# Diselenophosphate Ligands as a Surface Engineering Tool in PdH-Doped Silver Superatomic Nanoclusters

Yu-Rong Ni, Michael N Pillay, Tzu-Hao Chiu, Jagadeesh Rajaram, Ying-Yann Wu, Samia Kahlal, Jean-Yves Saillard, C W Liu

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Yu-Rong Ni, Michael N Pillay, Tzu-Hao Chiu, Jagadeesh Rajaram, Ying-Yann Wu, et al. Diselenophosphate Ligands as a Surface Engineering Tool in PdH-Doped Silver Superatomic Nanoclusters. Inorganic Chemistry, 2024, Inorganic Chemistry, 63 (5), pp.2766-2775. 10.1021/acs.inorgchem.3c04253. hal-04443357

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Distributed under a Creative Commons Attribution 4.0 International License
Diselenophosphate ligands as a surface engineering tool in PdH
doped silver superatomic nanoclusters

Yu-Rong Ni,[a] Michael N. Pillay,[a] Tzu-Hao Chiu,[a] Jagadeesh Rajaram,[a] Ying-Yann Wu,[a] Samia Kahlal,[b] and Jean-Yves Saillard,[* [b] and C. W. Liu*[a]

a. Department of Chemistry, National Dong Hwa University, Hualien, 97401, Taiwan, Republic of China. E-mail: chenwei@gms.ndhu.edu.tw

b. Univ Rennes, CNRS, ISCR-UMR 6226, F-35000, France. Email: jean-yves.saillard@univ-rennes.fr

KEYWORDS superatoms, diselenophosphates, silver, palladium, hydride

ABSTRACT. The first hydride-doped Pd/Ag superatoms stabilized by selenolates are reported: [PdHAg₁₉(dsep)₁₂] (dsep = Se₂P(O'Pr)₂) **1**, and [PdHAg₂₀(dsep)₁₂]⁺ **2**. **1** was derived from the targeted transformation of [PdHAg₁₉(dtp)₁₂] (dtp = S₂P(O'Pr)₂) by ligand exchange, whereas **2** was obtained from the addition of trifluoroacetic acid to **1**, resulting in a symmetric redistribution of the silver capping atoms. The transformations are all achieved while retaining an 8-electron superatomic configuration. VT-NMR attests to the good stability of the NCs in solution, and SCXRD reveals the crucial role the interstitial hydride plays in directing the position of the capping silver atoms. The total structures are reported alongside their electronic structures and optical properties. **1** and **2** are phosphorescent with a lifetime of 73 and 84 µs at 77 K,
respectively. The first antibacterial activity data for superatomic bimetallic Pd/Ag nanoclusters
are also reported.

**Introduction**

Crucial to the application of atomically precise nanoclusters (NCs) is the ability to controllably
modify their structural features and, in turn, their physicochemical properties. The search for new
configurations is motivated by the diversity of applications, which, *inter alia*, includes vital
renewable energy technologies¹,² and catalysis.³⁻⁷ Although creating novel architectures is
always essential, synthetic protocols that provide NCs with incremental atomically precise
changes (in nuclearity, symmetry, and surface nature) is of significant importance and often a
more challenging aspect of NC chemistry. Therefore, it is paramount to understand and control
these manipulations from the fundamental point of view and unlock the potential of NCs in
future technologies.

Coinage metals are of particular interest in the formation of ligand-protected NCs, particularly
because of their rich optical properties.⁸ Since our first report on silver superatomic icosahedral
frameworks,⁹ our priority has been to develop synthetic protocols for the optimal and precise
control of NC architecture and composition.⁹⁻¹¹ Ligand exchange-induced structure
transformation (LEIST) has been an efficient tool in fine-tuning NC morphology and has been
the subject of a review.¹² We have previously established the ability of diselenophosphate (dsep)
to completely substitute dithiophosphonates (dtp) in silver and copper frameworks,¹³ and have
expanded the methodology to group 10-doped NCs.¹⁴ Firstly, we isolated a Pd-doped NC of the
type [Pd@Ag₂₀{Se₂P(O'Pr)₂}₁₂], demonstrating the viability of LEIST surface modification in
these systems.¹⁴ This was followed by the isolation of a PtH derivative
[(PtH)@Ag₁₉{Se₂P(O'Pr)₂}₁₂] (structure confirmed by neutron diffraction), demonstrating a
remarkable configurational change of the metallic skeleton.¹⁵ Entrapping a PdH motif in superatomic coinage metal constructs has been realized only recently. It is exceedingly rare for silver-rich constructs,¹⁶⁻¹⁸ and so far unknown in the absence of dsep stabilizing ligands. Inserting PdH units into an Ag₁₂ icosahedron, itself surrounded by an outer capping layer, has been a focus of our current research.¹⁹ The stability of these closed-shell doped systems can be rationalized within the *superatom* model, with the dopant contributing to the stability of these assemblies.¹⁵,¹⁹ We report below the transformation of PdH-doped metallic frameworks with LEIST to yield unique structural conformations that would be otherwise inaccessible. Hydrides are well known for their invaluable electron-donor ability, thus reducing metal atoms to give rise to electron-precise mixed-valent *superatoms*. They can also behave as a dopant within the *superatom* core, thus contributing with one electron to the *superatom* count, ¹⁰,¹¹,¹³,¹⁵,²⁰⁻²³ whereas, in these systems, the Pd atom contribution to the *superatomic* count is zero.²⁴⁻²⁷ In this report, we provide experimental evidence for the ability of the hydride to dictate the configuration of the outer metal/ligand shell.

