Quantitative analysis of air-oxidation reactions of thiolate-protected gold nanoclusters

Abstract

The interaction of dioxygen (O₂) with inorganic nanomaterials is one of the most essential steps to understanding the reaction mechanism of O₂-related reactions. However, quantitative analyses for O₂-binding processes and subsequent oxidation reactions on the surface are still elusive, whereas the reaction of O₂ with molecules such as transition metal complexes has been widely explored. Herein, we have quantitatively evaluated reaction processes of air-oxidation reactions of atomically precise thiolate-protected Au₂₅ nanoclusters ([Au₂₅(SR)₁₈]⁻) as a model of O₂ activation by inorganic nanomaterials. Kinetic analyses on the air-oxidation reaction of [Au₂₅(SR)₁₈]⁻ revealed a controlling factor for O₂-activation processes, which could be finely tunable by the protecting thiolate ligands.

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Introduction

Dioxygen (O₂) activation in O₂ reduction and aerobic oxidations is one of the most fundamental and crucial reaction processes in chemistry. So far, many catalysts for O₂ activation have been developed by employing molecular catalysts^1–3 and inorganic nanomaterials, including metal nanoparticles (MNPs)^4–6 and metal nanoclusters (MNCs).^7–9 An understanding of the reaction mechanisms of O₂-activation processes at the atomic level is essential to develop more efficient catalysts. Hence, mechanistic insights into O₂ activation have also been investigated.^10–13 For molecular catalysts such as Fe, Co and Cu complexes or organocatalysts such as porphyrinoids, O₂-activation processes, including O₂-binding and subsequent reduction to form reactive O₂ species, were experimentally well-revealed through structural¹⁰ and kinetic¹¹ analyses based on experimental evidence. In contrast, because reaction systems with MNPs are relatively complicated due to the distributions of size and composition of MNPs, the understanding of the reaction mechanisms for the O₂-activation process at the atomic level is quite a difficult challenge. Thus, estimations of such a reaction mechanism have mainly relied on theoretical analysis until now.¹²

Compared with MNPs, MNCs with sizes of less than 2 nm are considered a desirable model to reveal detailed reaction mechanisms at the atomic level owing to their atomically precise structures.¹⁴ Among various MNCs, gold nanoclusters (AuNCs), such as thiolate-protected gold clusters (denoted as Au_n(SR)_m, SR = thiolate), are appropriate candidates to perform atomically precise analyses on O₂-activation processes using inorganic nanomaterials because of their tunable reactivities, easy preparation and good stability.^7,15,16 For example, the thiolate-protected Au₂₅ cluster anion ([Au₂₅(SR)₁₈]⁻) exhibited reactivity towards O₂, as seen in the catalytic oxygen reduction reaction (ORR).¹⁷ The catalytic activity of [Au₂₅(SR)₁₈]⁻ in the ORR could be tuned by the type of thiolate ligand^17a and hetero-atom doping.^17b In addition, [Au₂₅(SR)₁₈]⁻ which shows higher reactivity than other AuNCs with different sizes^17c is a promising material for practical use. Together with the ORR, the reactions of [Au₂₅(SR)₁₈]⁻ with O₂ are also important in the reductive activation of O₂ for substrate oxidation, which was seen in the aerobic oxidation of thiolates to form disulfide in Au₂₅ polymer films.¹⁸ In both cases, the interaction of O₂ on the surface of the Au₂₅ cluster and reduction of O₂, namely Au₂₅ cluster oxidation,^19,20 are proposed to be crucial processes as an initial step for O₂ activation. However, whereas some reports demonstrated the interaction between O₂ and [Au₂₅(SR)₁₈]⁻ using mass spectrometry²¹ or theoretical analysis,^13d,f,17d experimental evidence for the O₂ binding and/or activation by AuNCs is still limited. In particular, the air-oxidation reaction of [Au₂₅(SR)₁₈]⁻ was analysed just qualitatively,²² but quantitative analysis for O₂ activation by AuNCs, which is important to systematically evaluate the reactivities of AuNCs, has yet to be reported.

