Monoarsine-protected icosahedral cluster [Au₁₃(AsPh₃)₈Cl₄]⁺: comparative studies on ligand effect and surface reactivity with its stibine analogue

Abstract

Ligand shells of gold nanoclusters play important roles in regulating their molecular and electronic structures. However, the similar but distinct impacts of the homologous analogues of the protecting ligands remain elusive. The C_2v symmetric monoarsine-protected cluster [Au₁₃(AsPh₃)₈Cl₄]⁺ (Au₁₃As₈) was facilely prepared by direct reduction of (Ph₃As)AuCl with NaBH₄. This cluster is isostructural with its previously reported stibine analogue [Au₁₃(SbPh₃)₈Cl₄]⁺ (Au₁₃Sb₈), enabling a comparative study between them. Au₁₃As₈ exhibits a blue-shifted electronic absorption band, and this is probably related to the stronger π-back donation interactions between the Au₁₃ core and AsPh₃ ligands, which destabilize its superatomic 1P and 1D orbitals. In comparison to the thermodynamically less stable Au₁₃Sb₈, Au₁₃As₈ achieves a better trade-off between catalytic stability and activity, as demonstrated by its excellent catalytic performance towards the aldehyde–alkyne–amine (A³) coupling reaction. Moreover, the ligand exchange reactions between Au₁₃As₈ with phosphines, as exemplified by PPh₃ and Ph₂P(CH₂)₂PPh₂, suggest that Au₁₃As₈ may be a good precursor cluster for further cluster preparation through the “cluster-to-cluster” route.

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Introduction

The icosahedral Au₁₃ core is considered a fundamental building block which is commonly observed as the metal core in various structurally well-defined Au nanoclusters.^1–17 It was theoretically found that the electronic structures of the Au₁₃ unit might dictate the physicochemical properties of its assembled clusters.^18–21 It is therefore of importance to study the influencing factors in depth on the geometric and electronic structures of the individual icosahedral Au₁₃ clusters. In this context, the past 15 years have first witnessed a renewed interest in the typical phosphine-protected Au₁₃ clusters involving development of new synthetic routes (e.g., HCl-induced convergence),^6,7,22,23 exploration of formation mechanisms,^24,25 tuning of electronic properties (e.g., by heterometal doping^26–29), and optical properties (e.g., NIR emission^6,7,30 and chiroptical activity^31–33), as well as further expansion of their potential applications (e.g., photodynamic catalysis and nano-electronic devices).^34,35 More recently, comparative studies on several new types of Au₁₃ analogues protected by other pnictines (e.g., functionalized diphosphine,³⁶ phosphinous acid,³⁷ diarsine,³⁸ and monostibine^39–41) or mimics of pnictine (N-heterocyclic carbenes)^42–47 have also been extensively made both experimentally and theoretically.

The large number of studies on icosahedral Au₁₃ clusters showed that the stereoelectronic factors of the pnictine/NHC ligands have a non-negligible influence on their electronic structures.^14,48–54 These ligand-based influencing factors, however, are often intertwined with other factors such as the core geometry and cluster charge state. For example, in 2021, the Konishi group synthesized a pair of dipnictine-protected clusters [Au₁₃(dppe)₅Cl₂]³⁺ (dppe = Ph₂P(CH₂)₂PPh₂) and [Au₁₃(dpap)₅Cl₂]³⁺ (dpap = Ph₂As(CH₂)₃AsPh₂), and demonstrated that the Au₁₃ core geometry (deformation), rather than the coordinating P or As atoms, has a more pronounced effect on their absorption and photoluminescence properties.³⁸ In this sense, it is necessary to employ Au₁₃ cluster analogues, with essentially the same metal cores and ligand arrangements, as the comparable cases for better understanding the pnictine ligand effect. The monopnictine-protected Au₁₃ clusters may serve as good candidates for this purpose since the monopnictines are usually more fluxional in the coordination sphere, and the Au₁₃ core geometry is expected to be less affected by the ligands due to the absence of (possibly) strained Au–L′–Au staple/bridge moieties (L′ = bridging ligands). Unfortunately, few has been done with the monopnictine-protected Au₁₃ clusters, largely due to the lack of comparable analogue clusters with known crystal structures.

