Magnetic Modulation and Cation-Exchange in a Series of Isostructural

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Magnetic Modulation and Cation-Exchange in a Series of Isostructural (4,8)-Connected Metal−Organic Frameworks with Butterfly-like [M4(OH)2(RCO2)8] Building Units Ru-Xin Yao, Xia Xu, and Xian-Ming Zhang* School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, P. R. China S Supporting Information *

ABSTRACT: A series of isostructural metal-carboxylate frameworks [Me2NH2][M2(bptc)(μ3-OH)(H2O)2] (H4bptc = 3,3′,4,4′biphenyltetracarboxylic acid, M = Co, Co0.83Ni0.17, Co0.55Ni0.45, Co0.13Ni0.87, and Ni for 1−5, respectively) have been synthesized, which feature (4,8)-connected scu topological anionic frameworks constructed by a butterfly-like [M4(OH)2(RCO2)8] cluster and have three-dimensional (3D) channels filled by in situ generated [Me2NH2]+ cations. Flame atomic absorption spectroscopy (FAAS), scanning electron microscope energy-disperse X-ray spectroscopy (SEM-EDS), infrared spectroscopy (IR), Raman spectroscopy, and powder X-ray diffraction (PXRD) measurements reveal that 1 can selectively exchange alkali metal cations. Magnetic properties display that monometallic 1 and 5 are characteristic of a antiferromagnet and a ferromagnet, respectively. Incorporation of Co and Ni into the system produced heterometallic compounds 2, 3, and 4. Compound 2 with a very low ratio of Ni to Co shows antiferromagnetic behavior similar to 1, while compounds 3 and 4 with a large ratio of Ni to Co are characteristic of ferrimagnetic-like behavior. This work demonstrates that magnetic tuning from antiferromagnetic, ferrimagnetic-like, to ferromagnetic behaviors can be achieved by heterometallic substitution in isostructural magnetic frameworks. KEYWORDS: magnetic tuning, butterfly-like cluster, anionic framework, cation-exchange

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ligands.7−9 Natarajan and Drillon have reported a [Co4(μ3OH)2(μ2-H2O)2] cluster-based eight-connected bcu network, which shows interesting quasi-2D XY magnetic behavior and slow relaxation.10 The similarity of radius and coordination nature between Co(II) and Ni(II) ions indicates that it is possible to form [M4(μ3-OH)2] cluster-based heterometallic frameworks via metallic substitution. Furthermore, the difference of magnetic anisotropy between Co and Ni is significant, and thus, magnetic modulation in an isostructural Co−Ni doped system can be expected.11 Aromatic polycarbxylates have been widely used in design of neutral MOFs owing to their strong bonding ability as well as anionic nature.12 However, charged open frameworks have recently drawn great interest because of selective ion exchange and large adsorption enthalpy.13 Synthetically, construction of charged metal−carboxylate frameworks may borrow an organic template idea from traditional aluminosilicates.14 By using amines as templates, Gao et al. prepared multifunctional anionic magnetic metal formates and observed significant guestmodulated magnetic ordering temperature.15 We have been

INTRODUCTION Crystal engineering of metal−organic frameworks (MOFs) based on the self-assembly of second building units (SBUs) has been of considerable interest due to their unique architectures, intriguing topologies, and potential multifield applications.1 The current trend is to develop multifunctional materials with predefined and tunable properties.2 As structural and functional carriers, SBUs are mainly responsible for properties of MOFs, i.e., luminescence, magnetism, and catalysis.3 It has been demonstrated that spin carried clusters such as paddlewheel [M2(CO2)4] and trinuclear [M3O(CO2)6] can be used as SBUs for preparation of porous magnets.4 The butterfly-like tetranuclear [M4(μ3-O)2]n+ clusters, consisting of one edgesharing metal dimer “body” and two metal “wings” connected by μ3-O bridges, have attracted great attention as models in magnetic materials. Investigations on butterfly-like clusters reveal that an intracluster often presents competing antiferromagnetic and ferromagnetic interactions,5 and the overall magnetic behavior relies on the nature of the metal as well as the ligand. A series of discrete butterfly-like [M4(μ3-O)2] (M = V, Mn, Fe, and Cu) clusters have been documented, which exhibit interesting magnetic behaviors such as single molecular magnets.6 Recent interest has been transferred to explore the synergistic interaction of butterfly-like clusters via bridged © 2011 American Chemical Society

