Planar Cu2(ppz)2 Dimers as SBUs for Diverse Polyoxometalate

Oct 9, 2014 - E-mail: [email protected] (X.-M.Z.). ... framework constructed by Cu2(ppz)2 SBUs and [PMo12O40]6– anions, which also can be view...
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Planar Cu2(ppz)2 Dimers as SBUs for Diverse Polyoxometalate-Based Metal Organic Frameworks Zhi-Kai Qi,† Jun-Liang Liu,‡ Juan-Juan Hou,† Min-Min Liu,† and Xian-Ming Zhang*,† †

School of Chemistry & Material Science, Key Laboratory of Magnetic Molecules and Magnetic Information Material, Ministry of Education, Shanxi Normal University, Linfen 041004, P. R. China ‡ Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry&Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P. R. China S Supporting Information *

ABSTRACT: Five new polyoxometalate-based metal organic frameworks, [Cu4(ppz)4(PO4)]·[CuBr2] (1), [Cu4(ppz)4(MoO 4 ) 2 (H 2 O)]·2H 2 O (2), [Cu(Hppz)(Mo2 O 7 )] (3), [Cu2(ppz)2(Mo3O10)] (4), and [Cu6(ppz)6(PMo 3 V Mo 9 VI O 40 )] (5) (Hppz = 2-(1H-pyrazol-3-yl)pyrazine), have been hydrothermally synthesized and fully characterized, in which a planar Cu2(ppz)2 dimer acts as a secondary building unit (SBU) and provides up to six potential binding sites. In virtue of the strong Jahn−Teller (JT) effect of Cu(II) ions, the potential binding sites in the Cu2(ppz)2 SBU can facilely be occupied by auxiliary oxygen-donor anions and/ or outward pyrazine nitrogen atoms of neighboring SBUs. The auxiliary oxygen-donor anions in compounds 1−5 range from simple phosphate and molybdate, zonal Mo2O72− and Mo3O102− to a spherical three-electron reduced Keggin anion [PMo12O40]6−, respectively. Compound 1 contains an unprecedented 3D (4,6)connected cationic open framework [Cu4(ppz)4(PO4)]+ with 1D channels filled by linear CuBr2− anions, where the Cu2(ppz)2 SBUs and PO4 groups act as nodes and the outward Cu−N bonds and oxo bridges act as linkers. 2 possesses a complicated trinodal (3,4)-connected topological framework, in which μ3-MoO4 and Cu2(ppz)2 groups act as 3- and 4-connected nodes and the outward Cu−N bonds and double μ2-MoO4 groups act as linkers. Differing from 1 and 2, compound 3 shows a 2D organic− inorganic hybrid sheet constructed by a [Mo2O7]2− ribbon of cyclic hexameric edge-shared [MoO6]-octahedra and Cu2(ppz)2 SBUs. Similar to 3, compound 4 also shows 2D organic−inorganic hybrid sheets constructed by zigzag [Mo3O10]2− ribbons and Cu2(ppz)2 SBUs, and adjacent sheets are extended by weak Mo−N bonds into the 3D network. 5 has a 3D 6-connected pcu topological framework constructed by Cu2(ppz)2 SBUs and [PMo12O40]6− anions, which also can be viewed as cationic of the NbO topological [Cu6(ppz)6]6+ network with cavities filled by [PMo12O40]6− anions. Magnetic measurements show that there is strong antiferromagnetic coupling within the Cu2(ppz)2 dimer and only one unpaired electron within the three-electron reduced Keggin anion.

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INTRODUCTION Polyoxometalate-based metal organic frameworks (PMOFs) have aroused the interest of chemists in recent years on account of not only their intriguing architectures and aesthetic topologies but also their promising applications as functional materials in ferroelectric, catalysis, nonlinear optical devices, molecular magnetism, and so on.1−3 The design and selfassembly of well-defined coordination architectures with excellent properties can be based on the rational selection of the well-designed linkages (organic and metallic ligands) and specific metal nodes (transition and rare earth metal ions).4−6 Just as the literature reported, most of the explored and wellknown metal−organic frameworks (MOFs) could be constructed from mono-, di-, tri-, tetra-, penta-, or even higher nuclear metal clusters as secondary building units (SBUs) and single organic ligands or additional coligands as spacers.7,8 It is universally acknowledged that the introduction of polynuclear © 2014 American Chemical Society

clusters is of great significance for the construction of novel cluster-based MOFs.9,10 The symmetric paddle-wheel unit represents one of the most common dinuclear clusters generated by four carboxylates and two metal ions, which possess two square or tetragonal pyramids with a longer metal− metal separation.11,12 The synthesis and construction of the controllable networks including the paddle-wheel SBU can be directed through the linear, triangular, and quadrangular carboxylates and their derivatives.13,14 Meanwhile, dinuclear SBUs involving N-heterocycle ligands, such as pyrazole, pyrazine, imidazole, triazole, tetrazole, benzimidazole, azole or their derivatives, have been reported widely, but they do not commonly show a specific structural model between two metal Received: July 11, 2014 Revised: September 14, 2014 Published: October 9, 2014 5773