Two novel luminescent 8-electron superatoms: [PdHAg<sub>19</sub>(dsep)<sub>12</sub>] **1** and [PdHAg<sub>20</sub>(dsep)<sub>12</sub>](TFA) ([2](TFA)); TFA = trifluoroacetate), are isolated and characterized by single-crystal X-ray diffraction (SCXRD), X-ray photoelectron spectroscopy (XPS), ESI-TOF-MS, UV-vis, and NMR spectroscopy.

<figure><img src="image_2.png" /></figure>

**Scheme 1.** Ligand exchange protocol for the surface modification of Pd doped NC and the controlled addition of a single Ag atom.
## Results and discussion

The reaction of the diselenophosphate (dsep) ligand with [PdHAg<sub>19</sub>(dtp)<sub>12</sub>] in THF results in the formation of **1**, Scheme 1. The reaction is light-sensitive and must be performed in a reasonably dark environment. The conversion is not quantitative, and purification by column chromatography yields **1** in 54% yield. It should be noted that the direct co-reduction method in the presence of dsep as the ligand does not yield the target product. Once isolated, **1** shows moderate stability and is amenable to transformations similar to those we reported for the dtp analogs.<sup>19</sup> Briefly, the formation of **2** is induced by adding HTFA, which promotes the inter-cluster transfer of silver atoms. An 8-electron count is maintained throughout the conversion, and **2** is obtained in 62% yield.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Molecular representation of **1** (a) and **2** (b) (Atom color code: blue, P; orange, Se; purple, capping Ag (Ag<sub>cap</sub>); pale blue, capping Ag along C<sub>3</sub> axis (Ag<sub>cap</sub>'); green, icosahedron Ag; brown, Pd; yellow, H.)</figcaption></figure>
<table><caption>**Table 1.** Selected bond lengths and geometrical parameters for **1** and **2**.</caption><thead><tr><th></th><th>1</th><th>2</th></tr></thead><tbody><tr><td>CSM</td><td>0.12</td><td>0.09</td></tr><tr><td>Pd<sub>cen</sub>-Ag<sub>ico</sub>, Å</td><td>2.7589(8)-2.8895(8)<br>avg. 2.8042(8)</td><td>2.691(3)-2.876(3)<br>avg. 2.788(3)</td></tr><tr><td>Ag<sub>ico</sub>-Ag<sub>ico</sub>, Å</td><td>2.8427(9)-3.3021(9)<br>avg. 2.9490(8)</td><td>2.854(3)-3.002(4)<br>avg. 2.9312(3)</td></tr><tr><td>Ag<sub>ico</sub>-Ag<sub>cap</sub>, Å</td><td>2.8931(9)-3.0549(8)<br>avg. 2.9697(9)</td><td>2.880(3)-3.042(3)<br>avg. 2.964(3)</td></tr><tr><td>Ag<sub>ico</sub>-Ag<sub>cap</sub>', Å</td><td>2.9471(8)-2.9765(10)<br>avg. 2.9630(8)</td><td>2.917(3)-3.008(2)<br>avg. 2.963(3)</td></tr></tbody></table>

Single-crystal X-ray diffraction data collected for **1** and **2** were of sufficient quality that the position of the hydride could be assigned from the residual electron densities. Moreover, their positions were supported by density functional theory (DFT) calculations (vide infra). Selected bond lengths are given in Table 1, with molecular representations illustrated in Figure 1, S7-8. Refinement and crystallographic data are presented in Table S1. A common feature in both structures is a 12-atom silver icosahedron centered by a PdH unit. The encapsulated PdH motif distorts the Ag<sub>12</sub> icosahedron, highlighted by the continuous symmetry measure (CSM) values of 0.12 and 0.09 for **1** and **2**, respectively. The distortion is lower in **2** compared to **1** and significantly lower than in the dtp analog (2dtp), correlating to the increase in symmetry observed in solution NMR. Furthermore, an assessment of the Ag-Ag bond lengths given in Figure S5 emphasizes the significant distortion observed in **1**, with Ag···Ag contacts extending to 3.3 Å. The driving force behind the transformation of the metallic framework is related to the geometrical differences between the S-P-S and Se-P-Se moieties. The size difference between Se and S plays a crucial role in the ligand bite distance.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Metallic framework present in parent cluster [PdHAg<sub>19</sub>(dtp)<sub>12</sub>] (a) and the improved symmetric assembly present in **1** (b) after LEIST.</figcaption></figure>

Considering the distances between the two chalcogens, the Se...Se distances are ca 3.66 Å while the S...S distance is ca 3.39 Å. The seemingly trivial 0.27 Å difference plays a crucial role in allowing a more symmetrical arrangement of the capping atoms in the solid state. Also, it explains the lack of isomerization in the solution. Unlike the dtp analogs, which can only achieve these symmetric arrangements in solution, the dsep analogs stabilize C<sub>3</sub> configurations in the solid state, Figure 2.

