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One-pot synthesis of atomically precise chiral PtAg₄ nanoclusters

Yujie Zhang† ^a, Hongli Jia†^b, Bingzheng Yan^a, Jian Hou^a, Huifang Guo^a, Qi Li^a, Chengrui Xin^a, Simin Li^a, Jinrun Dong*^a and Hui Shen*^a
aCollege of Energy Materials and Chemistry, Inner Mongolia University, Hohhot 010021, China. E-mail: shen@imu.edu.cn; dongjrchem@imu.edu.cn
^bState Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, 38 Xueyuan Road, 100191 Beijing, China

First published on 28th July 2025

Despite significant advancements in the synthesis and application of bimetallic nanoclusters in the past decades, the rational design and fabrication of chiral bimetallic nanomaterials with atomic-level structural precision and tunable physicochemical properties remains a formidable challenge in nanoscience.^25–39 The incorporation of chirality into bimetallic architectures not only endows these systems with enhanced optical and catalytic properties but also unlocks their potential for enantioselective applications in pharmaceutical synthesis, asymmetric catalysis, and chiral sensing technologies.^40–43 Consequently, the development of innovative synthetic methodologies to construct well-defined chiral bimetallic nanoclusters with tailored functionalities represents a critical research direction for advancing both fundamental understanding and practical applications in nanomaterial science.^44–57

In this study, we present the synthesis and comprehensive characterization of a novel chiral bimetallic nanocluster, [PtAg₄Cl(BDPP)₄H]²⁺ (abbreviated as PtAg₄ hereafter), featuring a heterometallic Pt–Ag core coordinated with the chiral phosphine ligand BDPP (2,4-bis(diphenylphosphino)pentane). The synthesis of PtAg₄ is inspired from that of [PtAg₉(C₁₈H₁₂Br₃P)₇Cl₃](C₁₈H₁₂Br₃P), which proves a straight strategy for phosphine-stabilized Pt–Ag alloy clusters.⁵⁸ The strategic selection of platinum (Pt) and silver (Ag) leverages their synergistic electronic and catalytic properties, while the BDPP ligand not only imparts structural stability but also serves as a chiral inducer, enabling precise stereochemical control. Advanced characterization techniques—including single-crystal X-ray diffraction (SCXRD), high-resolution mass spectrometry (HRMS), and circular dichroism (CD) spectroscopy—were employed to elucidate the atomic-level structure and chiroptical properties of the nanocluster. Our findings not only advance the fundamental understanding of chiral bimetallic nanoclusters but also establish a robust platform for the rational design of functional nanomaterials with tailored catalytic and optical properties.

The PtAg₄ clusters were synthesized via a one-pot reaction (Scheme 1). Through the introduction of the chiral phosphorus ligand (R,R)- or (S,S)-2,4-bis(diphenylphosphino)pentane (BDPP, Fig. S1) as a chiral inducer, a novel chiral nanocluster, [PtAg₄Cl(BDPP)₄H]²⁺, was successfully constructed in a one-pot synthesis (see SI for details). In this system, silver nitrate (AgNO₃) served as the silver ion (Ag⁺) precursor, while sodium hexafluoroantimonate (NaSbF₆) played a crucial role in stabilizing the reaction system through charge regulation. The chiral BDPP phosphine ligand coordinates with metal ions, forming stable intermediates via its rigid structure and enantioselective induction. A controlled amount of sodium borohydride (NaBH₄) was employed as the reducing agent to initiate metal nucleation. The chiral configuration of the resulting nuclei was templated by the stereochemical influence of the BDPP ligand. As the reaction progressed, the nascent metal nuclei underwent gradual incorporation of Ag⁺ ions, which were subsequently reduced and deposited onto their surfaces, enabling controlled cluster growth. Concurrently, the BDPP ligand effectively suppressed uncontrolled aggregation through steric hindrance, while its chiral configuration directed the formation of an asymmetric metal core structure. The SbF₆⁻ functioned as a counterion to stabilize the cluster's surface charge. Through ether diffusion crystallization over a two-week period, high-quality yellow crystals suitable for X-ray analysis were obtained (Fig. S2). Before systematic investigation of the products, we measured their solubility in common solvents. It reveals that the products are soluble in most organic solvents, providing advantages for potential applications (Table S1).

