Synthesis of a Au₄₄(SR)₂₈ nanocluster: structure prediction and evolution from Au₂₈(SR)₂₀, Au₃₆(SR)₂₄ to Au₄₄(SR)₂₈

Abstract

We report the synthesis of a Au₄₄(SR)₂₈ nanocluster (SR = 4-tert-butylbenzenethiolate). Based on the structural rules learned from the known Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄ structures, we propose a plausible structure for Au₄₄(SR)₂₈, which is predicted to comprise a six-interpenetrating cuboctahedral Au₃₆ kernel protected by four dimeric staples and sixteen bridging thiolates, i.e. Au₃₆[Au₂(SR)₃]₄(SR)₁₆.

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Atomically precise metal nanoclusters have drawn significant research attention in recent years.^1–10 The structural evolution as a function of size constitutes an important issue in the nanocluster research,^1,11 which is the basis for obtaining deep insight into the size-dependent catalytic,^12,13 optical and electronic properties¹⁴ of metal nanoclusters. A number of thiolate-protected Au_n(SR)_m nanoclusters ranging from ca. a dozen to hundreds of gold atoms have been reported.^15–20 The reported Au_n(SR)_m structures have provided valuable information on the basic rules of structural construction, e.g. a highly symmetric Au kernel protected by staple-like Au-SR oligomers.²¹ But much effort is still needed in order to gain in-depth understanding of the structural evolution rules.^1,15,21–23 Recently, we reported the Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄ nanoclusters protected by 4-tert-butylbenzenethiolate (SPh-t-Bu, TBBT for short) and revealed their face centered cubic (fcc) structures.^24,25 These two nanoclusters share some common structural features (Fig. S1, ESI†): a gold kernel composed of interpenetrating cuboctahedra and a ligand shell composed of plain bridging thiolates and dimeric oligomers.

Herein, we report the synthesis of a third gold nanocluster in the TBBT ligand series, formulated as Au₄₄(SR)₂₈, and its predicted structure is based upon the structural rules learned from the Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄ structures.^24,25 A clear trend can be seen from the three formulae (28, 20), (36, 24) and (44, 28), i.e. the number of Au atoms uniformly increases by 8 and the number of thiolate ligands by 4 in the series. This clear trend, together with the insight into the structures of Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄, led us to propose a plausible structure for Au₄₄(SR)₂₈ and reveal the evolution in this series of medium-sized gold nanoclusters.

The synthesis of Au₄₄(SR)₂₈ nanoclusters was based on a one-phase method using ethyl acetate as the solvent (see the ESI† for details).²⁶ Briefly, HAuCl₄ was first reacted with 4 equivalents of HSPh-t-Bu to form colorless Au(I)SR complexes. Then, NaBH₄ (10 equivalents per Au) was added to reduce Au(I)SR to Au_n(SR)_m nanoclusters. The initially obtained polydispersed nanoclusters were subject to solvent-induced fractionation by diffusion of methanol into a toluene solution of the nanoclusters. After one month, a dark precipitate was formed; the precipitate was then separated from the supernatant, followed by electrospray ionization mass spectroscopy (ESI-MS) to determine the molecular weight of the nanocluster (Fig. 1A). In order to impart charge to the cluster, CsOAc was added to the cluster solution to form cluster–Cs⁺ adducts. A distinct peak at m/z = 13426.6 was detected. After careful comparison of possible (n, m) values for the formula of [Au_n(SR)_mCs]⁺, the peak was assigned to [Au₄₄(SPh-t-Bu)₂₈Cs]⁺ (theoretical FW: 13426.8, deviation: −0.2). Other combinations of (n, m) are ruled out (Table S1, ESI†) due to large discrepancies between the calculated formula weight and the experimental value. The Au₄₄(SR)₂₈ nanocluster is charge neutral since the addition of one Cs⁺ atom to the cluster forms the singly charged adduct. A small amount of Au₃₆(SR)₂₄ impurity was found to coexis with Au₄₄(SR)₂₈ (Fig. 1A, the asterisk-labeled peak). Note that in the MALDI-MS spectrum, no intact molecular peak corresponding to Au₄₄(SR)₂₈ was observed (Fig. S2 and inset, Table S2, ESI†); instead, a peak corresponding to one-ligand-lost Au₄₄(SR)₂₇ was observed together with other fragment peaks, similar to the case of Au₃₆(SR)₂₄.²⁷ The Au₄₄(SR)₂₈ formula is further confirmed by using thiophenol as the ligand. A peak at 11856.8 was observed in the ESI-MS spectrum (Fig. S3, ESI†), corresponding to [Au₄₄(SPh)₂₈Cs]⁺ (theoretical FW: 11855.7).

