Formation Mechanism of Au₂₀(SR)₁₆

Abstract

Understanding the formation mechanism is crucial for the synthesis and regulation of thiolate-protected gold nanoclusters at the atomic level. In this study, we elucidate the formation mechanism of a specially structured Au₂₀(SR)₁₆ cluster with a Au₈(SR)₈ ring. Utilizing density functional theory calculations, we propose an intermolecular coupling and hydrolysis reaction mechanism to elucidate the formation of Au₂₀(SR)₁₆ within the NaBH₄ system. Additionally, we formulate a molecular-like reaction equation to elucidate the size evolution of thiolate-protected gold nanoclusters, ranging from 0e ^– Au₁₂(SR)₁₂ to 4e ^– Au₂₀(SR)₁₆ clusters during the initial nucleation stage of growth. Notably, 0e ^– Au₁₂(SR)₁₂ exhibits a ring-in-ring structure, deviating from the conventional interlocked catenane ring. Furthermore, we delineate the formation pathway of 4e ^– Au₂₀(SR)₁₆, leveraging 0e ^– Au₁₂(SR)₁₂ as a seed based on the bottom-up reaction equation. These systematic investigations into the formation mechanism of 4e ^– Au₂₀(SR)₁₆ are pivotal for comprehending the NaBH₄ reduction and hydrolysis process and elucidating the structure–property relationship of Au₂₀(SR)₁₆. To the best of our knowledge, this represents the great theoretical exploration of the growth pathway from ring-in-ring structures to small gold clusters, offering valuable insights into the precise synthesis of gold clusters.

1. Introduction

In recent years, gold nanoparticles (AuNPs) have garnered significant attention across biomedical equipment, biosensors, catalysis, and medicine domains due to their distinctive electronic and chemical properties. − Various methods and technologies for synthesizing AuNPs have been documented, encompassing chemical, sonochemical, and photochemical approaches. Among these, chemical reduction emerges as a prevalent method due to its simplicity. This technique relies on the reduction of gold cluster precursors by reductive agents; for instance, using NaBH₄ to reduce the precursor of Au^(I)–SR complex facilitates the synthesis of small-sized thiolate-protected AuNPs. Through this approach, numerous well-defined thiolate-protected gold nanoclusters (abbreviated as Au _n (SR) _m ) have been successfully synthesized. − Nevertheless, despite the considerable volume of experimental studies on nanoparticle synthesis, little attention has been devoted to elucidating their mechanisms.

Understanding the formation mechanism of AuNPs is crucial for advancing the precise atomic synthesis of AuNPs with the desired size and composition. Early investigations into AuNPs formation proposed that gold ions (Au⁺) were initially reduced to gold atoms (Au⁰), which then aggregated to form nanoparticles. For instance, Anuradha et al. suggested that the initial step involved the reduction of gold ions into gold atoms, followed by their aggregation to form nanoparticles. Similarly, Polte et al. proposed that the synthesis process began with the reduction of Au^III to atoms, subsequent aggregation of these atoms, and condensation into nuclei to form AuNPs. However, this key assumption has often been found to contradict the standard reduction potential. Thus, the assumption that gold ions are first reduced to gold atoms, which then aggregate to form gold nanoclusters (AuNCs), is not valid for almost all common reducing agents.