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Table 1. Crystal Data and Structural Refinement Parameters of Compounds 1−5

a

compound

1

2

3

4

5

formula FW T/K crystal system space group Z a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 CCDC No. ρcalc/g cm−3 μ/mm−1 F(000) size/mm3 Tmax/Tmin S reflections data/para. R1a, wR2b[I > 2σ(I)] R1a, wR2b(all data) Δρmax/Δρmin (e A−3)

C28H20Br2Cu5N16O4P 1153.09 293(2) tetragonal P4/ncc 4 13.5883(9) 13.5883(9) 17.9216(14) 90 90 90 3309.1(4) 1009120 2.315 5.692 2248 0.15 × 0.13 × 0.10 0.5999/0.4823 1.072 6744/1698 1698/129 0.0608/0.1690 0.0785/0.1827 2.966/−2.198

C28H26Cu4Mo2N16O11 1208.67 293(2) triclinic P1̅ 2 10.4861(5) 13.2319(5) 13.5177(6) 99.295(3) 95.298(4) 93.921(3) 1836.42(13) 1009121 2.161 3.017 1173 0.15 × 0.12 × 0.08 0.7943/0.6603 1.020 13771/7511 7511/557 0.0489/0.0980 0.0759/0.1131 0.861/−0.811

C7H5CuMo2N4O7 512.57 293(2) triclinic P1̅ 2 5.1290(3) 9.7697(8) 12.5421(11) 110.797(8) 93.722(6) 90.436(6) 585.96(8) 1009122 2.905 3.949 488 0.20 × 0.15 × 0.12 0.6487/0.5056 1.043 3862/2396 2396/190 0.0360/0.0797 0.0454/0.0865 2.459/−0.762

C14H10Cu2Mo3N8O10 865.17 293(2) monoclinic C2/c 8 22.6870(18) 14.7113(8) 13.6766(10) 90 113.038(8) 90 4200.6(5) 1009123 2.739 3.811 3320 0.10 × 0.08 × 0.05 0.8323/0.7018 1.008 8998/4291 4291/337 0.0536/0.0795 0.1031/0.0964 1.170/−0.838

C42H30Cu6Mo12N24O40P 3074.39 293(2) hexagonal R3̅ 3 17.7586(5) 17.7586(5) 19.6912(6) 90 90 120 5378.0(3) 1009124 2.848 3.892 4389 0.25 × 0.15 × 0.10 0.6969/0.4429 1.119 3890/2444 2444/195 0.0377/0.0862 0.0423/0.0883 1.349/−1.268

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(F02 − Fc2)2]/∑[w(F02)2]}1/2, where w = 1/[σ2(F02) + (aP)2 + bP], P = (Fo2 + 2Fc2)/3.

centers.15−18 By virtue of a strong stereohindrance effect,19 the N-heterocycle donors possibly coordinate with metal centers to form planar SBUs, which can contain coordinately unsaturated metal centers with open binding sites. To the best of our knowledge, only a few of the tunable MOFs based on planar Nheterocycle dinuclear SBUs have been documented to date in the literature.20,21 On the other hand, metal ions play a fundamental role as electron acceptors in the construction of dinuclear clusters.22,23 Especially, copper has flexible coordination geometries depending upon the electronic configuration of metal centers.24 The cupric ions with a d9 configuration in an octahedral field are subjected to strong Jahn−Teller (JT) distortion at the axial sites, which can promote the generation of distorted “4 + 1” and “4 + 2” coordination geometries.25,26 Thus, Cu(II) ions as a Lewis acid can give unique characteristics, i.e., weak bonds, coordination flexibility, and polarity, to form polynuclear metal clusters with fabulous practical application. More recently, copper cluster-based MOFs have offered enormous potential applications in information storage, sensors, and displays for their promising magnetic performance.27 Luo et al.28 reported an isolated antiferromagnetic tetranuclear cluster Cu4(ppz)6(NO3)2 (Hppz = 2-(1H-pyrazol-3-yl)pyrazine), which consists of two planar Cu2(ppz)2 dimers bridged via an additional two ppz and two NO3− anions. Notably, the planar Cu2(ppz)2 dimer has four coordinately unsaturated sites at Cu(II) centers and two outward nitrogen binding sites, providing up to six binding sites. It is expected that tunable coordination frameworks with planar Cu2(ppz)2 units as SBUs can be created by the rational choice and design of coligands.29 The anionic polyoxometalate with flexible coordination modes can be viewed as versatile auxiliary groups, which can act as a Lewis base to link the cluster-based building units into novel

