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[Cu61(StBu)26S6Cl6H14]+: A Core–Shell Superatom Nanocluster with a Quasi-J36 Cu19 Core and an “18-Crown-6” Metal-Sulfide-like Stabilizing Belt Atanu Ghosh, Ren-Wu Huang, Badriah Alamer, Edy Abou-Hamad, Mohamed Nejib Hedhili, Omar F. Mohammed, and Osman M. Bakr ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00122 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 21, 2019
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ACS Materials Letters
[Cu61(StBu)26S6Cl6H14]+: A Core–Shell Superatom Nanocluster with a Quasi-J36 Cu19 Core and an “18-Crown-6” Metal-Sulfide-like Stabilizing Belt Atanu Ghosh, †# Ren-Wu Huang, †# Badriah Alamer, † Edy Abou-Hamad,II Mohamed Nejib Hedhili, II Omar F. Mohammed,‡ and Osman M. Bakr*,† †KAUST Catalysis Center (KCC) and ‡ Division of Physical Science and Engineering (PSE), II Core Labs, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia
ABSTRACT: Although core–shell copper metal nanoclusters are important emerging materials for practical applications and fundamental scientific research, their synthesis lags behind that of gold and silver nanoclusters – challenged by copper’s much lower half-cell reduction potential, M(I)/M(0). To overcome this synthetic hurdle, we introduce a simple reaction strategy, involving the mild reducing agent borane tert-butylamine complex, to produce a core-shell copper superatom nanocluster, [Cu61(StBu)26S6Cl6H14]+ (–StBu; tert-butyl thiolate), which is the largest Cu(0)-containing structurally-solved core-shell copper cluster to-date. The nanocluster exhibits a quasi-elongated triangular gyrobicupola (quasi-J36, J36: Johnson solid) Cu19 core and a shell held together by a novel “18-crown-6” metal-sulfidelike belt. Due to its stability, this cluster displays a single molecular ion peak in mass spectrometry measurements without any cluster fragmentation signals – a first observation of its kind for copper nanoclusters that paves the way for researchers to study nanocluster composition, charge, stability, and reaction mechanisms with atomic precision that only mass spectrometry could afford.
Atomically precise ligand-protected coinage-metal nanoclusters are fast-growing areas of research in materials science.1-10 These nanoclusters generally have a core–shell structure in which the core is composed of metal, and metal-ligands form the shell.12 These materials find applications in sensing, catalysis, cell imaging, and nanoelectronics.1-3, 11-14 However, core–shell coinage-metal nanoclusters are also important for fundamental scientific research. In this context, the synthesis of new and interesting structures is the primary goal. A large number of structurally characterized core–shell silver and gold nanoclusters have been reported in the literature, which have helped researchers identify structure–property relationships in these silver and gold nanomaterials.1-3 In stark contrast, only a handful of core–shell copper nanoclusters have been reported,15-18 although copper is considered a promising catalyst for photocatalytic water splitting, electrocatalytic CO2 reduction, and many organic reactions15, 19-24; and the synthesis of its clusters will offer new insight into the structural and surface chemistry of copper nanoparticles. Such core–shell nanoclusters generally have an M(0) character in the core. Unfortunately, in addition to its relative airsensitivity, the half-cell reduction potential, [M(I)/M(0)], of copper (0.52 V) is much lower than that of both silver (0.80 V) and gold (1.69 V).17, 25 Consequently, the syntheses of copper nanoclusters with zero oxidation state copper atoms in their core (particularly
with high nuclearity cores) faces more hurdles than its sister metals, and thus requires novel facile strategies for their synthesis. While single crystal diffraction (when possible) is the most powerful approach to elucidating the structure of the clusters, mass spectrometry techniques have proven to be tremendously useful (even when the crystal structure is unknown) in studying the composition, stability, and reaction pathways of nanoclusters with atomic precision in ways no other technique can afford. The literature is abound with examples from silver and gold nanoclusters1-3, 5, 10, 26-27 in the which the development of advanced mass spectrometric techniques has opened new avenues in investigating synthesis and conversion pathways,1-3, 6, 26, 28-29 intercluster reactions and their mechanism,27 gas-phase isomerism,30-32 the interaction of nanoclusters with macromolecules,5, 32 and even the calibration of mass spectrometers with thiol-protected gold and silver nanoclusters.33 Despite the paramount success of mass spectrometry in silver and gold nanocluster research, it has yet to be explored for copper nanoclusters. Unlike for silver and gold, the major