	Scheme 1 The one-pot synthetic prototype of PtAg4.

The single-crystal X-ray diffraction analysis unambiguously revealed the structural characteristics of the chiral nanoclusters. Both clusters crystallized in the chiral space group P2₁ (see Tables S2–S5), confirming their intrinsic chirality. The metal framework adopts an intriguing “shovel-like” architecture, comprising a PtAg₃ tetrahedral unit forming the scoop moiety and an AgCl unit serving as the handle (Fig. 1a, b, and Fig. S3–S6). The pronounced asymmetric arrangement of the metal core provides direct structural evidence for the chiral nature of the inner framework.

	Fig. 1 The chiral molecular structure of PtAg4. (a) The overall structure. (b) The chiral metal framework. (c) BDPP ligand arrangement.

Notably, the introduction of chiral phosphine ligands not only effectively induced the formation of the chiral core but also substantially enhanced the cluster's stability through coordination effects. Intriguingly, the BDPP ligand exhibits a dual coordination mode within the cluster (Fig. 1c): the upper ligand bridges two Ag atoms in a μ₂-fashion, forming a silver dimer unit via a μ₂-Cl bridge, while the lower ligand adopts a κ²-chelating mode to coordinate with a single Pt center. In the cluster, the Ag–Ag and Pt–Ag bond lengths fall within the range of 2.7882 to 2.8369 Å and 2.6850 to 2.8656 Å, which is consistent with those observed in [PtAg₁₈(dppp)₆Cl₈] and [PtAg₄₂(CCC₆H₄CH₃)₂₈].^59,60 The Cl⁻ ions exhibit tight coordination to the Ag centers, as demonstrated by their characteristically short bond distances.

The two enantiomers possess identical structural features and display perfect mirror symmetry. As illustrated in Fig. 1, both the metal core arrangement and ligand distribution exhibit exact mirror-image relationships, confirming their 100% enantiomeric purity. A detailed structural analysis demonstrates that the steric hindrance imposed by the two chiral carbon atoms on the propyl groups of (2R, 4R)-BDPP or (2S, 4S)-BDPP restricts the rotation of the alkyl groups, thereby dictating the absolute configuration of the metal framework. Furthermore, strong Ag–Cl interactions disrupt the potential centrosymmetric distribution of Cl atoms, effectively “locking” the cluster into specific enantiomeric configurations (R or S) and enabling precise enantioselective control.

To verify the elemental composition of the sample, we performed mass spectrometric characterization. The mass spectrum exhibited distinct ion signals, with a predominant peak A at m/z 1212 corresponding to the doubly charged molecular ion [PtAg₄Cl(BDPP)₄H]²⁺ (Fig. 2a), confirming the molecular structure of the major constituent. Furthermore, two additional characteristic peaks B and C were observed at higher m/z values, which were assigned to the molecular ions [Ag₂Pt(BDPP)₂H]⁺ and [Ag₃PtCl(BDPP)₂H]⁺, respectively (Fig. S7). Notably, the mass analysis revealed the incorporation of one hydride in the structure. To verify this finding, we synthesized deuteride analogues by replacing NaBH₄ with NaBD₄ as the reducing agent during cluster synthesis. As evidenced in Fig. S8, the PtAg₄Cl(BDPP)₄D]²⁺ peak exhibited precisely the expected 0.5 m/z shift, confirming our initial observation.

	Fig. 2 Characterization data of PtAg4. (a) ESI-MS spectra of PtAg4. (b) 31P NMR of PtAg4 in CD2Cl2. (c) UV/Vis spectra of PtAg4. (d) The XPS spectrum of the Ag 3d orbital in PtAg4.