It is noteworthy that recent density functional theory (DFT) calculations have shown that the formation of AuNCs involves the following steps: first, an aggregate with Au^(I)–Au^(I) and Au⁰–Au^I bonds is formed, and then this aggregate is reduced to an Au _n ⁰ species, which then serves as an intermediate for the formation of AuNCs. For example, Loh et al. employed ab initio calculations on gold nucleation, demonstrating the involvement of strong gold–gold atom coupling and water-mediated gold complexes in nanocrystal nucleation in aqueous solutions. Based on DFT study, Barngrover et al. showed how to form species containing Au⁰ without assuming the formation of Au⁰ atom. The precursors of AuNPs were AuCl₂ ^– , AuBr₂ ^– , AuI₂ ^– , AuClPH₃, and AuClSCH₃ ^– . At first, Au⁰ agglomerates, and then Au₂X₂ ^2– (X = Cl ^– , Br ^– , and I ^– ) and Au₃Cl₃ ^2– were formed. Larger clusters emerge through reactions between or among these clusters. Moreover, Mondal et al. analyzed the reduction mechanism of BH₄ ^–/AuCl₃OH^– via DFT, revealing that the Au⁰ atom was not an intermediate product and aggregation initiated from AuH₂ ^–. Furthermore, during AuNPs formation, the hydrolysis of the BH₄ ^– ligand leads to the formation of BH₃OH, with subsequent hydrolysis resulting in hydride ligand formation and loss of BH­(OH)₂. In aqueous solutions, BH­(OH)₂ serves as an intermediate that further hydrolyzes to yield BO₂ ^–. While numerous experimental and theoretical studies have confirmed the initial formation of Au^(I)–Au^(I) bonds followed by aggregation and reduction to Au _n ⁰ during AuNP formation, they often overlook the fundamental issue of AuNC nucleation or the formation of Au^(I)–Au^(I) bonds in the process of AuNC formation.

To comprehend the nucleation of AuNCs and the formation of Au^(I)–Au^(I) bonds within AuNCs, our research group has proposed an intermolecular reaction mechanism to elucidate the 2e ^– hopping phenomenon in the CO-directed synthesis of thiolate-gold clusters by DFT calculations. In this approach, two reaction steps were delineated. The first step was to propose a hieber base reaction, in which [Au^(I)–SR] _x species combines with CO and OH^– to form [(Au^(I)SR) _x COOH] ^– complex, which is same as reported by Xie et al. The second step involved [(Au^(I)SR) _x COOH] ^– species reacting with another homogenized [Au _y (SR) _y ] species to form a new intermediate species [Au _x+y (SR) _x+y ] by inserting the terminal [Au^(I)–COOH]⁻ group into the Au^(I)–S bond of the [Au _y (SR) _y ] species and then a decarboxylation reaction of [Au _x+y (SR) _x+y COOH] ^– species to release two [SR] ^– ligands, one CO₂ and one H₂O molecule, giving rise to a 2e ^– [Au _x+y (SR) _x+y–2] NC. Additionally, the size pathways for a series of small-sized Au­(I)-thiolate clusters also was proposed. Previous experimental studies have confirmed that NaBH₄-induced formation of Au₂₅ clusters and NaBH₄-induced size growth from Au₂₅ to Au₄₄ clusters also follows the 2e ^–-jump mechanism , _. For example, Xie and co-workers developed a novel NaOH-mediated NaBH₄ reduction method for the synthesis of Au₂₅ clusters. The reduction of NaBH₄ by NaOH was used to synthesize a monodisperse gold NC. In 2018, Xie and co-workers accurately determined the stoichiometry of the gold NP formation reaction during sodium borohydride reduction and identified a fine equilibrium reaction using real-time mass spectrometry: 32/x [Au­(SR)] _x + 8e ^– = [Au₂₅(SR)₁₈]⁻ + 7­[Au­(SR)₂]⁻; 8 electrons (from the reducing agent) are enough to react with every 32 gold atoms to form the high-purity [Au₂₅(SR)₁₈]⁻. Theoretically, Li et al. explored the nucleation and growth process of Au^(I)–SR oligomer conversion to gold NCs by NaBH₄ reduction and the mechanism based on the 2e ^–-jump mechanism. However, Au₂₀(SR)₁₆ has a special structure, so its formation process and mechanism are still unclear.

In this study, we employ DFT calculations to explore the formation pathway of 4e ^– Au₂₀(SR)₁₆ within the NaBH₄ system, elucidating the reduction and hydrolysis mechanism of NaBH₄ to comprehend the synthesis of 4e ^– Au₂₀(SR)₁₆. Drawing from the insights gained, we develop a molecular-like reaction equation to elucidate the initial nucleation stage of 4e ^– Au₂₀(SR)₁₆ formation. Through a 2e ^–-unit decomposition strategy, we identify several intermediates, including 0e ^– Au₁₂(SR)₁₂, 2e ^– Au₁₇(SR)₁₅, and 4e ^– Au₂₀(SR)₁₆. Notably, the structure of Au₁₂(SR)₁₂ exhibits a ring-in-ring configuration, consistent with prior findings by Che and co-workers. Leveraging the ring-in-ring configuration of 0e ^– Au₁₂(SR)₁₂ as the seed cluster and considering the 2e ^–-jump process, we propose a formation pathway for 4e ^– Au₂₀(SR)₁₆ in the NaBH₄ system, which yields a plausible trajectory. To the best of our knowledge, this is a great observation of the conversion between precursor species featuring a ring-in-ring configuration and Au^I-thiolate.

