The one/two atom size-reduction of [Au₂₃SCy₁₆]⁻ induced by the [Au₆(dppp)₄]²⁺ cluster

Abstract

The recent progress in atomically precise metal (Au, Ag etc.) nanoclusters has greatly enriched the molecular-level mechanistic understanding of metal nanomaterials. Herein, using two meta-stable (easy formation, easy transformation) clusters, i.e. [Au₂₃SCy₁₆]⁻ and [Au₆(dppp)₄]²⁺ (HSCy and dppp denote cyclohexanethiol and 1,3-bis(diphenylphosphino)propane), as the reaction precursors, the etching of Au₂₃ occurs smoothly, giving the one/two-atom size-reduced [Au₂₁SCy₁₂(dppp)₂]⁺ and [Au₂₂SCy₁₄(dppp)]²⁺ as the major products. Structural analysis and DFT calculations indicate that the active reaction site of Au₂₃ lies in the core–shell interference of the bi-capped icosahedral Au₁₅ core and the AuS₂ motifs. The fluorescence, band gap, and thermostability of the Au₂₁ cluster products are improved compared to that of the Au₂₃ precursors.

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Introduction

The atomic precision of noble metal nanoclusters (with single crystal X-ray diffraction, mass spectra etc.) makes it possible to elucidate the fantastic chemistry and the inherent structure–property correlations of nanomaterials at a molecular level.^1–3 The stimuli response (to pH changes, additives etc.⁴) represents one of the most appealing characteristics of metal nanoclusters, providing sound proof for the traditional theories (such as the Lamer/aggregative size growth),⁵ and opening the door for the practical applications in catalysis,^6,7 sensing^8,9 and bioclinics.¹⁰ In this scenario, the inter-cluster reactions have become a novel synthetic strategy to prepare atomically precise metal nanoclusters,¹¹ and to shed light on the dynamics of the cluster precursors.

So far, most of the reported inter-cluster reactions feature ligand exchange or/and metal exchange characteristics. Typically, the size-maintained ligand exchange occurs between two cluster analogs bearing different ligands, such as the reaction of [Au₂₅(PET)₁₈]⁻ with [Au₂₅(SBut)₁₈]⁻ (HPET and HSBut are short for 2-phenyl ethanethiol and 1-butanethiol),¹² and the reaction of [Au₂₅(SC₁₀H₂₁)₁₈]⁻ with [Au₂₅(SC₁₂H₂₅)₁₈]⁻ (linear alkyl thiolates in both cases).¹³ Meanwhile, the metal exchange has been widely reported in the interparticle reactions between two clusters of different metal components (or isotopic ones). For example, the size- and framework-maintained metal exchange occurs in the reaction of [Ag₂₅(DMBT)₁₈]⁻ with [Au₂₅(PET)₁₈]⁻,¹⁴ [Ag₇(H){S₂P(OⁱPr)₂}₆] with [Cu₇(H){S₂P(OⁱPr)₂}₆],¹⁵ and the isotopic exchange reactions of [^107/109Ag₂₅(DMBT)₁₈]⁻ (ref. 16) and [^107/109Ag₂₉(BDT)₁₂(TPP)₄³⁻].¹⁷ Of note, the inter-cluster reactions between two structurally distinct clusters have also been reported. In the pioneering studies, the reaction of [Au₂₅(FTP)₁₈]⁻ with [Ag₄₄(FTP)₃₀]³⁻,¹⁸ [Au₂₅(PET)₁₈]⁻ with [Ir₉(PET)₆]⁺ (ref. 19) and [Au₂₅(SBut)₁₈]⁻ with [Ag₅₁(BDT)₁₉(TPP)₃]³⁻ (ref. 20) each generates an alloy cluster product with the same framework as one of the precursors. The distinct metal components and the predominant doping processes in these reactions arise an interesting question as to the reaction mode between two same-metal clusters. To our knowledge, only one such reaction has been reported, i.e. the formation of [Ag₁₆(TBT)₈(TFA)₇(CH₃CN)₃Cl]⁺ and [Ag₁₇(TBT)₈(TFA)₇(CH₃CN)₃Cl]⁺ cocrystals via the reaction of [Ag₁₂(TBT)₈(TFA)₅(CH₃CN)]⁺ and [Ag₁₈(TPP)₁₀H₁₆]²⁺ (TBT = tert-butylthiolate, TFA = trifluoroacetate, CH₃CN = acetonitrile, TPP = triphenyl-phosphine).²¹ The structure of the co-crystalized Ag₁₆ and Ag₁₇ products are distinct from the precursors.

