# Structural Transformation of Metastable Two-Electron Superatom Au-Doped Cu-Rich Alloy Nanocluster

Rhone P. Brocha Silalahi, Samia Kahlal, Jean-Yves Saillard, C W Liu

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Rhone P. Brocha Silalahi, Samia Kahlal, Jean-Yves Saillard, C W Liu. Structural Transformation of Metastable Two-Electron Superatom Au-Doped Cu-Rich Alloy Nanocluster. Molecules, 2024, 29 (18), pp.4427. 10.3390/molecules29184427. hal-04718465

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Article

# Structural Transformation of Metastable Two-Electron Superatom Au-Doped Cu-Rich Alloy Nanocluster

Rhone P. Brocha Silalahi <fnref n="1" /> ID, Samia Kahlal <fnref n="2" />, Jean-Yves Saillard <fnref n="2" />,* ID and C. W. Liu <fnref n="1" />,* ID

**Abstract:** The ability to fabricate bimetallic clusters with atomic precision offers promising prospects for elucidating the correlations between their structures and properties. Nevertheless, achieving precise control at the atomic level in the production of clusters, including the quantity of dopant, characteristic of ligands, charge state of precursors, and structural transformation, have remained a challenge. Herein, we report the synthesis, purification, and characterization of a new bimetallic hydride cluster, $[AuCu_{11}(H)\{S_2P(O^iPr)_2\}_6(C \equiv CPh)_3] (AuCu_{11}H)$. The hydride position in $AuCu_{11}H$ was determined using DFT calculations. $AuCu_{11}H$ comprises a ligand-stabilized defective $fcc$ Au$@Cu_{11}$ cuboctahedron. $AuCu_{11}H$ is metastable and undergoes a spontaneous transformation through ligand exchange into the isostructural $[AuCu_{11}(Cl)\{S_2P(O^iPr)_2\}_6(C \equiv CPh)_3] (AuCu_{11}Cl)$ and into the complete cuboctahedral $[AuCu_{12}\{S_2P(O^iPr)_2\}_6(C \equiv CPh)_4]^+$ ($AuCu_{12}$) through an increase in nuclearity. These structural transformations were tracked by NMR and mass spectrometry.

**Keywords:** copper; gold; hydride; superatom; structural transformation

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**Citation:** Silalahi, R.P.B.; Kahlal, S.; Saillard, J.-Y.; Liu, C.W. Structural Transformation of Metastable Two-Electron Superatom Au-Doped Cu-Rich Alloy Nanocluster. *Molecules* **2024**, *29*, 4427. https://doi.org/10.3390/molecules29184427

Received: 31 July 2024

Revised: 10 September 2024

Accepted: 11 September 2024

Published: 18 September 2024

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**Copyright:** © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

Structural transformation in atomically precise metal nanoclusters (NCs) is important not only to investigate their structure–property relationships but also to serve as a unique platform for exploring structural evolution [1–3]. In the past decade, research in the field of structural NC transformations has broadened significantly, and many NCs resulting from structural evolutions have been synthesized [4–6]. The structural transformation of the metal core process can occur in certain metal NCs under suitable conditions, such as redox treatment [7,8], pH induction [9,10], ligand exchange [11,12], and elevated temperatures [13–17]. It can lead to the formation of structural isomers of various sizes [4–18]. While the structural transformation of pure and alloyed gold and silver clusters has been extensively studied, there has been limited research on their copper homologues [19–32]. A noticeable example, reported by Zhu et al., is the PPh₃ addition to $[Cu_{41}(SC₆H₃F₂)_{15}(P(PhF)₃)₆C₃H₂₅]^{2-}$ that induces the conversion into $[Cu_{14}(SC₆H₃F₂)₃(PPh₃)₈H₁₀]^+$, while the addition of P(PhF)₃ converts it into $[Cu_{13}(SC₆H₃F₂)₃(P(PhF)₃)₇H₁₀]$ [33]. Another remarkable example reported by Zang et al. is the step-by-step controlled preparation of two catalytically active carborane alkynyl-protected copper NCs, namely $[Cu_{13}(C₄B₁₀H₁₁)_{10}(PPh₃)₂(CH₃CN)₂]^{3+}$ and $[Cu_{26}(C₄B₁₀H₁₁)_{16}(C₂B₉H₁₀C₂)₂(PPh₃)₃(CH₃CN)]^{4+}$, through external ligand shell modifications and metal-core evolution [34]. We also reported the structural transformation under acidic conditions of the cuboctahedral $[Cu_{12}S\{S₂CNⁿBu}₂(C₂Ph)₄]$ into the fused bicuboctahedral $[Cu_{21}S₂\{S₂CNⁿBu₂}₉(C₂Ph)₆]$ [35] and the spontaneous transformation in a chlorinated solvent of $[Cu_{15}H₂\{S₂CNⁿBu₂}₆(C₂Ph)₄]^+$ into the $[Cu_{13}\{S₂CNⁿBu₂}₆(C₂Ph)₄]^+$ two-electron superatom [36].

<footnote n="1">¹ Department of Chemistry, National Dong Hwa University, Hualien 97401, Taiwan; rhone.p.brocha.silalahi@gmail.com</footnote>

<footnote n="2">² Univ Rennes, CNRS, Institut des Sciences Chimiques de Rennes-UMR 6226, F-35000 Rennes, France; kahlal@univ-rennes1.fr</footnote>

<footnote>* Correspondence: jean-yves.saillard@univ-rennes1.fr (J.-Y.S.); chenwei@gms.ndhu.edu.tw (C.W.L.); Tel.: +886-3-890-3607 (C.W.L.)</footnote>
It is to be noted that research on NC transformation has generally focused on stable species, while the analysis of the structural evolution of metastable NCs has only begun to receive attention in the last few years [37–41]. Capturing metastable clusters with a labile nature provides valuable information for understanding the pathways of metal NC transformations [42]. From this point of view, hydride-stabilized NCs constitute a privileged field of investigations [25–31,37–49]. Indeed, the coordination diversity and H-dissociation behavior in hydride-ligated NCs make them a dynamic system that can facilitate structural modifications [47]. The relatively weak M-H bond allows for the easy substitution of hydrides by other ligands, which is an effective way to obtain the evolution of intermediates [47,49]. In this context, by introducing a hydride ligand on two-electron superatoms, Au-Cu alloy NCs, the structural transformation pathway is expected to be particularly studied at the simplified hydride-binding site. To the best of our knowledge, no two-electron superatom Au-doped Cu-rich hydride has been reported so far, and hydride ligands are still rarely observed in alloy nanoclusters [50]. This work aims to obtain the isostructural Au-Cu bimetallic cluster and acquire insight into the underlying mechanism of transformation of a two-electron superatom, thereby enhancing its nuclearity.