2. Theoretical Models and Computation Method

In the investigation of the formation pathway of thiolate-ligand-protected gold cluster Au₂₀(SR)₁₆, we optimized the intermediate, transition state, and product structures of small-size [Au _n (SR) _m ] ^q NCs and [H–Au _x (SR) _x BH₃]⁻ complexes using DFT. We employed the generalized gradient approximation with the Perdew–Burke–Ernzerhof (PBE) functional and the double-ζbasis set with DFT Semicore pseudopots implemented in the DMol3 package. , To account for dispersion energy, we utilized the Tkatchenko and Scheffler methods, while the implicit conductor-like screening model solvation model was employed to incorporate solvation effects with water as the solvent. Geometric optimization was considered converged when the energy change reached 1.0 × 10^–5 Hartree, the gradient reached 1.0 × 10^–3 Hartree/Å, and the displacement reached 5.0 × 10^–3 Å.

Unless stated otherwise, the thiolate group SR was represented by 4-tert-butylbenzenethiol­(−SPh-t-Bu). The configurations of thiolate-gold clusters, including Au₂₀(SR)₁₆ and Au₁₂(SR)₁₂, were adopted from experimental crystal structures, with SR ligands standardized to SPh-t-Bu.

3. Result and Discussion

3.1. Intermolecular Coupling and Hydrolysis Reaction Mechanisms

The distinctive structures of Au₂₀(SR)₁₆ have piqued our interest, prompting an exploration of its formation mechanism to elucidate its structure-performance correlation. First, based on the 2e ^–-jump mechanism, we propose an intermolecular coupling and hydrolysis reaction mechanism to understand the formation of gold NCs from Au^(I)-SR oligomers by NaBH₄ reduction. Scheme shows the idea of the intermolecular coupling and hydrolysis reaction mechanism: the synthesis of [Au _n (SR) _m ] ^q with the assistance of a NaBH₄ reductant can be explained by two reaction steps. In the first step, the interaction between BH₄ ^– and the Au­(I)-precursors such as [Au^(I)–SR] _x leads to the formation of [BH₄–Au _x (SR) _x ]⁻. In [BH₄–Au _x (SR) _x ]⁻, the H atom is tightly bonded to the Au atom. Due to the strong interaction between gold and hydride, the BH₃ group only weakly interacts with the [HAu _x (SR) _x ]⁻ fragment, lead to the formation of [H–Au^(I)–(Au^(I)SR) _x−1–SR–BH₃]⁻ complex. In the second step, the [H–Au^(I)–(Au^(I)SR) _x−1–SR–BH₃]⁻ complex engages with another homoleptic species (Au^(I)–SR) _y to form an Au^(I)–Au^(I) bond, leading to the creation of a new intermediate species, [H–(Au^(I)–SR) _x+y –BH₃]⁻. Afterward, because of the weak interaction between BH₃ and [H–(Au^(I)–SR) _x+y ]⁻ fragments, the [H–(Au^(I)–SR) _x+y –BH₃]⁻ species undergoes hydrolysis; the BH₃ unit may either detach to bind with the water molecule or transfer to the adjacent SR group. The exposed hydride terminal further nucleophilically attacks the adjacent Au^(I) atom or binds with the terminal SR, liberating two HSR ligands, three H₂ molecules, and one [BO₂]⁻ ion, resulting in the formation of a 2e ^– [Au _x+y (SR) _x+y–2] NC featuring either an Au₃ core (NC I-A) or an Au₄ core (NC I–B). Apparently, this process not only follows the previously proposed 2e ^–-jump mechanism , but also explains the formation of Au^(I)–Au^(I) and the nucleation of gold NCs.