Also of note is the effect of the kernel composition on the position of the capping atoms. The inclusion of the interstitial hydride not only plays a crucial role in the NC stabilization but also has a significant structural effect on the metallic shell arrangement. Comparing Ag<sub>20</sub> shells in Pd@Ag<sub>20</sub>(dtp<sub>12</sub>)<sup>28</sup>, Pd@Ag<sub>20</sub>(dsep)<sub>12</sub><sup>14</sup>, PdH@Ag<sub>20</sub>(dtp)<sub>12</sub><sup>19</sup>, and PdH@Ag<sub>20</sub>(dsep)<sub>12</sub> (**2**), we found the hydride to be directive towards a particular Ag<sub>20</sub> framework. In the absence of a hydride dopant, crystal growth of Pd@Ag<sub>20</sub>(dsep)<sub>12</sub> results in a co-crystal consisting of two distinct C<sub>3a</sub> and C<sub>3b</sub> metallic frameworks, Figure 3.<sup>14</sup>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** The different metallic frameworks of **2dtp** ($C_{3a}$) and **2** ($C_{3b}$), illustrating the transformation driven by the ligand nature and the encapsulated hydride.</figcaption></figure>

In the case of **2**, the $C_{3b}$ arrangement is exclusively obtained, while the $C_{3a}$ structure is obtained in the case of $PdH@Ag_{20}(dtp)_{12}$.¹⁹ The Pd-H bond axis dictates the orientation of the $C_3$ rotational axis, thus affording fewer possible positions to occupy for the remaining six capping atoms while retaining the symmetric arrangement.

Combining this effect with the limited reach of the dtp ligand, the occupation of adjacent triangular faces is necessary to passivate the assembly. In **2**, the dsep ligands allow the capping atoms to occupy faces further away, resulting in the $C_{3b}$ structure. Furthermore, the arrangement of the capping atoms in **2** creates several equivalent interstitial sites for occupation by the hydride, which relates to the dynamic behavior observed in VT-NMR. The change in the configuration and the surface of the cluster have a profound effect on the physical properties of the NC, resulting in both NCs being emissive at 77 K.
In our investigations of the isomeric distributions in the thiolate analogs, we found the solid state confined to one single configuration, whereas a complex isomer mixture was identified in solution from low-temperature NMR experiments.¹⁹ In stark contrast to their dtp analogs, the dsep species are substantially more rigid in solution at low temperatures. In the present series, no competing isomerization was identified in solution up to 213 K. In both cases, the dsep ligand induces the formation of the more symmetrical isomer, which is substantially more rigid in solution, as evident in the VT NMR, shown in Figure 4.

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

**Figure 4.** (a) ³¹P{¹H} NMR (CD₂Cl₂) (left) of **1** indicating the lability of the ligand environment at 293 K, and ¹H NMR (right) with quartet resonance observed at 213 K. Color-coded ligand environments corresponding to the four phosphorus resonances observed in **1** (b) and the single ligand environment observed in **2** (c). (d) ³¹P{¹H} NMR (CD₂Cl₂) (left) indicating the pseudo symmetrically equivalent ligand environment maintained over the range of 293 – 213K, and dynamic hydride environment observed in the ¹H NMR (right) due to the movement of the hydride within the Ag₁₂ icosahedron.
The low-temperature ³¹P NMR spectrum of **1** shows four non-equivalent resonances (Figure 4a), which correspond directly to its solid-state structure of *C*₃ symmetry, wherein four distinct phosphorus environments are present, Figure 4b. In **2**, a single ³¹P resonance is observed during the VT-NMR (Figure 4d), indicating that the twelve dsep ligands are engaged in the same coordination mode (μ₃: η₂, η₁), Figure 4c. The pair of selenium satellite peaks at 213 K corresponds to the highly symmetrical configuration of **2**, with two chemically and magnetically inequivalent Se atoms bonded to the same P atom, Figure 1d. The ³¹P NMR is significantly downfield compared to dtp analogs, with resonances observed at 63.63 (¹J<sub>PSe</sub> = 633.36 Hz) and 63.26 (¹J<sub>PSe</sub> = 635.44 Hz) for **1** and **2**, respectively, Figure S4a,b. The integral values of the ¹H NMR signals in both **1** and **2** correlate to a ratio of 12 dsep ligands to a single hydride, Figure S2. Hydride resonances for **1** and **2** are significantly different, -7.2 ppm and -10.7 ppm, respectively. The 3.5 ppm disparity in the hydride signal further emphasizes the susceptibility of the interstitial hydride environment to the nature of the external silver shell. Thus, not only do the hydride resonances confirm their presence, but they also provide a diagnostic tool for differentiating metallic arrangements. The hydride resonance assignments are confirmed by ²H NMR of their deuterated derivatives ¹D and ²D, which resonate at -7.1 and -10.6 ppm, respectively, Figure S3.

At ambient conditions, the single broad hydride peak in **1** and **2** shows no definition. However, a reduction in temperature results in the formation of a quartet at -6.8 ppm for **1**. The quartet resonance has been established as resulting from the hydride coupling with three equivalent Ag atoms defining an icosahedron face while occupying an interstitial position inside a PdAg₃ tetrahedron.¹⁹ In contrast, the hydride resonance in **2** does not form a quartet at low temperatures, and the peak maintains a multiplet.
This is due to the equivalent (symmetrically and energetically) sites for the hydride to reside. The magnetically equivalent ³¹P nuclei indicate the formation of a Tₕ-symmetric metallic skeleton, which is, rigorously speaking, impossible because of the non-point PdH central unit. However, in solution and on an NMR time scale, the hydride couples with all the triangular Ag₃ faces of the icosahedron, indicating it is able to migrate even at low temperatures. The TFA⁻ anion in 2 was further confirmed by ¹⁹F NMR, with a single resonance observed at -76.5 ppm, Figure S4c.