functional PMOFs.30 Molybdenum has a wide variety of stereochemistries and oxidation states, and molybdates can exist in a MoO42− ion and polymeric chain, ribbon, and layer-like anions in different pH values.31 Moreover, molybdates can form heteropolyanions in the presence of additional high valence elements such as P, Si, and As for famous Keggin-type structures,32 which further enrich its structure chemistry.33 On the basis of the above considerations, the stable antiferromagnetic planar dinuclear Cu2(ppz)2, which has six potential binding sites from unsaturated Cu centers and outward nitrogen atoms, has been chosen as SBUs in combination with phosphate and molybdate coligands to construct coordination frameworks. Herein, we present five novel Cu2(ppz)2 dimer-based 2D and 3D coordination frameworks with polyoxo anions as coligands, namely, [Cu4(ppz)4(PO4)]·[CuBr2] (1), [Cu4(ppz)4(MoO4)2(H2O)]· 2H2O (2), [Cu(Hppz)(Mo2O7)] (3), [Cu2(ppz)2(Mo3O10)] (4), and [Cu6(ppz)6(PMo3VMo9VIO40)] (5). In compounds 1− 5, the potential coordination sites of Cu2(ppz)2 SBUs are bonded by diverse polyoxo groups ranging from tetrahedral PO43− and MoO42−, zonal [Mo2O7]2−, [Mo3O10]2−, and spherical Keggin-type [PMo12O40] ligands.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals were analytically pure from commercial sources and used without further purification. Elemental analyses were performed on a Vario EL-II analyzer. FTIR spectra were recorded from KBr pellets in the range of 4000−400 cm−1 on a PerkinElmer Spectrum BX FT-IR spectrometer. Powder X-ray diffraction (PXRD) data were collected in a Bruker D8 advance diffractometer. The magnetic measurements were studied with a Quantum Design SQUID MPMS XL-5 instrument. The UV−vis diffuse reflection curves were performed on the powder samples in the 5774

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room temperature through a TU-1901 double beam spectrophotometer. X-ray Crystallographic Study. Crystal data of 1−5 were collected using an Xcalibur, Eos, Gemini diffractometer at 293(2) K using Mo Kα radiation (λ = 0.71073 Å). The program SAINT was used for integration of the diffraction profiles. All the structures were solved by direct methods and refined by full-matrix least-squares methods (SHELXL-97). All non-hydrogen atoms were refined anisotropically. All hydrogen atoms of organic ligands were generated theoretically onto the specific carbon and nitrogen atoms and refined isotropically with fixed thermal factors. The correctness of compounds 1−5 has been carefully checked by PLATON. Further details for structural analysis are summarized in Table 1, and the selected bond distances and angles are listed in Table S1 (Supporting Information). Synthesis of [Cu4(ppz)4(PO4)]·[CuBr2] (1). A mixture of Hppz (0.029g, 0.2 mmol), CuBr2 (0.048g, 0.2 mmol), KH2PO4 (0.018g, 0.1 mmol), and H2O (6 mL) was sealed in a 15 mL Teflon-lined stainless autoclave under autogenous pressure, heated to 150 °C and held for 96 h, and cooled to room temperature. After filteration, dark green quadrate block crystals were generated in about 72% yield based on Cu(II) salts. Anal. Calcd for C28H20Br2Cu5N16O4P (%): C, 29.14; H, 1.73; N, 19.43; Found: C, 29.22; H, 1.52; N, 19.59. IR data (KBr, cm−1): 3429s, 1024s, 1408s, 1191s, 1131s, 775s, 983m, 588m, 1606w, 1541w, 843w, 549w. Synthesis of [Cu4(ppz)4(MoO4)2(H2O)]·2H2O (2). A mixture of Hppz (0.029g, 0.2 mmol), CuBr2 (0.048g, 0.2 mmol), (NH4)2MoO4 (35 mg, 0.1 mmol), and H2O (6 mL) was placed in a 15 mL Teflonlined stainless autoclave under autogenous pressure that was heated to 150 °C and held for 96 h. After cooling to room temperature and filteration, green block crystals were obtained in about 10% yield based on Cu(II) salts. Anal. Calcd for C28H26Cu4Mo2N16O11 (%): C, 27.82; H, 2.17; N, 18.54; Found: C, 28.43; H, 2.22; N, 18.82. IR data (KBr, cm−1) 3400s, 3119s, 1635w, 1597w, 1545m, 1406s, 1291w, 1235w, 1180s, 1148s, 1135s, 1043m. Synthesis of [Cu(Hppz)(Mo2O7)] (3). A mixture of Hppz (0.029g, 0.2 mmol), CuBr2 (0.048g, 0.2 mmol), MoO3 (46 mg, 0.3 mmol), and H2O (6 mL) was placed in a 15 mL Teflon-lined stainless autoclave that was heated to 160 °C and held for 96 h. After cooling to room temperature and filteration, dark green block crystals were generated in about 48% yield based on Cu(II) salts. Anal. Calcd for C7H5CuMo2N4O7 (%): C, 16.39; H, 0.98; N, 10.93; Found:C, 16.23; H, 1.12; N, 10.57. IR data (KBr, cm−1) for 3: 3429s, 1632s, 960s, 881s, 605s, 1406m, 1136m, 835m, 733m, 654m, 1076w, 789w, 538w. Synthesis of [Cu2(ppz)2(Mo3O10)] (4). A solution (6 mL water) of Hppz (0.029 mg, 0.2 mmol), CuBr2 (0.048g, 0.2 mmol), and (NH4)6Mo7O24·4H2O (100 mg, 0.08 mmol) was placed in a 15 mL Teflon-lined stainless autoclave and heated to 150 °C and held for 72 h. After cooling to room temperature and filteration, dark green block crystals were gained in about 45% yield based on Cu(II) salts. Anal. Calcd for C14H10Cu2Mo3N8O10 (%): C, 19.39; H, 1.27; N, 12.93; Found: C, 19.22; H, 1.22; N, 13.27. IR data (KBr, cm−1) for 4: 3429s, 1638s, 1411s, 604s, 1129m, 894m, 2368m, 2942m, 2850w, 831w, 533w. Synthesis of [Cu6(ppz)6(Mo12PO40)] (5). A solution (6 mL water) of Hppz (0.029g, 0.2 mmol), CuBr2 (0.048g, 0.2 mmol), and (NH4)3PMo12O40·xH2O (129 mg, 0.07 mmol) was sealed in a 15 mL Teflon-lined stainless autoclave under autogenous pressure, heated to 150 °C and held for 96 h, cooled to room temperature, and filtrated. Dark green block crystals were obtained in about 15% based on Cu(II) salts. Anal. Calcd for C42H30Cu6Mo12N24O40P (%): C, 16.41; H, 0.98; N, 10.93; Found: C, 16.05; H, 1.18; N, 11.37. IR data (KBr, cm−1) for 5: 3429s, 1632s, 952s, 790s, 1405m, 1060m, 2926w, 2855w, 859w, 593w.