## 2. Results and Discussion

**AuCu<sub>11</sub>H** was first prepared by a one-pot procedure with mixed solvent THF-CH<sub>3</sub>CN (1:1). The synthetic procedure of **AuCu<sub>11</sub>H** differs from that reported in **AuCu<sub>11</sub>Cl** [25] and **AuCu<sub>12</sub>** [26]. The synthesis protocol involves the reduction of copper and gold precursors with the reducing agent NaBH<sub>4</sub> in the presence of alkynyl, trimethylamine, and dithio-phosphate (dtp) ligands (Scheme 1). The yellowish-green mixture immediately turned yellow after addition of the reducing agent and finally dark brown after stirring for 4 h at ambient temperature. The resulting mixture was evaporated by a rotavapor to give a dark brown solid. This solid was purified by an aluminum oxide column to afford an orange-red precipitate of **AuCu<sub>11</sub>H** in 13% yield.

<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Synthesis route for AuCu<sub>11</sub>H.</figcaption></figure>

The deuterated derivatives [AuCu<sub>11</sub>(D){S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>6</sub>(C≡CPh)<sub>3</sub>] (AuCu<sub>11</sub>D) were also synthesized to support the existence of the hydride in the clusters. Synthetic details are provided in the supporting information. Electrospray ionization mass spectrometry (ESI-MS) in a positive mode was performed to verify the molecular formula of AuCu<sub>11</sub>H. The ESI-MS spectra of AuCu<sub>11</sub>H (Figure 1 and Table 1) show two prominent peaks at *m/z* = 2542.03 and 2642.37 Da, which correspond to the molecular weight of AuCu<sub>11</sub>H with an adduct of one copper ion, [AuCu<sub>11</sub>(H){S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>6</sub>(C≡CPh)<sub>3</sub> + Cu<sup>+</sup>]<sup>+</sup> (*m/z* calc. = 2542.34 Da), and the molecular ion [AuCu<sub>12</sub>{S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>6</sub>(C≡CPh)<sub>4</sub>]<sup>+</sup> (*m/z* calc. = 2642.37 Da), respectively. The smaller peak can be ascribed to [AuCu<sub>11</sub>(Cl){S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>6</sub>(C≡CPh)<sub>3</sub> + Cu<sup>+</sup>]<sup>+</sup> at *m/z* = 2575.96 Da (*m/z* calc. = 2575.31 Da). The measured isotopic pattern fully matches with the simulated pattern (Figure 1 (inset) and Figure S1). To confirm the number of hydride(s), a controlled preparation with NaBD<sub>4</sub> was performed to obtain the deuterated AuCu<sub>11</sub>D. The ESI-MS analysis of AuCu<sub>11</sub>D depicts two intense peaks at *m/z* = 2543.37 Da (*m/z* calc. = 2543.35 Da) and 2478.43 Da (*m/z* calc. = 2478.41 Da) (Figure S2), which correspond to the molecular weight of AuCu<sub>11</sub>D with a copper adduct ion, [AuCu<sub>11</sub>(D){S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>6</sub>(C≡CPh)<sub>3</sub> + Cu<sup>+</sup>]<sup>+</sup>, and neutral [AuCu<sub>11</sub>(D){S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>6</sub>(C≡CPh)<sub>3</sub>], respectively. The mass spectra also display the molecular ion [AuCu<sub>12</sub>{S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>6</sub>(C≡CPh)<sub>4</sub>]<sup>+</sup> at *m/z* = 2642.40 Da (*m/z* calc. = 2642.37 Da) (Figure S2). A distinct shift of *m/z* = 1.34 is observed between the mass of AuCu<sub>11</sub>H and
**AuCu<sub>11</sub>D**, confirming that the total number of hydrides in **AuCu<sub>11</sub>H** is one. The theoretically determined isotopic patterns of [AuCu<sub>11</sub>D + Cu<sup>+</sup>]<sup>+</sup>, [AuCu<sub>11</sub>D], and [AuCu<sub>12</sub>]<sup>+</sup> show excellent agreement with the experiment, as depicted in Figures S2 (inset) and S3, respectively.

<figure><img src="image_5.png" /><figcaption>**Figure 1.** ESI-MS spectra of [AuCu<sub>11</sub>H + Cu<sup>+</sup>]<sup>+</sup>. The inset shows the comparison between the experimental data (top) and the simulated (bottom) isotope patterns for [AuCu<sub>11</sub>H + Cu<sup>+</sup>]<sup>+</sup>.</figcaption></figure>

<table><caption>**Table 1.** The positive-mode ESI-MS of [AuCu<sub>11</sub>H + Cu<sup>+</sup>]<sup>+</sup>.</caption><thead><tr><th>No.</th><th>Cluster Code</th><th>m/z Exp.</th><th>m/z Cacl.</th></tr></thead><tbody><tr><td>1.</td><td>[AuCu<sub>11</sub>H + Cu<sup>+</sup>]<sup>+</sup></td><td>2542.03</td><td>2542.34</td></tr><tr><td>2.</td><td>[AuCu<sub>11</sub>Cl + Cu<sup>+</sup>]<sup>+</sup></td><td>2575.96</td><td>2575.31</td></tr><tr><td>3.</td><td>[AuCu<sub>12</sub>]<sup>+</sup></td><td>2642.37</td><td>2642.37</td></tr></tbody></table>