The composition of clusters **1** and **2** were confirmed by ESI-TOF-MS spectrometry, and a good correlation between experimental and simulated isotopic distribution patterns was observed, Figure 5. The mass spectrum of **1** shows a prominent *m/z* peak at 5949.8566 Da assigned to the silver adduct [1 + Ag]⁺ (calc. 5949.9058 Da). Attempts to prepare alternative adducts were unsuccessful, with the addition of tertiary butyl ammonium or cesium cations prior to analysis not yielding the desired adducts and consistently indicating the formation of [1 + Ag]⁺. This further emphasizes the propensity of **1** to add an additional Ag atom and the preference of the dsep ligand to bind to silver. The cationic [2]⁺ molecular ion peak is observed at *m/z* 5949.8784 Da (calc. 5949.9058 Da).

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Experimental (black) and simulated (green) ESI-TOF-MS spectrums for **1**(a) and **2**(b).</figcaption></figure>
The minimal fragmentation observed in the spectra is a testament to the substantial stability of the dsep derivatives, which show remarkable resistance to fragmentation in solution even under the harsh conditions required for ionization.

The deuterated derivatives follow a similar fragmentation pattern with adduct formation
[1<sub>D</sub>+Ag]<sup>+</sup> *m/z* 5950.0186 Da (calc. 5950.8991 Da) and [2<sub>D</sub>]<sup>+</sup> *m/z* 5950.8874 Da (calc. 5950.8991 Da), Figure S1. Importantly, the increase in the *m/z* ratio confirms the inclusion of a single hydride within both **1** and **2**.

**Optical properties**

The contributions from the 12 dsep ligands dominate the optical properties, with both NCs
having relatively similar absorption, irrespective of the modified kernel. The UV-vis spectra
share a common peak centered *ca* 410 nm, with minimally different additional bands at 500 and
485 nm for **1** and **2**, respectively (Figure 6).

<figure><img src="image_8.png" /><figcaption>**Figure 6.** UV-vis absorption and emission spectra for **1-2** (λex = 448 nm) recorded in 2-MeTHF. DFT-simulated phosphorescence spectrum of the C<sub>3b</sub> isomer of **2**.</figcaption></figure>
The second band significantly differs from dtp variants in profile, which contained a major band at 405 nm and shoulder peaks at 455 and 462 nm for **1** and **2**, respectively.¹⁹ Intriguing is the influence of the hydride on the emissive state, considering that the dtp analogs containing hydride-doped cores were found to be non-emissive.¹⁹ The additional silver atom results in a marginal red shift in the $\lambda_{em}$ from 725 nm in **1** to 739 nm in **2**. The lifetime of the emission for **1** and **2** is 73 and 84 µs, respectively, Figure S6. A comparison of the emission wavelength ($\lambda_{em}$) for structural-related NCs containing related dopants is presented in Table 2. In broad terms, the dsep analogs can achieve relatively blue-shifted emission wavelengths compared to their dtp counterparts.

<table><caption>**Table 2.** Emission trend in superatomic doped clusters</caption><thead><tr><th>NC</th><th>Dopant</th><th>Ligand</th><th>λ<sub>em</sub>/nm</th><th>Ref</th></tr></thead><tbody><tr><td>[PtHAg<sub>19</sub>{Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>]</td><td>PtH</td><td>dsep</td><td>636</td><td>15</td></tr><tr><td>[PdAg<sub>20</sub>{Se<sub>2</sub>P(O<sup>n</sup>Pr)<sub>2</sub>}<sub>12</sub>]</td><td>Pd</td><td>dsep</td><td>702</td><td>14</td></tr><tr><td>[PdAg<sub>20</sub>{Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>]</td><td>Pd</td><td>dsep</td><td>712</td><td>14</td></tr><tr><td>[PdHAg<sub>19</sub>{Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>]</td><td>PdH</td><td>dsep</td><td>725</td><td>This work</td></tr><tr><td>[PdHAg<sub>20</sub>{Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>]<sup>+</sup></td><td>PdH</td><td>dsep</td><td>739</td><td>This work</td></tr><tr><td>[PdAg<sub>20</sub>{S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>]</td><td>Pd</td><td>dtp</td><td>741</td><td>14</td></tr><tr><td>[PdAg<sub>20</sub>{S<sub>2</sub>P(O<sup>n</sup>Pr)<sub>2</sub>}<sub>12</sub>]</td><td>Pd</td><td>dtp</td><td>748</td><td>28</td></tr><tr><td>[AuAg<sub>19</sub>{S<sub>2</sub>P(O<sup>n</sup>Pr)<sub>2</sub>}<sub>12</sub>]</td><td>Au</td><td>dtp</td><td>782</td><td>20</td></tr><tr><td>[AuAg<sub>20</sub>{S<sub>2</sub>P(O<sup>n</sup>Pr)<sub>2</sub>}<sub>12</sub>]<sup>+</sup></td><td>Au</td><td>dtp</td><td>745</td><td>20</td></tr><tr><td>[Ag<sub>20</sub>{S<sub>2</sub>P(O<sup>n</sup>Pr)<sub>2</sub>}<sub>12</sub>]</td><td>Ag</td><td>dtp</td><td>835</td><td>20</td></tr><tr><td>[PdAg<sub>21</sub>{S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>]<sup>+</sup></td><td>Pd</td><td>dtp</td><td>842</td><td>19</td></tr></tbody></table>
This indicates the precision that can be afforded by the addition of single silver atoms to the metal framework, controllably moving the emission wavelength over a few nm.

