Communication pubs.acs.org/IC
Bottom-Up Self-Assembly of the Sphere-Shaped Icosametallic Oxo Clusters {Cu20} and {Cu12Zn8} Juan Chen, Hulan Zhou, and Feng Xu* State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry & Chemical Engineering, Hunan University, Changsha 410082, P. R. China S Supporting Information *
ABSTRACT: A discrete nanospheric icosametallic cluster comprised of 20 Cu ions (1) was self-assembled from facile synthesis. Adjustment of the synthesis by the choice of ligands gave rise to another cluster (2) with an intact icosacupric core and improved stability. Referring to the synthesis of 1 and 2, a heterometallic cluster (3), which contains 12 Cu II and 8 Zn II , was designed and characterized by single-crystal X-ray diffraction, combined with elemental analysis, energy-dispersive X-ray, X-ray photoelectron spectroscopy, thermogravimetric analysis, and element mapping. The magnetic measurements of 2 and 3 and the scanning electron microscopy images and UV−visible diffuse-reflectance measurements of metal oxides from 2 and 3 indicate that isolation of {Cu12M8} is a new synthetic route to materials with engineered properties.
Figure 1. (a) Ball-and-stick representation of the discrete icosametallic cluster {Cu20} 1. (b) Structure of the {Cu20} core templated by CO32− with the surrounding acetate. n-Propylamine and water ligands are removed for clarity. (c) Space-filling model of 1. Color scheme: Cu, blue; O, red; N, green; C, gray.
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he molecular self-assembly of highly symmetric molecules, such as wheel- and sphere-shaped clusters, has been proved to be a key strategy in the design and development of functional nanomaterials.1 The nanospheric clusters comprised of 20 metal centers ({M20}) may exhibit such a structural prototype. Winpenny’s group reported the first example of {M20} made of 12 Cu and 8 La ions ({Cu12La8}).2 Mal and Kortz extensively investigated the synthesis and magnetic, catalytic, and electrochemical properties of an extraordinary polyanion, [Cu20Cl(OH)24(H2O)12(P4W48O184)]25− ({Cu20⊂W48}) with a core of {Cu20}.3 Lately, Wang et al. synthesized two discrete clusters of [M20(OH)12(maleate)12(Me2NH)12](BF4)3 (OH)·nH2O (M = Co, Ni), resembling the structure of the {Cu20} core.4 Although the above works support our hypothesis of the possible isolation of a self-assembled discrete {Cu20} from the solution, the facile synthesis and mechanism remain a mystery. Herein we report the discovery of a discrete homometallic {Cu 2 0 } compound of [Cu 2 0 (OH) 2 4 (CH 3 COO) 8 (npropylamine)10CO3](CH3COO)6·15H2O (1) via a one-pot procedure (Figure 1a), which led to the rational design of the {Cu20} derivative, [Cu20(OH)24(CH3COO)6(iso-propylamine)8 CO 3 ](CH 3 COO) 8 ·10H 2 O (2) and the heterometallic [Cu12Zn8(OH)24(CH3COO)12CO3](CH3COO)2· 4CH3COOH·12H2O (3) cluster. Compound 1 was obtained from the reaction of Cu(OAc)2 with n-propylamine in wet acetonitrile at room temperature, followed by filtration and crystallization under slow evaporation (formed in ∼50% yield). Fragile crystals of 1 led us to replace the © XXXX American Chemical Society
n-propylamine ligands by the sterically more congested and less toxic isopropylamines, giving rise to the more air-stable compound 2 with an intact {Cu20} core (Figure 1b) in high yield (ca. 60%). To further investigate the system favoring the formation of the nanoscale {M20} ensembles, a mixture of Cu(OAc)2 and Zn(OAc)2 was used to react with isopropylamine in wet acetonitrile at room temperature, resulting in isolation of compound 3 with a well-defined {M20} core (Figure 2a) in ∼10% yield (based on Cu). It should be noted that another two icosametallic clusters, {Cu20} and {Cu12Mg8}, were revealed by Winpenny et al. very recently during our studies.5 The fact that the reported syntheses differ greatly from those of 1−3 further substantiates our initial idea that the self-assembly of {M20} from the solution is favored. Single-crystal X-ray analysis on compounds 1−3, which crystallized in the triclinic P1̅, tetragonal P42/n, and cubic Im3m ̅ space groups, respectively, revealed the shared structure of a cage-shaped {M20} core. The {M20} core can be further viewed as a {Cu12} cuboctahedron with its eight triangular faces capped by eight CuII or ZnII ions from an outer cube. Thus, the structures of compounds 1−3 can be represented by {Cu12@M8} (M = Cu2+, Zn2+), in comparison with the {M8@M12} in {Ni20} and {Co20} (Figure S1).4 It is of interest to notice that the Received: February 26, 2016