The <sup>1</sup>H NMR spectrum of AuCu<sub>11</sub>H is shown in Figure 2. The presence of the hydride is confirmed with a chemical shift at -1.49 ppm (Figure 2a). The peak integration ratios indicate six dtp and three alkynyl ligands to one hydride. The signals at 4.69-5.06 and 1.07-1.43 ppm in a 1:6 intensity ratio are attributed to the -OCH and CH<sub>3</sub> protons of the dtp isopropyl groups, respectively (Figure 2a). In addition to the isopropyl peaks, the spectrum also shows the two peaks of the alkynyl protons at 7.39-7.64 ppm. The corresponding deuterated resonance in the <sup>2</sup>H NMR spectra of AuCu<sub>11</sub>D confirms the position of the hydride with relatively similar resonances observed at -1.41 ppm. (Figures 2b and S4). The <sup>31</sup>P{<sup>1</sup>H} spectrum of AuCu<sub>11</sub>H displays three signals at 104.4, 100.5, and 99.9 ppm attributed to three types of di-isopropyl dithiophosphate ligands in a symmetrical structure AuCu<sub>11</sub>H (Figures S5 and S6). In the <sup>13</sup>C NMR spectrum, the peaks at 23.2 and 77.2 ppm are assigned to the isopropyl group of dithiophosphates, followed by the peaks at 125.2, 128.9, 132.3, 134.1, and 77.2 and 137.4 ppm corresponding to the aromatic carbons of the phenyl rings and the carbon atoms of the C≡C functional groups, respectively (Figure S7). A similar synthetic procedure was conducted to obtain two derivatives of AuCu<sub>11</sub>H by using di-n-propyl and di-isobutyl dithiophosphate ligands and 4-ethylanisole. All their spectroscopic data are consistent with the proposed formula (see Materials and Methods section, Figures S8-S20).
<figure><img src="image_6.png" /><figcaption>**Figure 2.** (a) The ¹H NMR spectrum of **AuCu₁₁H** in CDCl₃. (b) The ²H NMR spectrum of **AuCu₁₁H** in CHCl₃. The asterisk symbol signifies the hydride peak.</figcaption></figure>

Due to the inherent instability of AuCu₁₁H in solution, it was not possible to obtain a single crystal of AuCu₁₁H. Nevertheless, a comparison of the NMR data collected for this compound with those of the isoelectronic AuCu₁₁Cl [25] suggests strongly that both NCs have a similar structure. This hypothesis is also supported by the fact that the composition of the two compounds differs only by the nature of one ligand (H vs. Cl). Then, a geometry optimization of AuCu₁₁H by means of density functional theory (DFT) at the PBE0/Def2-TZVP was performed (see computational details), starting from that of the AuCu₁₁Cl parent, and indeed the converged structure presented similar features as its chloride homologue. Other structural arrangements were also tested, including several with different hydride positions on different metal atoms (including Au) and in different coordination modes, but the global minimum was found to remain definitely the same, i.e., both AuCu₁₁H and AuCu₁₁Cl are isostructural, including with respect to the location of the H vs. Cl ligand (see Figure 3). The secondary minimum of lowest energy was found to be at a much higher energy (ΔE = 1.40 eV) to allow considering any equilibrium in solution. In this isomer, the hydride is shifted on the opposite side of the vacancy and bridges an AuCu₂ triangle (see SI). The AuCu₁₁ core describes an Au-centered cuboctahedron of which one vertex is missing. This defective fcc Au@Cu₁₁ core is decorated with four (μ₂, μ₂) and two (μ₂, μ₁) dtp ligands and three (μ₃-η¹) alkynyl ligands (Figure 3a,b). The hydride bridges one of the Cu-Cu edges, which borders the core vacancy. This location makes sense in the way that the hydride (or chloride in the case of AuCu₁₁Cl) is linked to the two copper atoms that would be the least connected metals if this ligand was missing. It is noteworthy that the hydride ligand in AuCu₁₁H becomes closer to the central gold atom than the chloride in AuCu₁₁Cl (Figure 3c,d). Nevertheless, the Au...H distance (2.851 Å) is of the order of magnitude of a van der Waals contact, whereas the corresponding Cu-H distances of AuCu₁₁H (1.633 Å) correspond to standard Cu-(μ₂-H) bonds. Other average computed metrical data are provided in Table 2. They are comparable to those previously calculated for AuCu₁₁Cl [25], also reported in Table 2 for comparison.
AuCu<sub>11</sub>H

a)

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

c)

<figure><img src="image_8.png" /></figure>

AuCu<sub>11</sub>Cl

b)

<figure><img src="image_9.png" /></figure>

d)

<figure><img src="image_10.png" /><figcaption>**Figure 3.** The DFT-optimized structure of AuCu<sub>11</sub>H and the X-ray structure of AuCu<sub>11</sub>Cl [41]: (a,b) overall structures; (c,d) the Au@Cu<sub>11</sub> cuboctahedral core with the bridging hydride-chloride ligand. All the phenyl rings of alkynyls and the alkyl group of dtp ligands are omitted for clarity. Color labels: dark red = Cu; orange = Au; yellow = S; magenta = P; pink = H; green = Cl, gray = C.</figcaption></figure>

<table><caption>**Table 2.** Selected DFT-computed data for AuCu<sub>11</sub>H and AuCu<sub>11</sub>Cl<sup>a</sup>.</caption><thead><tr><th></th><th></th><th>AuCu<sub>11</sub>H</th><th>AuCu<sub>11</sub>Cl<sup>a</sup></th></tr></thead><tbody><tr><td colspan="2">HOMO-LUMO gap (eV)</td><td>3.51</td><td>3.59</td></tr><tr><td rowspan="4">Interatomic distances (Å) and<br>Wiberg indices (avg. values)</td><td>Au-Cu</td><td>2.790<br>[0.062]</td><td>2.814<br>[0.060]</td></tr><tr><td>Cu-Cu</td><td>2.777<br>[0.040]</td><td>2.789<br>[0.041]</td></tr><tr><td>Cu-H/Cl</td><td>1.633<br>[0.273]</td><td>2.272<br>[0.198]</td></tr><tr><td>Natural atomic charges<br>(avg. values)</td><td>Au<br>Cu<br>H/Cl</td><td>-0.69<br>0.73<br>-0.53</td><td>-0.73<br>0.72<br>-0.73</td></tr></tbody></table>

