Anion-Directed Assembly of a Bimetallic Pd/Ag Nanocluster: Synthesis, Characterization, and HER Activity

1. Introduction

The pursuit of atomically precise nanomaterials has led to the rapid expansion of nanocluster (NC) research [1,2,3]. These ultrasmall metal clusters, typically composed of a few to hundreds of atoms, bridge the gap between individual atoms and bulk materials. Unlike their bulk counterparts, NCs exhibit unique size-dependent properties from quantum confinement effects and the high proportion of surface atoms [4]. Among bimetallic systems, the combination of palladium and silver has long been exploited for their resistance to tarnish, a common problem with traditional sterling silver. On the nanoscale, the combination of metals can lead to synergistic effects, where the resulting properties are not merely a sum of the individual components but expanded functionalities compared to their monometallic counterparts. The controlled synthesis of Pd-doped Ag NCs with well-defined structures remains a significant challenge. Achieving precise control over the size, composition, and atomic arrangement is crucial for tailoring their properties and optimizing their performance.

Anion-directed synthesis has emerged as a powerful tool for manipulating the structure and composition of NCs. Anions are not limited to capping agents but can actively direct the arrangement of metal atoms as templates, influencing the overall morphology of the NC [5,6,7,8]. Halides have provided this service, and a recent publication highlights their crucial role in forming vertex-sharing Pd and Pt-doped clusters [9,10,11,12]. The selective use of sulfides in assembling atomically precise nanoclusters was first exploited by Fenske and co-workers, who reported a series of large metallic frameworks and exploited the cooperative stabilization of chalcogens and ligands in large silver assemblies very early on [13,14,15]. Various NC morphologies have been identified due to the versatility of the sulfide [13,14], including the recently reported [PdAg₁₆(S)₂{S₂P(OR)₂}₁₂] [16,17,18]. However, rare is the combination of two different anions, and this report highlights the combination of sulfide and hydride. Considering the ability of hydrides to occupy interstitial sites within Pd/Ag metallic frameworks, we pursued the isolation of NC containing both hydride and sulfide [19,20].

We report a novel molecule [Pd(H)Ag₁₃(S){S₂P(OⁱPr)₂}₁₀] (PdHAg₁₃S), featuring a distinct linear S-Pd-H axis encapsulated by a silver cage and dithiophosphate (dtp) passivating layer. A suite of characterization techniques, including NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and computational studies, are applied to elucidate the NC’s composition, structure, and bonding. Furthermore, we explore the catalytic activity of this novel construct for the hydrogen evolution reaction (HER), a key process in sustainable energy production.

2. Results and Discussion

The cleavage of the P-S bond from the dtp ligand and the generation of the sulfide anions were indicated by our previous report [17,18]. Tuning the stoichiometric ratios of the starting materials and reducing reagent LiBH₄, we are able to isolate a novel NC with two hetero anions of the type [Pd(H)Ag₁₃(S){S₂P(OⁱPr)₂}₁₀] (PdHAg₁₃S), with known by-products [Ag₇(H){S₂P(OⁱPr)₂}₁₂] and [PdAg₂₀{S₂P(OⁱPr)₂}₁₂], Scheme 1 [19,20].

The purity and composition of the isolated NC was confirmed by mass spectroscopy. The ESI-TOF-MS spectrum indicates the formation of both silver and sodium adducts during ionization with peaks obtained at m/z 3782.1853 Da assigned to [PdHAg₁₃S+Ag]⁺ (Calc: m/z 3782.3404 Da) and m/z 3697.9587 Da for [PdHAg₁₃S+Na]⁺ (Calc: m/z 3697.4614Da), Figure 1. The cluster is prone to decomposition during the ionization process, with the peaks assigned to several smaller silver clusters, Figure S1. The XPS analysis further establishes the cluster’s compositions and verifies that Pd(0) and the Ag approaching Ag(I) are based on their distinctive binding energies (Figure S2).

This bimetallic nanocluster exhibits a unique palladium-centered Ag₉ kernel, and Pd@Ag₉ metallic framework (Figure 2a). The central Ag₅ motif is sandwiched between Pd and S atoms and capped by two Ag₄ motifs (Figure 2b). Notably, the µ₅ hydride resides within a square pyramidal cavity formed by Pd and four Ag atoms (Figure 2c). This arrangement, where a hydrogen atom occupies an interstitial site within a metal framework, is characteristic of interstitial hydride systems [2]. The axis connecting the S, Pd, and H atoms (S-Pd-H) is almost perpendicular to the plane of the Ag₅ motif, deviating by 5.92°. This moderate tilt contributes to the C₁ symmetry observed in the solid state. The Pd-S bond, measuring 2.26 Å, is notably shorter than in traditional molecular complexes. The passivating layer contains three η₃ (μ₂, μ₁) and eight η₄ (μ₂, μ₂) ligands, resulting in an overall neutral configuration [1].

