Communication pubs.acs.org/IC
Crystal Structures of Two New Gold−Copper Bimetallic Nanoclusters: CuxAu25−x(PPh3)10(PhC2H4S)5Cl22+ and Cu3Au34(PPh3)13(tBuPhCH2S)6S23+ Sha Yang, Jinsong Chai, Tao Chen, Bo Rao, Yiting Pan, Haizhu Yu, and Manzhou Zhu* Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui University, Hefei, Anhui 230601, People’s Republic of China S Supporting Information *
rodlike Ag−Au alloy clusters and cyclic 37-atom triicosahedral Ag−Au structures were also reported.18,13 Unlike the vast studies on nobel-metal NCs, the copper NCs (either the monometallic or alloy NCs) have rarely been reported.19−21 To the best of our knowledge, only a few copper NCs have been reported. For example, Zheng and co-workers recently identified the precise structure of the bimetallic Au13Cux (x = 2, 4, 8) NCs.22 They found that the Cu atoms are all facecapped in the clusters and are triply coordinated by thiolate or pyridyl groups. Quite recently, their group reported another bimetallic Au−Cu alloy, i.e., [Au12+nCu32(SR)30+n]4− (n = 0, 2, 4, 6).23 In these alloy NCs, most Cu atoms occupy the peripheral positions (similar to the aforementioned Au13Cux) and the Cu atoms are either triply or doubly coordinated by the ligands. Inspired by these observations, we suggest that novel gold− copper bimetallic NCs, with Cu occupying both the peripheral and internal sites, could be synthesized. In addition, the Cu atoms could show different coordination modes, i.e., triple, double, or even monocoordinated modes (with the ligands). Herein, we report the synthesis and crystal structures of two new gold−copper bimetallic NCs, Cu x Au 25−x (PPh 3 ) 10 (PhC2H4S)5Cl22+ (NC-1 for short) and Cu3Au34(PPh3)13(tBuPhCH2S)6S23+ (NC-2 for short). The precise atomic structures of both NCs have been determined by single-crystal X-ray crystallography. The metal frameworks in both NCs correspond to the vertex-sharing polyicosahedra, and thus both structures could be referred to as “clusters of clusters” (Figure 1). The NC-1 core was assembled by two vertex-sharing icosahedral M13, and both the top and waist sites were partially occupied by Cu atoms. The Cu atoms are monocoordinated with chlorine or thiol ligands. In contrast, the NC-2 core can be describe as an assembly of three M13 icosahedra clusters sharing three vertexes in a cyclic fashion. All three Cu atoms in NC-2 occupy the internal positions of the cluster core. Specifically, all of these Cu atoms are monocoordinated by the bare S atom. The synthetic details are provided in the Supporting Information (SI). The overall synthetic processes of NC-1 and NC-2 are similar. In a typical reaction, the synthesis of the gold− copper bimetallic NCs, NC-1 and NC-2, includes two steps. First, the triphenylphosphine-stabilized gold nanoparticles were synthesized (described in the SI). Then, gold particles were reacted with a CuISR complex to generate the target NCs (SR =
ABSTRACT: Herein, we report the synthesis and atomic structures of the cluster-assembled CuxAu25−x(PPh3)10(PhCH2CH2S)5Cl22+ and Cu3Au34(PPh3)13(tBuPhCH2S)6S23+ nanoclusters (NCs). The atomic structures of both NCs were precisely determined by single-crystal X-ray crystallography. The CuxAu25−x(PPh3)10(PhC2H4S)5Cl22+ NC was assembled by two icosahedral M13 via a vertexsharing mode. The Cu atom partially occupies the top and waist sites and is monocoordinated with chlorine or thiol ligands. Meanwhile, the Cu3Au34(PPh3)13(tBuPhCH2S)6S23+ NC can be described as three 13-atom icosahedra sharing three vertexes in a cyclic fashion. The three Cu atoms all occupy the internal positions of the cluster core. What is more important is that all three Cu atoms in Cu3Au34 are monocoordinated by the bare S atoms. The absorption spectra of the as-synthesized bimetallic NCs reveal that the additional metal doping and different cluster assemblies affect the electronic structure of the monometallic NCs.