Herein, we report quantitative analyses of the air-oxidation reactions of AuNCs based on kinetic analysis for the first time. For a model of AuNCs, we chose [Au₂₅(SR)₁₈]⁻ because of its reversible redox properties²⁰ and a variety of choices for protecting ligands.^14,23 The Au₂₅ cluster anions are composed of an Au₁₃⁵⁺ core surrounded by six [Au₂(SR)₃]⁻ units, and the oxidation reactions of the gold cluster occur in the Au₁₃ core rather than in staple moieties.^19b Evaluations of the reactivities of [Au₂₅(SR)₁₈]⁻ with O₂ were conducted in the presence of protons (Scheme 1). The addition of protons can promote the electron transfer reaction to reduce O₂ through proton-coupled electron transfer (PCET)²⁴ and this reaction system has a big advantage of facile quantitative analysis of the reactivity, as seen in the mechanistic analysis of O₂ reduction by organic molecules^3c,25 or transition metal complexes.²⁶ We have controlled redox potentials and O₂-affinities of [Au₂₅(SR)₁₈]⁻ by changing the thiolate (SR) ligands and revealed controlling factors in air-oxidation reactions of [Au₂₅(SR)₁₈]⁻ to form neutral [Au₂₅(SR)₁₈]⁰.

Scheme 1 Reaction scheme for air-oxidation reactions of [Au₂₅(SR)₁₈]⁻ employed in this work. R groups were omitted for clarity. Atom labels: grey: gold in the Au₁₃ core, yellow: gold on the staple, and green: sulfur.

Results and discussion

Synthesis and characterisation of thiolate-protected Au₂₅ cluster anions

Synthesis of the phenylethanethiolate-protected Au₂₅ cluster anion (TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻, TOA⁺ = tetraoctylammonium) was conducted through the reduction of Au(I)–thiolate complexes according to the previous report.²⁷ Other Au₂₅ clusters (TOA⁺[Au₂₅(SR)₁₈]⁻) with different thiolate ligands (phenylmethanethiol (PhC₁SH), thiophenol (PhSH) and 4-methoxythiophenol (MeOPhSH)) were synthesized using ligand-exchange reactions. Phenylmethanethiolate-protected Au₂₅ cluster anions (TOA⁺[Au₂₅(SC₁Ph)₁₈]⁻) were successfully synthesized through the ligand-exchange reaction of TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻ with excess amounts of PhC₁SH in CH₂Cl₂. TOA⁺[Au₂₅(SPh)₁₈]⁻ and TOA⁺[Au₂₅(SPhOMe)₁₈]⁻ were prepared by the ligand-exchange reaction of the cyclohexanethiolate-protected Au₂₃ cluster (TOA⁺[Au₂₃(SCy)₁₆]⁻, SCy = cyclohexanethiolate) with PhSH or MeOPhSH (see the ESI† for detailed synthetic methods).²⁸ A series of thiolate-protected Au₂₅ cluster anions were characterised using UV-vis absorption spectroscopy, ¹H NMR, ESI-MS and X-ray absorption fine structure (XAFS) measurements (Fig. 1 and S1–S6†). In negative-mode ESI-MS spectra (Fig. S1†) of TOA⁺[Au₂₅(SR)₁₈]⁻, peaks from [Au₂₅(SR)₁₈]⁻ were clearly observed as well as the results of ¹H NMR spectroscopy (Fig. S2–S5†), supporting the high purity of the products. UV-vis absorption spectra of all products in tetrahydrofuran (THF) showed similar spectral features with a thiolate-protected Au₂₅ cluster anion,²² that is, the absorption bands at around 400 nm and 700 nm and a shoulder peak at ca. 800 nm (Fig. 1). The absorption peak at around 700 nm, which is mainly derived from a Au₁₃ core-to-core transition,²⁹ was slightly red-shifted from TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻ (682 nm) to TOA⁺[Au₂₅(SPh)₁₈]⁻ (702 nm). These shifts of absorption bands were caused by the differences in the electronic effects of thiolate ligands.³⁰ By contrast, the shape of absorption bands in the higher energy region was clearly different depending on the SR ligands because of the direct contribution of protecting ligands to the electronic transitions.²⁹ The structural features of a series of Au₂₅ cluster anions were compared using Au-L₃ edge XAFS measurements at 10 K with transmission mode (Fig. S6 and Table S1†). Extended XAFS (EXAFS) analysis showed that structures of all Au₂₅ cluster anions employed in this work showed similar structural parameters (Table S1†). Moreover, the solid-state structure of TOA⁺[Au₂₅(SPh)₁₈]⁻ was clarified using single-crystal X-ray crystallographic analysis (Fig. 2 and S7†). The anionic part in TOA⁺[Au₂₅(SPh)₁₈]⁻ has a Au₁₃ icosahedral core (Au₁₃⁵⁺) with six dimeric staple moieties ([Au₂(SR)₃]⁻), which are typical components for the Au₂₅ cluster anion. The average Au–Au and Au–S bond lengths, summarized in Table S2,† were comparable to those of previously reported TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻.¹⁶