## Computational analysis

DFT calculations were performed on **1** and **2** at the BP86/Def2-TZVP level of calculations (see Computational Details). For the sake of computational limits, the dsep ligands were replaced by $Se_2PH_2$, a simplification which has been proved to be reasonable in many past investigations.¹⁰, ¹¹, ¹³-¹⁶, ¹⁹-²², ²⁸ The optimized geometries of **1** and **2** are in good agreement with their experimental counterparts and fully confirm the hydride locations (compare Tables 2 and 3).

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

**Figure 7.** The metal frameworks of the computed isomers of **1** (top) and **2** (bottom), with their free energies at 298 K (ΔG). The $C_{3a,b}$ and $C_{1-c}$ labels refer to the definitions given in reference 19 in the main text. The vertical arrows illustrate the structure relationships between the $C_{3a}$ and $C_{3b}$ structures of **1** and **2** by addition on **1** of an $Ag_{cap'}$ atom (in blue) on the $C_3$ axis.
Following our investigation on the related dithiolates **1dtp** and **2dtp**,¹⁹ we also computed various
possible isomers of **1** and **2**, all differing from the topology of their protecting outer shell made
of 12 dsep ligands and 7 and 8 capping Ag(I) atoms, respectively. As for their dtp analogs, the
free energy differences between the computed isomers are small, lying in a range of 4.2 and 8.3
kcal/mol in the case of **1** and **2**, respectively (Figure 7). The most stable isomer of **1** (labeled C₃b)
is of C₃ ideal symmetry (ligand configuration considered). Its Ag₁₂@Ag₇ silver framework can
be described a made of an icosahedron inscribed in a cube of which one vertex is vacant. The
encapsulated Pd-H unit is aligned along the C₃ axis, which contains the missing vertex. This
structure differs from the SCXRD one, which is also of C₃ symmetry (labeled C₃a in Figure 7) but
has a different arrangement of the 7 outer metals (Figures 2 and 8) and is computed to be only
3.2 kcal/mol higher in free energy. Adding one supplementary Ag(I) capping atom on the C₃ axis
of these two structures generates two isomers of **2**, still of C₃ symmetry, which differ in energy
by only 1.3 kcal/mol. Alternative C₁ structures generated from related species¹⁹ lie at somewhat
higher energy (see Figure 7). The most stable isomer of **2** is of C₃ symmetry (labeled C₃b) and
depicts an Ag₁₂ icosahedron inscribed in an Ag₈ cube, with the Pt-H unit along one cubic
diagonal and corresponds to the SCXRD structure of **2**. Hence, the encapsulated hydride lies
within a PdAg₃ tetrahedron, of which the three Ag atoms depicts one of the eight icosahedron
faces, which are capped by an outer Ag(I) atom. Moving this hydride inside one of the 20 PdAg₃
tetrahedra which are not capped by an Ag(I) atom leads to an additional isomeric structure of C₁
symmetry (labeled C₁d) which is computed to be almost isoenergetic with the global C₃ minimum
(Figure 7). This result suggests that all the PdAg₃ tetrahedral sites in **2** are statistically nearly
equiprobable for H-occupation and that the encapsulated hydride is likely moving nearly freely
from one to another.
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Kohn-Sham frontier orbital diagram of **1** and **2** (isomeric forms corresponding to their SCXRD structures ($C_{3a}$ and $C_{3b}$ for **1** and **2**, respectively)). The 1P and 1D orbitals are plotted in Figure S10.</figcaption></figure>

<table><caption>**Table 3.** Selected DFT-computed data for the optimized $C_3$ geometries of **1** ($C_{3a}$) and **2** ($C_{3b}$) which correspond to their SCXRD structures. Average distances are in Å and their corresponding Wiberg bond indices (WBI) into brackets.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">HOMO<br>-LUMO<br>gap<br>(eV)</th><th colspan="5">NAO charges (avg.)</th><th colspan="6">Distances [WBI] (avg.)</th></tr><tr><th>H</th><th>Pd</th><th>Ag<sub>ico</sub></th><th>Ag<sub>cap</sub></th><th>Pd-H</th><th>Ag<sub>ico</sub>-H</th><th>Pd<sub>cen</sub>-Ag<sub>ico</sub></th><th>Ag<sub>ico</sub>-Ag<sub>ico</sub></th><th>Ag<sub>ico</sub>-Ag<sub>cap</sub></th><th>Ag<sub>ico</sub>-Ag<sub>cap'</sub></th></tr></thead><tbody><tr><td>1</td><td>1.63</td><td>-0.44</td><td>-0.94</td><td>+0.33</td><td>+0.66</td><td>1.710<br>[0.142]</td><td>1.994<br>[0.108]</td><td>2.887<br>[0.133]</td><td>3.005<br>[0.082]</td><td>3.104<br>[0.044]</td><td>3.091<br>[0.030]</td></tr><tr><td>2</td><td>1.78</td><td>-0.45</td><td>-0.94</td><td>+0.32</td><td>+0.67</td><td>1.706<br>[0.120]</td><td>1.987<br>[0.123]</td><td>2.887<br>[0.137]</td><td>3.000<br>[0.082]</td><td>3.082<br>[0.037]</td><td>3.102<br>[0.030]</td></tr></tbody></table>
This is consistent with the variable temperature ¹H and ³¹P NMR data, which indicate a highly symmetrical cluster (in average) at the NMR time scale (see above). As also found for their dtp analogues,¹⁹ the various isomers computed for **1** and **2** exhibit similar metrics and electronic structure. The Kohn-Sham orbital diagrams of the C₃ geometries that correspond to their SCXRD structures are provided in Figure 8, and their 1P and 1D orbitals are plotted in Figure S9. As their dtp analogs they can be described as 8-electron *superatoms* containing an icosahedral [(PdH)@Ag₁₂]⁵⁺ core with 1S² 1P⁶ 1D⁰ configuration. In both cases, the 1P and 1D levels constitute the cluster frontier orbitals (Figures 8 and S9). Whereas the electron of the encapsulated hydrogen is part of the 8-electron count, its 1s orbital is barely involved in the construction of the *superatomic* orbitals. Rather, it interacts preferentially with the 4d<sub>22</sub> orbital of Pd (assuming z://Pd-H), making a fairly localized Pd-H bond. The same situation was found for the related dtp species.¹⁹