As illustrated in Fig. 2b, ³¹P nuclear magnetic resonance (³¹P NMR) spectroscopic analysis provided further insights into the ligand coordination environment. The experimental data revealed that the ³¹P NMR spectrum of the cluster system in solution displayed four well-resolved resonance signals at 17.98, 17.89, 16.68, and 16.58 ppm, respectively, clearly indicating the existence of four chemically distinct phosphorus environments within the molecular structure. The remarkably close chemical shifts of these signals (Δδ < 1.4 ppm) suggest only minor variations in the electronic environments surrounding the phosphorus atoms. These subtle chemical shift differences further corroborate the presence of slight local environmental variations within the molecular framework, which is in excellent agreement with the proposed structural model.

Complementary UV/Vis spectroscopic studies (Fig. 2c) demonstrated that the PtAg₄ cluster exhibits two characteristic absorption features at 360 nm and 430 nm. The higher-energy absorption at 360 nm can be rationally assigned to ligand-to-metal charge transfer (LMCT, P → Pt/Ag) transitions involving the BDPP ligand. Meanwhile, the broad lower-energy absorption band centered at 430 nm may arise from either localized surface plasmon resonance (LSPR) of the silver core or ligand-modified d → sp electronic transitions within the cluster system.^61–63 To further study the absorption characteristics of this cluster over a longer range and explore its potential applications, we examined its optical absorption properties in the far-infrared region and conducted ultraviolet-visible diffuse reflectance spectroscopy (Fig. S9 and S10). XPS analysis (Fig. 2d and Fig. S11, S12) reveals that the binding energies of Ag 3d_5/2 and Pt 4f_7/2 in PtAg₄ clusters are 368.5 eV and 71.5 eV, respectively, indicating that Ag is in the oxidized state while Pt is in the reduced state.^59,64,65

Circular dichroism (CD) spectroscopy unequivocally confirmed the chiral nature of the enantiomers. The optically pure PtAg₄ solution demonstrated nearly ideal mirror-image CD signals between 280 and 500 nm (Fig. 3a), with distinct Cotton effects observed at 360 nm and 430 nm. These characteristic peaks showed excellent correspondence with the absorption spectrum. Quantitative analysis of chiroptical activity was performed by calculating the anisotropy factor (g) across the spectral range using the equation g = ΔA/A = θ [mdeg]/(32980 × A), which yielded a maximum g value of 5.8 × 10⁻³ at 430 nm (Fig. 3b), indicating significant chiral discrimination at this wavelength.

	Fig. 3 Optical properties of PtAg4 enantiomers. (a) Circular dichroism spectra. (b) Corresponding anisotropy factors.

In summary, chiral PtAg₄ clusters stabilized by the phosphine ligand 2,4-bis(diphenylphosphino)pentane (BDPP) and chloride were successfully synthesized, and their molecular structure was unequivocally determined. The synthetic methodology employed is straightforward, providing a versatile and efficient strategy to access a broader range of analogous structures. The deliberate incorporation of the chiral BDPP ligand not only induces structural chirality but also establishes a well-defined molecular framework for fine-tuning the nanocluster's physicochemical properties, underscoring the critical role of ligand engineering in nanocluster design. This work not only expands the synthetic toolkit for constructing functional chiral nanoclusters but also opens new avenues for investigating bimetallic synergistic effects at the nanoscale. Future studies can build upon these findings to rationally design tailored nanoclusters with optimized catalytic, optical, or sensing capabilities.

H. Shen acknowledges the financial support from the National Key R&D Program of China (2023YFB3507100), National Natural Science Foundation of China (22301149), Natural Science Foundation of Inner Mongolia (2025JQ026), Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (NJYT23035) and start-up funding of Inner Mongolia University (10000-23112101/043 and 23600-5233710). S. Li acknowledges the financial support from the Natural Science Foundation of Inner Mongolia (2025QN02068) and Start-Up Funding of Inner Mongolia University (10000-A24202027).

Conflicts of interest

Data availability

CCDC 2077403 and 2077404 contain the supplementary crystallographic data for this paper.^66,67

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