The calculated ¹H NMR chemical shift (see SI for computational details) of the hydride (1.3 ppm) is in a satisfying agreement with the experimental value (−1.49 ppm), owing to the fact that a discrepancy of ~3 ppm is not uncommon for metal-bound hydrides in these types of clusters [51]. The computed chemical shifts of the ligand protons match well with the experiment, with averaged value of 7.3, 7.4, and 7.8 ppm (aromatic protons), 4.8 ppm (–OCH), and 0.8 ppm (CH₃), as well as the ¹³C chemical shifts (88.4 and 140.4 (C≡C), 135.6, 128.4, 127.6, and 126.5 (aromatic carbons), and 70.3 and 20.2 (isopropyl carbons)).

<footnote>a From ref. [25].</footnote>
The KohnSham orbital diagram of AuCu<sub>11</sub>H (Figure 4) is typical of the two-electron superatoms based on a centered complete (closo-type) [52] or defective (nido-type) [25] cuboctahedron. It is characterized by a 1S HOMO with a dominant Au character. A peculiarity of this diagram is the fact that only two among the 1P orbitals are the lowest unoccupied levels, the third one being the LUMO+5. The somewhat higher energy of the latter could be due to some antibonding (weak) hydride admixture.

<figure><img src="image_11.png" /><figcaption>**Figure 4.** The Kohn-Sham frontier orbital diagram of AuCu<sub>11</sub>H. The orbital contribution (in %) is given in the order Au/Cu<sub>11</sub>/H/ligands.</figcaption></figure>

The UV-vis spectrum of AuCu<sub>11</sub>H is shown in Figure 5a. There are three weak bands at 424, 487, and 533 nm, and another highly structured one at 318 nm. Figure 5b compares the UV-vis absorption spectra of AuCu<sub>11</sub>H and AuCu<sub>11</sub>Cl. AuCu<sub>11</sub>Cl also displays four absorption peaks at 328, 432, 485, and 540 nm [25]. These similarities in the UV-vis spectra also support the fact that both NCs are isostructural. Their multiband absorption profiles are typical of superatom-type clusters (Figure S21) [25,26,29]. The TD-DFT-simulated spectrum of AuCu<sub>11</sub>H is shown in Figure S22. Owing to the shapeless nature of its experimental counterpart, the agreement is satisfying. A similar agreement was found for AuCu<sub>11</sub>Cl [25]. The broad band of lowest energy results from a combination of HOMO→LUMO and HOMO→LUMO+1 transitions; hence, it is of superatomic 1S→1P nature, whereas the high-intensity peak at high energy (322 nm) is found to be of MLCT character, resulting from transitions between Cu(3d) levels and π*(phenyl) combinations.
<figure><img src="image_12.png" /><figcaption>**Figure 5.** (a) The experimental UV-vis absorption spectrum of AuCu<sub>11</sub>H in MeTHF. (b) Comparison of UV-vis absorption spectra of AuCu<sub>11</sub>H and AuCu<sub>11</sub>Cl in MeTHF.</figcaption></figure>

The photoluminescence (PL) properties of the two-electron superatomic AuCu<sub>11</sub>H and AuCu<sub>11</sub>Cl relatives were analyzed and compared (Figure 6). Both NCs are emissive at ambient temperatures in MeTHF. The excitation spectrum of AuCu<sub>11</sub>H, shown in Figure S23, is closely related to its absorption UV-vis spectrum. Upon exciting at 437 nm, AuCu<sub>11</sub>H (in MeTHF) emits at 633 nm, with a PL quantum yield (QY) of 32.7%. On the other hand, AuCu<sub>11</sub>Cl emits at 606 nm, with a ten times lower PL QY of 3.3% [25]. Interestingly, the emission band of AuCu<sub>11</sub>H is close to that of AuCu<sub>12</sub> at 637 nm, with a QY of 32% [26]. Thus, the PL QY for the two-electron Au-doped Cu-rich alloy clusters follows the order AuCu<sub>12</sub> > AuCu<sub>11</sub>H > AuCu<sub>11</sub>Cl. The PL lifetime of AuCu<sub>11</sub>H is 1.71 µs (Figure S24), whereas that of AuCu<sub>11</sub>Cl and AuCu<sub>12</sub> were determined to be 3.75 and 3.20 µs. Such a PL lifetime quenching is attributed to the hydride-chloride exchange and reveals the instability of AuCu<sub>11</sub>H in solution. Nevertheless, the high QY of AuCu<sub>11</sub>H (k<sub>rad</sub> = 0.19 µs<sup>-1</sup>) might originate from a similar value of the radiative rate constant as AuCu<sub>12</sub> (k<sub>rad</sub> = 0.10 µs<sup>-1</sup>) (Table S1) [25,26].

<figure><img src="image_13.png" /><figcaption>**Figure 6.** (a) The emission spectrum of AuCu<sub>11</sub>H. (b) Comparison emission spectra of AuCu<sub>11</sub>H, AuCu<sub>11</sub>Cl, and AuCu<sub>12</sub> in MeTHF at ambient temperature.</figcaption></figure>