The time-dependent-DFT (TD-DFT) simulated UV-Vis spectra of **1** and **2**, considered in their C₃ SCXRD configurations, were simulated at the CAM-B3LYP/Def2-TZVP level (see Computational Details) and are shown in Figure S10. They exhibit the same double band shape as their experimental counterparts (Figure 6), although the respective intensities of the two bands are switched in the case of **1**. In the simulated spectra, the highest energy band is centered at 364 and 373 nm for **1** and **2**, respectively, and the low energy band, at 487 and 453 nm. These values are in a satisfying agreement with experiment. As in the case of the dtp analogues,¹⁹ the two peaks are of 1P → 1D nature, being the result of a single-band splitting caused by the lowering of spheroidal symmetry induced by the presence of an encapsulated hydrogen atom. The DFT-simulated phosphorescence emission spectrum of **2**, which accounts for vibronic transitions (see Computational details), is shown in Figure 6. Its peak at 764 nm is in good agreement with the
experimental value of 739 nm. The emissive triplet state can be described as resulting from an
electron transfer from the doubly degenerate 1P<sub>x,y</sub> HOMO to the 1D<sub>z2</sub> LUMO (Figure 9).

Antibacterial activity

The combined series of dsep and dtp analogs for 8-electron superatomic doped clusters
provides an opportunity to evaluate their antibacterial activity. The utilization of silver for
antimicrobial purposes predates the advent of pharmaceutical antibiotics. Furthermore,
commercializing silver nanoparticles for antimicrobial activity has become common practice.
Several publications have reported the viability of silver and a combination of stabilizing agents
as viable candidates for antibacterial.<sup>29-31</sup>

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Assessment of the antibacterial properties of a series of Pd/Ag superatomic NC.</figcaption></figure>
The bimetallic system Pd/Ag has also recently been the subject of antibacterial activity in
nanoparticle form.<sup>32,33</sup> However, it should be noted that studies involving atomically precise NC,
which have fully identified structural features, are exceedingly rare in literature. The critical
factor to the antibacterial activity has been the ability to release Ag<sup>+</sup> at a specific rate and
location.<sup>34</sup> Intuitively, using atomically precise constructs for such an application should be ideal
since these assemblies lie at the junction of bulk NCs and precise molecular constructs. The
ability to precisely modify the surface and size of the NCs allows engineering NCs for particular
cell membranes or environments. In this preliminary investigation, we chose two model bacteria,
Escherichia coli (E. coli) and Bacillus subtilis (BS), and performed a zone of inhibition (ZOI)
assay to determine the comparative antibacterial activity of each NC and their dtp analogs.<sup>35</sup> The
assay results are summarized in Figure 9 and Table S2. To provide a basis for comparison,
several related NCs and ligands are tested under the same conditions, Table S2. The ligands dtp
and dsep also have antibacterial properties, contributing to the NCs' effectiveness. Notably, the
monometallic [Ag<sub>20</sub>{Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>] has superior performance than bulk nanoparticles prepared
from traditional wet methods.<sup>36</sup> The performance is comparable to Ag nanoparticles, usually
derived from a combination of silver precursors with natural extracts<sup>37,38,39</sup> and Ag
composites.<sup>40,41</sup> The inclusion of Pd/PdH into the kernel results in a marked increase in the
antibacterial efficacy, *ca* 2.5-fold increase in the ZOI over [Ag<sub>20</sub>{Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>]. The NC does
not outperform the commercial antibiotic ciprofloxacin. However, knowledge of the atomically
precise structure provides a robust platform for a future structural investigation into the complex
interactions in these systems.
## Conclusion

An expansion of a series of superatomic Pd-doped silver NCs has been achieved by controlled transformation of the passivating shell. The dsep ligands profoundly affect both the NC structures, which in turn greatly influences their properties. VT-NMR demonstrates the rigidity of the dsep ligand, with only one configurational isomer existing at low temperatures. Importantly, these isomers directly correspond to the metallic configurations observed in the solid state. The stability afforded by the dsep ligands contrasts with the complex isomeric distribution observed for the dtp relatives in solution and the dtp inability to stabilize the most symmetrical arrangements in the solid state. The ligand nature heavily influences the optical properties, and the 12 dsep ligands surrounding the metallic framework induce relatively similar absorption profiles for 1-2. The shorter wavelength emissions come at the cost of reduced lifetimes. However, the ability to move an emission wavelength a few nanometers with a precise modification will be crucial for future applications. The complete series showed antibacterial activity against E. coli and Bacillus subtilis. Although not a commercially viable system for antibacterial activity, we hope such information provides a basis for future investigations into this avenue of research.