their precursors (Scheme S1, Supporting Information). Advantages of this synthesis procedure mainly include the ability to get large crystals of high quality suitable X-ray analysis, being beneficial to obtain compound structural information and explore their optical, electrical, and magnetic properties. However, the introduction of electron-active and crystallization-easy polyoxometalate coligands in the hydrothermal conditions can bring about unusual results for crystal morphology, color, yield, and so on. On the other hand, the rigidity polytopic N-donor for Hppz is made up of both heterocyclic pyrazole (pz) and pyrazine (pr) linked by the C(sp2)−C(sp2) bond, which has the potential to provide four coordination sites in both chelate and bridging modes, as shown in Scheme 1. A large number of experiments Scheme 1. Coordination Modes of ppz and Potential Open Binding Mode of Cu2(ppz)2 SBU

confirmed that the hydrothermal reaction of CuBr2 and Hppz in the presence of additional polyoxometalate groups could generate the planar dinuclear Cu2(ppz)2 SBUs due to stability of the chelate effect of the ppz ligand and formation of a stable planar Cu2N4 six-atom ring. The Cu2(ppz)2 SBUs have six potential open binding sites from unsaturated Cu(II) ions and outward pyrazine nitrogen atoms. The pr-N sites of ppz can grow out via coordination interactions of Cu−N, and the remaining unsaturated sites of Cu centers can be bound to additional oxo-donors such as tetrahedral PO43− and MoO42−, zonal [Mo2O7]2− and [Mo3O10]2− to spherical α-Keggin-type [PMo12O40]6− anions. All Cu(II) centers in 1−5 maintain a square pyramid “4 + 1” coordination mode. Two Cu square pyramids of Cu2(ppz)2 SBUs are cis-arranged in 1, 2, and 4, whereas they are trans-arranged in 3 and 5. Crystal Structural Descriptions. X-ray crystallography reveals that compound 1 crystallizes in the tetragonal centric space group P4/ncc, and the asymmetric unit consists of one independent Cu(II) ion, one deprotonated ppz ligand, and a quarter of a PO43− group and a CuBr2− anion, where Cu(2), Br(1), Br(2), and P(1) atoms are situated in the crystallographic 4-fold axis. As shown in Figure 1a, Cu(1) shows a highly distorted square-pyramidal geometry, completed by four nitrogen atoms from three different ppz groups and one oxygen atom from PO43− ions, in which O(1), N(1a), N(2), and N(3) form the basal quadrangular plane and N4b is located at the 4fold axis with a longer bond. The Cu1−N4b bond length of 2.408(7) Å is longer than other Cu(1)−L (L = O, N) distances from 1.926(5) to 2.058(6) Å, and the cis and trans L−Cu(1)−L angles are in the ranges of 80.1(2)−96.6(2) and 167.0(2)− 174.8(2)σ2, indicating an axially elongated square pyramid due to the strong JT effect. It is worth noting that ppz adopts a tridentate coordination mode to bridge two Cu(II) atoms in N, N′-chelating and monodentate fashion to form a Cu2(ppz)2 SBU, in which there is a stable planar Cu2N4 six-membered ring with a Cu(1)···Cu(1a) separation of 4.0048(3) Å. Especially, the cis-arranged dinuclear coordination geometries, which result from the existence of the crystallographic C2 axis in the middle of the six-membered ring, can be further connected with the