AuCu<sub>11</sub>H is metastable in solution, spontaneously converting into AuCu<sub>11</sub>Cl and AuCu<sub>12</sub> (Scheme 2). The transformation process was monitored by both time-dependent NMR spectroscopy in different types of solvents and ESI-MS analysis. A non-chlorinated solvent was first employed for the investigation. As shown in Scheme 2, a direct conversion of AuCu<sub>11</sub>H into AuCu<sub>12</sub> occurs in acetone. The ¹H NMR chemical shift of the isopropyl groups in AuCu<sub>11</sub>H gradually merges with time and shows good agreement with the chemical shift of the isopropyl group in AuCu<sub>12</sub> (Figures S25 and S26). No hydride peak can be identified in the ¹H NMR spectrum after 24 h. Moreover, whereas the time-dependent ³¹P{¹H} NMR spectra of AuCu<sub>11</sub>H exhibited single sharp peaks at 104.4,
100.5, and 99.9 ppm, respectively, the sharp resonances at 104.4 and 99.9 coalesced into a single sharp peak of 100.5 ppm, which correlates to the ³¹P{¹H} chemical shift of **AuCu₁₂** (Figures S27 and S28). This is also supported by the ESI-MS spectrometry, as depicted in Figures S29 and S30. Thus, during the conversion, **AuCu₁₁H** undergoes aggregation and is accompanied by the rearrangement of the core structure from a vacancy defect Au@Cu₁₁ cuboctahedron to a full **Au@Cu₁₂** cuboctahedron after the successive addition of one copper acetylide unit.

<figure><img src="image_14.png" /><figcaption>**Scheme 2.** Structural transformation of **AuCu₁₁H** to **AuCu₁₂** via **AuCu₁₁Cl** in solution.</figcaption></figure>

In contrast, the conversion of **AuCu₁₁H** in a chlorinated solvent leads to the formation of **AuCu₁₂** via **AuCu₁₁Cl** (Scheme 2). The ¹H NMR spectra identified that the hydride peak gradually vanished after 48 h (Figure S31). As shown in Figure S32, the intensity of the ³¹P{¹H} peaks of **AuCu₁₁H** at 104.4 and 99.9 ppm decreases with time, followed by a new peak at 97.8 ppm, which corresponds to the ³¹P{¹H} chemical shift of **AuCu₁₁Cl** (Figure S33). These time-dependent spectra suggest that the presence of the chloride ion from the chlorinated solvent triggered the hydride-chloride exchange, and the hydride tends to dissociate, releasing H⁺ and being replaced by the chloride ion [25]. Hydride and chloride exchange slowly, giving rise to a weak **AuCu₁₁Cl** signal at *m/z* = 2578.32 Da (*m/z* calc = 2578.30 Da) in the mass spectrum (Figures S34 and S35). Furthermore, this transformation is followed by the degradation of **AuCu₁₁Cl** to **AuCu₁₂** through a partial aggregation process [25]. These results are supported by the ³¹P{¹H} NMR resonance at 97.8 ppm, which shifts to 100.7 ppm within 96 h, i.e., to the chemical shift of **AuCu₁₂**. We thus propose that transformation in the chlorinated solvent allows the metastable **AuCu₁₁H** to be the key precursor in the formation of **AuCu₁₂** through **AuCu₁₁Cl**. The time-dependent ¹H NMR of **AuCu₁₁H** in the chlorinated solvent shows the characteristic of a hydride ligand, indicating less stability of **AuCu₁₁H** in the solution. Based on this result, the hydride in **AuCu₁₁H** acts as an electron-withdrawing hydride, as shown by the dissociation of H⁻ and subsequently followed by the replacement of the chloride ligand to generate **AuCu₁₁Cl**. In addition, **AuCu₁₁Cl** grows via partial aggregation to form **AuCu₁₂** following the sequential addition of one copper acetylide unit (Scheme S1). This study shows that Au-Cu bimetallic NCs grow by creating a hydride bimetallic complex and then exchanging ligands with H⁻ dissociation.

**3. Materials and Methods**

**3.1. General Remarks**

All chemicals were used without further purification and purchased from commercial sources as follows: phenylacetylene, sodium borohydride, sodium borodeuteride, and all the required anhydrous solvents were purchased from Sigma Aldrich. All reactions were conducted in a N₂ atmosphere utilizing typical Schlenk procedures. [Cu(CH₃CN)₄](PF₆) [53], Au(PPh₃)Cl [54], and [NH₄][S₂P(OR)₂] (R = ¹Pr, ²Pr, and ³Bu) [55] were synthesized accord-
ing to the methodology documented in the literature and then described. NMR spectra were acquired using a Bruker Advance DPX300 FT-NMR spectrometer running at 400 MHz for ¹H, 121.5 MHz for ³¹P{¹H}, 100.61 MHz for ¹³C{¹H}, and 46.1 MHz for ²H. The ¹H NMR chemical shift was referenced to residual CDCl₃ (δ = 7.26 ppm) and deuterated acetone (δ = 2.04 ppm). The ¹³C{¹H} NMR spectra were referenced to deuterated acetone (δ = 29.8 and 206.3 ppm). The ³¹P{¹H} NMR spectra were referenced to external 85% H₃PO₄ at δ = 0 ppm. The chemical shift (δ) is expressed in ppm, and processing was carried out with Bruker's Topspin 2.1 software. The ESI-mass spectrometry was obtained using a Fison Quattro Bio-Q (Fisons Instruments, VG Biotech, U.K.). The UV-visible absorption spectra were acquired using a Perkin Elmer Lambda 750 spectrophotometer with quartz cells featuring a 1 cm path length. An Edinburgh FLS920 spectrophotometer captured the photoluminescence excitation and emission spectra using the Xe900 lamp as the excitation source. The PL decays were assessed utilizing an Edinburgh FLS920 spectrometer equipped with a gated hydrogen arc lamp and a scattering solution to characterize the instrument response function.

## 3.2. General Synthesis

The clusters [AuCu₁₁(H){S₂P(O'iPr)₂}₆(C≡CPh)₃], [AuCu₁₁(H){S₂P(O"Pr)₂}₆(C≡CPh)₃], and [AuCu₁₁(H){S₂P(O'Bu)₂}₆(C≡CPh)₃] can be prepared by following general procedures, and a detailed preparation method is given for cluster [AuCu₁₁(H){S₂P(O'iPr)₂}₆(C≡CPh)₃] (AuCu₁₁H).