Fig. 1 (A) ESI-MS and (B) UV-vis spectrum of Au₄₄(SR)₂₈ nanoclusters.

The optical spectrum of Au₄₄(SR)₂₈ exhibits an absorption band at 380 nm and a broad platform from 600 to 800 nm (Fig. 1B). The energy gap derived from the optical spectrum is ∼1.45 eV. It is worth noting that Price et al.^28a previously observed an ∼8.7 kDa species etched from ∼22 kDa clusters and suggested a formula of [Au₄₄(SPh)₂₈]²⁻ but with no precise mass determined. Recently Jiang et al.^28b predicted two structures protected by monomeric and dimeric staples.

The most interesting aspect about this Au₄₄(SR)₂₈ nanocluster is its relationship with the recently reported Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄.^24,25 In this series, the number of gold atoms increases by eight from Au₂₈ to Au₃₆ to Au₄₄, and the number of thiolate ligands increases by four from Au₂₈(SR)₂₀ to Au₃₆(SR)₂₄ to Au₄₄(SR)₂₈. The Au₈(SR)₄ increment in the series inspired us to consider the inherent structural connection among these clusters.

A careful examination of the crystal structures of Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄ reveals some interesting structural rules shared by the two nanoclusters (Table 1). (i) With respect to the kernel structure, both nanoclusters have fcc-type kernels composed of interpenetrating cuboctahedra. Au₂₈(SR)₂₀ has a rod-like Au₂₀ kernel composed of two interpenetrating cuboctahedra with two centres (i.e. twelve-coordinate gold atoms resembling the bulk fcc case), whereas Au₃₆(SR)₂₄ has a Au₂₈ kernel composed of four interpenetrating cuboctahedra with four centres (Au₄) arranged into a tetrahedron. (ii) Both the Au₂₀ rod kernel and the Au₂₈ tetrahedral kernel are enclosed by well-defined crystal faces: the Au₂₀ kernel has four trapezoid-shaped {111} facets on the front and back sides of the rod, while the Au₂₈ kernel also has four triangular {111} facets on the four surfaces of the truncated tetrahedron (Table 1, blue shadowed). Besides, the Au₂₀ rod kernel exposes eight square facets corresponding to {100}, and the Au₂₈ tetrahedral kernel possesses twelve squares (Table 1, yellow shadowed). (iii) Regarding the thiolate protecting modes, both Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄ nanoclusters have dimeric staples (–SR-Au-SR-Au-SR–) protecting the {111} facets in a one-on-one fashion (Fig. S4, ESI†), and bridging thiolates (–SR–) residing on the edges of square {100} facets with one –SR– ligand per square (Fig. S5, ESI†). Hence the formulae of Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄ can be deduced as Au₂₀[Au₂(SR)₃]₄(SR)₈ and Au₂₈[Au₂(SR)₃]₄(SR)₁₂, respectively (Table 1). It is thus obvious that the eight-gold-atom difference between Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄ is added into the kernel (Fig. 2A), and the four-thiolate difference is due to the difference in the number of bridging thiolates. The number of dimeric staples is kept the same in the structural evolution from Au₂₈(SR)₂₀ to Au₃₆(SR)₂₄.