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RESULT AND DISCUSSION Synthesis Chemistry and Binding Modes of Cu2(ppz)2 SBU. Compounds 1−5 were hydrothermally synthesized by treatment of CuBr2 and Hppz in the presence of coligands or 5775

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Figure 1. Coordination environment of Cu1 and the planar Cu2(ppz)2 SBU (a), each PO4 group being connected to four Cu2(ppz)2 SBUs (b), 6connected Cu2(ppz)2 SBU (c), and the 3D porous network with channels filled by linear CuBr2 anions (d) in 1.

for Cu(2) and Cu(3). With regard to Cu(1), one oxygen atom from μ3-MoO4 and three nitrogen atoms from two different ppz groups form the equatorial plane, while the apical position is occupied by a water molecule with a long bond. The O(1w)− Cu(1)−N(3) angle of 107.51(19)° is larger than the expected value of 90° for a regular square pyramid.22 The Cu(2) and Cu(3) sites have a similar coordination geometry, being bonded by three nitrogen atoms and one molybdate oxygen atom in the equatorial plane and one additional nitrogen at the axial position. The Cu(4) ion also has a square-pyramidal geometry, being coordinated by three nitrogen atoms from two ppz and one oxygen atom from molybdate in the equatorial plane and an oxygen at the 4-fold axis. The distance ranges of Cu−L bonds in the basal quadrangular plane for the four Cu(II) atoms are 1.936(5)−2.024(5), 1.971(4)−2.088(5), 1.951(4)−2.058(4), and 1.972(4)−2.052(4) Å, respectively. Nevertheless, the axial longer bonds are Cu(1)−O(1w) of 2.392(4) Å, Cu(2)−N(12b) of 2.290(4) Å, Cu(3)−N(5) of 2.311(5) Å, and Cu(4)−O(2d) of 2.383(4) Å, due to the strong JT effect of Cu (II) ions. The cis and trans angles of L− Cu(1)−L (L = O, N) are in the ranges of 79.80(19)− 107.51(19)° and 155.17(19)−175.95(19)°, indicating that the equatorial planes are perfectly noncoplanar, which is an usual mode in the reported compounds.34

nearest groups via the O−P−O linkers to form one tetranuclear unit (Figure S1, Supporting Information). The μ 4 -PO 4 tetrahedral group as the 4-connected node is linked to four Cu2(ppz)2 SBUs via coordination interactions of Cu and O in the equatorial plane (Figure 1b). Each planar Cu2(ppz)2 SBU as a six-connected node connects with two μ4-PO4 units and four adjacent Cu2(ppz)2 SBUs via the axially elongated bonds (Figure 1c). Thus, the complicated connectivity in 1 can be simplified as an unusual binodal (4,6)-connected net with Schläfli symbol {42.64}{49.66}2 (Scheme S2, Supporting Information). Interestingly, as shown in Figure 1, the overall structure of 1 includes a cationic 3D network featuring 1D channels along the c axis, which are filled by linear CuBr2 anionic groups as the charge compensators (Figure S2, Supporting Information). Therefore, it is conceivable that, if the phosphates in 1 are replaced by other functional polyoxometalates to coordinate with SBUs, a variety of characteristic structures can be created hopefully. Compound 2 crystallizes in the triclinic space group P1̅, and the asymmetric unit has 61 non-hydrogen atoms, as shown in Figure 2a. All atoms localize in general positions. There are four distinct copper ions, four ppz groups, two different MoO4 units, and three water molecules. As shown in Figure 2a, all four unique Cu(II) centers have a JT distorted square-pyramidal geometry, i.e., [CuN3O2] for Cu(1) and Cu(4), and [CuN4O] 5776