### 3.2.1. Synthesis of [AuCu₁₁(H){S₂P(O'iPr)₂}₆(C≡CPh)₃] (AuCu₁₁H)

In a Flame-dried Schlenk tube [NH₄][S₂P{O'iPr}₂] (30 mg, 0.15 mmol), Cu(CH₃CN)₄(BF₄) (130 mg, 0.30 mmol) and Au(PPh₃)Cl (4 mg, 0.1 mmol) were suspended in a mixture of THF (15 cm³) and CH₃CN (15 cm³) followed by the continuous addition of phenylacetylene (40 µL, 0.3 mmol) and triethylamine (40 µL, 0.3 mmol). After stirring for 5 min, NaBH₄ (6 mg, 0.15 mmol) was added to the mixture. The resulting mixture was stirred at room temperature for 4 h. The solvent was evaporated under vacuum, and the residue was dissolved in DCM and washed with water (3 × 15 mL). After separation, the organic layer was passed through aluminum oxide, and the orange-red residue was washed with ether and ethyl acetate to get rid of the impurities. The precipitate was then dissolved in methanol (3 × 15 mL) to collect the orange-red solution. Finally, the solvent was evaporated to dryness under vacuum to obtain a pure orange-red powder of AuCu₁₁H. The yield was 13% based on Cu.

Similarly, the deuterium analog [AuCu₁₁(D){S₂P(O'iPr)₂}₆(C≡CPh)₃] was synthesized by reacting NaBD₄ (0.0042, 0.15 mmol), resulting in the formation of [AuCu₁₁(D){S₂P(O'iPr)₂}₆(C≡CPh)₃] (15%, based on Cu).

**[AuCu₁₁H]:** ESI-MS: *m/z* 2542.37 Da (calcd. *m/z* 2542.34 Da) for [AuCu₁₁(H){S₂P(O'iPr)₂}₆(C≡CPh)₃ + Cu⁺]⁺. ¹H NMR (400 MHz, CDCl₃): 7.39 (8H, d, C₆H₅), 7.64 (7H, d, C₆H₅), 4.69–5.06 (12H, m, OCH), 1.07–1.43 (72H, dd, CH₃), and −1.49 (1H, s, µ₂-H) ppm. ³¹P NMR (121.5 MHz, CDCl₃): 104.45, 100.49, and 99.90 ppm. ¹³C NMR (100.61 MHz, d₆-acetone): 205.33 (CH₃COCH₃), 29.84 (CH₃COCH₃), 134.08 (o-C₆H₆), 132.27 (o-C₆H₆), 128.85 (p-C₆H₆), 125.17 (p-C₆H₆), 77.80 (C≡C), 76.18 (C≡C), 34.13 (OCH(CH₃)₂), and 23.22 (OCH(CH₃)₂).

**[AuCu₁₁D]:** ESI-MS: *m/z* 2543.37 Da (calcd. *m/z* 2543.35 Da) for [AuCu₁₁(D){S₂P(O'iPr)₂}₆(C≡CPh)₃ + Cu⁺]⁺. ¹H NMR (400 MHz, CDCl₃): 7.35 (8H, d, C₆H₅), 7.63 (7H, d, C₆H₅), 4.67–5.05 (12H, m, OCH), and 1.05–1.42 (72H, dd, CH₃). ³¹P NMR (121.5 MHz, CDCl₃): 104.45, 100.50, and 99.91 ppm. ²H NMR (400 MHz, CHCl₃): −1.41 (1D, s, µ₂-D) ppm.

### 3.2.2. [AuCu₁₁(H){S₂P(OnPr)₂}₆(C≡CPh)₃]

**[AuCu₁₁(H){S₂P(O"Pr)₂}₆(C≡CPh)₃]:** ESI-MS: *m/z* 2542.37 Da (calcd. *m/z* 2542.34 Da) for [AuCu₁₁(H){S₂P(O"Pr)₂}₆(C≡CPh)₃ + Cu⁺]⁺; *m/z* 2578.32 Da (calcd. *m/z* 2578.30 Da) for [AuCu₁₁(Cl){S₂P(O"Pr)₂}₆(C≡CPh)₃ + Cu⁺]⁺; *m/z* 2642.39 Da (calcd. *m/z* 2642.37 Da) for [AuCu₁₂{S₂P(O"Pr)₂}₆(C≡CPh)₄]⁺. ¹H NMR (400 MHz, CDCl₃): 7.48–7.70 (15H, C₆H₅),
3.94–4.26 (24H, OCH₂), 1.27–1.73 (24H, CH₂), 0.72–1.01 (36H, CH₃), and –1.48 (1H, µ₂-H) ppm. ³¹P NMR (121.5 MHz, CDCl₃): 109.7, 106.2, and 105.9 ppm.

[AuCu₁₁(D){S₂P(O"Pr)₂}₆(C≡CPh)₃]: ESI-MS: m/z 2543.44 Da (calcd. m/z 2543.35 Da) for [AuCu₁₁(D){S₂P(O"Pr)₂}₆(C≡CPh)₃ + Cu⁺]⁺; m/z 2642.46 Da (calcd. m/z 2642.37 Da) for [AuCu₁₂{S₂P(O"Pr)₂}₆(C≡CPh)₄]⁺. ¹H NMR (400 MHz, CDCl₃): 7.37–7.59 (15H, C₆H₅), 3.99–4.26 (24H, OCH₂), 1.34–1.76 (24H, CH₂), and 0.75–0.98 (36H, CH₃). ³¹P NMR (121.5 MHz, CDCl₃): 110.1–106.3, and 105.4 ppm. ²H NMR (400 MHz, CHCl₃): –1.40 (1D, s, D) ppm.