Received: September 24, 2011 Revised: November 27, 2011 Published: December 13, 2011 303

dx.doi.org/10.1021/cm202876r | Chem. Mater. 2012, 24, 303−310

Chemistry of Materials

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Figure 1. Photos of the crystals of 1−5 showing regular color change from purple, red, gray, greenish, to green. 3.38; N, 2.33. IR data (KBr, cm−1): 3441(b), 3136(b), 2799(w), 2493(w), 1626(s), 1594(vs), 1482(w), 1404(vs), 1098(w), 843(w), 796(w), 691(w). [Me2NH2][M2(bptc)(OH)(H2O)2] (2−4). A procedure similar to 1 was employed to synthesize heterometallic MOFs 2−4 except that the cobalt source was replaced by a mixture of Co(CH3COO)2 and Ni(CH3COO)2 with a ratio 6:1, 4:5, and 1:7, respectively. The red, gray, and greenish block crystals (Figure 1) were collected with ca. 45−60% yield based on H4bptc. The relative molar ratio of Co to Ni in the heterometallic crystals 2−4 determined by FAAS are 4.2:1, 1:1.2, and 1:6.6, respectively, which roughly match the EDS analyzed values (Supporting Information, Figure S3 and Table S1). The formulas of 2−4 were determined by a combination of the above different techniques. Elemental analysis, IR, and PXRD data also support the formulas of 2−4. [Me2NH2][Ni2(bptc)(OH)(H2O)2] (5). Green platelike crystals of 5 in yield of 53.4% (based on H4bptc) were prepared with N(Et)3 as a pH modulator using a similar procedure. Anal. Calcd (%) for C18H19Ni2NO11 (5): C, 39.80; H, 3.50; N, 2.58. Found: C, 40.21; H, 3.40; N, 2.41. IR data (KBr, cm−1): 3416(b), 31112(b), 2472(w), 1629(s), 1567(vs), 1402(s), 1082(w), 982(m), 849(m), 701(w). X-ray Crystallography. X-ray single-crystal diffraction data for 1− 5 were collected on a Bruker Smart APEX CCD diffractometer at 298(2) K using Mo Kα radiation (λ = 0.71073 Å). The program SAINT was used for integration of the diffraction profiles, and the program SADABS was used for absorption correction. All the structures were solved by direct method using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares technique with SHELXL.19 All nonhydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of organic ligand were generated theoretically onto the specific carbon and refined isotropically with fixed thermal factors. Hydrogen atoms of the water molecules could not be determined and are not included in the model. All 1−5 crystallize in the monoclinic space group P21/c, and the asymmetric unit consists of crystallographically independent M(1) and M(2) metal sites. In heterometallic compounds 2−4, we set the compositions of both metal sites according to the SEM-EDS measured ratio of Ni to Co. Further details for structural analysis are summarized in Table 1 and selected bond lengths and angles are shown in Table S3 of the Supporting Information.

pursuing porous magnet materials by using paramagnetic metal hydroxide SBUs and N- or O-containing ligands16 and have presented several porous frameworks with intriguing magnetic behaviors such as solvomagnetic effect, single-chain magnet, and metamagnetism. In the ongoing research of porous magnetic frameworks, multifunctional 3,3′,4,4′-biphenyltetracarboxylic acid (H4bptc) attracts our attention. H4bptc has four carboxylate groups allowing for construction of topologically diverse materials, and its phenyl rings are able to twist around the C−C single bond. Surprisingly, a search of the CSD database revealed that only several 2D brickwall or interpenetrated networks with a bptc ligand have been reported.17 By in situ hydrolysis of solvent DMF (DMF = N,N′dimethylformamide) into dimethylammonium cations,18 we present herein a series of novel anionic open frameworks [Me2NH2][M2(bptc)(μ3-OH)(H2O)2] (M = Co, Co0.83Ni0.17, Co0.55Ni0.45, Co0.13Ni0.87, and Ni for 1−5, respectively), which feature a butterfly-like [M4(OH)2(RCO2)8] cluster based on a (4,8)-connected scu topological anionic network with 3D channels filled by [Me2NH2]+ cations. As Co ions are gradually replaced by Ni ions, magnetic properties of 1−5 transform from antiferromagnet, ferrimagnet-like, to ferromagnet. Moreover, [Me2NH2]+ cations in 1 can be selectively exchanged with alkali-metal cations.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. 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 4000−400 cm−1 on a Perkin-Elmer Spectrum BX FT-IR spectrometer. Raman spectra were collected on a Invia Raman microscope spectrometer with an excitation wavelength of 514.5 nm. Powder Xray diffraction (PXRD) data were collected in a Bruker D8 advance diffractometer. The magnetic measurements were made with Quantum Design SQUID MPMS XL-5 instruments. The diamagnetic correction for each sample was applied using Pascal’s constants. The relative molar ratio of Co to Ni in the mixed-metal crystals 2−4 and the amount of exchanged alkali metal cations were determined on a Thermo S2 by flame atomic absorption spectroscopy analysis (FAAS). Heating and 1:1 (v/v) nitric acid were required for digestion of the crystal samples. Scanning electron microscope Energy-disperse X-ray spectroscopy analysis (SEM-EDS) were measured on a JSM-7500F equipped with an EDAX CDU leap detector. The thermogravimetric analyses (TGA) were carried out in air atmosphere using SETARAM LABSYS equipment at a heating rate of 10 °C/min. Syntheses. [Me2NH2][Co2(bptc)(OH)(H2O)2] (1). A mixture of Co(CH3COO)2·4H2O (0.100 g, 0.40 mmol), H4bptc (0.033 g, 0.20 mmol), DMF (1 mL), and H2O (5 mL) was stirred and adjusted to pH = 5 with a 2 M KOH solution, then sealed in a 15 mL Teflon-lined stainless autoclave at 150 °C for 5 days. After it was cooled to room temperature and subjected to filtration, purple platelike crystals of 1 in yield of 62.3% (based on H4bptc) were recovered. Anal. Calcd (%) for C18H19Co2NO11 (1): C, 39.76; H, 3.50; N, 2.58. Found: C, 40.02; H,