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structural extension. The Cu2(ppz)2 SBUs and μ3-MoO42− groups can be viewed as 4-connected and 3-connected nodes, respectively, and the double μ2-MoO4 groups and the Cu−N bonds act as linkers. The overall structure in 2 is a complicated 3D trinodal topological framework with the Schläfli symbol {4.6.8}{4.63.82}{64.82} (Figure 2c,d). By contrast, compound 3 also crystallizes in the triclinic space group P1̅, and the asymmetric unit includes one crystallographically independent Cu(II) ion, one Hppz, and one Mo2O72− ligand. All the non-hydrogen atoms are situated at the general positions. As shown in Figure 3a, the Cu(1) site affords a distorted CuN3O2 square-pyramidal geometry, with O(1), N(1a), N(2), and N(3) atoms coordinated in the equatorial plane and the O(3) atom at the axial position. The axial Cu(1)−O(3b) bond length is 2.489(4) Å, resulting from the strong JT effect.38 The O(3b)−Cu(1)−N bond angles are in the range of 90.032(3)−101.583(3)°, while the trans L− Cu(1)−L (L = O, N) angles in the square subface range from 168.58(19) to 173.0(2)°. The ppz groups merely adopt a tridentate coordination mode (Scheme 1-I). Similarly, there are Cu2(ppz)2 SBUs that show an inversion rather than 2-fold rotation symmetry. Different from 1 and 2, two crystallographically equivalent Cu(II) polyhedra are trans-arranged with a Cu(1)···Cu(1a) separation of 3.94 Å. All Mo(VI) sites in 3 show distorted MoO6 octahedra with Mo−O distances in agreement with known polyoxometalate compounds (Table S1).39 The inorganic Mo2O7 ribbon is constructed by fusion of cyclic hexamers of edge-shared [MoO6] octahedra. The whole 2D layer-like structure of 3 is constructed by linear [Mo2O7]2− ribbons and discrete Cu2(ppz)2 SBUs, as shown in Figure 3c. The adjacent layers in 3 can be further extended into a 3D supramolecular array via hydrogen bonds (Figure S5, Supporting Information). Crystal-structural analysis exhibits that compound 4 crystallizes in the monoclinic space group C2/c with an asymmetric unit containing one Cu2(ppz)2 SBU and one Mo3O102− ribbon. As shown in Figure 4a, both Cu sites are fivecoordinate but show different distortion. Cu(1) shows slightly JT distorted CuN3O2 square-pyramidal geometry with the axial Cu(1)−O(5c) bond of 2.363(6) Å. The range of O(5c)− Cu(1)−N bond angles vary from 90.6(3) to 93.6(3)°, and the O(5c)−Cu(1)−O(1) bond angle is 104.2(2)°. Compared with Cu(1), the Cu(2) site is more distorted and close to triangular bipyramid geometry, with Cu−N distances of 1.929(7)− 2.010(7) Å and Cu−O distances of 2.103(5)−2.116(6) Å. The cis L−Cu(2)−L (O, N) bond angles range from 80.9(3) to 98.4(3)°, while the trans L−Cu(2)−L (O, N) bond angles are in the range of 127.8(3)−176.1(3)°. The Cu(1)···Cu(2) distance in the Cu2(ppz)2 SBU is 3.90 Å. The inorganic Mo3O102− zigzag ribbon consists of corner and edge-shared MoO5 square pyramids. The adjacent Mo3O102− ribbons are linked by Cu2(ppz)2 SBUs into a layered structural motif, as shown in Figure 4c. It should be noted that there are weak coordination interactions between Mo and outward N atoms of Cu2(ppz)2 SBUs with Mo−N distances of 2.534 and 2.658 Å, which results from the trans influence defined as “the tendency of a ligand to selectively weaken the bond trans to itself”.40 After considering this kind of Mo−N weak bonds, Mo(2) and Mo(3) sites show highly distorted octahedral geometry and the adjacent layered are further extended into the overall 3D structure. Compared to 1−4, compound 5 formulated as [Cu6(ppz)6(PMo12O40)] is a bulk dodecanuclear spherical α-Keggin-type

Figure 2. Coordination environments of Cu atoms (a), the μ2- and μ3MoO4 ligands (b), the 3D network structure (c), and (3,4)-connected topological net (d) in 2. Note: yellow balls represent SBUs involving Cu(1) and Cu(2); red balls represent SBUs involving Cu(3) and Cu(4); and green balls represent μ3-MoO4.

The ppz groups in 2 adopt types I and II two different coordination modes, as shown in Scheme 1, which can bridge two adjacent Cu(II) atoms to form cis-arranged Cu2(ppz)2 SBUs with a Cu(1)···Cu(2) distance of 3.9701(2) Å and a Cu(3)···Cu(4) distance of 3.9371(2) Å. Two SBUs can be further linked via an O−Mo−O connector into tetranuclear Cu-[2 + 2] grids (Figures S2 and S3, Supporting Information). Besides, the axial longer Cu−N bonds in Cu(2)N4O and Cu(3)N4O square pyramids can play an important role for 5777

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Figure 3. Coordination environments of metals showing planar structure of SBU (a), the [Mo2O7]2− ribbon of fused cyclic hexamers (b), and 2D sheet structure (c) in 3.

[Cu6(ppz)6]6+ network with large cavities (Figure 5e). Actually, the cavities are filled by three-electron reduced α-Keggin [PMo12O40]6− anions that are composed of 12 MoO6 octahedra and one interstitial PO4 groups with a tetrahedral symmetry. It is worth noting that the P(1) atom is located centrally at the 6fold axis and is surrounded by eight highly distorted oxygen atoms. The Mo−Ob bond lengths are in the range of 1.863(8)− 1.978(8) Å, while the average Mo−Ot bond length is 1.702 Å. In 5, each Cu2(ppz)2 SBU is linked to two Keggin groups and four Cu2(ppz)2 SBUs, while each Keggin group is exclusively connected to six Cu2(ppz)2 SBUs. Thus, considering Cu2(ppz)2 SBUs and Keggin groups as six-connected nodes, the whole network of 5 can be defined as an intriguing pcu topology (412· 63) (Scheme 2; Figure S7, Supporting Information). If Keggin groups are neglected, the topology of the cationic [Cu6(ppz)6]6+ network will be the well-known NbO topology (64·82).