Fig. 1 UV-vis spectra of TOA⁺[Au₂₅(SR)₁₈]⁻ (0.010 mM) in THF. SR = SC₂Ph (black), SC₁Ph (green), SPh (blue) and SPhOMe (red).

Fig. 2 A thermal ellipsoid plot (50% probability) of TOA⁺[Au₂₅(SPh)₁₈]⁻. The TOA cation, solvent molecules, and hydrogen atoms were omitted for clarity. Atom labels: grey: gold in the Au₁₃ core, yellow: gold on the staple, green: sulfur, and light grey: carbon.

To determine the redox potentials of Au₂₅ clusters, cyclic voltammetry (CV) and differential pulse voltammetry on a series of Au₂₅ cluster anions were performed in THF containing TBAPF₆ as an electrolyte (Fig. S8†). The cyclic voltammogram of TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻ showed two reversible redox waves at E_1/2 = +0.06 V and +0.41 V vs. SCE (Fig. S8a†). These two redox waves could be assigned to the redox couples of Au₂₅⁻/Au₂₅⁰ and Au₂₅⁰/Au₂₅⁺, respectively.²⁰ The first E_1/2 values were positively shifted with decreasing electron-donating abilities of thiolate ligands;³⁰ for TOA⁺[Au₂₅(SC₁Ph)₁₈]⁻, the first oxidation wave was observed at +0.16 V vs. SCE owing to the weaker electron-donating ability of SC₁Ph than that of SC₂Ph. The largest shift of E_1/2 (=+0.26 V vs. SCE) was seen for TOA⁺[Au₂₅(SPh)₁₈]⁻ (Fig. S8d†) with the weakest electron-donating ligands in this work. It should be noted that the E_1/2 value in the redox couple of Au₂₅⁻/Au₂₅⁰ for TOA⁺[Au₂₅(SPhOMe)₁₈]⁻ (E_1/2 = +0.17 V vs. SCE) is almost the same as that for TOA⁺[Au₂₅(SC₁Ph)₁₈]⁻ (Table 1 and Fig. S8b, c†). As well as the redox properties of [Au₂₅(SR)₁₈]⁻, the affinity with O₂ should be one of the essential factors to determine reactivities with O₂. The O₂-affinities of inorganic nanomaterials would be affected by surface environments including structures of protecting ligands. Thus, the two kinds of [Au₂₅(SR)₁₈]⁻, with the same redox potentials but different surface ligands, could provide meaningful insights into the correlation between the O₂-affinity and the reactivities of Au₂₅ clusters.

Table 1 Summary of redox potentials of TOA⁺[Au₂₅(SR)₁₈]⁻ in THF containing 0.1 M TBAPF₆ under Ar

SR	E 1/2 (Au25^−/Au25^0) V, vs. SCE	E 1/2 (Au25^0/Au25^+) V, vs. SCE
SC2Ph	+0.06	+0.41
SC1Ph	+0.16	+0.50
SPhOMe	+0.17	+0.41
SPh	+0.26	+0.54

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Air-oxidation reactions of thiolate-protected Au₂₅ clusters