bottleneck in studying the known copper nanoclusters that are protected with alkynyl and phosphine ligands lies in the appearance of numerous fragmented peaks in the mass spectrum, limiting the use of mass spectrometry in exploring the materials diverse chemistry.15-18, 34 Given that thiol ligands form strong bonds with metals, we anticipated that thiols could be very important to synthesizing stable copper nanoclusters and could enable thorough physical characterization by displaying a coherent mass spectrum. We report a novel synthetic strategy for producing thiol-protected core–shell copper nanoclusters. This strategy has yielded the nanocluster [Cu61(StBu)26S6Cl6H14]+, which produces only a molecular ion peak (without any fragmentation) in its mass spectrum. The cluster was structurally analyzed by single-crystal X-ray diffraction (SCXRD) and complementary analytical tools, revealing the largest Cu(0)-containing core-shell copper cluster reported to date. The cluster is composed of a quasi-elongated triangular gyrobicupola (quasi-J36, J36: Johnson solid)35-36 Cu19 core and a giant Cu42(StBu)26S6Cl6 shell. In other metals, 19-atom metal cores, occur only in much higher nuclearity clusters (e.g. Ag19 in ligand-protected Ag141 and Ag210 clusters) and showcase a completely different structure and symmetry from that of Cu19 core observed in our work.37-38 Previously reports on copper clusters exhibited cores that are trigonal (Cu3),16 tetrahedral (Cu4),15 centered-icosahedral (Cu13),17-18 or centered-cuboctahedral (Cu13).34 Thus the present observation implies that the quasi-J36 core (Cu19) is a new member in the family of protected
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nanoclusters. The shell, Cu42(StBu)26S6Cl6, contains a ring-like macrocycle or belt, (Cu2S)6, and a pair of tricorne-like caps, Cu15(StBu)13Cl3. The structure of the belt resembles that of “18crown-6”,39 which has not yet been observed neither among copper nanoclusters nor among the well-established silver and gold nanoclusters. In a typical synthesis, a chloroform solution of Cu(I)–StBu thiolate (–StBu; tert-butylthiolate) is reduced by using a mild reducing agent, borane tert-butylamine complex, at room temperature (23°C). During the synthesis, the color of the solution changes from light yellow to dark red. The as-synthesized cluster solution is mixed with ethanol and kept for crystallization at room temperature, which produces dark-red crystals after one week. The crystals are washed with methanol and used for further characterization. Details of the synthesis and purification process are provided in the experimental section of the Supporting Information. A control experiment with a strong reducing agent (NaBH4) was performed to elucidate the crucial role of the mild reducing agent, borane tert-butylamine complex, in this synthesis (Figure S1). Nanoclusters synthesized with borane tert-butylamine complex were more stable than ones synthesized via NaBH4, which enabled the clusters crystallization The availability of high-quality crystals prompted us to elucidate the structure with SCXRD measurement. The as-synthesized cluster crystallizes in a monoclinic crystal system with space group C2/c (Table S1). The full structure, to the extent by which SCXRD could reveal, of the cluster is shown in Figure 1. The full cluster formula, [Cu61(StBu)26S6Cl6H14]+, was derived using SCXRD complimented by mass spectrometry (vide infra), which enabled us to ascertain the hydride count as well as the overall superatomic charge. The sulfide (S2-) and chloride ions (Cl-) in the cluster should derive from the cleavage of C–S and C–Cl bonds of the thiol and chloroform during the reaction process, respectively. In the structure, 19 copper atoms form a unique core structure (Cu19) (Figure 2a), which is further protected by a giant metal-ligand shell, Cu42(StBu)26S6Cl6 (Figure 2b). The quasi-J36 Cu19 core has quasiD3d symmetry, featuring a C3 axis, three C2 axes, and three symmetry planes passing through the C3 axis and equally dividing the angle between two adjacent C2 axes. Here 18 Cu atoms are situated on the vertices of the quasi elongated triangular gyrobicupola which consists of 6 rhombuses and 18 (2+2×8) triangles (Figures 2a and S2a). The center of this structure is occupied by a copper atom – completing the Cu19 core (Figures 2a and S2b). It is worth mentioning that an elongated triangular gyrobicupola of Cu(I) atoms was previously reported.40 a)
b)
Figure 1. Molecular structure of the Cu61(StBu)26S6Cl6H14 cluster along two different directions: a) top view and b) side view. Hydrogen atoms are omitted for clarity. Color legend: green, light blue, and brown, copper atoms of the Cu19 core, (Cu2S)6 belt, and tricorne-like cap, Cu15(StBu)13Cl3, respectively; yellow, sulfur; pink, sulfide; dark blue, chlorine; and gray, carbon.