To our knowledge, only four monophosphine-protected Au₁₃ clusters have been reported, i.e., [Au₁₃(PMe₂Ph)₁₀Cl₂]³⁺, [Au₁₃(PMePh₂)₈Cl₄]⁺, [Au₁₃(POct₃)₈Cl₄]⁺ (POct₃ = trioctylphosphine) and [Au₁₃(L)₈Cl₄]⁺ (L = P(CH₂CH₂CO₂CH₃)₃), and the last of these could further convert to Au₂₅ nanoclusters by reacting with thiols.^2,3,7,16 In contrast, much less is known about their heavier analogues. In 2018, we reported the monostibine-protected Au₁₃ cluster [Au₁₃(SbPh₃)₈Cl₄]⁺ (Au₁₃Sb₈),³⁹ which was found more amenable to ligand exchange and also more catalytically active than PPh₃-protected Au₁₁ clusters.^40,41 Systematic studies on monoarsine-protected Au₁₃ clusters have lagged behind, despite the structurally similar cluster [Au₁₆(AsPh₃)₈Cl₆] published long ago.⁵⁵ We are thus intrigued by further synthesis of the AsPh₃-protected Au₁₃ clusters, so as to better understand the heavier pnictine ligand effect on their structures and reactivities by comparative studies with their heavier analogue Au₁₃Sb₈.

Herein, we present the synthesis, characterization and reactivity of the monoarsine-protected cluster [Au₁₃(AsPh₃)₈Cl₄]⁺ (Au₁₃As₈) which is isostructural with its heavier analogue Au₁₃Sb₈, in terms of the core geometry and ligand arrangement. Comparative studies between them revealed that the blue-shift electronic absorption band of Au₁₃As₈ is probably related to the stronger π-back donation interactions between the Au₁₃ core and AsPh₃ ligands, which destabilize its superatomic 1P and 1D orbitals. Moreover, Au₁₃As₈ is thermodynamically more stable than its stibine counterparts due to the moderate Au–As coordination bond strength, but its surface arsine ligands are kinetically more labile than phosphines. The unique characteristic of Au₁₃As₈ enables a better trade-off between its catalytic stability and activity, and also makes it an ideal candidate for the precursor cluster in the preparation of other clusters via the “cluster-to-cluster” strategy.

Results and discussion

Synthesis and characterization

Briefly, Au₁₃As₈ was prepared by direct reduction of (Ph₃As)AuCl with 0.5 equivalent of NaBH₄ in dichloromethane (DCM)–ethanol mixed solvent (Scheme 1). The identity of the precursor (Ph₃As)AuCl was first confirmed prior to reduction (Fig. S1†). After reduction, the crude Au₁₃As₈ could be easily precipitated from its DCM solution by addition of hexane and is proved to be fairly pure by comparison of its IR spectrum with that of the purer crystalline Au₁₃As₈, which was obtained from its DCM–hexane solution by slow evaporation. In addition, the primary byproduct in this reaction, mainly found in the supernatant separated in the step of using hexane to precipitate the crude Au₁₃As₈, was identified as (Ph₃As)₃AuCl by X-ray diffraction analysis (Fig. S2†). This finding provides strong evidence that even minor alterations to the reaction conditions, such as the type of solvent or quantity of reducing agent used, can have a considerable impact on the structure of the resulting product.⁵⁵

Scheme 1 Synthetic route to Au₁₃As₈.

Crystallographic data showed that Au₁₃As₈ crystallized in a monoclinic lattice with the P2₁/n space group (Table S1†). Two crystallographically independent molecules of Au₁₃As₈, with almost the same bond parameters, are present in the unit cell (Fig. S3†). Each comprises an icosahedral Au₁₃ core, eight AsPh₃ ligands and four Cl⁻ ligands (Fig. 1). As is the case with its heavier analogue Au₁₃Sb₈,³⁹ the ligands in Au₁₃As₈ also arranged in an approximate C_2v symmetry. The radial Au–Au bond lengths range from 2.726(2) Å to 2.810(2) Å (2.757(2) Å on average), while the peripheral ones are in the range of 2.808(2)–2.985(2) Å (2.900(2) Å on average). The Au–Cl bond lengths fall in the range of 2.31(1)–2.37(1) Å. The Au–As bond lengths range from 2.392(4) Å to 2.414(6) Å, which is longer than that in its precursor complex (Ph₃As)AuCl (2.334(1) Å). As expected, these Au–Au and Au–Cl bond lengths in Au₁₃As₈ are also quite close to the corresponding ones in Au₁₃Sb₈ (Table S2†).