Air-oxidation reactions of TOA⁺[Au₂₅(SR)₁₈]⁻ were monitored by UV-vis spectroscopic measurements. First, we evaluated the stability of TOA⁺[Au₂₅(SR)₁₈]⁻ (0.010 mM) in air-saturated THF at 298 K for 24 h (Fig. S9†). The lack of spectroscopic changes indicates that all TOA⁺[Au₂₅(SR)₁₈]⁻ are highly stable in the absence of any proton sources. Jin and co-workers reported photo-mediated oxidation reactions of [Au₂₅(SC₂Ph)₁₈]⁻ under ambient light exposure,^13d but such oxidation reactions were not observed in our reaction system. Thus, the light irradiation from the UV-vis spectroscopic device could not induce the photo-induced oxidation reaction and we have excluded the light-mediated effect from our reaction system. By contrast, in the presence of a Brønsted acid like trifluoroacetic acid (TFA, 0.25 mM), the absorption at 445 nm and around 800 nm of TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻ slightly decreased and a new absorption band at around 630 nm appeared (Fig. 3a). In addition, the absorption peak at around 400 nm became sharper. Such spectral change was also observed even in the presence of a small amount of TFA (0.050 mM, Fig. S10†). The UV-vis absorption feature of the final product indicates the formation of [Au₂₅(SC₂Ph)₁₈]⁰, a one-electron oxidized product of [Au₂₅(SC₂Ph)₁₈]⁻ (eqn (1)).²² The formation of [Au₂₅(SC₂Ph)₁₈]⁰ was also confirmed by ¹H NMR (Fig. S11†) and CV measurements of [Au₂₅(SC₂Ph)₁₈]⁻ (Fig. S12†). In the ¹H NMR spectrum, air-oxidation of TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻ in THF-d₈ gave a new peak at 5.17 ppm (Fig. S11a and b†), which was in good agreement with that from [Au₂₅(SC₂Ph)₁₈]⁰ (Fig. S11c†), supporting the formation of an one-electron-oxidized Au₂₅ cluster. In the electrochemical measurements on TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻, the open circuit potential (OCP) was determined to be −0.03 V vs. SCE in the absence of TFA, which was more negative than E_1/2 (Au₂₅⁻/Au₂₅⁰) of [Au₂₅(SC₂Ph)₁₈]⁻ (+0.06 V vs. SCE). Upon addition of TFA, OCP shifted to +0.21 V vs. SCE, which was positioned between E_1/2 (Au₂₅⁻/Au₂₅⁰) and E_1/2 (Au₂₅⁰/Au₂₅⁺). Moreover, the negative potential sweep from +0.21 V showed a clear reduction wave assigned with the reduction of [Au₂₅(SC₂Ph)₁₈]⁰ to [Au₂₅(SC₂Ph)₁₈]⁻. This electrochemical evidence strongly supports the formation of the neutral Au₂₅ cluster. The resulting [Au₂₅(SC₂Ph)₁₈]⁰ formed under UV-vis spectroscopic conditions could be reduced to [Au₂₅(SC₂Ph)₁₈]⁻ quantitatively (Fig. S13†) by reductants such as sodium borohydride (NaBH₄). The reversible behaviour of Au₂₅⁻/Au₂₅⁰ conversion was also observed even at a high TFA concentration (10 mM, Fig. S14†). This complete reversibility indicates that no side reactions such as decompositions of Au₂₅ clusters occur, suggesting that [Au₂₅(SC₂Ph)₁₈]⁻ is desirable as a highly durable redox catalyst.

d[Au₂₅(SR)₁₈]/dt = k_obs[[Au₂₅(SR)₁₈]⁻] = k[TFA][[Au₂₅(SR)₁₈]⁻] (2)

Fig. 3 (a) UV-vis spectral change of TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻ (0.010 mM) in air-saturated THF containing TFA (0.25 mM) at 298 K. Inset: the time profile of absorbance at 600 nm. (b) [TFA] dependence of k_obs in the air-oxidation reaction of TOA⁺[Au₂₅(SR)₁₈]⁻ in air-saturated THF at 298 K. SR = SC₂Ph (black), SC₁Ph (green), SPhOMe (red) and SPh (blue). [O₂] = 2.2 mM.³¹