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RESULTS AND DISCUSSION Crystal Structures. Single-crystal X-ray diffraction analyses reveal that complexes 1−5 are isostructural (Figure S1, Supporting Information), and thus, only 1 is discussed herein in detail. The purity of all compounds was confirmed by comparison of experimental PXRD patterns with the simulated pattern derived from the X-ray single crystal data (Figure S2, Supporting Information). Complex 1 crystallizes in the monoclinic space group P21/c. and the asymmetric unit of 1 consists of two crystallographically independent Co(II) ions, one bptc, one μ3-OH group, two coordinated water molecules, and one [Me2NH2]+ cation (Figure 2). The torsion angle of the two phenyls of bptc is 149.9°, in agreement with semirigid nature of bptc. Both Co centers adopt octahedral geometries. Co(1) is coordinated by two μ3-OH oxygen atoms, one water 304



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Table 1. Crystallographic Data and Structure Refinement for 1−5 compd

1

2

3

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalc (g cm−3) μ (mm−1) F(000) crystal size (mm) reflections Rint Tmax /Tmin data/parameters S R1a, wR2b[I > 2σ(I)] R1, wR2 (all data) Δρmax/Δρmin (eÅ−3) compd

C18 H19Co2NO11 543.20 monoclinic P21/c 13.5820(4) 10.8173(3) 14.7037(5) 90 110.6280(10) 90 2021.77(11) 4 1.785 1.705 1104 0.26 × 0.12 × 0.08 11552/4364 0.0167 0.8757/0.6656 4364/289 1.035 0.0310, 0.0848 0.0344, 0.0878 0.745, −0.279

C18H19Co1.66Ni0.34NO11 543.09 monoclinic P21/c 13.5674(5) 10.7942(4) 14.6837(5) 90 110.6380(10) 90 2012.42(13) 4 1.776 1.750 1085 0.23 × 0.18 × 0.09 12240/4384 0.0161 0.8584/0.6891 4384/289 1.075 0.0257, 0.0759 0.0276, 0.0769 0.728, −0.260

C18H19Co1.09Ni0.91NO11 542.96 monoclinic P21/c 13.5146(8) 10.7692(6) 14.6278(8) 90 110.6310(10) 90 1992.4(2) 4 1.793 1.831 1088 0.16 × 0.12 × 0.06 12082/4353 0.0502 0.8981/0.7582 4353/289 0.988 0.0428, 0.0966 0.0665, 0.1096 0.663, −0.409 5

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalc (g cm−3) μ (mm−1) F(000) crystal size (mm) reflections Rint Tmax /Tmin data/parameters S R1a, wR2b[I > 2σ(I)] R1, wR2 (all data) Δρmax/Δρmin (eÅ−3) a

4 C18H19Co0.25Ni1.75NO11 542.78 monoclinic P21/c 13.5186(6) 10.7685(5) 14.6323(7) 90 110.6300(10) 90 1993.51(16) 4 1.792 1.924 1091 0.31 × 0.21 × 0.10 9974/4348 0.0272 0.8309/0.5869 4348/289 1.016 0.0361, 0.0910 0.0459, 0.0965 0.754, −0.316