The construction of the Cu19 core is illustrated in Figure S3. Here, six copper atoms (Cu6) are arranged in a chair conformation, and two such Cu6 conformations are stacked vertically to form a Cu12 cage (Figures S3a-b). The center of this cage is occupied by a copper atom and results in a centered cage (Cu13) (Figure S3c). Subsequently, two triangles (Cu3) are connected from the top and bottom sides of the Cu13 centered cage and form the Cu19 core (Figures S3d-e). Here, the Cu–Cu bond distances range from 2.486 to 2.726 Å, which are comparable with the bond distances of the reported copper clusters (Table S2).17, 34 The construction of the core can also be viewed as shown in Figure S4. The giant shell, Cu42(StBu)26S6Cl6, is composed of one ring-like macrocycle or belt, (Cu2S)6 (Figure 2c), and two equivalent tricorne-like caps, Cu15(StBu)13Cl3 (Figure 2d). For the belt, twelve copper and six sulfur atoms form a structure with D3d symmetry that resembles the structure of “18-crown-6” (Figure 2c).37 Here, the Cu–Cu and Cu–S bond distances are in the ranges of 2.737– 2.764 Å and 2.163–2.184 Å, respectively. These Cu–Cu bonds are longer than those in the Cu19 core. In addition, six copper atoms form a Cu6 plane (Figure S5). The remaining six copper atoms belong to another plane. Similarly, three sulfur atoms form an S3 plane with D3h symmetry (Figure S5). This “18-crown-6”-like metal sulfide structure, (Cu2S)6, is observed for the first time among structurally characterized core–shell copper clusters and is rare among silver and gold clusters. The formation of the tricornelike cap, Cu15(StBu)13Cl3, is shown in Figure S6. The cap contains a crown structure, Cu9(StBu)4Cl3, which is further connected to three dimeric staple motifs (–SR–Cu–SR–Cu–SR–) with C3v symmetry. Such dimeric staple motifs have not been previously observed in core–shell copper clusters, although they are quite common in silver and gold clusters. Two types of Cu–S bonds are observed in Cu15(StBu)13Cl3. The bond distances range from 2.283 to 2.292 Å when the copper atoms are connected to the chlorine atom. However, the distances range from 2.149 to 2.192 Å when the copper atoms are not connected to the chlorine atoms. The bonding interaction between the (Cu2S)6 belt and one of the tricorne-like caps, Cu15(StBu)13Cl3, is shown in Figures 2e and S7. The six S atoms [two terminal S atoms from each dimeric staple motif (–S–Cu–S–Cu–S–)] of Cu15(StBu)13Cl3 are connected to the six copper atoms of the Cu6 plane of the belt (Figures 2e and S5). In contrast, the three sulfur atoms of the (Cu2S)6 belt, which belongs to the same S3 plane (Figures 2e and S5), are weakly attached to the six copper atoms of the three dimeric staple motifs, with the Cu–S distances varying from 2.538 to 2.608 Å. Another tricorne-like cap structure is connected to the atoms of the other available Cu6 and S3 planes (Figure 2e) of the belt in a similar manner. The bonding interactions between the (Cu2S)6 belt and core (Cu19) are presented in Figures 2f and S8. The copper and sulfur atoms of the belt are connected to only 12 copper atoms of the core; those atoms belong to the Cu12 cage. The bonding between the Cu12 cage and (Cu2S)6 belt is shown in Figures S9a and a1. Each sulfur atom of the belt is connected to two copper atoms of the core (Figures S9b and b1). The Cu–S bond distances range from 2.318 to 2.332 Å, which are longer than the Cu–S bond distances in the (Cu2S)6 belt (2.163–2.184 Å). Furthermore, the 12 copper atoms of the belt are only connected to the six copper atoms of the core, with the Cu– Cu bond distances in the range of 2.697–2.727 Å, which are longer than those in the core (2.486–2.726 Å) (Figures S9c and c1). The bonding interaction between the core and one of the tricorne-like caps, Cu15(StBu)13Cl3, is shown in Figures 2g and S10. Clearly, the three dimeric staple motifs (–S–Cu–S–Cu–S–) of Cu15(StBu)13Cl3 have no bonding interaction with the core. Only the cap, Cu9(StBu)4Cl3, is connected to the core through its chlorine and
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ACS Materials Letters copper atoms. Three chlorine atoms of each cap are connected to three copper atoms, which belong to the Cu3 triangle of the Cu19 core (Figure S3d). Here, the Cu–Cl bond distances (2.390–2.460 Å) are shorter than those in the cap (2.673–2.734 Å). Alternatively, the full structure can be viewed as shown in Figure S11.