## Experimental

### Materials and Characterization

Commercially sourced sodium borohydride (NaBH₄, 98 %), trifluoroacetic acid (HTFA, 99.5 %), and sodium borodeuteride (NaBD₄, 98 %) were applied without further purification. [PdHAg₁₉{S₂P(O'Pr)₂}₁₂]¹⁹ and NH₄[Se₂P(O'Pr)₂]⁴² was prepared following the method outlined in the literature. These reagents are considered corrosive and should be used with caution. All
reactions were carried out under an inert atmosphere, using standard Schlenck line techniques.

¹H (400 MHz), ³¹P{¹H} (161.97 MHz), and ²H (61.42 MHz) NMR spectra were recorded on the Bruker Advance II 400 MHz spectrometer. VT-³¹P{¹H} (242.94 MHz) and VT-¹H (600 MHz) NMR spectra were recorded on Bruker AV-600 BBO probe spectrometer. ¹⁹F NMR (470.59 MHz) spectrum was recorded on the AVIIHD-500 BBFO probe. The chemical shift (δ) and coupling constants (J) are reported in ppm and Hz, respectively. ESI-MS spectra were captured on the AB SCIE X QSTAR® XL High-Resolution Electrospray Mass Spectrometer. At 298 K, the Agilent Cary-60 and Perkin Elmer Lambda 750 UV-visible spectrometers were used to detect the optical absorption spectra. On an Edinburgh FLS920 fluorescence spectrometer, the lifetime decays and PL spectra were monitored in an EPR tube with a liquid cryogenic system at 77 K. A PHI 5000 VersaProbe-Scanning ESCA Microprobe on an X-ray Photoelectron Spectrometer was used to record the XPS spectra.

## Synthesis of [PdHAg<sub>19</sub>{Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>], 1

[PdHAg<sub>19</sub>{S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>] (0.01 g, 0.002 mmol) and NH<sub>4</sub>[Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>] (0.0078 g, 0.07 mmol) were dissolved in THF (15 ml). The brown solution was stirred at room temperature for 10 min resulting in a mulberry colored solution. The solvent is removed in vacuo to obtain a residue of NH<sub>4</sub>[S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>] and **1**. The crude product was purified by column chromatography (alumina stationary phase) and **1** was eluted with diethyl ether in 54% yield. ¹H NMR (600 MHz, CD<sub>2</sub>Cl<sub>2</sub>, δ, ppm, 293 K): 4.99 (septet, CH, 24H), 1.43 (d, CH<sub>2</sub>, 144H); -7.18 (¹J<sub>HAg</sub> = 10.35 Hz). ³¹P{¹H} NMR (242.94 MHz, CD<sub>2</sub>Cl<sub>2</sub>, δ, ppm, 293 K): 63.63 (¹J<sub>PSe</sub> = 633.36 Hz). ESI-MS (*m/z*): exp. 5949.8566 Da (calc. 5949.9058 Da for [**1** + Ag]<sup>+</sup>). UV-Vis [λ<sub>max</sub> in nm, (ε in M<sup>-1</sup>cm<sup>-1</sup>)]: 414
(41134), 500 (26042). Elemental Anal. C₇₂H₁₆₉Ag₁₉O₂₄P₁₂PdSe₂₄ [calc.: C%, 14.8 and H%, 2.92]: C%, 14.39 and H%, 2.77.

(1D): The deuteride analogue was prepared under the same conditions with the parent cluster substituted for the deuterated analog [PdDAg₁₉{S₂P(O'Pr)₂}₁₂], yield 71%. ²H NMR (61.42 MHz, CHCl₃, δ, ppm): -7.11. ESI-MS (m/z): exp. 5950.0186 Da (calc. 5950.8991 Da for [1D + Ag]⁺).

## Synthesis of [PdHAg₂₀{Se₂P(O'Pr)₂}₁₂]CF₃COO, 2

A solution of [PdHAg₁₉{Se₂P(O'Pr)₂}₁₂] (0.03 g, 0.005 mmol) in hexane (50 ml) is treated with 0.4 µl HTFA at ambient temperature. After stirring for 15 min, the reaction mixture was allowed to stand for 20 min, resulting in the separation of an orange precipitate from a colorless solvent. The precipitate is consolidated and washed with hexane, yield: 62%. ¹H NMR (600 MHz, CD₂Cl₂, δ, ppm, 293 K): 4.97 (septet, CH, 24H), 1.44 (d, CH₂, 144H); -10.70 (¹JHAg = 9.70 Hz). ³¹P{¹H} NMR (242.94 MHz, CD₂Cl₂, δ, ppm, 293 K): 63.26 (¹JPSe = 635.44 Hz). VT-³¹P{¹H} NMR (242.94 MHz, CD₂Cl₂, δ, ppm, 213 K): 64.88 (¹JPSe = 579.47 Hz, 684.06 Hz). ¹⁹F NMR (470.59 MHz, CD₂Cl₂, δ, ppm, 298 K): -76.50. ESI-MS (m/z): exp. 5949.8784 Da (calc. 5949.9058 Da for [2]⁺). UV-Vis [λmax in nm, (ε in M⁻¹cm⁻¹)]: 408 (401294), 486 (225676). Elemental Anal. C₇₄H₁₆₉Ag₂₀F₃O₂₆P₁₂PdSe₂₄ [calc.: C%, 14.66 and H%, 2.81]: C%, 14.65 and H%, 2.73.