Table 1 Structure evolution of the three clusters. Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄ are based on crystal structures. The Au₄₄(SR)₂₈ structure is proposed

	Au28(SR)20	Au36(SR)24	Au44(SR)28
Central atoms
Kernel
Surface structure of the kernel Number of {111} faces
Surface structure of the kernel Number of {100} squares
Number of dimeric staples	4	4	4
Number of bridging thiolates	8	12	16
Overall formula	Au20[Au2(SR)3]4(SR)8	Au28[Au2(SR)3]4(SR)12	Au36[Au2(SR)3]4(SR)16

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Fig. 2 Kernel evolution. (A) Au₂₀ in Au₂₈(SR)₂₀ to Au₂₈ in Au₃₆(SR)₂₄; (B) Au₂₈ in Au₃₆(SR)₂₄ to Au₃₆ in Au₄₄(SR)₂₈.

Based on the structural rules learned from Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄, we propose a plausible structural model for Au₄₄(SR)₂₈ which possesses a Au₃₆ kernel composed of six interpenetrating cuboctahedra, in which the six centres (Au₆) are arranged into an edge-sharing bi-tetrahedron (Table 1, right). The Au₃₆ kernel can be evolved from the Au₂₈ kernel of Au₃₆(SR)₂₄ by adding eight more gold atoms to the bottom of the Au₂₈ tetrahedron (Fig. 2B). The Au₃₆ kernel also exhibits four triangular {111} facets (Table 1, blue shadowed), as in Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄. Besides, 16 squares can be found on the surface of the proposed Au₃₆ kernel, with 4 on the top and bottom, 6 on the left, and another 6 on the right side of the kernel (Table 1, yellow shadowed). The surface protecting motifs of the Au₃₆ kernel can be deduced from the above rule (iii), that is, one dimeric staple per {111} face and one bridging thiolate per {100} square face; hence, overall four dimeric staples and sixteen bridging thiolates protect the four {111} faces and sixteen {100} faces of the Au₃₆ kernel. The formula of Au₄₄(SR)₂₈ can thus be given as Au₃₆[Au₂(SR)₃]₄(SR)₁₆.

Positioning of the dimeric staples and bridging thiolates in Au₄₄(SR)₂₈ can also be obtained through comparison with Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄ (Fig. S4 and S5, ESI†). Fig. 3A illustrates the positions of the dimeric staples: on the top of the blue diamond unit on each {111} facet is a dimeric staple, with its two sulfur ends bonded to the two farthest vertices of the diamond unit. The positions of the 16 bridging thiolates are shown in Fig. 3B, with the green bond in each square spanned by a bridging thiolate. In this way, all the 30 gold atoms on the surface of the Au₃₆ kernel are bonded to at least one thiolate group, hence the kernel is well protected. The Au₄₄(SR)₂₈ structure is predicted to be chiral as in the case of Au₂₈(SR)₂₀ due to the rotary arrangement of the dimeric staples and bridging thiolates. The Au₄₄S₂₈ framework exhibits a D₂ symmetry. Crystallization trials have not been successful, thus the proposed structure remains to be verified in future work. Of note, the [Ag₄₄(SR)₃₀]⁴⁻ cluster²⁹ has recently been structurally characterized,^30,31 and the structure is quite different from the predicted structure of [Au₄₄(SR)₂₈]⁰ due to different bonding mode of Ag-SR compared to Au-SR.

Fig. 3 Proposed positioning of 4 dimeric staples (A) and 16 bridging thiolates (B) on the surface of the Au₃₆ kernel.

In summary, we have synthesized Au₄₄(SR)₂₈ nanoclusters. Based upon the structural features learned from the crystal structures of Au₂₈(SR)₂₀ and Au₃₆(SR)₂₄, we have proposed a plausible structural model for Au₄₄(SR)₂₈. The three nanoclusters constitute an intriguing series that exhibits some remarkable structural trends in terms of the kernel and surface motifs.

R.J. is grateful for the financial support by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-11-1-9999 (FA9550-11-1-0147) and the Camille Dreyfus Teacher-Scholar Awards Program.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc47089j

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