Fig. 1 Crystal structure of Au₁₃As₈ with all hydrogen atoms omitted (left: side view; right: top view). Au: orange, As: magenta, Cl: green, C: grey.

The mass spectrum of Au₁₃As₈ shows a major peak at m/z = 5151.7672, attributed to the molecular ion [Au₁₃(AsPh₃)₈Cl₄]⁺ (calcd = 5151.7738), which matches well with its calculated one; the isotopic interval of 1.0 amu support its +1 charge (Fig. 2). Besides, the ³¹P NMR of the product shows a heptet resonance ascribable to PF₆⁻ (Fig. S4†), and its IR spectrum also shows the characteristic absorptions of PF₆⁻ at 839 and 557 cm⁻¹ (Fig. S5†). All these indicate that Au₁₃As₈ bears +1 charge, and the counterion is most probably PF₆⁻, though it fails to be appropriately located crystallographically, probably due to its severe disorder.

Fig. 2 ESI-MS(+) spectrum of Au₁₃As₈ in CH₂Cl₂. Inset: the experimental (black trace) and calculated (magenta trace) isotopic patterns of the Au₁₃As₈ ion (A) [Au₁₃(AsPh₃)₈Cl₄]⁺ and its fragment ion (B) [Au₁₃(AsPh₃)₇Cl₄]⁺.

The ¹H NMR spectrum of Au₁₃As₈ shows three doublet resonances in the lower magnetic field (ca. 7.10–7.50 ppm) region in an integration ratio of 1 : 2 : 1, and with a pattern quite akin to that of Au₁₃Sb₈ (Fig. S6†). This suggests the presence of three types of chemically non-equivalent AsPh₃ ligands, which is consistent with its solid-state structure; this also further confirms that the icosahedral Au₁₃ clusters are stereochemically more rigid than the phosphine-protected Au₁₁ clusters.^56,57

Theoretical calculations

Since the Au₁₃As₈ and Au₁₃Sb₈ are isostructural either in the solid state or in solution, they constitute a good comparable pair for comparatively studying the heavier pnictine ligand effect on their structures and reactivities. As shown in Fig. 3, the experimental electronic absorptions for Au₁₃As₈ appear at 346 and 439 nm, which are blue-shifted by 6 and 15 nm, respectively, as compared to those for Au₁₃Sb₈ at 352 and 454 nm. Interestingly, the absorptions for the previously reported monophosphine-protected Au₁₃ clusters are further blue-shifted (Fig. S7†).^2,3,7 Given the more similar structures of Au₁₃As₈ and Au₁₃Sb₈, TD-DFT calculations for them were performed at the same level of theory, to reveal the stereoelectronic influences of the pnictine ligands on the absorptions; the metal cores in both after structure relaxation are more of C_2v symmetry, probably transferred from the ligand shells and reflected by the continuous symmetry measure (CSM) value of 0.00 for both.⁵⁸ The optimized Au–Au bond lengths in Au₁₃As₈ are also slightly longer than those in Au₁₃Sb₈ (Table S3†), which is consistent with the crystallographic data. The calculated UV-vis spectra of them are well matched with their respective experimental ones, despite a small blue-shift, probably due to the overestimation of the excitation energies for the functional employed (Fig. S8 and S9†). Thus, the calculated peaks at 335 and 417 nm for Au₁₃As₈ (340 and 435 nm for Au₁₃Sb₈) should correspond to its experimental ones at 346 and 439 nm (352 and 454 nm for Au₁₃Sb₈), respectively.

Fig. 3 Experimental (solid lines) and TD-DFT calculated (dash lines) UV-vis spectra for Au₁₃As₈ (magenta traces) and Au₁₃Sb₈ (cyan traces) in CH₂Cl₂.