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s a new class of nanomaterial, the atomically precise, noblemetal nanoclusters (NCs) have been extensively studied in recent years and have shown great potential in biolabeling and sensing, medical therapy, and catalysis.1−6 With single-crystal Xray crystallography and theoretical studies,7−12 the structures of many gold phosphine NCs have been well characterized. Specifically, the clusters of clusters (i.e., the assembly of the metal cluster building blocks) have recently been widely explored. Among the various structural building blocks, the icosahedral motif (such as the 13-atom icosahedral structure) has been frequently observed in organic ligand-protected (such as phosphine, thiolate, or mixed ligands) NCs. In particular, Teo et al. recently identified a common growth mode termed the cluster of clusters via vertex sharing.13 The icosahedral 13-atom building blocks pack into larger clusters such as M25 (linear vertex sharing of double M13 clusters), M36 (cyclic vertex sharing by three M13 clusters), and M46 (tetrahedral vertex sharing by four M13 clusters). Recently, both the synthesis and structural characterization of the noble-metal cluster of clusters (and especially the gold NCs) have been significantly developed. For example, the synthesis and crystal structures of biicosahedral [Au25(PPh3)10(SR)5Cl2]2+ NCs were first reported.14−16 The triicosahedral [Au37(PPh3)10(SC2H4Ph)10X2]+ NCs were also reported soon afterward.17 In addition to these gold NCs, the © XXXX American Chemical Society
Received: August 29, 2016
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DOI: 10.1021/acs.inorgchem.6b02016 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 1. Crystal structures of (a) CuxAu25−x(PPh3)10(PhC2H4S)5Cl22+ and (b) Cu3Au34(PPh3)13(tBuPhCH2S)6S23+ NCs. Color labels: dark green, Au; red, S; violet, P; light green, Cl; blue, Cu (partial occupancy); stick, C. The H atoms are not shown.
Figure 2. Core structure of the CuxAu25−x(PPh3)10(PhC2H4S)5Cl22+ NC. Color labels: dark green/magenta, Au; red, S; violet, P; light green, Cl; blue, Cu/Au (partial occupancy).
2-phenylethanethiol for NC-1 and 4-tert-butylbenzylmercaptan for NC-2). X-ray photoelectron spectroscopy measurements and thermal gravimetric analysis (TGA) are employed to estimate the composition of the copper thiolate complex (Figures S5 and S6 and Table S4). Detailed descriptions are given in the SI. The Cu3Au34 NC was first characterized by electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) in positive mode (Figure S3). The spectrum clearly shows the trivalent action of Cu3Au34 fragmentation (i.e., loss of one phosphine) occurring in the ESI process. Similarly, the loss of phosphine in ESI-TOF-MS was also reported for [Au20(PPhpy2)10Cl4]2+ and [Au24(PPh3)10(SC2H4Ph)5X2]+ in the recent studies of Wang et al. and Jin et al.24,25 The prominent peak fragmentation of cluster ions is at m/z 3724.7437 ([Cu3Au34(PPh3)12(StBuCH2Ph)6S2]3+), and this isotopic pattern is in excellent agreement with the simulation (Figure S3, inset). TGA was used to confirm the composition. As shown in Figure S4A, TGA reveals a weight loss of 43.51%, which is very close to the theoretical value (43.10%) for CuxAu25−x. Meanwhile, the weight loss of Cu3Au34 is 39.90% (Figure S4B). These data are in good agreement with the calculated loss (39.78%) according to the formula. In addition, the accurate atomic ratios of both Au−Cu alloy NCs were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The ICP-AES test reveals that the Au/Cu atomic ratio matches well with the one obtained by single-crystal X-ray diffraction (see Table S3 for details). The black bar crystals of NC-1 were grown by the vapor diffusion of diethyl ether into the CH2Cl2 solution of clusters, and the single