rep.*, 2019, **9**, 15900.</footnote>

<footnote n="24">24 F. Guérard, Y.-S. Lee, K. Baidoo, J.-F. Gestin and M. W. Brechbiel, *Chemistry – A European Journal*, 2016, **22**, 12332-12339.</footnote>

<footnote n="25">25 M. Berdal, S. Gouard, R. Eychenne, S. Marionneau-Lambot, M. Croyal, A. Faivre-Chauvet, M. Chérel, J. Gaschet, J.-F. Gestin and F. Guérard, *Chem. Sci.*, 2021, **12**, 1458-1468.</footnote>

<footnote n="26">26 K. Matsuoka, H. Obata, K. Nagatsu, M. Kojima, T. Yoshino, M. Ogawa and S. Matsunaga, *Org. Biomol. Chem.*, 2021, **19**, 5525-5528.</footnote>

<footnote n="27">27 L. Dziawer, P. Koźmiński, S. Męczyńska-Wielgosz, M. Pruszyński, M. Łyczko, B. Wąs, G. Celichowski, J. Grobelny, J. Jastrzębski and A. Bilewicz, *RSC Adv.*, 2017, **7**, 41024-41032.</footnote>

<footnote n="28">28 T. J. Robilotto, J. Bacsa, T. G. Gray and J. P. Sadighi, *Angewandte Chemie International Edition*, 2012, **51**, 12077-12080.</footnote>
29 S. Lindegren, T. Bäck and H. J. Jensen, *Applied Radiation and Isotopes*, 2001, **55**, 157–160.

30 R. Savka and H. Plenio, *Dalton Trans.*, 2015, **44**, 891–893.

31 J. Champion, M. Seydou, A. Sabatié-Gogova, E. Renault, G. Montavon and N. Galland, *Phys. Chem. Chem. Phys.*, 2011, **13**, 14984–14992.

32 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, Williams, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman and D. J. Fox, *Gaussian 16 Rev. A* **03 2016**.

33 C. Adamo and V. Barone, *The Journal of Chemical Physics*, 1999, **110**, 6158–6170.

34 Y. Demidov and A. Zaitsevskii, *Chemical Physics Letters*, 2018, **691**, 126–130.

35 K. A. Peterson, *The Journal of Chemical Physics*, 2003, **119**, 11099–11112.

36 D. Figgen, K. A. Peterson, M. Dolg and H. Stoll, *The Journal of Chemical Physics*, 2009, **130**, 164108.

37 F. Weigend and R. Ahlrichs, *Phys. Chem. Chem. Phys.*, 2005, **7**, 3297–3305.

38 F. Weigend and A. Baldes, *The Journal of Chemical Physics*, 2010, **133**, 174102.

39 V. Barone and M. Cossi, *J. Phys. Chem. A*, 1998, **102**, 1995–2001.

40 V. Barone, M. Cossi and J. Tomasi, *The Journal of Chemical Physics*, 1997, **107**, 3210–3221.
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<figure><img src="image_13.png" /></figure>

In the search for alternatives to the traditional astatoaryl bond, a Au(I)-NHC complex was successfully radiolabelled with ²¹¹At by halide exchange, and showed promising stability in phosphate buffered saline. Mass spectrometry experiments coupled with relativistic DFT calculations support the superiority of Au(I) over Rh(I) and Ir(I) and give insights into the role of the NHC ligand in order to access stable ²¹¹At-compounds.