anion contained multilevel-structured material, and the incorporation of Keggin anions into structure dramatically contributes to the complexity and functionality.36 Compound 5 crystallizes in the hexagonal space group R3̅, and the asymmetric unit contains one independent copper, one ppz, and one-sixth of [PMo12O40] groups, where P(1) sits on the crystallographic 6-fold axis and O(7) and O(8) are considered to be severely disordered. As shown in Figure 5a, Cu(1) with a JT distorted square-pyramidal geometry is coordinated to four nitrogen atoms from ppz ligands and one equatorial oxygen atom from the [PMo12O40] unit. The apical N(4) atom related N(4f)−Cu(1)−Lequator (L = O, N) bond angles of 87.9(2)− 101.9(2)° and Cu(1)−N(4f) distance of 2.356(6) Å indicate an apparent distortion of square pyramid. Two Cu square pyramids in the Cu2(ppz)2 SBU are trans-arranged. Remarkably, the Cu2(ppz)2 SBUs can be mutually linked together via axial Cu−N coordination interactions to form a 3D cationic 5778

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Figure 4. Coordination environments of metal ions (a), the [Mo3O10]2− ribbon (b), 2D layered motif (c), and extended 3D structure via weak Mo− N bonds (d) in 4.

with the applied field, and no saturation is observed (Figure S9a, Supporting Information). The magnetization value at the highest field of 50 kOe is 0.0156 Nβ, in agreement with antiferromagnetic coupling. The magnetic susceptibilities have been fitted by a well-known isotropic dinuclear mode for the Cu(II) dimer (effective spin of 1/2) by the following expressions

It is also should be mentioned that there are intra- and intermolecular nonclassic C−H···N and C−H···O hydrogen bonding interactions in 1−5, which play an important role in the stabilization (Figures S4, S6, and Table S2, Supporting Information). Besides, syntheses and structural characterization of compounds 1−5 also enrich and develop the chemsitry of PMOFs.37−39 Magnetic Properties. Variable-temperature magnetic susceptibility measurements for 1, 3, 4, and 5 are performed on crystalline samples at a dc field of 1000 Oe between 1.8 and 300 K on a Quantum Design SQUID MPMS XL-5, and the purity of samples used for measurement has been confirmed by powder X-ray diffraction (Figure S8, Supporting Information). As shown in Figure 6a, the χmT value of 1 at 300 K is 2.03 cm3 K mol−1, which is larger than the expected value of 1.50 cm3 K mol−1 four spin-only Cu(II) ions (assuming g = 2 and S = 1/2) per Cu5 unit. BVS calculations as well as coordination geometry confirm that the linear coordinate Cu(2) atom by two bromides is monovalent with a d10 configuration.35 Upon cooling the temperature, the χmT product monotonously decreases and reaches to the value of 0.014 cm3 K mol−1 at 2 K, characteristic of strong antiferromagnetic (AFM) behavior. The fielddependent magnetization at 2 K increases slowly and linearly

χm =

Nβ 2g 2 ⎡ 6 exp( −2J /kT ) ⎤ ⎥ ⎢ 3k(T − θ ) ⎣ 1 + 3 exp( −2J /kT ) ⎦

χdimer =

2zJ 1 − χm Nβ 2g 2

(1)

(2)

The best fit to the experimental magnetic data gives the following parameters: g = 2.01, J = −87.9 cm−1, zJ = 12.9 cm−1, TIP = 156 × 10−6 cm3 mol−1, IMP = 0.3%. The large J value of −87.9 cm−1 indicates the strong AFM coupling between two Cu ions in Cu2(ppz)2 SBUs. The temperature dependence of χmT for 3 and 4 manifests extremely similar curves, which is characteristic of AFM coupling between two Cu(II) centers in SBUs (Figure 6b,c). The χmT products, 0.396 cm3 K mol−1 per Cu unit for 3 and 5779

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Figure 5. Coordination environments of metal ions (a), the polyhedral view of each Cu2(ppz)2 linked to two Keggin groups (b), and each Keggin linked to six Cu2(ppz)2 dimers (c), and the Keggin-type groups showing dually distorted PO4 group (d), and 3D MOFs viewed along the c axis without (e) and with (f) Keggin groups in 5. 5780

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On the basis of structural analysis, there are six Cu(II) ions and three “blue electrons” from the reduced Keggin [PMo12O40] anion. It is well-known that even “blue electrons” will be paired in the reduced Keggin [PMo12O40] anion, and thus one may expect that there is only one unpaired electron in the three-electron reduced [PMo12O40]6− group. The χmT product for 5 at room temperature is 2.73 cm3 K mol−1, as shown in Figure 6d, which is close to the theoretical value for noncoupled six Cu(II) ions and one unpaired “blue electron”. With the temperature decreasing, the χmT value reduces to 0.386 cm3 K mol−1 at 2 K, which is close to the contribution of one unpaired electron. The M−H curve at 2 K is typical of paramagnetism, and the magnetization value at the highest field of 50 kOe is 0.923 Nβ, in agreement with the expected 1.0 Nβ for one unpaired “blue electron” (Figure S9a, Supporting Information). Thermal Analyses and Optical Band Gap Study. Thermogravimetric analyses for 1−5 in a flowing nitrogen stream and under 1 atm pressure at the heating rate of 10 °C min−1 are performed on polycrystalline samples, as shown in Figure S11 (Supporting Information). The TGA curve of 1 shows that its structure is thermally stable up to 350 °C. The weight decreases with the increase of temperature, and the remnant weight is 35.9% at the measured highest temperature of 1000 °C. For 2, the weight loss of ca. 3.2% in the temperature range of 125−185 °C is in good agreement with the removal of lattice water molecule (calc. 2.9%), and the dehydrated product maintains good thermal stability from 185 to 328 °C. As the temperature rises, the weight loss continues due to the removal of oxygen and ppz groups. The residue of

