2 Ground

Communication pubs.acs.org/IC

An Anionic Heptacopper(II) Oxo-Cluster {CuII7} with an S = 7/2 Ground State Wei Meng,†,‡ Feng Xu,*,† and Weijian Xu*,† †

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry & Chemical Engineering, Hunan University, Changsha, 410082, Peoples’ Republic of China ‡ Department of Chemistry & Environmental Engineering, Hunan City University, Yiyang, 413000, Peoples’ Republic of China S Supporting Information *

recombination process in an aqueous medium. By the utilization of enantiopure forms of K2Sb2L2 as the starting material, Jacobson et al. reported a series of sandwich-type clusters with a multimetallic core of high valence metal ions, such as Fe(III)/Mn(II), V(V)/Mn(II), Ni(II)/Cr(III), Fe(III), and Fe(II)/Fe(III).6 However, pure divalence late transition metals sandwiched by {Sb3(μ3-O)} have not yet been reported. Herein, we present an anionic cluster with a heptacopper(II) core, [Cu7Sb6(μ3-OH)2(μ4-O)6L6]6−, which was synthesized in an aqueous medium at room temperature and isolated as the hydrated sodium salt Na6[Cu7Sb6(μ3−OH)2(μ4-O)6L6]·24H2O (1). To the best of our knowledge, compound 1 is the first heptacopper(II) polyoxoanion that consists of a planar core ({Cu7}) sandwiched between two {Sb3(μ3-O)} layers. We further explored the profound effects of various factors on the synthesis of 1 and discovered another new dicopper(II) oxocluster, namely Na4[Cu2Sb12(μ3-O)6(μ3-OH)2(μ4-O)3L6]· 19H2O (2), under subtly different reaction conditions with 1. Hereby, the synthesis of 1 and 2 features the choice of ligands (K2Sb2L2) together with the introduction of the facile and green synthetic route arising from the newly established polyoxopalladate and polyaurate chemistry, in lieu of the conventional hydrothermal synthesis. Polyanion 1 is formed by mixing Cu(OAc)2 and rac-K2Sb2L2 in a HOAc-NaOAc buffer at room temperature, followed by the addition of 2 M K2CO3 into the mixture and crystallization under slow evaporation. To examine the influence of synthetic factors on the self-assembly of 1, we set out to explore similar reactions under a series of different reaction conditions by changing the types and concentration of reactants and buffer solutions, pH, and reaction temperature. We found slight changes in reaction conditions impact the formation of another new cage compound 2, comprising a {Sb12} cage and a {Cu2} dinuclear core. The choice of the HOAc-NaOAc buffer solution at a pH of 5.5 is crucial for the successful synthesis of both compounds 1 and 2. When the reactions were performed in other buffer (such as Na 2 CO 3 −NaHCO 3 , NaH 2 PO 4 − Na2HPO4, HCOOH−HCOONa), neither 1 nor 2 was obtained. By comparing the different syntheses between 1 and 2, we found K2CO3 plays a determining role in the synthesis of the title compound 1. The crystallization of the polyanion 1 was only successful by the introduction of K2CO3 during the reaction; otherwise, the polyanion 2 would

ABSTRACT: An anionic heptacopper(II) oxo-cluster {Cu7} was obtained from a simple reaction in aqueous solution without heating. In a similar reaction, another cage compound {Sb12} with an encapsulated {Cu2} core was produced, implying the structural versatility of copper(II)-oxide assemblies constructed from potassium antimonyl tartrate as the ligand in aqueous solution. The ferromagnetic S = 7/2 ground state of {Cu7} is confirmed by magnetization measurements.