Au₁₃As₈ and Au₁₃Sb₈ possess a similar electronic structure, with a HOMO–LUMO gap of 3.32 and 3.09 eV, respectively. The HOMO, HOMO − 1 and HOMO − 2 orbitals are typical of 1P superatomic orbitals while LUMO through LUMO + 4 show 1D characteristics (Tables S4 and S5†). The long-wavelength absorptions (417 nm for Au₁₃As₈ and 435 nm for Au₁₃Sb₈) are mainly ascribable to metal-to-metal charge transfer (MMCT) transitions from 1P to 1D shell, while the shorter-wavelength absorptions (335 nm for Au₁₃As₈ and 340 nm for Au₁₃Sb₈) are mainly attributed to ligand-to-metal charge transfer (LMCT) transitions to 1P orbitals from higher-lying occupied orbitals (HOMO − 3 to ca. HOMO − 10). In addition, the absorptions below 300 nm for both result from the even higher-energy transitions, such as HOMO − n (n > 15) to 1D, or HOMO − n (n < 5) to LUMO + n (n > 10); the phenyl rings in the ligands contributed significantly to HOMO − n (n > 15) orbitals with energy lower than −8.2 eV, and LUMO + n (n > 10) orbitals with energy higher than −2.2 eV (Fig. 4a, b and Tables S6, S7†).

Fig. 4 Total density-of-states (TDOS) and partial density-of-states (PDOS) spectra of (a) Au₁₃As₈, (b) Au₁₃Sb₈, and (c) their selected Kohn–Sham molecular orbitals showing the populations of atomic orbitals.

As shown in Fig. 4c, on average, the 1P orbitals of Au₁₃As₈ comprise Au 5d/6sp orbitals (ca. 64%), As 4sp orbitals (ca. 13%), Cl 3sp orbitals (ca. 9%) and C 2sp orbitals (ca. 14%). The 1D orbitals are also mainly composed of Au atomic orbitals (Au : As : Cl : C = 72 : 13 : 3 : 12). The proportions of Au and As in orbitals from HOMO − 3 to HOMO − 10 decreased to a certain extent (48% for Au and 7% for As), but those of Cl and phenyl rings increased (21% for Cl and 24% for C). This means that, for a given type of cluster [Au₁₃(ER₃)₈Cl₄]⁺ (E = pnictogen), the R substituent of ER₃ and the inorganic Cl⁻ may less affect the longer-wavelength absorptions resulting from 1P to 1D transitions, due to their less contributions to 1P/D orbitals; in other words, the shorter-wavelength absorptions are more susceptible to the stereoelectronic property of the ER₃ ligand. Consistent with this are the experimental UV-vis spectra of [Au₁₃(L)₈Cl₄]⁺ (L = PPh₂Me; P(CH₂CH₂CO₂CH₃)₃),^3,16 which show almost the same absorption peaks at 432 nm in the lower-energy region but obviously different peaks at 342 nm and 333 nm, respectively, in the higher-energy region (Fig. S7†).

The orbital composition pattern of Au₁₃Sb₈ is akin to that of Au₁₃As₈. The 1P orbitals are composed of Au 5d/6sp (ca. 63%), Sb 5sp (ca. 13%), Cl 3sp (ca. 13%) and C 2sp (ca. 12%) orbitals. The 1D orbitals also mainly comprise Au atomic orbitals (Au : Sb : Cl : C = 67 : 17 : 4 : 11). The proportions of Au and As from HOMO − 3 to HOMO − 10 also decreased (44% for Au and 6% for Sb), but those of Cl and phenyl rings increased (29% for Cl and 20% for C). In comparison, the superatomic 1P, 1D and the higher-lying occupied orbitals (e.g. HOMO − 3 to HOMO − 10) in Au₁₃As₈ all up-shifted relative to their counterparts in Au₁₃Sb₈, except for the HOMO in Au₁₃As₈, which is marginally lower in energy by 0.025 eV than that in Au₁₃Sb₈.

Previous studies with thiol/alkynyl-protected Au₂₅ clusters based on the jellium model assumed that the more electronegative (usually X-type) ligands bonded to the Au₁₃ core might lead to the reduction of potential volume for the confinement of Au₁₃ core valence electrons, and result in the destabilization of the superatomic 1P orbitals.¹⁵ In this work, the L-type two-electron-donor pnictine ligands are more suitable to be classified by overall electron-donating ability. AsPh₃ is a stronger σ-donating and also a better π-accepting ligand than SbPh₃;⁵⁹ it is thus envisioned that both interactions may influence the superatomic cluster orbitals. The π-back donation from the Au core to the π-acid ligand is not uncommon, for instance, Au(5d) → CO(π*) in the carbonyl-protected Au cluster,⁶⁰ and Au(5d) → N=C(π*) in the guanine-protected Au₁₃ cluster.⁶¹