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DOI: 10.1021/acs.inorgchem.6b00487 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
solubility in both acetonitrile and methanol, crystals of 3 can be readily dissolved in acetonitrile with controlled concentration and recrystallization occurs on the Si substrate after quick solvent evaporation. The as-made material is ready for scanning electron microscopy (SEM) imaging without further gold-spraying treatment. Figure 3a presents an SEM image of a large number
Figure 2. Representation of the structures of (a) the {Cu12M8} core templated by CO32−, with the surrounding ligands removed for clarity, (b) the {Cu12@M8} core, and (c) the pristine {Cu12} cuboctahedron. Color scheme: Cu, blue; M(Cu/Zn), purple; O, red; N, green; C, gray.
Figure 3. SEM images of the homogeneous oxide samples from 3 (a) and 2 (b) with a relatively uniform particle-size distribution of 100 nm.
cuboctahedron of the {Cu12@M8} cage has a great resemblance to that of the newly found polyoxopalladate clusters {Pd12M} (M = Pd2+, Ce4+, Na+, etc.) with a {Pd12X8} (X = As, P) moiety.6 As such, it is presumed that the formation of a {Cu12} cage is followed by the addition of eight other metal ions in the construction of discrete {M20}. The proposed mechanism corresponds well with three observations: (1) in the synthesis of 3, the composition of the products obtained in different batches from the solution remains unchanged in spite of the continuing crystal growth and the resulting decreased Cu/Zn ratio in the solution; (2) the isolation of {CuxM20−x} (0 ⩽ x < 12) in different reactions has not succeeded yet; (3) our preliminary results indicate the possible synthesis of other mixed copper− transition-metal nanospheric icosanuclear clusters, such as CoII and NiII. Moreover, it should be noted that the correlation between CuII and PdII may be supported again by comparing the minimal structural repeating unit {Pd6} in the giant wheelshaped polyoxopalladate {Pd84} with the discrete {Cu6} cluster, suggesting the underlying rich and yet rudimentary chemistry of polynuclear copper complexes.7,8 The structures of 1−3 feature an encapsulated CO32− as the template at the center of the clusters. It is not unusual that the system at high amine concentration fixes CO2 from air to form CO32−-encapsulated clusters.9 The template anion, such as Cl−, Br−, I−, and N3−, was found to be crucial in the formation of {Cu20⊂W48},3 whereas NO3− was employed in the fabrication of {Cu12La8}.2 By a close comparison, the structures of {Pd12M}, templated by Pd2+, Ce4+, or Na+,6 corroborate the role of templates in the assembly of the pristine {M12} cuboctahedron. The heterometallic structure of 3 has been further confirmed by elemental analysis, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy, energy-dispersive X-ray, and element mapping (Figures S10−S12). The TGA curve of compound 3 indicates its facile thermal decomposition, and the residual weight (52.3% at 500 °C) is very close to the expected composition for Cu12Zn8O20 (51.2%). X-ray diffraction patterns of the residue show the major oxide phases are hexagonal ZnO and monoclinic CuO (syn-tenorite) in the space groups P63mc and C2/c, respectively,10 verifying the elemental composition of 3. Further characterization of the oxide samples from 3 at 500 °C indicates that {Cu12M8} represents a new candidate to synthesize mixed-metal oxides of homogeneous composition at the atomic length scale, as discussed by Winpenny et al.2 By virtue of its great
of crystals with a relatively uniform particle-size distribution of 100 nm. Element mappings (Figure S21) give the spatial distribution of Zn and Cu in the sample, suggesting the formation of a Cu12Zn8O20 solid solution. Thereafter, the homogeneous CuO sample from 2 was prepared and characterized under similar conditions (Figure 3b). Further, the observed UV−visible diffuse-reflectance spectra of as-prepared Cu12Zn8O20, CuO, and bulk ZnO are presented in Figure S22. Given their similar structures and different numbers of CuII d9 ions in 2 and 3, it is interesting to investigate the magnetic behavior of both compounds. The magnetic susceptibilities of 2 and 3, measured in an applied field of 1000 Oe over a temperature range of 2−300 K, are shown in Figure 4 in the form
Figure 4. Plots of experimental χMT versus T for compounds 2 (■) and 3 (□) and χM−1 versus T for compounds 2 (▲) and 3 (Δ) in the temperature range 0−300 K at a field of 1000 Oe. The red solid lines present the fitting results of 2 and 3 with modified Curie−Weiss law.