To perform the kinetic analysis in the formation of [Au₂₅(SC₂Ph)₁₈]⁰, the pseudo-first-order rate constant (k_obs) was determined to be (1.6 ± 0.1) × 10⁻³ s⁻¹ at 298 K from the time course of the UV-vis spectral change at 600 nm (Fig. 3a, inset). A linear correlation was observed between the TFA concentration and k_obs (Fig. 3b, black), indicating that only one proton was involved in the oxidation reaction. The second-order rate constant (k in eqn (1) and (2)) could be determined to be (5.65 ± 0.06) M⁻¹ s⁻¹ from the slope. It should be noted that no oxidation reaction of [Au₂₅(SC₂Ph)₁₈]⁻ occurred in deaerated THF (Fig. S15†). Therefore, O₂ should be an oxidant for [Au₂₅(SC₂Ph)₁₈]⁻ in this reaction system. In addition, no change in optical properties of [Au₂₅(SC₂Ph)₁₈]⁻ upon addition of TFA under the degassed conditions suggests no interaction between the Au₂₅ cluster and TFA (Fig. S15†). Therefore, TFA should play a role in acceleration of Au₂₅ cluster air-oxidation, namely O₂ reduction through PCET, in which an electron transfer from [Au₂₅(SC₂Ph)₁₈]⁻ to O₂ and a proton comes from a Brønsted acid.²⁴ Other TOA⁺[Au₂₅(SR)₁₈]⁻ (SR = SC₁Ph, SPh and SPhOMe) were also oxidized to form corresponding neutral [Au₂₅(SR)₁₈]⁰ in air-saturated THF containing TFA, as observed by the UV-vis spectral change (Fig. S16†). In addition, each [Au₂₅(SR)₁₈]⁰ could be fully recovered to the corresponding anionic species by the addition of NaBH₄, indicating high durability of these Au₂₅⁻/Au₂₅⁰ redox reaction systems (Fig. S17†). Kinetic analyses were performed to determine k values (Fig. 3b), which are summarized in Table 2. The oxidation reaction of [Au₂₅(SC₂Ph)₁₈]⁻ with the lowest one-electron oxidation potential (+0.06 V vs. SCE, Table 1) showed the highest k value, whereas the lowest k value was confirmed in TOA⁺[Au₂₅(SPh)₁₈]⁻ with the highest E_1/2 among TOA⁺[Au₂₅(SR)₁₈]⁻ in this work. The reactivities of TOA⁺[Au₂₅(SC₁Ph)₁₈]⁻ and TOA⁺[Au₂₅(SPhOMe)₁₈]⁻ were almost the same despite the different types of SR moieties (benzylic and aromatic thiolate, respectively). Considering the similar E_1/2 values between TOA⁺[Au₂₅(SC₁Ph)₁₈]⁻ and TOA⁺[Au₂₅(SPhOMe)₁₈]⁻ (Table 1), the redox potential of TOA⁺[Au₂₅(SR)₁₈]⁻ should be one of the main controlling factors to determine the reactivities with O₂.

Table 2 Summary of rate constants (k and k₂) and equilibrium constants (K) in the air oxidation of TOA⁺[Au₂₅(SR)₁₈]⁻ in THF in the presence of TFA^a

SR	k, M^−1 s^−1	k 2, s^−1	K, mM^−1
a At 298 K. [Au25(SR)18]^− = 0.010 mM.
SC2Ph	5.65 ± 0.06	9.9 ± 0.3	0.20 ± 0.01
SC1Ph	2.93 ± 0.08	4.2 ± 0.1	0.26 ± 0.03
SPhOMe	3.19 ± 0.01	3.67 ± 0.03	0.34 ± 0.01
SPh	1.51 ± 0.02	1.35 ± 0.01	0.55 ± 0.01