1. NaBH₄ Reduction Mechanism for Formation and Size Growth of [Au _n (SR) _m ] ^q .

The further reaction of 2e ^– [Au _x+y (SR) _x+y−2] clusters with [Au^(I)–SR] _z species realizes the size growth of gold NCs as shown in Scheme . For example, 2e ^– Au _x+y (SR) _x+y−2 clusters can be grown into larger-sized 4e ^– Au _x+y+z (SR) _x+y+z−4(NC II-A) by reaction with [Au^(I)–SR] _z species. The intermolecular coupling and hydrolysis reaction mechanisms adequately account for the formation of Au^(I)–Au^(I) bonds and nucleation of gold NCs during [Au _m (SR) _n ] ^q formation. It is argued that this mechanism provides a theoretical basis for a deeper understanding of how AuNCs nucleate and how Au^(I)–Au^(I) bonds are formed during [Au _n (SR) _m ] ^q formation.

3.2. Formation Pathway of 4e ^– Au₂₀(SR)₁₆

The special structure of the Au₂₀(SR)₁₆ cluster has attracted our attention. As shown in Figure , the Au₂₀(SR)₁₆ cluster consists of an Au₇ core, an Au₈(SR)₈ ring, and two Au­(SR)₂ and an Au₃(SR)₄ ligand motifs. Unlike other cluster structures, the Au₂₀(SR)₁₆ cluster has a special Au₈(SR)₈ ring ligand. Therefore, the study of the formation mechanism of Au₂₀(SR)₁₆ clusters is imminent in order to further understand the relationship between their structures and properties.

1.

Structural sketch of Au₂₀(SR)₁₆. The R group in the SR ligand is not displayed for the sake of clarity.

Since the intermolecular coupling and hydrolysis reaction mechanisms of NaBH₄-directed [Au _n (SR) _m ] ^q formation and size growth follow the same 2e ^– hopping mechanism, the formation of 4e ^– Au₂₀(SR)₁₆ clusters can be regarded as a 2e ^– growth process. In order to obtain possible intermediates in the formation of Au₂₀(SR)₁₆, the previously proposed ‘2e ^–-unit dissociation strategy was used to reverse the dissociation of 4e ^– Au₂₀(SR)₁₆ clusters. As depicted in Figure , the 4e ^– Au₂₀(SR)₁₆ cluster yields a 2e ^– intermediate cluster, Au₁₇(SR)₁₅, by dissociating an Au₃(SR)₃ stapler unit. The core of the obtained intermediate cluster, Au₁₇(SR)₁₅, consists of a triangular Au₃. Subsequently, 2e ^– Au₁₇(SR)₁₅ dissociates an Au₅(SR)₅ stapler unit, releasing two [SR]⁻ ligands, three H₂ molecules, and one [BO₂]⁻ ion, resulting in the formation of a 0e ^– Au₁₂(SR)₁₂ cluster. Interestingly, the newly derived Au₁₂(SR)₁₂ structure differs from the conventional alkane Au₁₂(SR)₁₂ structure in that it consists of a four-membered ring Au₄(SR)₄ and an eight-membered ring Au₈(SR)₈, as opposed to the conventional alkane Au₁₂(SR)₁₂ structure, which consists of two six-membered rings, Au₆(SR)₆, interspersed with each other. This finding aligns with the structure of the experimentally synthesized Au₁₂(SR)₁₂ reported by Che and co-workers. Therefore, we hypothesize that Au₂₀(SR)₁₆ clusters can be formed by NaBH₄ reduction with Au₁₂(SR)₁₂ in a ring-in-ring configuration as the precursor.

2.

2e ^–-unit decomposition pathway of 4e ^– Au₂₀(SR)₁₆ from 0e ^– Au₁₂(SR)₁₂ is depicted. The large cyan atoms represent Au atoms in foreign Au atoms in Au₅(SR)₅ motifs, while the small cyan balls represent S atoms in foreign Au atoms in Au₅(SR)₅ motifs. The large yellow atoms denote Au atoms, and the small red atoms denote S atoms. The R group in the SR ligand is not displayed for clarity.