Scheme 2. Topologically Simplified View of the Structure of 5 Showing 4-Connected Cu2(ppz)2 SBUs Based NbO Net and 6-Connected Cu2(ppz)2 and Keggin Groups Based pcu Net

0.72 cm3 K mol−1 per Cu2 unit for 4 at room temperature, can be obtained, which are close to the expected spin-only values of 0.375 and 0.75 cm3 K mol−1 for S = 1/2 and g = 2, respectively. As the temperature decreases, the χmT product continuously decreases to the values of 0.0058 and 0.0024 cm3 K mol−1 at 2K. Moreover, the magnetization value of 3 and 4 at 2 K increases linearly with the applied field (Figure S9b,c, Supporting Information).

Figure 6. Temperature dependence of χmT per Cu5 unit in 1 (a), Cu unit in 3 (b), Cu2 unit in 4 (c), and Cu6Mo12 unit in 5 (d) measured under an applied field of 1000 Oe. 5781

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53.1% is reached at 1000 °C, which may be the mixture of molybdenum and cupreous oxides (calc. 52.5%). Obviously, 3 and 4 show similar thermal stability up to 350 °C, which may be due to similar solid structures. A continuous weight loss of ca. 44.3% and 44.8% occurs in the temperature range of 350− 600 °C, respectively, indicating the removal of ppz ligands. Interestingly, the TGA curve of 5 exhibits two distinct weight loss steps. The first weight loss step of 37.6% in 343−458 °C corresponds to the elimination of ppz groups (calcd. 38.2%). The second weight loss occurs after 880 °C, and a stable residue up to 1000 °C is not obtained. The solid-state UV−vis spectra of 1, 3, and 4 were investigated at room temperature, and the results are plotted in Figure S12 (Supporting Information). The band gap energies (Eg) were calculated from the formula Eg = 1240/λg (eV), where λg stands for the wavelength in the minima of the second derivatives of the optical absorption curve. It is found that the band gaps for 1, 3, and 4 are 2.25, 2.13, and 2.15 eV, respectively, indicating that they are narrow gap semiconductors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program 2012CB821701), the Ministry of Education of China (No. IRT1156), and NSFC (20925101).

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CONCLUSIONS To sum up, five new polyoxometalate-based coordination polymers based on planar Cu2(ppz)2 SBUs and flexible oxo coligands have been hydrothermally synthesized and fully characterized. Systematic studies have demonstrated that the planar Cu2(ppz)2 SBU is a stable and easily formed species due to N,N′-chelation interaction of the ppz group and the stability of the Cu2(NN)2 six-membered ring. Because of the strong JT effect, Cu(II) atoms from the Cu2(ppz)2 SBU tend to show a square-pyramidal coordination geometry, which means that each Cu2(ppz)2 SBU should have four potential binding sites at Cu centers and two outward pyrazine nitrogen sites to be exploited in the formation of coordination frameworks. By using planar Cu2(ppz)2 dimers as basic SBUs in combination with simple phosphate and molybdate, five polymers have been hydrothermally prepared, namely, zonal Mo 2 O 7 2− and Mo3O102− to the spherical three-electron reduced Keggin anion [PMo 12 O 40 ] 6− , (4,6)-connected open framework [Cu4(ppz)4(PO4)]+ with channels filled by linear CuBr2− anions, the complicated trinodal (3,4)-connected topological framework [Cu4(ppz)4(MoO4)2(H2O)], organic−inorganic hybrid sheets [Cu(Hppz)(Mo 2 O 7 )], and [Cu 2 (ppz) 2 (Mo3O10)] as well as the 6-connected pcu topological framework [Cu6(ppz)6(PMo3VMo9VIO40)]. To be noted, compounds 1, 3, and 4 show semiconducting behavior confirmed by solid UV−vis spectra. Magnetic measurements show that there is strong antiferromagnetic coupling within the Cu2(ppz)2 dimer and only one unpaired electron within the three-electron reduced Keggin anion. This work also enriches and develops the chemistry of PMOFs.

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AUTHOR INFORMATION

REFERENCES

S Supporting Information *

Additional figures for crystallographic data for 1−5 in CIF format, hydrothermal synthesis, crystal shape, IR spectra and magnetic data. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Fax: +86 357 2051402. E-mail: [email protected] (X.-M.Z.). 5782

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