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Häkkinen and co-workers demonstrated that small molecules such as O₂ were able to access the surface of ligand-protected AuNCs, which would play an important role in small molecule activation.³² Then, to examine the interaction of the surface of [Au₂₅(SR)₁₈]⁻ with O₂ during air-oxidation reactions, kinetic analyses were performed by changing the concentration of O₂ in THF (Fig. 4). The pseudo-first-order rate constants (k_obs) of the air-oxidation reaction of TOA⁺[Au₂₅(SR)₁₈]⁻ showed saturation behaviour on increasing the concentration of O₂, suggesting the existence of a pre-equilibrium between TOA⁺[Au₂₅(SR)₁₈]⁻ and O₂ (eqn (3)). Based on eqn (4),³³ pre-equilibrium constants (K), namely O₂ binding constants with Au₂₅ cluster anions, and second-order rate constants (k₂) were determined and are summarized in Table 2. In the case of TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻, k₂ and K were (9.9 ± 0.3) s⁻¹ and (0.20 ± 0.01) mM⁻¹ at [TFA] = 0.50 mM, respectively. When changing the concentration of TFA to 2.0 mM, the K value was retained (0.18 ± 0.04 mM⁻¹), whereas the k₂ value increased ((44 ± 4) s⁻¹) (Fig. S18†). No influence of the acid concentration on the O₂ binding constant suggests that protons are not involved in the pre-equilibrium between the Au₂₅ cluster and O₂ (eqn (3)). The highest O₂ binding constant (0.55 ± 0.01 mM⁻¹) was observed in the air-oxidation reaction of TOA⁺[Au₂₅(SPh)₁₈]⁻, whereas TOA⁺[Au₂₅(SC₂Ph)₁₈]⁻ showed the lowest O₂-binding properties (0.20 ± 0.01 mM⁻¹). In other words, Au₂₅ cluster anions protected by rigid aromatic thiolate ligands showed higher O₂ binding constants than those with flexible aliphatic thiolate ligands. The affinity of O₂ would be regulated by the electronic effects and/or the steric effects of R moieties of thiolate ligands. In mononuclear metal complexes, more electron-donating ligands increased the O₂-binding constants,³⁴ whereas an opposite tendency was observed in our system, in that more electron-donating ligands such as SC₂Ph gave reduced K values. This difference between MNCs and transition metal complexes would be caused by the binding position of O₂: in mononuclear metal complexes, because both the O₂-binding and the redox reactions generally take place at the same sites, namely the metal centres (Fig. 5a), the affinity of O₂ should be affected by the electronic states of metal centres. Although experimental evidence for O₂ binding sites is insufficient due to the weak electronic interaction between O₂ and AuNCs^13d and further spectral and structural analyses for O₂-bound Au₂₅ clusters are needed, some theoretical reports discussed that O₂ would bind with the gold atom of staple Au₂(SR)₃ moieties of [Au₂₅(SR)₁₈]⁻ in an end-on coordinating fashion rather than the Au₁₃ icosahedral core.^13d,17d It should be noted that the one-electron oxidation of [Au₂₅(SR)₁₈]⁻ occurs at the Au₁₃ core, not at the staple Au₂(SR)₃ units.^19b Therefore, we could think that the O₂-binding and the redox sites are different in the ligand-protected MNCs, suggesting almost no electronic effects on the O₂ affinity (Fig. 5b). To evaluate the steric effects of protecting ligands, ligand flexibility such as free rotation of alkyl chains should also be considered.³⁵ We compared the half cone angles (θ) of [Au₂₅(SC₂Ph)₁₈]⁻ and [Au₂₅(SPh)₁₈]⁻ based on the atomic structures revealed by X-ray analysis. The θ value is defined as the averaged C–Au–S angle, where C is the carbon atom at the ortho-position of the phenyl ring of SC₂Ph or SPh ligands (Fig. S19†). The θ (63.2°) for [Au₂₅(SC₂Ph)₁₈]⁻ is larger than that for [Au₂₅(SPh)₁₈]⁻ (50.3°), indicating the sterically more crowded environment on the surface of [Au₂₅(SC₂Ph)₁₈]⁻. Based on the above discussion, it would be reasonable to explain that the steric effects of thiolate moieties, their bulkiness or flexibility, are dominant factors in determining the O₂-affinity with a Au₂₅ cluster anion. This conclusion suggests that O₂ would be favourably bound at a less steric gold(I) ion at the staple moiety rather than at sterically crowded metal-core gold atoms, which does not contradict with the theoretical analysis.^13d,17d

Fig. 4 [O₂] dependence of k_obs in oxidation reactions of TOA⁺[Au₂₅(SR)₁₈]⁻ in THF ([TFA] = 0.50 mM) at 298 K. SR = SC₂Ph (black), SC₁Ph (green), SPhOMe (red) and SPh (blue).

Fig. 5 Schematic illustrations of the interaction and reduction of O₂ with (a) transition metal complexes and (b) [Au₂₅(SR)₁₈]⁻ through PCET. In (b), only one staple moiety (Au₂SR₃ unit) is shown for clarity. Atom labels: grey: gold in the Au₁₃ core, yellow: gold on the staple, and green: sulfur.