Inspired by the inter-particle reaction of the two Ag clusters bearing totally different ligands, herein we chose [Au₂₃SCy₁₆]⁻ and [Au₆(dppp)₄]²⁺ (abbreviated as Au₂₃ and Au₆) as the reactants. Both of them are meta-stable (easy formation, easy transformation). The single crystal structure of Au₆ ²² and Au₂₃ ²³ has been reported, demonstrating their stability during synthesis and under crystallization conditions. But on the other hand, Au₆ easily react with the Au(I) complex or Ag⁺ salt to generate [Au₈(dppp)₄Cl₂]²⁺,²⁴ or [Au₇(dppp)₄]³⁺.²⁵ While upon heating or oxidation with H₂O₂, Au₆ easily converts to [Au₁₁(dppp)₅]³⁺ or [Au₈(dppp)₄Cl₂]²⁺.²⁶ Similarly, rich chemistry has been reported for the Au₂₃ clusters. The addition of different thiolate ligands (TBBzT/TBBT/2-NPT) results in the size-growth of Au₂₃ to Au₂₄/Au₂₅/Au₂₈,^27,28 while the addition of phosphine ligand results in a distinct size-reduction of Au₂₃ → Au₂₂ (ref. 29 and 30)/Au₂₁.³⁰ Meanwhile, the addition of MSCy (M = Ag/Au) complexes results in the formation of heavily Ag-doped alloy (AuAg)₂₅ ³¹ and Au₂₈,³² respectively. Of note, the Au₂₃ → Au₂₈ conversion has also been regulated by oxidation³³ and photooxidation³⁴ conditions. In this context, the reaction of Au₆ with Au₂₃ clusters will aid the elucidation on the relative stability of the two cluster precursors, and shed light on the inherent structure–activity relationships therein.

In this study, the inter-cluster reaction of [Au₂₃SCy₁₆]⁻ with [Au₆(dppp)₄]²⁺ were conducted. In an equimolar reaction of Au₂₃ and Au₆, two main products, i.e. [Au₂₁SCy₁₂(dppp)₂]⁺ (Au₂₁), and [Au₂₂SCy₁₄(dppp)]²⁺ (Au₂₂ for short) were identified and characterized by ESI-MS and UV-vis etc. The framework of Au₂₃ is largely maintained in Au₂₂ and Au₂₁, while the one or two groups of Au(SCy)₂ motifs were each replaced by a dppp ligand. With the combination of DFT and structural analysis, the active etching site on the Au₂₃ precursor was found to be the Au(core)-S(on AuS₂ motif) bonds. Meanwhile, replacing the AuS₂ motifs with the dppp ligands results in significantly higher luminescence, a relatively larger O1–R1 gap, and higher thermal stability.

Experimental

Materials

All reagents were commercially available and used without further purification: dichloromethane (CH₂Cl₂, HPLC grade, ≥99.9%), methanol (MeOH, HPLC grade, ≥99.9%), n-hexane (n-Hex, HPLC grade, ≥98.0%) HAuCl₄·4H₂O (≥99.99%, metal basis), and cyclohexane-thiol (HSCy, ≥98%), sodium borohydride (NaBH₄, ≥98%), tetrabutylammonium bromide (TOABr, ≥99%), 1,3-bis(diphenylphosphine)propane (≥98%) were purchased from Shanghai Macklin Biochemical Co., Ltd. All glassware was thoroughly cleaned with aqua regia (HCl/HNO₃ 3/1 v/v), rinsed with copious amounts of pure water, and then dried in an oven before use.

Synthesis of [Au₂₃SCy₁₆]⁻ and [Au₆(dppp)₄]²⁺

[Au₂₃SCy₁₆]⁻ and [Au₆(dppp)₄]²⁺ were prepared via the previously reported methods,^35,36 and verified by UV-vis, and ESI-MS analysis (please see supporting information Fig. S1 and S2†). Briefly, Au₂₃ was synthesized by adding HSCy, TOAB, and NaBH₄ into the aqueous solution of HAuCl₄ in methanol, and [Au₆(dppp)₄]²⁺ was formed via the reaction of HAuCl₄, dppp and NaBH₄ under room temperature in ethanol.

Characterization

UV-vis absorption spectra were recorded on a UV-6000PC instrument. All fluorescence spectra were obtained using a HORIBA FluoroMax-4P fluorescence spectrophotometer.

Electrospray ionization mass spectrometry measurement was recorded using a Waters Xevo G2-XS QT mass spectrometer.