The ³¹P{¹H} NMR spectrum displays a single resonance at 102.32 ppm at ambient temperature (Figure S3a), indicating a time-averaged equivalence of all phosphorus atoms resulting from the dynamic processes. However, upon cooling to 213 K, the peak splits into five distinct resonances, correlating to non-equivalent phosphorus environments observed in the solid state (Figure S4a). The disorder observed in the solid state is also present in the solution, as evidenced by ¹H NMR data. A multiplet resonance was observed at -3.98 ppm (Figure S3b), confirming the presence of a hydride. Notably, the integration ratio of the ¹H NMR spectrum further supports the assignment of ten dtp ligands per single hydride and correlates to isopropyl substituents. The multiplicity observed indicates that the hydride couples to several Ag nuclei due to the dynamic nature of the hydride. This observation is consistent with the dynamic flipping of the S-Pd-H axis observed in the solid-state structure. Furthermore, the variable temperature NMR (VT-NMR) experiments indicate that the hydride resonance remains broad at low temperatures (213 K), suggesting residual mobility (Figure S4b). In solution, the dynamic behavior of this encapsulated unit, coupled with the fluxionality of the ligand shell, is expected to play a crucial role in the cluster’s effectiveness as an electrocatalyst for hydrogen evolution reactions (HER).

A key goal in tailoring NC is the tuning of optical properties, and incorporating sulfide yields an emissive NC. Comparing previously reported silver-rich palladium NC, PdHAg₁₃S has the shortest emissive wavelength reported for chalcogen-passivated NCs [19,20,21,22]. The optical spectrum for PdHAg₁₃S, with an absorption maximum centered at 356 nm (Figure 3), is similar to the previously reported [PdAg₁₆S₂{S₂P(OⁱPr)₂}₁₂], 365 nm [18]. However the emission wavelength is blue shifted (λem = 681 nm, τ = 121.1 µs) (Figure S5) compared to [PdAg₁₆S₂{S₂P(OⁱPr)₂}₁₂] (λem = 808 nm, τ = 92.3 µs).

The DFT-optimized geometry was performed on a simplified model for PdHAg₁₃S, [Pd(H)Ag₁₃(S)(S₂PH₂)₁₀] to gain insights into its structure and electronic properties. The optimized geometry obtained from these calculations is in good agreement with the structure determined by single-crystal X-ray diffraction (SCXRD) (see Table 1 and the corresponding XYZ file in Table S1).

Analysis of the computed data supports the view of describing the whole NC as made of a quasi-linear [SPdH]^3- 14-electron complex stabilized by a shell of 13 Ag(I) centers, Table 2 and Figure S6. Two significantly different Ag(I) environments can be distinguished as nine Ag(I) atoms coordinated to the Pd(0) atom (Ag_ker) and four outer Ag(I) atoms (Ag_out) anchored by a μ₇ sulfide. NAO atomic charges (Table 2) support this assignment. Four Ag_ker atoms form a square pyramid, with the Pd atom encapsulating an interstitial μ₅ hydride. The Pd-H and Pd-S bonds exhibit significant covalent character, as evidenced by their relatively large Wiberg bond indices (WBIs, Table 2). The Ag-S(dtp) bonds also show substantial covalent character. In addition to these covalent interactions, metallophilic interactions between the silver atoms contribute to the stability of the NC, as exemplified by the Ag…Ag WBI values which are typical for such type of interaction (Table 2). The Kohn-Sham orbital diagram (Figure S6) provides further insights into the electronic structure. The highest occupied molecular orbitals (HOMOs) are primarily composed of 4d(Pd) and 3p(S), whereas the lowest unoccupied molecular orbitals (LUMOs) are 5p(Pd) and 5s/5p(Ag_ker). Time-dependent DFT (TD-DFT) calculations were performed to simulate the UV-vis spectrum. The calculated UV-vis spectrum is in reasonable agreement with the experimental spectrum, Figure S7. It shows a low-energy band attributed to the HOMO-1 → LUMO+2, HOMO-1 → LUMO+1 and HOMO-2 → LUMO transitions, thus of 4d(Pd)/3p(S) → 5s/5p(Ag_ker) nature. These transitions involve charge transfer from the Pd and S atoms to the Ag atoms.