peak at m/z 6613.50. Upon expansion, the peak shows a characteristic separation of R >L = 1 (Figure S12), which confirms the presence of singly charged (1+) species. The assigned cluster composition for the corresponding peak is [Cu61(StBu)26S6Cl6H14]+. The exact match between the experimental and simulated isotopic distributions further supports the assigned composition (inset, Figure 3a). We were also able to detect doubly charged species at m/z 3306.75 (Figure S13). The existence of multiple charge states in mass spectra has been previously observed for Ag44(SR)30 (SR = 4-fluorothiophenol, pmercaptobenzoic acid) and [Ag40(SPhMe2)24(PPh3)8](NO3)2 clusters.30, 41-42 The availability of a clean mass spectrum allows us to study the systematic fragmentation of the cluster (Figure S14). The fragmentation pathway is similar to the widely studied ligand protected Au25 cluster.43 The presence of the counter ion (Cl-) is also confirmed by ESI MS (Figure S15).
The presence of hydrides in a copper cluster is a rather common phenomenon.16-18 However, it is notoriously difficult to detect those hydrides by SCXRD. Therefore, we used mass spectrometry to confirm the final composition of the cluster. ESI MS measurement of a toluene solution of the cluster was performed using a Bruker MicroTOF-II mass spectrometer. The sample preparation and instrument conditions are detailed in the experimental section. The signal was observed only in the positive mode. The ESI MS spectrum over the range of m/z 1000–10000 is presented in Figure 3a, which confirms the presence of a single a)
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available Cu6 Plane available S3 Plane
Figure 2. a) Quasi-J36 Cu19 core, side view. b) Metal-ligand shell, Cu42(StBu)26S6Cl6, top view. c) Metal sulfide macrocycle or belt, (Cu2S)6, which resembles the structure of “18-crown-6”. d) Tricorne-like cap, Cu15(StBu)13Cl3. e) Bonding interaction between the belt, (Cu2S)6, and one of the tricorne-like caps, side view. f) and g) Bonding interactions of the core (Cu19) with the belt and one of the tricornelike caps, respectively. Carbon and hydrogen atoms are omitted for clarity. Cu green, sky blue, and brown; S yellow; sulfide pink; and Cl dark blue.
To further confirm the presence as well as the number of hydrides in the cluster composition, high-resolution solid-state NMR coupled with 2D SS-NMR was used. The 1H MAS NMR spectrum of the recrystallized sample displays two major signals at 1.4 and 4.1 ppm that can be attributed to –CH3 of tert-butyl thiolate and hydrides, respectively (Figure S16). 1H-1H double-quantum (DQ) measurements confirm our assignments, in which a strong autocorrelation peak is observed for the –CH3 groups of tert-butyl thiolate, whereas the proton resonance at 4.1 ppm displays no autocorrelation (Figure S17). Notably, to have an autocorrelation peak in the DQ spectrum, a minimum of two protons should be present, which would prove that the peak at 4.1 ppm is a single proton. The integration ratio for these two peaks is (1:16.3) close to the expected value (1:16.7) assigned by mass spectrometry. It should be noted that two other small peaks are observed at 2.4 and 6.6 ppm, which can be attributed to –CH3 and aromatic protons of toluene, respectively.