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olynuclear metal oxo-clusters have been the subject of extensive studies in various fields including inorganic synthesis, catalysis, electrochemistry, magnetism, and materials.1 Despite their versatile architectures and the resulting properties, it is still confined to designing and developing novel discrete oxo-clusters built from different metal ions, due to the complexity of metal salts’ hydrolysis and the lack of synthetic strategies. Recently, high-nuclearity transition metal oxo-cluster anions based on Pd and Au among late transition metals have redefined the area,2 while those based on other late transition metals still remain underexplored. The present work focuses on the paramagnetic Cu(II), which has found a wide use in magnetochemistry and materials such as catalysts in numerous industrially relevant processes and purifiers for automobile tail gas.3 It has proved its validity to fabricate anionic polynuclear copper oxo-clusters in an aqueous medium by employment of inorganic ligands as scaffolds, including examples of graceful {Cu20 ⊂ W48} and {Cu14 ⊂ W36} clusters.4 Although a number of discrete oxo-clusters sandwiched by lacunary polyoxometalates are reported with mono-, bi-, tri-, tetra-, penta-, and hexanuclear copper cores,5 to synthesize this type of sandwichtype cluster with a heptanuclear or even larger copper core is still challenging. To circumvent the nuclearity limitation, from the inorganic ligand pool, K2Sb2L2 (H4L = tartaric acid), namely dipotassium bis(μ-tartrato)-diantimony(III), is applied in our work of discovering new polyoxocuprates through a combination of two reasons. First, a few forms of K2Sb2L2 are commonly available due to the existence of two chirality centers in each tartrate ligand, offering a rich chemistry of tartratebased coordination compounds with favored ligand conformation. Second, K2Sb2L2 has demonstrated its ability to form the Sb3(μ3-O) tartrate scaffold ({Sb3(μ3-O)}), which possesses three bridging oxo groups, by a decomposition and © XXXX American Chemical Society

Received: September 27, 2015

A

DOI: 10.1021/acs.inorgchem.5b02206 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

described as distorted [4 + 1] coordinated, with the apical bond (ca. 2.33 Å) to the μ4-oxide ion. Pentacoordination for Cu(II) is not unusual in coordination chemistry.10 The structure of 2 bears the resemblance to the cage cluster [NiCrSb12(μ3-O)8(μ4-O)3L6]3−, which was reported by Jacobson and synthesized in H2O/DMF by hydrothermal reaction.7 The cage cluster of 2 ([Cu2 ⊂ Sb12]) comprises two {Sb3(μ3O)} scaffolds, an equatorial belt of {Sb6} and an encapsulated {Cu2} core linked through bridging oxide ligands (Figure 2).

crystallize from the solution. The dual roles of K2CO3 in the synthesis are to control the final pH and adjust the coordination ability of the tartrate ligands with Cu2+ and Sb3+ ions.7 In order to further explore the role of K2CO3, we tried to replace it by equivalent amounts of other potassium salts (such as KCl, KNO3) or carbonates (such as Li2CO3, Na2CO3, Cs2CO3) during the reaction. However, no crystal growth was observed after a few days. Besides, it is presumed that the clusters’ larger negative charge by using the divalent copper(II) ions, instead of other high valence metal ions in the literature, contributes to the successful synthesis and crystallization of 1 and 2 from the aqueous solution. Compound 1 crystallizes out in the triclinic space group P1̅. The structure of 1 consists of a planar heptacopper(II) core ({Cu7}) sandwiched between two {Sb3(μ3-O)} layers (Figure 1). In 1, all seven copper atoms are linked together by six

Figure 2. A ball-and-stick representation of [Cu2Sb12O11L6]4− cluster in 2. The encapsulated {Cu2} core is represented by two octahedra which share a face (highlighted in bright blue color). Color scheme: Cu, blue; Sb, green; O, red; C, gray.

Single-crystal X-ray diffraction studies reveal that the structure of the {Cu2} core features two distorted face-sharing octahedra, each with three Cu−(μ3-O) bonds and three Cu−(μ4-O)−Cu bridging interactions. These Cu−(μ4-O)−Cu angles (79.6°, 78.7°, 79.4°) are found to be smaller than 97°, consistent with the observed and reported ferromagnetism of the clusters (Figure S13, Supporting Information).8 Moreover, the short Cu−Cu distance of 2.69 Å is inferior to the sum of the van der Waals radii of two Cu atoms (∼2.80 Å), implying possible Cu− Cu interactions (Figure 2). Magnetic susceptibilities of 1 and 2 were measured in a temperature range of 2−300 K with field of 1 kOe. Plots of the temperature dependence of χMT vs T for 1 and 2 are shown in Figures 3, S13, and S14. The room temperature χMT value for 1 (3.25 cm3 K mol−1) is higher than the calculated spin-only

Figure 1. A ball-and-stick representation of the {Cu7} cluster in 1. Color scheme: Cu, blue; Sb, green; O, red; C, gray.