### 3.2.3. [AuCu₁₁(H){S₂P(O'Bu)₂}₆(C≡CPhOCH₃)₃]

[AuCu₁₁(H){S₂P(O'Bu)₂}₆(C≡CPhOCH₃)₃]: ESI-MS: m/z 2800.59 Da (calcd. m/z 2800.56 Da) for [AuCu₁₁(H){S₂P(O'Bu)₂}₆(C≡CPhOCH₃)₃ + Cu⁺]⁺; m/z 2834.52 Da (calcd. m/z 2834.52 Da) for [AuCu₁₁(Cl){S₂P(O'Bu)₂}₆(C≡CPhOCH₃)₃ + Cu⁺]⁺; m/z 2932.64 Da (calcd. m/z 2932.66 Da) for [AuCu₁₂{S₂P(O'Bu)₂}₆(C≡CPhOCH₃)₄]⁺. ¹H NMR (400 MHz, CDCl₃): 6.86–7.57 (12H, C₆H₅OCH₃), 3.71–4.00 (24H, OCH), 3.81 (3H, C₆H₅OCH₃), 1.55–1.72 (12H, CH₂), 0.76–1.00 (36H, CH₃), and –1.63 (1H, µ₂-H) ppm. ³¹P NMR (121.5 MHz, CDCl₃): 109.6, 105.6, and 104.9 ppm.

[AuCu₁₁(D){S₂P(O'Bu)₂}₆(C≡CPhOCH₃)₃]: ESI-MS: m/z 2801.55 Da (calcd. m/z 2801.57 Da) for [AuCu₁₁(D){S₂P(O'Bu)₂}₆(C≡CPhOCH₃)₃ + Cu⁺]⁺; m/z 2672.52 Da (calcd. m/z 2671.70 Da) for [AuCu₁₁(D){S₂P(O'Bu)₂}₆(C≡CPhOCH₃)₃]. ¹H NMR (400 MHz, CDCl₃): 6.86–7.57 (12H, C₆H₅OCH₃), 3.71–4.00 (24H, OCH), 3.81 (3H, C₆H₅OCH₃), 1.55–1.72 (12H, CH₂), and 0.76–1.00 (36H, CH₃). ³¹P NMR (121.5 MHz, CDCl₃): 109.6, 105.6, and 104.9 ppm. ²H NMR (400 MHz, CHCl₃): –1.74 (1D, s, D) ppm.

### 3.3. Computational Details

Geometry optimizations were performed by DFT calculations with the Gaussian 16 package [56] using the PBE0 functional [57] and the all-electron Def2-TZVP basis set from the EMSL Basis Set Exchange Library [58]. All the optimized geometries were characterized as true minima on their potential energy surface by harmonic vibrational analysis. The Wiberg bond indices were computed with the NBO 6.0 program [59]. The UV-visible transitions were calculated by means of TD-DFT calculations [60]. Only singlet–singlet, i.e., spin-allowed, transitions were computed. The UV-visible spectra were simulated from the computed TD-DFT transitions and their oscillator strengths by using the SWizard program [61], each transition being associated with a Gaussian function of half-height width equal to 2000 cm⁻¹. The compositions of the molecular orbitals were calculated using the AOMix program [62], and the NMR chemical shifts were calculated by using the gauge-including atomic orbital (GIAO) method [63]. All calculations correspond to the molecule in vacuum.

## 4. Conclusions

We report here the synthesis and characterization of the bimetallic hydride cluster AuCu₁₁H. The combination of spectroscopic methods and DFT calculations allowed establishing that AuCu₁₁H has a similar structure as its AuCu₁₁Cl relative. Its ligand-protected metallic core is composed of a defective Au@Cu₁₁ cuboctahedron. AuCu₁₁H is metastable and, in a chlorinated solvent, undergoes a spontaneous ligand-exchange transformation into AuCu₁₁Cl and aggregates into the AuCu₁₂ NC of increased nuclearity.