The chemical equation depicting the ‘2e ^–-unit dissociation’ process of 4e ^– Au₂₀(SR)₁₆ is provided in Table , revealing that the formation of Au₂₀(SR)₁₆ also adheres to the 2e ^– growth mechanism. By utilizing Au₁₂(SR)₁₂ as the seed cluster and considering the formation mechanism of [Au _n (SR) _m ] ^q facilitated by NaBH₄, a concise molecular-like reaction equation for the bottom-up formation of 4e ^– Au₂₀(SR)₁₆ can be expressed as follows: Au₁₂(SR)₁₂ + Au₃(SR)₃ + Au₅(SR)₅ + 2­[BH₄]⁻ + 4H₂O → Au₂₀(SR)₁₆ + 6H₂ + 4H­(SR) + 2­[BO₂]⁻.

1. Chemical Equation of ‘2e ^– -Unit Decomposition’ Processes.

the chemical equation of ‘2e – -unit decomposition’ processes:

$$4e^{-}{\mathrm{Au}}_{20}(\mathrm{SR}{)}_{16}\to 2e^{-}{\mathrm{Au}}_{17}(\mathrm{SR}{)}_{15}+[{\mathrm{Au}}^{(\mathrm{I})}-\mathrm{SR}{]}_3+[{\mathrm{BH}}_4{]}^{-}+2{\mathrm{H}}_2\mathrm{O}-[{\mathrm{BO}}_2{]}^{-}-3{\mathrm{H}}_2-2\mathrm{HSR}$$

$$2e^{-}{\mathrm{Au}}_{17}(\mathrm{SR}{)}_{15}\to 0e^{-}{\mathrm{Au}}_{12}(\mathrm{SR}{)}_{12}+[{\mathrm{Au}}^{(\mathrm{I})}-\mathrm{SR}{]}_5+[{\mathrm{BH}}_4{]}^{-}+2{\mathrm{H}}_2\mathrm{O}-[{\mathrm{BO}}_2{]}^{-}-3{\mathrm{H}}_2-2\mathrm{HSR}$$

the stepwise molecular-like formation equations for 4e – Au20(SR)16 cluster:

$$0e^{-}[{\mathrm{Au}}^{(\mathrm{I})}-\mathrm{SR}{]}_{12}+[{\mathrm{Au}}^{(\mathrm{I})}-\mathrm{SR}{]}_5+[{\mathrm{BH}}_4{]}^{-}+2{\mathrm{H}}_2\mathrm{O}\to 2e^{-}{\mathrm{Au}}_{17}(\mathrm{SR}{)}_{15}+3{\mathrm{H}}_2+2\mathrm{HSR}$$

$$2e^{-}{\mathrm{Au}}_{17}(\mathrm{SR}{)}_{15}+[{\mathrm{Au}}^{(\mathrm{I})}-\mathrm{SR}{]}_3+[{\mathrm{BH}}_4{]}^{-}+2{\mathrm{H}}_2\mathrm{O}\to 4e^{-}{\mathrm{Au}}_{20}(\mathrm{SR}{)}_{16}+3{\mathrm{H}}_2+2\mathrm{HSR}$$

the bottom-up formation equations for 4e – Au20(SR)16 cluster:

$$0e^{-}[{\mathrm{Au}}^{(\mathrm{I})}-\mathrm{SR}{]}_{12}+[{\mathrm{Au}}^{(\mathrm{I})}-\mathrm{SR}{]}_5+[{\mathrm{Au}}^{(\mathrm{I})}-\mathrm{SR}{]}_3+2[{\mathrm{BH}}_4{]}^{-}+4{\mathrm{H}}_2\mathrm{O}\to 4e^{-}{\mathrm{Au}}_{20}(\mathrm{SR}{)}_{16}+6{\mathrm{H}}_2+4\mathrm{HSR}+2[{\mathrm{BO}}_2{]}^{-}$$