internal μ4-oxo groups which also connect with six antimony atoms from the upper and lower {Sb3(μ3-O)} layers. This interstitial μ4-O templates each of the six tetrahedra in the form of {Cu3Sb(μ4-O)} that share two Cu−Cu edges with two neighboring tetrahedra and a vertex with five other tetrahedrals (Figure S1, Supporting Information). The distance between the edge-sharing copper ions ranges from 3.05 to 3.11 Å, whereas the other Cu−Cu distances lie between 3.07 and 3.09 Å. Further, these six {Cu3Sb(μ4-O)} tetrahedra are capped by six tratrate ligands. In the peripheral ligand shell, each of the six tartrate anions ligates ions in the {Cu7} and the {Sb3(μ3-O)} layers via four pendant oxygen atoms, therefore acting as a bridging “staple” ligand. It is intriguing to find that the compact {Cu7} planar aggregate resembles the Anderson−Evans structure type (Figure S2).8 This type is by and large observed in several coordination complexes of high valence metal ions but rare in those of divalent transition metal cations.9 As such, {Cu7} can be viewed as a {Cu6} cage with an encapsulated central Cu(II) ion. The central Cu(II) ion is surrounded by six μ4-O atoms with the Cu−O bond length ranging from 2.02 to 2.23 Å. As shown in Figure S2, the central Cu(II) ion octahedron shares six edges with the other six Cu(II) polyhedrons. Four of the six outer Cu(II) ions are hexacoordinated, each with four “normal” Cu−O bonds (ca. 1.94−2.30 Å) and two long Cu−O bonds (ca. 2.69−2.74 Å). The elongation of the Cu−O axial bonds can be explained by Jahn−Teller distortions, which is typical for copper(II) d9. The other two outer Cu(II) ions are best

Figure 3. χMT (○) vs T plots for 1, with the field dependence of magnetization for 1 (inset). The red line represents the best fit using the spin-Hamiltonian. B

DOI: 10.1021/acs.inorgchem.5b02206 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry value for seven isolated copper(II) centers (2.62 cm3 K mol−1; S = 1/2, g = 2). Upon cooling, the χMT product steadily increases to reach a maximum of 7.80 cm3 K mol−1 at 3 K, indicating dominating ferromagnetic interactions. This is followed by the decrease of χMT to 7.43 cm3 K mol−1 at 2 K, related to antiferromagnetic intercluster interactions. The maximum suggests a ground-state spin (S) value for the complex of S = 7/2 (g = 2). The above data were fitted to the spin Hamiltonian in eq 1 using the program PHI (Figure 3, Supporting Information),11 which gives g = 2.15, J1 = 25.5 cm−1, J2 = 25.8 cm−1, J3 = −0.9 cm−1, J4 = 1.6 cm−1, and J5 = 6.5 cm−1. Five exchange pathways can be seen clearly in the asymmetric unit (Figure S15, Supporting Information). Magnetization curves for 1 and 2 at temperatures below 2 K reveal that the magnetization, M, increases with the external magnetic field, H (Figures 3 and S13 and S14, Supporting Information). For 1, the magnetization increases rapidly and reaches a saturation of 7.1 Nβ at 80 kOe, consistent with the expected value calculated from the Brillouin function (S = 7/2 and g = 2.0).

Foundation of Hunan Province (14JJ3068), and the Fundamental Research Funds for Central Universities.

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H = −2J1(S1S4 + S1 S4 ) − 2J2 (S2S4 + S2 S4 ) ′ ′ − 2J3(S3S4 + S3 S4 ) − 2J4 (S1S2 + S1S2 + S1 S3 ′ ′ + S1 S2 + S1S3 ) − 2J5(S2S3 + S2 S3 ) (1) ′ ′ ′ ′ ′ In summary, we prepared compound 1, which is the first example of a heptacopper(II) cluster capped by two {Sb3(μ3O)} scaffolds and six tartrate ligands from the aqueous medium under mild conditions. Another new cluster (2) comprised a {Sb12} cage, and an encapsulated {Cu2} core was synthesized under subtly different reaction conditions with 1, implying the structural versatility of copper(II)-oxide assemblies constructed from a K2Sb2L2 ligand in aqueous solution. Furthermore, the magnetic properties of 1 (S = 7/2) and 2 were investigated, indicating their ferromagnetic behavior. The work to design and synthesize new K2Sb2L2-based polynuclear copper oxoclusters and the chiral derivatives of 1 and 2 is in progress.

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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.5b02206. Experimental data, additional structure, characterization data, and tables (PDF) Crystallographic information for complex 1 (CIF) Crystallographic information for complex 2 (CIF) Accession Codes

CCDC 1409012 (1) and 1409013 (2).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *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 (21301057), the Natural Science C

DOI: 10.1021/acs.inorgchem.5b02206 Inorg. Chem. XXXX, XXX, XXX−XXX