Hydrogen is widely regarded as a highly promising energy source because of its environmental sustainability and zero-emission properties. The electrocatalytic HER plays a crucial role in renewable and clean energy initiatives. Noble metals like Pt and Pd are known for their exceptional electrocatalytic efficiency in HER, largely due to their optimal hydrogen binding energy, favorable Gibbs free energy for atomic hydrogen adsorption, and low activation barriers for hydrogen desorption. Although silver is typically considered one of the least reactive metals in HER processes, the cost-effectiveness and abundance of silver-based nanomaterials have sparked interest in investigating their potential for hydrogen evolution [23,24,25]. The HER activity of PdHAg₁₃S was compared with related sulfide containing [Pd₆Ag₁₄(S){S₂P(OPr)₂}₁₂] (Pd₆Ag₁₄S) [21], and [PdAg₁₆(S)₂{S₂P(OⁱPr)₂}₁₂] (PdAg₁₆S₂) [18], by linear sweep voltammetry (LSV) in a three-electrode electrochemical cell containing an argon saturated 0.5 M H₂SO₄ solution, (Figure 4a). Pd₆Ag₁₄S and PdHAg₁₃S exhibit a higher HER activity than PdAg₁₆S₂. The overpotentials to reach a current density of -10 mAcm^-2 were found to be -290, -345, and -542 mV for Pd₆Ag₁₄S, PdHAg₁₃S, and PdAg₁₆S₂, respectively, indicating that Pd₆Ag₁₄S is the most efficient H₂-producing electrocatalyst in the series. The Tafel slope is used to evaluate the kinetics of the HER, Pd₆Ag₁₄S, PdHAg₁₃S, and PdAg₁₆S_2, which are 129, 137, and 182 mV/dec, respectively (Figure 4b). A lower Tafel slope typically indicates more efficient electron transfer, leading to faster reaction rates. Pd₆Ag₁₄S, and PdHAg₁₃S clusters, with Tafel slopes of 129 and 137 mV/dec, have relatively more efficient HER kinetics compared to PdAg₁₆S₂ (182 mV/dec). Furthermore, the Tafel slopes of Pd₆Ag₁₄S and PdHAg₁₃S are closer to the theoretical Tafel slope of the Volmer step (120 mV/dec), indicating proton adsorption is the rate-determining step. The improved efficiency of the silver sites compared to monometallic clusters [26] is due to the synergistic effect of the Pd atom encapsulated by the silver metallic framework. For instance, Pd₆Ag₁₄S exhibits a slightly lower overpotential and Tafel slope for HER compared to PdHAg₁₃S, likely due to the higher number of Pd atoms present in the cluster. However, in comparison to open Pd sites, such as those found in [PdHCu₁₁{S₂P(OⁱPr)₂}₆(C≡CPh)₄] [27]_, the performance is significantly lower. Further highlighting that the defect engineering or ligand modifications may help reveal the reactions on Pd of these clusters, which potentially enhance their activity for HER.

3. Materials and Methods

All reactions were conducted using standard Schlenk protocols under an inert N₂ environment. All chemicals were used as supplied from commercial sources, including palladium(II) acetate (PdOAc₂, 98%) and lithium borohydride (LiBH₄, 2 M in THF). [Ag(MeCN)₄]BF₄ [28] and NH₄ [S₂P(OⁱPr)₂] [29] were prepared according to previous reports. NMR and VT-NMR spectra were recorded on a Bruker Advance II 400 spectrometer, operating at 400 MHz for ¹H and 161.97 MHz for ³¹P{¹H}. ESI-TOF-MS spectra were recorded on a Fison Quattro Bio-Q (Fisons Instruments, VG Biotech, UK). The XPS spectra were recorded using a PHI 5000 VersaProbe-Scanning ESCA Microprobe on an X-ray Photoelectron Spectrometer. The absorption spectrum was recorded on an Agilent Cary-60 spectrometer. The PL spectra and lifetime decays were measured in an EPR tube with a liquid cryogenic system at 77 K and were recorded on a HORIBA FluoroMax plus spectrometer.

SCXRD Analysis

Paratone oil was used to coat and mount a single crystal onto a glass fiber. Data were recorded with a graphite mono-chromated Mo-Kα radiation source (λ=0.71073 Å) fitted on a Bruker APEX II CCD diffractometer operating at 100 K. Data analysis was done with SADABS and SAINT, for adsorption corrections on raw data frames [30,31]. SHELXL-2018/3 package integrated into SHELXL/PC V6.14. [32,33,34] was used to solve the structure and then refined using least-squares versus F². Anisotropic refinements were applied to all non-hydrogen atoms. Structure refinement details are summarized below in Table 3.

Computational Methods

The geometry optimization was performed by density functional theory (DFT) calculations with the Gaussian16 package, [35] with a BP86 functional [36,37] and the all-electron Def2-TZVP basis set from the EMSL Basis Set Exchange Library [38,39]. The optimized geometries were characterized as true minima through vibrational analysis. The natural atomic orbital (NAO) charges and the Wiberg bond indices (WBI) were computed with the NBO6.0 program [40,41] on single-point calculations performed with the BP86 functional and the Def2-SVP basis set for computational limitations [42,43]. The UV-visible transitions were calculated by means of time-dependent DFT (TD-DFT) calculations, with the CAM-B3LYP functional [44] 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 Multiwfn program [45]. The compositions of the molecular orbitals were calculated using the AOMix program [46].