of χMT versus T and 1/χM versus T. The χMT value of complex 2 at 300 K, 6.20 emu·K/mol, is lower than the theoretical spin-only (g = 2) value for 20 noninteracting CuII metal centers of 7.5 emu· K/mol. Upon cooling, a decrease of χMT suggests the presence of overall antiferromagnetic interaction in 2. After a minimum value of 1.95 emu·K/mol around 10 K is reached, χMT increases rapidly B
DOI: 10.1021/acs.inorgchem.6b00487 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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to a maximum value of 3.16 emu·K/mol at 2 K. For 3, the χMT value of the material is 3.5 emu·K/mol at room temperature, which is lower than the calculated spin-only value for 12 CuII centers (4.5 emu·K/mol; S = 1/2, g = 2). As the temperature is lowered, the χMT value decreases gradually to a minimum of 0.49 emu·K/mol at 2 K. As elaborated by Sun et al., the combination of possible magnetic coupling pathways in {M20} clusters is numerous and of variety because of the presence of many short Cu···Cu fragments bridged by hydroxido anions in the title compounds.11 It is rather difficult to find a model to fit the curves and determine the exact structural origin of the couplings. Yet the temperature dependence of the reciprocal susceptibility (1/χM) for both 2 and 3 can be fitted nicely to the Curie−Weiss law [1/ χM = (T − θ)/C] in the range of 50−300 K, with C = 7.57 emu· K/mol and θ = −69.46 K for 2 and C = 4.55 emu·K/mol and θ = −94.97 K for 3. More detailed magnetic studies of 2 and 3, by virtue of their great solubility in acetonitrile and methanol, respectively, and other {Cu12M8} clusters with different transition metals are in progress.5 In summary, we have successfully isolated the discrete icosametallic cluster 1 and designed the synthesis of 2 with improved stability. The heterometallic cluster 3 was further obtained and characterized unambiguously in reference to the synthesis of 1 and 2. The great solubility of 2 and 3 in acetonitrile and methanol respectively is expected to be more convenient for further solution studies.5 It is presumed that {Cu12M8} is preceded with the formation of a pristine {Cu12} cuboctahedron, which is templated by CO32−. The obtained homogeneous Cu12Zn8O20 sample from 3 demonstrates its potential as a new precursor to mixed-metal oxides. The work to engineer the {Cu12M8} clusters with different metal compositions, search for other highly symmetric polycopper(II) clusters, and explore the host−guest chemistry of polyoxotungstates encapsulated with {Cu12M8} is underway.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00487. Crystallographic data of complexes 1−3, experimental data, and additional structures, characterization data, and tables (PDF) CCDC 1438513 (1) (CIF) CCDC 1438514 (2) (CIF) CCDC 1438515 (3) (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21301057), the Natural Science Foundation of Hunan Province (Grant 14JJ3068), and the Fundamental Research Funds for Central Universities. C
DOI: 10.1021/acs.inorgchem.6b00487 Inorg. Chem. XXXX, XXX, XXX−XXX