To get more insight into this, the natural bond orbital (NBO) analyses were performed. The qualitative natural population analyses (NPA) show that the Au₁₃ core in Au₁₃As₈ is less negatively charged than that in Au₁₃Sb₈ (−0.50 vs. −1.43, Table S9†), as is also the case with the Mulliken charge analysis (−1.95 vs. −2.27, Table S10†). This implies that, given the stronger σ-donating ability of AsPh₃, a stronger electron back donation from the Au₁₃ core to AsPh₃ may contribute to the relatively lower electron density for the Au₁₃ core in the case of Au₁₃As₈. As expected, the second-order perturbation theory analyses in NBO basis for Au₁₃As₈ show strong interactions between the central Au lone pair orbitals and the radial Au–As σ* orbitals, as reflected by the large second-order perturbation stabilization energies E(2) (Table S11†). In comparison, this type of interaction in Au₁₃Sb₈ is much weaker (Table S12†). This is probably associated with the fact that the As 4sp orbitals involved in the Au–As σ* orbitals are less diffuse, lower in energy, and thus more electrophilic, compared to the Sb 5sp orbitals in Au–Sb σ* orbitals. Since the stronger π-back donation interactions between Au and Au–As σ* orbitals would reduce the Au–As bond orders more significantly, the Wiberg bond indices for the Au–As bonds in Au₁₃As₈ are indeed smaller (ca. 0.46) than those for the Au–Sb bonds in Au₁₃Sb₈ (ca. 0.54), whereas those for the innocent Au–Cl bonds in both are almost the same (ca. 0.42) (Table S13†). In addition, other types of π-back donation interactions between surface Au lone pairs and As/Sb–C σ* orbitals are rather weak and comparable for both Au₁₃As₈ and Au₁₃Sb₈ (Tables S11 and S12†). Based on these, the upshift of 1P orbitals in Au₁₃As₈ relative to that in Au₁₃Sb₈ is tentatively accounted for by the stronger π-back donation interactions between central Au and radial Au–As σ* orbitals, by which the better π-accepting L-type ligand AsPh₃ exerts a similar impact on the Au₁₃ core valence electrons to that for the case of the more electronegative X-type ligand.

On the other hand, the correlations of 1D orbitals with the electronegativity of ligands are rather complicated, though they shift to the same direction as 1P orbitals in most cases but lack an obvious trend in the shift amount.¹⁵ In this work, the 1D orbitals in Au₁₃As₈ up-shifted more (by 0.156 eV on average) than 1P orbitals (by 0.031 eV on average). That is to say, the 1D orbitals are more susceptible to the electronic factors of the pnictine ligands. In view of this, more comparative studies, including with the lighter phosphine-protected analogue clusters, may be of interest to better explain the behavior of 1D orbitals with respect to the integrated σ-donating/π-accepting parameters of the L-type pnictine ligands.

Optical properties

The solution photoluminescence (PL) of Au₁₃As₈ emits at ca. 802 nm, which is red-shifted in wavelength and also higher in intensity, as compared to the main emission of Au₁₃Sb₈ at ca. 740 nm (Fig. 5). This is presumably due to the heavy metal effect of Sb, which facilitates the reverse intersystem crossing (RISC) process from the T₁ state to the S₁ state and therefore leads to the blue-shift and lower intensity of the emission for Au₁₃Sb₈.^62,63 The previously reported two phosphine-protected clusters [Au₁₃(L^P)_nCl₄]⁺ (L^P = trioctylphosphine (POct₃), n = 8; L^P = Ph₂P(CH₂)₃PPh₂, n = 4) both emit at ca. 775 nm.⁷ Considering the likely dual emissions of Au₁₃Sb₈, if the shoulder at ca. 825 nm is of the same phosphorescence character as the emissions of Au₁₃As₈ and [Au₁₃(L^P)_nCl₄]⁺, the emission wavelengths would be in the order of Au₁₃Sb₈ > Au₁₃As₈ > [Au₁₃(L^P)_nCl₄]⁺.

Fig. 5 Photoluminescence spectra for Au₁₃As₈ (magenta line, λ_ex = 460 nm) and Au₁₃Sb₈ (cyan line, λ_ex = 470 nm) in CH₂Cl₂ (ca. 2.0 mg mL⁻¹) at room temperature.