Results and discussion

The reaction of [Au₂₃SCy₁₆]⁻ and [Au₆(dppp)₄]²⁺

The reaction of [Au₂₃SCy₁₆]⁻ and [Au₆(dppp)₄]²⁺ was conducted in a 1 : 1 molar ratio. In detail, 20 mg [Au₂₃SCy₁₆]⁻ was dissolved in 15 mL of DCM, and then 8.3 mg of [Au₆(dppp)₄]²⁺ was added. Stirring for about 3 hours, the solution colour changed from brownish black to crimson black (Fig. 1a). The crude product was then concentrated and purified by preparative thin-layer chromatography (abbreviated as PTLC) with DCM/Hex (1 : 2) for the first time, and by PTLC with DCM/MeOH (10 : 1) for the second time (Fig. 1b inset).

Fig. 1 The UV-vis spectra of the Au₂₃, Au₆ precursors and the solution after reacting for 3 hours (a), inset: digital photo of the reaction solution before (left) and after reaction (right); and the UV-vis of the different band components after PTLC separation (b) insets: digital photo of the band distribution.

According to Fig. 1b, the UV-vis spectra of the DCM solution of the two bands are very similar, each of which shows a prominent peak at ∼570 nm, and a shoulder peak at ∼460 nm. Meanwhile, ESI-MS characterization of the band I component shows a prominent cluster peak at m/z = 3179.37, corresponding to [Au₂₂SCy₁₄(dppp)]²⁺ (Fig. 2a). The ESI-MS of the band II component shows a cluster peak at m/z = 6343.40 (Fig. 2b), corresponding to a cluster formula of [Au₂₁SCy₁₂(dppp)₂]⁺ (Au₂₁ for short). For both Au₂₂ and Au₂₁, the isotopic pattern is in excellent agreement with the theoretical one (Fig. 2a and b inset).

Fig. 2 The ESI-MS and the correlation of the experimental isotopic pattern with the theoretical one (inset) of Au₂₂ (a) and Au₂₁ clusters (b).

Of note, the UV-vis spectra of the formed Au₂₁ and Au₂₂ clusters are very similar to the reported spectra of [Au₂₂SCy₁₄(dppp)], and [Au₂₁SCy₁₂L₂]⁺ (L = dppm/dppe/CDPE).³⁰ But the conversion details and the components of the Au₂₁ and Au₂₂ clusters are distinct from the reported ones. First, the reported Au₂₁ and Au₂₂ clusters were formed via the Au^IL (L = diphosphate) etching of Au₂₃ precursor, while the inter-cluster reaction of Au₂₃ and Au₆ was used in this study. Second, the charge state of the Au₂₂ clusters in this study is distinct from the reported one (+2 vs. 0). Besides, the Au₂₁ cluster co-protected by SCy and dppp ligands was not reported,³⁰ and the etching of Au₂₃ with Au^Idppp generates Au₂₂ cluster exclusively in the early study. Herein, using Au₆ as a dppp-donating reagent, the Au₂₁ cluster co-protected by SCy and dppp ligand was gained as a main product. Nevertheless, given the similarity in the UV-vis of the formed Au₂₂/Au₂₁ cluster with the reported ones, and the plausibility of using UV-vis absorption curve to determine the cluster frameworks,^35,36 we anticipated that the framework of the formed Au₂₁ and Au₂₂ clusters is similar to the reported ones. Accordingly, the structure of the [Au₂₃SCy₁₆]⁻ has been largely maintained after the reaction.

As shown in Fig. 3, the structure of Au₂₃ could be viewed as protecting the bicapped icosahedral Au₁₅ core with two Au₃S₄, two AuS₂ staple motifs, and four bridging thiolate ligands. Replacing one or two AuS₂ motifs with one/two dppp ligands generates the structure of Au₂₂/Au₂₁. On the basis of the structural analysis, we performed density functional theory (DFT) calculations on the bond dissociation energy (BDE) of the Au–S bonds (see ESI† for the details of the computational method). The detailed results are given in Fig. S3,† and the BDE of the Au–S bonds between Au₁₅ core and S on AuS₂ is remarkably lower than all other ones, while the Au^cap–S bond (Fig. 3) is slightly lower than that of the Au^core–S (8.1 vs. 10.8 kcal mol⁻¹). According to the calculation results, both bonds could be easily broken under experimental conditions due to the low energy demands.

Fig. 3 The size-conversion of the [Au₂₃SCy₁₆]⁻ and [Au₆(dppp)₄]²⁺ clusters with the related structures.