Finally, we considered the controlling factors in determining the reactivities of Au₂₅ clusters. As seen in the comparison of the kinetic parameters (K and k₂) of [Au₂₅(SC₂Ph)₁₈]⁻ with those of [Au₂₅(SPh)₁₈]⁻ (Table 2), the electronic effects of Au₂₅ clusters, evaluated using redox potentials (E_1/2), are a dominant factor in regulating the reaction with O₂. Indeed, a linear relationship was observed between k₂ values and E_1/2 (Au₂₅⁻/Au₂₅⁰), as shown in Fig. S20.† However, interestingly, a different aspect could be seen when the influences of O₂ affinity on the reactivities were extracted from the kinetic parameters of TOA⁺[Au₂₅(SC₁Ph)₁₈]⁻ and TOA⁺[Au₂₅(SPhOMe)₁₈]⁻: a higher K value (0.34 ± 0.01 mM⁻¹) was observed in TOA⁺[Au₂₅(SPhOMe)₁₈]⁻, whereas the k₂ value of TOA⁺[Au₂₅(SPhOMe)₁₈]⁻ (3.67 ± 0.03 s⁻¹) was slightly lower than that of TOA⁺[Au₂₅(SC₁Ph)₁₈]⁻ (4.2 ± 0.1 s⁻¹), resulting in comparable reactivities of these two Au₂₅ clusters. Thus, the steric effects of protecting ligands could also contribute to tuning the reactivities of the Au₂₅ clusters against O₂. For ca. 3 nm gold nanoparticles, the reactivities with O₂ were simply correlated with O₂-binding constants in pre-equilibrium,^12c which were regulated using the accessibility to active sites. In contrast, the oxidation reactivities of [Au₂₅(SR)₁₈]⁻ with O₂ could be controlled over both steric and electronic effects. This dual effect on the reactivity enables us to construct finely tunable reaction systems by protecting ligand design, which should be important for precisely controlled catalytic systems based on inorganic nanomaterials.

Conclusions

We have compared the air-oxidation reactivities of a series of thiolate-protected Au₂₅ nanoclusters bearing different ligands. Kinetic analysis for the oxidation of Au₂₅⁻ to Au₂₅⁰ has quantitatively revealed that the O₂-affinity was regulated by the accessibility of surface gold atoms surrounded by protecting ligands, similar to other inorganic nanomaterials. On the other hand, the total reactivities with O₂ can also be tuned by the electronic states of Au₂₅ clusters such as transition metal complexes. This duality in controlling factors for reactivities allows us to construct precisely controlled reaction systems based on metal nanoclusters, which can be achieved by the rational design of protecting ligands. These results shed new light on ligand-protected MNCs with sizes of less than 2 nm to construct finely tunable reaction systems, in contrast to the case of reaction systems based on larger MNPs. Ligand designs, considering ligand–ligand or ligand–substrate interactions, should be effective for more precise control over the reactivities of MNCs.

Data availability

Additional experimental data supporting this article are included in the ESI.† Reasonable requests for additional information can be made to the corresponding authors.

Author contributions

W. S. designed the research, performed the synthesis and characterisation and wrote the manuscript. R. T. performed EXAFS measurements using the synchrotron. Y. M. and N. T. performed single crystal X-ray diffraction measurement and analysis. S. X. contributed to the discussion and editing the manuscript. T. T. supervised the research. All authors have approved the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

XAFS measurements were performed at the BL01B1 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). This work was financially supported by the MEXT/Japan Society for the Promotion of Science (JSPS), KAKENHI, for Scientific Research (S) (grant no. JP19H05634 and JP24H00053) (T. T.), a JSPS Research Fellowship (grant no. 20J01921), JSPS KAKENHI for Early-Career Scientists (grant no. JP23K13765), International Collaboration Research Project of the Institute for Chemical Research, Kyoto University (grant no. 2024-119), and the Tokuyama Science Foundation (W. S.). We thank Ian McNaught, PhD, from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2299526. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc02995j

‡ Present Address: University of Hyogo, Himeji, 671-2280, Japan.

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