Drawing from the aforementioned reaction equation for the formation of Au₂₀(SR)₁₆ and the intermolecular coupling and hydrolysis reaction mechanism, we conducted a detailed exploration of the formation pathway of 4e ^– Au₂₀(SR)₁₆ in the NaBH₄ system using DFT calculations. The presence of [BH₄]⁻ in NaBH₄ can induce the rupture of the Au^(I)–S bond in the Au₅(SR)₅ ring, leading to the formation of [H–Au₅(SR)₅–BH₃]⁻species. This is consistent with the report by Li et al. As illustrated in Figure , this species initially reacts with 0e ^– Au₁₂(SR)₁₂ to generate an intermediate [H–Au₁₇(SR)₁₇–BH₃]⁻ featuring an Au^(I)–Au^(I) bond, achieved by the insertion of the terminal [Au^(I)–H]⁻ from the [H–Au₅(SR)₅–BH₃]⁻ species into the Au–S bond in Au₁₂(SR)₁₂. The reaction energy of this step is −1.67 eV, indicating that the embedding of [H–Au₅(SR)₅-BH₃]⁻ into Au₁₂(SR)₁₂ is spontaneous. Subsequently, the [H–Au₁₇(SR)₁₇–BH₃]⁻ intermediate undergoes reduction and hydrolysis reactions, yielding a 2e ^– Au₁₇(SR)₁₅ NC while releasing two HSR species, one [BO₂]⁻ species, and three H₂ molecules. The reaction energy of this step is −0.13 eV. This is similar to the Au:S ratios of Au:SR = 17:14 and Au:SR = 20:14 intermediate substances monitored during the reduction of [Au₂₅(SR)₁₈]⁻ by NaBH₄ reported by Xie and co-workers. This further confirms that the possible intermediate in the formation of Au₂₀(SR)₁₆ is an Au₁₇(SR)₁₅ cluster.

3.

Formation pathways of the 4e ^– Au₂₀(SR)₁₆ cluster (SR = −SPh-t-Bu). Bright blue atoms represent newly added gold atoms in the gold core during size evolution, and orange atoms depict the original Au atoms in the Au₁₂(SR)₁₂ cluster. Large purple-red atoms denote the Au atoms in the foreign Au₃(SR)₃ motifs and Au₅(SR)₅, while small purple-red atoms represent the Au atoms in these motifs. Red atoms indicate the active S atoms in the Au₁₂(SR)₁₂ cluster, with the blue atom representing the boron atom and brown atoms symbolizing hydrogen atoms. The R group in the SR ligand is omitted for the sake of clarity.

The size growth pathway from 2e ^– Au₁₇(SR)₁₅ to 4e ^– Au₂₀(SR)₁₆ is depicted in Figure . Initially, 2e ^– Au₁₇(SR)₁₅ undergoes a reaction with [H–Au₃(SR)₃–BH₃]⁻ to generate a [H–Au₂₀(SR)₁₈–BH₃]⁻ complex featuring an Au^(I)–Au^(I) bond, facilitated by the insertion of the terminal [Au^(I)–H]⁻ from the [H–Au₃(SR)₃–BH₃]⁻ species into the Au^(I)–S bond in Au₁₇(SR)₁₅. This reaction exhibits an energy of −0.27 eV. Subsequently, [H–Au₂₀(SR)₁₈–BH₃]⁻ releases one [BO₂]⁻ ion, two HSR ligands, and three H₂ molecules, leading to the formation of a 4e ^– Au₂₀(SR)₁₆ cluster with an Au₇ core. The energy of this step is −3.52 eV (Table ). Consequently, utilizing Au₁₂(SR)₁₂ as the seed cluster, the overall reaction energy for the formation of the 4e ^– Au₂₀(SR)₁₆ cluster via NaBH₄ reduction is −5.59 eV, indicating thermodynamic feasibility and spontaneous occurrence of this process. It further confirms that Au₂₀(SR)₁₆ clusters can be formed by NaBH₄ reduction of Au₁₂(SR)₁₂ clusters in the ring-in-ring configuration.