Au₁₃As₈ in its crystalline state exhibits a PL emission over a wider range of wavelengths (from ca. 650 nm to ca. 850 nm), and the emission maximum at ca. 740 nm is blue-shifted by ca. 60 nm compared to that in solution. The PL emission of Au₁₃Sb₈ in the crystalline state displays a similar but also blue-shifted weak dual-emission-like pattern to that in solution, with an emission peak centered at ca. 805 nm and a shoulder at ca. 710 nm (Fig. S10†). In general, the PL blue-shift for both can be partially attributed to the rigidity enhancement of the structures in the solid state. In addition, the largely broadened and blue-shifted solid PL spectrum of Au₁₃As₈, somewhat resembling that of Au₁₃Sb₈, also suggest that a similar dual-emission phenomenon to that of Au₁₃Sb₈ may be present in its solid/crystalline state.

Thermal stability

The Au₁₃As₈ is rather stable in the solid state, but gradually decomposes with time in solution at room temperature, although such a process is much slower than for the case of Au₁₃Sb₈. Lower temperature could effectively inhibit the decomposition of Au₁₃As₈; its UV-vis monitoring spectra showed no essential decomposition even after 12 days when its DCM solution was stored at 4 °C (Fig. S11†). Thermogravimetric analysis (TGA) of Au₁₃As₈ at a heating rate of 10 °C min⁻¹ under N₂ purge (60 mL min⁻¹) showed that the rapid weight loss started at ca. 200 °C, whereas that for Au₁₃Sb₈ was observed at 155 °C (Fig. S12†).³⁹ These results suggest that Au₁₃As₈ is more stable than Au₁₃Sb₈ either in solution or in the solid state but most probably still less stable than the phosphine-protected analogue clusters; this is believed to result from the moderate Au–As bond strength.

Surface reactivity-catalysis

Since the electron-rich Au₁₃ core was speculated to activate the C=O bond,^36,64,65 the pnictine-protected Au₁₃ clusters may find uses in the aldehyde/ketone-involved catalytic reactions. Au₁₃Sb₈ was found to be able to efficiently catalyze the aldehyde–alkyne–amine (A³) coupling reaction, but the drawback of lower catalytic stability could also not be neglected.⁴⁰ To better realize the trade-off between stability and activity, the catalytic performance of Au₁₃As₈ towards the same A³ coupling reaction was tested (Table 1). It was found that, despite its relatively stronger Au–As coordination bonds, Au₁₃As₈ could also achieve a comparable conversion ratio to Au₁₃Sb₈ after 12 h, probably associated with different reaction kinetics. More importantly, Au₁₃As₈ is catalytically more stable than Au₁₃Sb₈; it can be simply recovered (and also purified) from the reaction mixture by precipitation with hexane (Fig. S13†), and can be reused for five cycles with only slight decrease in catalytic activity (Fig. S14 and S15†). This finding further underlines the important role of ligands in tuning the catalytic performance and may provide some implications for the design of more practically useful ligand-protected metal nanocluster catalysts.

Table 1 A³ coupling reactions catalyzed by Au₁₃As₈ or Au₁₃Sb₈^a

Catalyst^b	NMR yield (%)
	Cycle 1	Cycle 2	Cycle 3	Cycle 4	Cycle 5
a Benzaldehyde (1.0 mmol), piperidine (1.2 mmol), phenylacetylene (1.3 mmol), solvent-free reaction. b With respect to the amount of benzaldehyde. c Given the obvious decomposition in each cycle for Au13Sb8, no further trial was made after the second cycle due to the marked decrease in yield.
Au13As8	73	74	69	64	62
Au13Sb8	81	32	—	—	—

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Surface reactivity-ligand exchange