**Supplementary Materials:** The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules29184427/s1, Figure S1. Comparison between the experimental data (top) and the simulated (bottom) isotope patterns of peaks (a) [AuCu₁₁Cl+Cu⁺]⁺ and (b) [AuCu₁₂]⁺; Figure S2. ESI-MS spectra [AuCu₁₁D+Cu⁺]⁺. The insets show the comparisons between the experimental data (top) and the simulated (bottom) isotope patterns for [AuCu₁₁D+Cu⁺]⁺; Figure S3. Comparisons between the experimental data (top) and the simulated (bottom) isotope patterns of peaks (a) [AuCu₁₁D] and (b) [AuCu₁₂]⁺; Figure S4. ¹H NMR spectrum of AuCu₁₁D in CDCl₃; Figure S5. ³¹P{¹H} NMR spectrum of AuCu₁₁H in CDCl₃; Figure S6. ³¹P{¹H} NMR spectrum of AuCu₁₁D in CDCl₃; Figure S7. ¹³C NMR spectrum of AuCu₁₁H in *d*₆-acetone; Figure S8. The ESI-MS spectrum
of [AuCu₁₁(H)S₂P(O''Pr)₂]₆(C≡CPh)₃+Cu⁺]⁺ (I), [AuCu₁₁(Cl)S₂P(O''Pr)₂]₆(C≡CPh)₃+Cu⁺]⁺ (II), and [AuCu₁₂[S₂P(O''Pr)₂]₆(C≡CPh)₄]⁺ (III); Figure S9. Comparisons between the experimental data (top) and the simulated (bottom) isotope patterns of peaks (a) [AuCu₁₁(H)S₂P(O''Pr)₂]₆(C≡CPh)₃+Cu⁺]⁺ (I), (b) [AuCu₁₁(Cl)S₂P(O''Pr)₂]₆(C≡CPh)₃+Cu⁺]⁺ (II), and (c) [AuCu₁₂[S₂P(O''Pr)₂]₆(C≡CPh)₄]⁺ (III); Figure S10. The ESI-MS spectrum of [AuCu₁₁(D)S₂P(O''Pr)₂]₆(C≡CPh)₃+Cu⁺]⁺ (I) and [AuCu₁₂[S₂P(O''Pr)₂]₆(C≡CPh)₄]⁺ (II); Figure S11. Comparisons between the experimental data (top) and the simulated (bottom) isotope patterns of peaks (a) [AuCu₁₁(D)S₂P(O''Pr)₂]₆(C≡CPh)₃+Cu⁺]⁺ (I) and (b) [AuCu₁₂[S₂P(O''Pr)₂]₆(C≡CPh)₄]⁺ (II); Figure S12. The ESI-MS spectrum of [AuCu₁₁(H)S₂P(O'IBu)₂]₆-(C≡CPhOCH₃)₃+Cu⁺]⁺ (I), [AuCu₁₁(Cl)S₂P(O'IBu)₂]₆(C≡CPhOCH₃)₃+Cu⁺]⁺ (II), and [AuCu₁₂[S₂P(O'IBu)₂]₆-(C≡CPhOCH₃)₄]⁺ (III); Figure S13. Comparisons between the experimental data (top) and the simulated (bottom) isotope patterns of peaks (a) [AuCu₁₁(H)S₂P(O'IBu)₂]₆(C≡CPhOCH₃)₃+Cu⁺]⁺ (I), (b) [AuCu₁₁(Cl)S₂P(O'IBu)₂]₆-(C≡CPhOCH₃)₃+Cu⁺]⁺ (II), and (c) [AuCu₁₂[S₂P(O'IBu)₂]₆(C≡CPhOCH₃)₄]⁺ (II); Figure S14. The ESI-MS spectrum of [AuCu₁₁(D)S₂P(O'IBu)₂]₆(C≡CPhOCH₃)₃+Cu⁺]⁺ (I) and [AuCu₁₀[S₂P(O'IBu)₂]₆(C≡CPhOCH₃)₃]⁺ (II). Inset: Comparisons between the experimental data (top) and the simulated (bottom) isotope pattern of peak (a) [AuCu₁₁(D)S₂P(O'IBu)₂]₆(C≡CPhOCH₃)₃+Cu⁺]⁺ (I); Figure S15. ¹H NMR spectrum of [AuCu₁₁(H)S₂P(O''Pr)₂]₆(C≡CPh)₃] in CDCl₃; Figure S16. ³¹P{¹H} NMR spectrum of [AuCu₁₁(H)S₂P(O''Pr)₂]₆(C≡CPh)₃] in CDCl₃; Figure S17. ²H NMR spectrum of [AuCu₁₁(D)S₂P(O''Pr)₂]₆(C≡CPh)₃] in CHCl₃; Figure S18. ¹H NMR spectrum of [AuCu₁₁(H)S₂P(O'IBu)₂]₆(C≡CPhOCH₃)₃] in CDCl₃; Figure S19. ³¹P{¹H} NMR spectrum of [AuCu₁₁(H)S₂P(O'IBu)₂]₆(C≡CPhOCH₃)₃] in CDCl₃; Figure S20. ²H NMR spectrum of [AuCu₁₁(D)S₂P(O'IBu)₂]₆(C≡CPhOCH₃)₃] in CHCl₃; Figure S21. Comparison of the UV-vis absorption spectra of AuCu₁₁H, AuCu₁₁Cl, and AuCu₁₂ in 2-MeTHF solution; Figure S22. The TD-DFT-simulated UV-vis spectrum of AuCu₁₁H, with individual transitions as vertical bars of lengths proportional to their oscillator strengths; Figure S23. (a) The UV-vis absorption spectrum of AuCu₁₁H. (b) The excitation spectrum of AuCu₁₁H in 2-MeTHF solution at ambient temperature; Figure S24. The emission lifetime of cluster AuCu₁₁H at ambient temperature; Figure S25. Structural transformation of AuCu₁₁H into AuCu₁₂ in d₆-acetone monitored by time-dependent ¹H NMR spectroscopy; Figure S26. ¹H NMR spectrum of AuCu₁₂ in d₆-acetone; Figure S27. Structural transformation of AuCu₁₁H into AuCu₁₂ in d₆-acetone monitored by time-dependent ³¹P{¹H} NMR spectroscopy; Figure S28. ³¹P{¹H} NMR spectrum of AuCu₁₂ in d₆-acetone; Figure S29. Positive-mode ESI-MS spectra of structural transformation of AuCu₁₁H in acetone; Figure S30. Isotope pattern of (a) AuCu₁₁H and (b) AuCu₁₂ in structural transformation; inset shows experimental (black) and simulated (red) isotopic distribution pattern; Figure S31. Structural transformation of AuCu₁₁H into AuCu₁₂ via AuCu₁₁Cl in CDCl₃ monitored by time-dependent ¹H NMR spectroscopy; Figure S32. Structural transformation of AuCu₁₁H into AuCu₁₂ via AuCu₁₁Cl in CDCl₃ monitored by time-dependent ³¹P{¹H} NMR spectroscopy; Figure S33. ³¹P{¹H} NMR spectrum of AuCu₁₁Cl in CDCl₃; Figure S34. Positive-mode ESI-MS spectra of structural transformation of AuCu₁₁H in CH₂Cl₂; Figure S35. Isotope pattern of (a) AuCu₁₁H, (b) AuCu₁₁Cl, and (c) AuCu₁₂ in structural transformation; inset shows experimental (black) and simulated (red) isotopic distribution pattern; Figure S36. ¹H NMR spectrum of AuCu₁₁Cl in d₆-acetone; Table S1. The 298 K absorption, emission, and lifetime of AuCu₁₁H, AuCu₁₁Cl, and AuCu₁₂; Scheme S1. A proposed mechanism of chloride-hydride ligand exchange, and aggregation within the transformation core of AuCu₁₁H to AuCu₁₂ via AuCu₁₁Cl in chloride-containing solvent; Table S2. Cartesian coordinates of the DFT-optimized structure of AuCu₁₁H and its high-energy isomer.

**Author Contributions:** Synthesis, R.P.B.S.; writing—original draft preparation, R.P.B.S.; conceptualization, J.-Y.S. and C.W.L.; software, S.K. and J.-Y.S.; validation, S.K. and J.-Y.S.; formal analysis, S.K. and J.-Y.S.; investigation, S.K. and J.-Y.S.; data curation, S.K. and J.-Y.S.; writing—review and editing, J.-Y.S. and C.W.L.; supervision, C.W.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Science and Technology Council of Taiwan (113-2123-M-259-001) and the GENCI computing resource (grant A0090807367).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.
**Acknowledgments:** The authors acknowledge the National Dong Hwa University and the University of Rennes.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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