Electrocatalytic Measurements

Electrode preparation: The catalyst ink was prepared by sonicating the mixture of the nanocluster and CH₂Cl₂ for 20 min. 3 nmol of the catalyst ink was loaded onto a carbon paper (1 cm²⁾, which was air-dried overnight before use.

Electrochemical measurements: Measurements were performed on an Admiral Instruments: Squidstat Plus Potentiostat. A three-electrode electrochemical cell consisting of a catalyst-immobilized carbon paper electrode, a Pt foil anode (3 cm²), and an Ag/AgCl reference electrode. The electrolyte solution of 0.5 M H₂SO₄ was degassed with Ar gas for 30 min before the electrochemical measurements. For comparison, a benchmark Pt/C electrode was prepared by drop casting 200 μL of a catalyst ink, made by dispersing 10 mg of Pt/C with 200 μL of 5 wt% Nafion solution, in 800 μL of isopropyl alcohol, onto carbon paper. Electrode potential measured was converted to the RHE scale using the following equation: E_RHE = E_Ag/AgCl + 0.197 + 0.059 pH (1)

Synthesis of [Pd(H)Ag₁₃(S){S₂P(OⁱPr)₂}₁₀], PdHAg₁₃S

[NH₄] [S₂P(OⁱPr)₂] (0.26 g, 1.13 mmol), [Pd(OAc)₂] (0.025 g, 0.11 mmol), [Ag(CH₃CN)₄]BF₄ (0.74 g, 2.06 mmol), NEt₃ (100 µl, 0.717 mmol) and THF (20 mL) were added in the reaction flask for 20 min. LiBH₄ (0.3 mL, 0.59 mmol) was added with vigorous stirring at 253 K. The color of the solution gradually changed from orange to brown. After 4 hours, the mixture was evaporated, and the residue was extracted with DCM and washed water. Column chromatography with Al₂O₃ was used to purify the yellow targeted product with mixed solvent DCM and ether (v:v=5:1), [Pd(H)Ag₁₃(S){S₂P(OⁱPr)₂}₁₀], 0.86 % yield. The byproducts further used the solvent ratio (10:1) and (1:1) to yield [Ag₇(H){S₂P(OⁱPr)₂}₆] in 18.6% and [PdAg₂₀{S₂P(OⁱPr)₂}₁₂] in 31.6%, based on Pd. ¹ H NMR (400 MHz, (CD₃)₂CO, δ, ppm, 298 K): 4.94 (septet, CH, 20H), 1.39 (d, CH₃, 120H); -3.98(¹J_HAg = 13.2 Hz). VT ¹H NMR (400 MHz, (CD₃)₂CO, δ, ppm, 213 K): -3.66. ³¹P{¹H} NMR (161.97 MHz, (CD₃)₂CO, δ, ppm, 298 K): 102.32. VT ³¹P{¹H} NMR (161.97 MHz, (CD₃)₂CO, δ, ppm, 213 K): 104.21, 102.57, 102.46, 102.25, 101.69. ESI-MS (m/z): exp. 3782.1853 (calc.3782.3404 for [PdHAg₁₃S +Ag]⁺) and exp. 3697.9587 (calc. 3697.4614 for [PdHAg₁₃S +Na]⁺). UV-Vis [λ_max in nm, (ε in M^-1cm^-1)]: 356 (66023). XPS (Calc.: Pd%0.95, Ag%12.38, S%20.00, P%9.52, C%57.14): Pd%0.94, Ag%12.40, S%20.07, P%9.50, C%57.09, and (binding energy, eV): Ag 3d_5/2, 367.55; Ag 3d_3/2, 373.55; Pd 3d_5/2, 337.31; Pd 3d_3/2, 342.57.

4. Conclusions

This study successfully demonstrated the anion-directed synthesis of a novel palladium-doped silver nanocluster featuring a unique linear S-Pd-H unit which can be described as a [SPH]^3- anionic complex encapsulated within. Incorporating sulfide and hydride anions was crucial in directing the morphology. Comprehensive characterization, including NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and computational studies, provided detailed insights into the NC’s composition, structure, and electronic properties. The NC exhibits promising catalytic activity for the hydrogen evolution reaction (HER), underscoring the potential of anion-directed synthesis as a powerful strategy for creating bimetallic NCs with tailored structures and properties for catalytic applications. Future research will explore the detailed mechanism of HER cata by this NC and investigate the effects of ligand modification and alloying with other metals on its catalytic performance. Ultimately, this work opens new avenues for the design and synthesis of bimetallic nanoclusters with enhanced properties.