The exquisite “cluster-to-cluster” synthetic strategy based on ligand exchange has recently become increasingly prevailing.^47,66,67 Taking Au₁₃ clusters as examples, reactivity studies on Au₁₃Sb₈ or its bromo-analogue [Au₁₃(SbPh₃)₈Br₄]⁺ had shown that they could readily react with glutathione (GSH) or 1-adamantanethiol (S-Adm) to give the achiral Au₂₅(SG)₁₈ or chiral Au₁₈(SAdm)₈(SbPh₃)₄Br₂, respectively.^39,41 The Wang group also developed a new method of synthesizing thiol-protected Au₂₅ clusters through reactions of [Au₁₃(L′′)₈Cl₄]⁺ (L′′ = P(CH₂CH₂CO₂CH₃)₃) with appropriate thiols.¹⁶ In analogy to these stibine or phosphine-protected Au₁₃ clusters, Au₁₃As₈ may also possess the potential as a precursor cluster in the syntheses of other new clusters by reacting with stronger coordinating ligands. Thus, the reactions of Au₁₃As₈ with monophosphine PPh₃ or diphosphine Ph₂P(CH₂)₂PPh₂ (dppe) at room temperature were carried out as a proof-of-concept in this work. As shown in Scheme 2, two well-known clusters [Au₁₁(PPh₃)₈Cl₂]⁺ and [Au₁₃(dppe)₅Cl₂]³⁺ were obtained in the cases of PPh₃ and dppe, respectively, and their structures were determined by comparison of their UV-vis, ¹H NMR and MS spectra with those from literature (Fig. S16–S20†).^7,68 These preliminary trials confirm the potential use of Au₁₃As₈ in the “cluster-to-cluster” syntheses. In comparison, the reactions with Au₁₃As₈ proceed more slowly than with Au₁₃Sb₈, indicating that it is more likely for the former to realize more controlled derivatization or functionalization by ligand engineering.

Scheme 2 Reactions of Au₁₃As₈ with phosphines.

Conclusions

In this work, a monoarsine-protected icosahedral cluster [Au₁₃(AsPh₃)₈Cl₄]⁺ (Au₁₃As₈) was prepared. It is isostructural with its previously reported stibine analogue [Au₁₃(SbPh₃)₈Cl₄]⁺ (Au₁₃Sb₈), which provides an opportunity to better comparatively study the heavier pnictine ligand effect on their electronic structures, optical properties, and surface reactivities. The influencing mechanism of the L-type pnictine ligands on the Au₁₃ superatomic orbitals may be different from those of the X-type ligands (e.g., organic thiol, selenol, alkynyl). The superatomic 1P and 1D orbitals of Au₁₃As₈ are up-shifted relative to those of Au₁₃As₈, which is tentatively ascribed to the stronger π-back donation interactions between the Au₁₃ core and AsPh₃ ligands. In addition, compared to the thermodynamically less stable Au₁₃Sb₈, a better trade-off between stability and activity was realized with Au₁₃As₈, as reflected by its good catalytic performance towards the A³ coupling reaction. Finally, the ligand exchange reactions between Au₁₃As₈ with phosphines, as exemplified by PPh₃ and Ph₂P(CH₂)₂PPh₂, suggest that Au₁₃As₈ may be a good precursor cluster and may find more use in the preparation of other clusters via the prevailing “cluster-to-cluster” strategy. Taken together, the heavier organometalloidal arsines or organometallic stibines, especially the latter, are not simply “weaker organic phosphines”, and they are more likely to exert distinct ligand effects on the structures and properties of metal nanoclusters. Since there has been only a limited number of heavier pnictine-protected metal nanoclusters, more diverse syntheses of such type of clusters, as well as explorations of their physicochemical properties and applications, are of much interest and deserve deeper scrutiny.

Data availability

All experimental and computational data associated with this article have been included in the main text and ESI.†

Author contributions

Y.-Z. L. and D. S. conceived the manuscript; J. H. Y. performed the experiments; J.-H. Y., Z.-R. Y., J. X., J.-G. W., M. A., T.-D. L. and Y.-Z. L. analyzed data, prepared figures and provided conceptual contributions; J. H. Y., Y. Z. L. and D. S. wrote the manuscript with contributions from all co-authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from the Natural Science Foundation of China (no. 22001139, 22178184, and 52072190), Natural Science Foundation of Shandong Province (no. ZR2022MB099), the Taishan Scholar Project of Shandong Province of China (no. tsqn201812003 and ts20190908), the Fok Ying Tong Education Foundation (171009), the National Postdoctoral Innovative Talents Support Program (No. BX2021171), the China Postdoctoral Science Foundation (No. 2021M700081), and the Science, Education and Industry Integration Pilot Project Program from Qilu University of Technology (Shandong Academy of Science) (no. 2022PY066 and 2022JBZ02-04). Dr M. Azam also acknowledges the financial support through the Researchers Supporting Project (RSP2023R147) at King Saud University, Riyadh, Saudi Arabia.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, computational and crystallographic details, and spectroscopic characterization including IR, ESI-MS, UV-vis, and NMR. CCDC 2244318 and 2244690. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc01311a

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