2. Reaction Equation and Reaction Energy of Each Step in the Formation Path of 4e ^– Au₂₀(SR)₁₆ .

the reaction equation and reaction energy of 4e –Au20(SR)16
$${\mathrm{Au}}_{12}{(\mathrm{SR})}_{12}+{[\mathrm{H}-{\mathrm{Au}}_5{(\mathrm{SR})}_5-{\mathrm{BH}}_3]}^{-}\to {[\mathrm{H}-{\mathrm{Au}}_{17}{(\mathrm{SR})}_{17}-{\mathrm{BH}}_3]}^{-}$$	ΔE = −1.67 eV
$${[\mathrm{H}-{\mathrm{Au}}_{17}{(\mathrm{SR})}_{17}-{\mathrm{BH}}_3]}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Au}}_{17}{(\mathrm{SR})}_{15}+{[{\mathrm{BO}}_2]}^{-}+3{\mathrm{H}}_2+2\mathrm{HSR}$$	ΔE = −0.13 eV
$${\mathrm{Au}}_{17}{(\mathrm{SR})}_{15}+{[\mathrm{H}-{\mathrm{Au}}_3{(\mathrm{SR})}_3-{\mathrm{BH}}_3]}^{-}\to {[\mathrm{H}-{\mathrm{Au}}_{20}(\mathrm{SR})18-{\mathrm{BH}}_3]}^{-}$$	ΔE = −0.27 eV
$${[\mathrm{H}-{\mathrm{Au}}_{20}{(\mathrm{SR})}_{18}-{\mathrm{BH}}_3]}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Au}}_{20}{(\mathrm{SR})}_{16}+{[{\mathrm{BO}}_2]}^{-}+3{\mathrm{H}}_2+2\mathrm{HSR}$$	ΔE = −3.52 eV

The formation pathway of 4e ^– Au₂₀(SR)₁₆ can generally be delineated into four stages. Initially, the Au₅(SR)₅ species reacts with [BH₄]⁻ to yield the [H–Au₅(SR)₅–BH₃]⁻ complex through a NaBH₄ reduction reaction. Subsequently, in the second stage, the [H–Au₅(SR)₅–BH₃]⁻ complex interacts with [Au^(I)–(SR)]₁₂ species, resulting in the formation of the [H–Au₁₇(SR)₁₇–BH₃]⁻ intermediate containing an Au^(I)–Au^(I) bond. The third stage involves the hydrolysis reaction of the [H–Au₁₇(SR)₁₇–BH₃]⁻ species, leading to the simultaneous release of two HSR species, three H₂ molecules, and one [BO₂]⁻ ion, thereby producing the 2e ^– Au₁₇(SR)₁₅ NC. Finally, the fourth stage entails a 2e ^– → 4e ^– size growth process. Throughout the formation process of 4e ^– Au₂₀(SR)₁₆, NaBH₄ plays a pivotal role. Its strong reducibility facilitates the formation of Au^(I)–Au^(I) bonds and the nucleation of AuNCs, which are critical stages in the formation of Au^(I)-thiolate clusters.

4. Conclusions

This study proposes intermolecular coupling and hydrolysis reaction mechanisms for the formation of Au^(I)-thiolate clusters in the NaBH₄ system. Through this mechanism, the formation pathway and mechanism of Au₂₀(SR)₁₆ in the NaBH₄ system are elucidated. Utilizing a ‘2e ^–-unit decomposition’ strategy for 4e ^– Au₂₀(SR)₁₆, a molecular-like reaction equation for the bottom-up formation of 4e ^– Au₂₀(SR)₁₆ is established, and intermediates including 0e ^– Au₁₂(SR)₁₂ and 2e ^– Au₁₇(SR)₁₅ clusters are identified. Structural analysis reveals that Au₁₂(SR)₁₂ comprises an eight-membered ring Au₈(SR)₈ and a four-membered ring Au₄(SR)₄, which is the key precursor for the formation of Au₂₀(SR)₁₆ clusters. Furthermore, employing DFT calculations, a pathway is proposed to elucidate the bottom-up formation of 4e ^– Au₂₀(SR)₁₆. The DFT calculation results indicate that the formation process of the 4e ^– Au₂₀(SR)₁₆ cluster using 0e ^– Au₁₂(SR)₁₂ as the seed cluster is spontaneous. Additionally, the formation pathway of the Au₂₀(SR)₁₆ cluster elucidates the nucleation of AuNCs and the formation of Au^(I)–Au^(I) bonds in detail. This research provides a more coherent path for the nucleation and size growth of Au₂₀(SR)₁₆ in the NaBH₄ system, thereby confirming the significance of NaBH₄’s reducibility in the formation of Au₂₀(SR)₁₆. We believe that this work holds significant implications for enhancing our understanding of the nucleation and size growth of NCs.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c10303.

• Structural coordinates involved in the formation pathways of the 4e ^– Au₂₀(SR)₁₆ cluster (PDF)

The authors declare no competing financial interest.