(2D): The deuterated analog was prepared under the same conditions with the precursor cluster substituted for the deuterated analog [PdDAg₁₉{S₂P(O'Pr)₂}₁₂], yield: 73%. ²H NMR (61.42
MHz, CHCl<sub>3</sub>, δ, ppm): -10.637. ESI-MS (m/z): exp. 5950.8874 Da (calc. 5950.8991 Da for [2<sub>D</sub> + Ag]<sup>+</sup>).

## Computational Details

Geometry optimizations were carried out within the DFT formalism with the Gaussian 16 package,<sup>43</sup> using the BP86 functional<sup>44,45</sup> and the Def2-TZVP basis set.<sup>46,47</sup> All the optimized geometries were computed without any symmetry constraint and characterized as true minima by vibrational analysis. The natural atomic orbital (NAO) charges and Wiberg bond indices were computed with the NBO 6.0 program.<sup>48</sup> The UV-visible transitions were calculated on the above-mentioned optimized geometries by means of TD-DFT calculations, with the CAM-B3LYP functional<sup>49</sup> and the Def2-TZVP basis set. The UV-visible spectra were simulated from the computed TD-DFT transitions and their oscillator strengths by using the SWizard program,<sup>50</sup> each transition being associated with a Gaussian function of half-height width equal to 2000 cm<sup>-1</sup>.

The phosphorescence emission spectra were simulated from DFT calculations on the optimized singlet ground and first excited triplet states, using the Adiabatic Hessian method within the Franck-Condon principle, which accounts for vibrational mode mixing and a proper description of both optimized ground and excited states potential energy surfaces. The triplet state was optimized at the unrestricted BP86/Def2-TZVP level (assuming the corresponding singlet structure as the starting geometry) and characterized as a true minimum. The lowest normal modes in the vibronic treatment were neglected in order to obtain sufficient spectrum progression. We employed the class-based pre-screening to limit the number of terms involved in
the vibronic calculation with the following settings: C<sup>max</sup><sub>1</sub> = 70, C<sup>max</sup><sub>2</sub> = 70, N<sup>max</sup><sub>1</sub> = 100 × 10<sup>8</sup>.<sup>51,52</sup>

The vibronic plot was realized using the VMS software.<sup>53</sup>

X-ray Crystallography

Single crystals were covered in paratone, mounted on a glass fiber, and cooled in a nitrogen cold
stream to 100 K. Data were collected on a Bruker APEX II CCD diffractometer using graphite
monochromated Mo Kα radiation (λ = 0.71073 Å). Absorption corrections were performed with
SADABS,<sup>54</sup> and the integration of raw data frames was done with SAINT.<sup>55</sup> The structures were
solved by direct methods and refined by least-squares against *F*<sup>2</sup> using the SHELXL-2018/3
package,<sup>56</sup> incorporated in SHELXTL/PC V6.14.<sup>57</sup> All non-hydrogen atoms were refined
anisotropically.

ASSOCIATED CONTENT

**Supporting Information.** NMR spectra, ESI-MS spectra, X-ray crystallography refinement parameters, and xyz coordinate file. (.DOC)

Accession Codes

CCDC 2310294 and 2310295 contain the complete crystallographic data for this publication.
The data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by e-mail at
data_request@ccdc.cam.ac.uk, or by contacting the Cambridge Crystallographic Data Center, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION
**Corresponding Author**

*C. W. Liu – Department of Chemistry, National Dong Hwa University, Hualien 974301, Taiwan; orcid.org/0000-0003-0801-6499, E-mail: chenwei@gms.ndhu.edu.tw

**Funding Sources**

National Science and Technology Council in Taiwan (112-2123-M-259-001)

**ACKNOWLEDGMENT**

This work was supported by the National Science and Technology Council in Taiwan (112-2123-M-259-001)

**ABBREVIATIONS**

NC atomically precise nanocluster, dsep diselenophosphate, dtp dithiophosphate, LEIST Ligand exchange-induced structure transformation.

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<figure><img src="image_12.png" /><figcaption>TOC</figcaption></figure>

Two novel superatomic PdH doped silver superatoms, [PdHAg<sub>19</sub>{Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>] **1**, and [PdHAg<sub>20</sub>{Se<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>] **2** are reported. Derived from the ligand exchange transformation of [PdHAg<sub>19</sub>{S<sub>2</sub>P(O<sup>i</sup>Pr)<sub>2</sub>}<sub>12</sub>], a stable 8-electron superatomic configuration is maintained. The interstitial hydride plays a crucial role in directing the metal framework and results in the isolation of highly symmetric arrangement. VT-NMR indicates the solid-state configuration is maintained in solution. The electronic structure is investigated computationally, with optical and antibacterial properties reported.