crystals of NC-2 were obtained via crystallization in CH2Cl2/hexane over 2−3 days. The structures of NC-1 and NC-2 are all determined by X-ray crystallography (Figure 1). As revealed by single-crystal analysis, the crystal structures of both NC-1 and NC-2 share a monoclinic space group P121/n1. In each unit cell of NC-1, one CuxAu25−x and two SbF6 counteranions are contained. The rodlike core framework of NC1 is shown in Figure 2. This cluster has a CuxAu25−x metal core, which is protected by 10 PPh3, 5 −SC2H4Ph ligands, and 2 Cl atoms. Aside from that, CuxAu25−x metal core can be viewed as two icosahedral M13 units sharing one vertex (Figure 2, left). There are four pentagonal metal layers (1−4 in Figure 2, left) arranged in a staggered−eclipsed−staggered configuration. In the first layers, 1 and 1′, these two apex metal atoms are coordinated by Cl atoms. Both apex sites (blue metal in Figure 2) show partial occupancy of Au and Cu. The M−S bond lengths
are 2.199 and 2.243 Å, respectively. A total of 10 PPh3 ligands coordinate with the top of the peripheral Au atoms in layers 2 and 2′, and the Au−P bond lengths range from 2.287 to 2.328 Å. The five thiol ligands bridging the 3 and 3′ layers draw the two icosahedral M13 units together, resulting in the overlapped layer 4. The average Au−S (Cu−S) bond length is 2.327 Å. The M−M bond lengths in the core range from 2.652 to 3.042 Å, which are similar to the Au−Au bond lengths of [Au 25(PPh3 )10(SCnH2n+1)5Cl2]2+ (range from 2.70 to 3.00 Å). For NC-2, one Cu3Au34 and three SbF6 counteranions existed in each unit cell, so the formula of NC-2 is [Cu3Au34(PPh3)13(tBuPhCH2S)6S2]3+. As shown in Figure 3,
Figure 3. Front (A) and side (B) views of the Cu3Au34(PPh3)13(SCH2Ph-tBu)5S23+ NC. Color labels: green/yellow, Au; red, S; violet, P; blue, Cu/Au (half-occupancy).
NC-2 is constituted by a Cu3Au34 core, 13 PPh3 ligands, 6 SCH2Ph-tBu ligands, and 2 S atoms. Meanwhile, Cu3Au34 could be alternatively viewed as the assembly of three icosahedral M13 building blocks. As shown in Figure 3, each group of adjacent M13 units shares a vertex Au atom, and the three shared Au atoms are arranged in a cyclic fashion (i.e., three vertex Au atoms in total). Therefore, 3 × 13 − 3 = 36 metal atoms are located in the metal core (green and blue atoms in Figure 3). In particular, the last Au atom (the yellow atom in Figure 3) is a formal AuI because it is coordinated with one bare S atom and one PPh3 ligand. The similar extra Cl−AgI structure was also previously reported by Teo and co-workers.13,26,27 The other PPh3 ligands coordinate with the top Au atoms, and the average Au−P bond length is 2.301 Å. Herein, the two kinds of S (bare S atoms and S ligands) in NC-2 are noteworthy. As shown in Figure 4a, the bare S atoms coordinate with the M atoms in the center of NC-2 and results in the M3−S group. These metal sites (blue atoms in Figure 4) are B
DOI: 10.1021/acs.inorgchem.6b02016 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
assigned to Ph-tBuCH2+Cl− (calculated formula weight with one Cl− missing, m/z 147.12; Figures S7 and S8). Accordingly, we suggest that dissociation of the C−S bond of Ph-tBuCH2SH occurs to account for the release of the free S atom. The optical absorption spectra of NC-1 and NC-2 are shown in Figure S9. For comparison, the spectrum of the rodlike Au25(PPh)10(SC2H4Ph)5Cl2 (Au25 for short) was also provided. As shown in Figure S9, these spectra are all similar. Nevertheless, in Figure S9, the spectra of CuxAu25−x are different from those of Au25 on two points. The first difference is the weaker absorption of the two main peaks (centered at 415 and 685 nm). The second one is the appearance of a new shoulder peak at 740 nm (Figure S9, red line). Meanwhile, the spectral profile of NC-2 (Cu3Au34) in the wavelength range of