Additional investigations, namely, 13C CP-MAS and 2D 1H-13C HETCOR NMR spectroscopy, were also conducted (Figure S18). The 13C CP/MAS NMR spectrum shows signals at 36, 43, and 48 ppm, and a clear correlation is observed in the 2D 1 U13C HETCOR NMR spectrum between these three carbon atoms and – CH3 (the methyl) protons at 1.4 ppm, which indicates the presence of three different –CH3 groups . Furthermore, the proton peak at 4.1 ppm shows no correlations with any carbon signal, which means that this proton does not have any carbon atom connectivity and can be undoubtedly assigned to hydrides. Small signals are also detected at 128 and 20 ppm and show a correlation between the aromatic protons at 6.6 ppm and methyl protons at 2.4 ppm, respectively, corresponding to a small amount of toluene. A control experiment was further conducted to avoid the confusion associated with the absorbed-water peak in the 1H MAS NMR spectrum (Figure S19). Currently, the exact positions of these
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hydrides in the cluster structure is still undetermined. Neutron diffraction studies may reveal these positions in the near future. The recrystallized cluster was used for X-ray photoelectron spectroscopy (XPS) analysis. The XPS survey spectrum of the cluster (Figure S20a) confirms the existence of all the expected elements (C, S, Cu, and Cl). High-resolution XPS spectra of C 1s, Cu 2p, S 2p, and Cl 2p are shown in Figures S20 b-e. The highresolution Cu 2p spectra of the Cu(I)–StBu thiolate and the cluster are compared in Figure S21. No significant differences are observed, as the Cu 2p3/2 binding energies of Cu(0) and Cu(I) are almost similar. Cu LMM Auger spectra were gathered to obtain information about the oxidation state of the copper atoms in the cluster (Figure 3b). The Cu LMM Auger spectrum of the Cu(I)– StBu thiolate (red trace) shows one main peak at 916.3 eV accompanied by two shoulders at approximately 912.1 eV and 918.8 eV, which are similar to those obtained for Cu–SR bonds in the Cu(I) oxidation state.44 In contrast, the cluster shows (blue trace), in addition to the peaks observed for the Cu(I)–StBu thiolate, a sharp peak at 918.4 eV accompanied by a shoulder at 921.1eV, which corresponds to Cu(0).44 This finding implies that the copper atoms in the cluster are in mixed valent states [between Cu(0) and Cu(I)]. A comparison of the Cu LMM spectra of the cluster and copper metal further confirms the existence of a partial Cu(0) character in the cluster (Figure S22). 45
trace) and the cluster (blue trace). c) UV–vis spectrum of the toluene solution of the cluster. Inset: A photograph of the cluster in toluene solution. The zero absorbance point of the cluster is observed to be at 625 nm. In summary, we developed a new strategy for synthesizing the first thiol-protected core–shell copper nanoclusters. The mild reducing agent borane tert-butylamine complex plays a crucial role in this process. The cluster was characterized using different analytical tools, namely, SCXRD, ESI MS, UV–Vis, XPS, and NMR. The cluster, [Cu61(StBu)26S6Cl6H14]+, exhibits a novel structure with a quasi-J36 Cu19 core – the largest known Cu(0) containing core for any structurally-determined copper clusters – and an “18-crown6” metal-sulfide-like, (Cu2S)6, belt in the shell. Because of the strong Cu–S bond in the shell, the cluster yields a single peak in its mass spectrum, corresponding to the molecular ion. The availability of only a molecular ion peak in the mass spectrum will enable the diverse chemistry and reactivity of the cluster to be examined with the atomic precision that only advanced mass spectrometry-based techniques can afford. Our findings will pave the way for the cluster community to synthesize a diverse array of core–shell copper nanoclusters and explore their various properties with advanced characterization tools.
The Jelliumatic electron count of the cluster ([Cu61(StBu)26S6Cl6H14]+, n = 61 (metals) - 26 (StBu ligands) -12 (S2-) – 6 (Cl-) - 14 (hydrides) – 1 (charge) = 2) confirms that it is a 2-electron superatom system. 1
ASSOCIATED CONTENT
The UV–Vis spectrum of the cluster in toluene solution shows two peaks at 335 nm and 440 nm (Figure 3c). These peaks may be attributed to interband electronic transitions of Cu clusters. The color of the solution is dark brown (inset, Figure 3c), and the solid sample is stable at room temperature for more than a month.
AUTHOR INFORMATION
Supporting Information Crystal structure, XPS, ESI MS, and NMR
Corresponding Author *Email:
[email protected]. Author Contributions
[Cu61(StBu)26S6Cl6H14]1+
a)
#These authors contributed equally.
Notes
Experimental Simulated
The authors declare no competing financial interest.
ACKNOWLEDGMENT 6594
6608
6622
* *
Cu(I) Cluster Cu(I)-StBu thiolate
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Kinetic energy (eV)
8000
REFERENCES
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Cu(0)
Visible light photograph
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440 nm
Absorbance
b)
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355 nm
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This work was supported by King Abdullah University of Science and Technology (KAUST).
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Figure 3. a) Positive-mode ESI MS spectrum of the cluster over the mass range of m/z 1000–10000. The peak at m/z 6613.50 corresponds to [Cu61(StBu)26S6Cl6H14]+. Inset: The exact match between the experimental (black) and simulated (red) isotopic distribution supports the assignment. The features indicated by black asterisks are due to the addition and loss of Cu–StBu thiolate. b) Comparison of the Cu LMM spectra of Cu(I)–StBu thiolate (red
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