Given the reaction mechanism, PTLC monitoring on the target reaction system (Fig. S4†) indicates the rapid formation of Au₂₂ and Au₂₁ within the first 10 minutes. After that, the amount of the Au₂₂ slightly diminished in the following reaction time. By contrast, the amount of Au₂₁ gradually increased, associated with the continuous reduction of both Au₂₂ and the Au₆ components. The results imply the easy replacement of the first AuS₂ motif, but the relatively difficult replacement of the second AuS₂ motif on the Au₂₃ precursor. In other words, the reactivity for the ligand exchange of AuS₂ to dppp has been greatly reduced after the first time exchange.

Fluorescence

Albeit the similar framework, [Au₂₃(SCy)₁₆]⁻ showed a weak emission peak at 703 nm, while [Au₂₂SCy₁₄(dppp)]²⁺ and [Au₂₁SCy₁₂(dppp)₂]⁺ showed approximately 2-fold emission enhancement, with a tiny redshift of the emission maximum wavelength (708 nm for Au₂₂ and 707 nm for Au₂₁, Fig. 4).

Fig. 4 Emission spectra of Au₂₃, Au₂₂ and Au₂₁ under excitation upon 365 nm light irradiation.

Differential pulse voltammetry (DPV)

The differential pulse voltammetry (DPV) curves of both Au₂₃ precursor and the Au₂₂/Au₂₁ product clusters (Fig. 5) feature molecular-like electrochemical characteristics, with distinctive oxidation and reduction potentials. In detail, the first oxidation peaks (O1) of the Au₂₃, Au₂₂ and Au₂₁ clusters are at 0.13, 0.23 and 0.66 V, respectively. The first reduction peaks (R1) are observed at −0.78, −1.00 and −0.80 V, respectively. Accordingly, the R1–O1 gap of the Au₂₃, Au₂₂ and Au₂₁ clusters are 0.91, 1.23 and 1.46 V, respectively. The enlarged gap of Au₂₂ and Au₂₁ than that of Au₂₃ demonstrates the enhanced electrochemical stability induced by the AuS₂ → dppp exchange.

Fig. 5 DPV spectra of Au₂₃ (a), Au₂₂ (b) and Au₂₁ (c) in 0.1 M Bu₄NPF₆–CH₂Cl₂ solutions that are degassed for 15 min and blanketed with N₂ at room temperature. Open-circuit voltage is 0.204 V.

Thermal stability

Associating with the size-reduction, the thermal stability of the clusters has been significantly improved. Following the recent studies,^37,38 the stability of the Au₂₁, Au₂₂ and Au₂₃ clusters under heating conditions were tracked with the UV-vis spectrum. As shown in Fig. 6, upon heating at 60 °C, the characteristic peak on UV-vis spectra of Au₂₂ and Au₂₁ maintains after even 18 hours, and the Au₂₁ cluster is even stable after 60 hours. By contrast, the characteristic peak of Au₂₃ attenuated within 1 hour, and becomes very weak after 18 hours. Herein, both the higher thermal stability and the co-presence of thiolate and diphosphine ligands will be helpful for the future application of the Au₂₂/Au₂₁ clusters.

Fig. 6 Thermal stability test of Au₂₃ (a), Au₂₂ (b) and Au₂₁ (c) clusters at 60 °C in toluene.

Conclusions

Herein, the inter-cluster reaction of [Au₂₃SCy₁₆]⁻ and [Au₆(dppp)₄]²⁺ clusters were conducted. [Au₂₁SCy₁₂(dppp)₂]⁺, and [Au₂₂SCy₁₄(dppp)]²⁺ were identified as the main products. The preliminary mechanistic insights with the combination of experimental and DFT calculations indicates the Au–S bond in the core–shell interference, and especially the Au–S bond of AuS₂ motif and the core structure, is the active reaction site. After incorporating the diphosphine ligands, the Au₂₂/Au₂₁ clusters show stronger luminescence than the Au₂₃ precursor. Meanwhile, both the electrochemical and thermo-stability tests indicate the higher stability of the Au₂₁ than that of the Au₂₃ precursors. The enhanced stability of the produced clusters might show high potential in future applications.

Author contributions

All authors have given approval to the final version of the manuscript. L. Z.: experiments, data analysis, manuscript draft; D. F.: investigation, formal analysis; M.C.: initial experiments; S. H. and Y. S.: theoretical calculation; H. Y.: conceptualization, supervision, review & editing, funding acquisition; M. Z.: conceptualization, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge financial support from National Science Foundation of Anhui Province (2108085J08), The University Synergy Innovation Program of Anhui Province (GXXT-2021-023).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of the [Au₂₃SCy₁₆]⁻[TOA]⁺ and [Au₆(dppp)₄]²⁺Cl₂ nanocluster (PDF) and density functional theory calculation on the Au–S bond dissociation energy. See DOI: https://doi.org/10.1039/d3ra01606d

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