C18H19Ni2NO11 542.76 monoclinic P21/c 13.5204(7) 10.7639(5) 14.6348(7) 90 110.5320(10) 90 1994.54(17) 4 1.807 1.953 1112 0.25 × 0.19 × 0.08 9602/4354 0.0282 0.8594/0.6410 4354/289 1.032 0.0394, 0.1001 0.0530, 0.1090 0.942, −0.365

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

molecule, and three bis-monodentate carboxylate oxygen atoms from three different bptc groups. Co(2) is ligated by one μ3OH oxygen atom, one water molecule, one monodentate carboxylate oxygen atom (O3d), and three bis-monodentate carboxylate oxygen atoms from three different bptc groups. The Co−Ocarboxylate distances are in the range of 2.0254(14)− 2.1436(15) Ǻ ,and the Co−Owater lengths are 2.2726(19) Å and 2.1900(18) Å. The cis- and trans-O−Co−O angles are in the range of 81.20(6)−102.37(6)° and 166.30(7)−176.59(6)°,

respectively. The edge-sharing Co(1) and Co(1a) dimer is bridged to Co(2) and Co(2a) by μ3-OH, forming a butterflylike tetranuclear cobalt cluster [Co4(μ3-OH)2] in which Co(1) and Co(2) can function as body and wing, respectively (Figure 3a). Three adjacent Co ions within the butterfly-like [Co4(μ3− OH)2] cluster are arranged into isosceles triangles, and the two isosceles triangle are related by an inversion center. The Co···Co distances are 3.5218(4) Å for Cobody−Cobody and 3.3802(5) and 3.1349(4) Å for Cobody−Cowing; the Co−O−Co 305



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[Co5(O2C)8(μ−OH)2] cluster.20 Compound 1 is the first porous scu topological network with butterfly-like tetranuclear clusters as 8-connected nodes. As shown in Figure 3d, 1 possess 3D channels along the a, b, and c axes, which are filled by organic cations [Me2NH2]+ for charge-compensation and space-filling. Calculation with PLATON reveals that the free volume of channels is 448.3 Å3 per unit cell, or 22.5% of the total volume. Cation-Exchange. Because isostructral 1−5 have anionic open frameworks with channels occupied by [Me2NH2]+ cations, 1 was selected to perform ion exchange experiments. A typical procedure of ion exchange is described herein. Freshly prepared crystals of 1 (20 mg) were immersed into saturated MCl (M = Li+, Na+, K+) solution of deionized water (4 mL) and methanol (2 mL). The mixture was then heated in a 10 mL capped vial at 60 °C for 3 days. The exchanged products were filtration and further rinsed several times with water−methanol (v/v = 2:1). Organic cations [Me2NH2]+ in 1 can be selectively exchanged by alkali-metal cations, as confirmed by FAAS, SEMEDS, IR, Raman spectra, and PXRD. The elemental analyses and exchange yield are given in Table S2 of the Supporting Information. Within a single step ion-exchange, about 51, 75, and 36% of organic cations were exchanged by Li+, Na+, and K+, respectively. Complete exchange of [Me2NH2]+ can be achieved by multistep ion-exchange. Interestingly, competitive ion-exchange experiments indicate that 1 has a strong preference for Na+ rather than Li+ or K+. For example, in an ion-exchange experiment with an equimolar amount of alkalimetal cations Li+, Na+, and K+, only Li+ and Na+ ions were exchanged. The exchange yield of Na+ was 4.2 times that of Li+. In another competitive ion-exchange experiment with the initial

Figure 2. View of coordination environment of cobalt atoms and organic [Me2NH2]+ cation in 1.

angles are 98.27(6), 112.57(6)°, and 116.92(7)°. The bptc with three syn−syn bidentate and one monodentate carboxylate groups is coordinated to seven cobalt ions. Each bptc ligand is linked to four butterfly-like [Co4(OH)2] clusters, while each cluster is connected to eight bptc groups (Figure 3b,c). The [Co4(OH)2] cluster serves as an 8-connected node, and the slightly twisted biphenyl group of bptc acts as a 4-connected node. Overall, the structure is a rare 3D (4,8)-connected scu topological anionic open framework with a short Schläf li symbol of {(44·62)2(416·612)} (Figure 3e,f). Only several networks with (4,8)-connected scu topology have been documented in which 8-connected nodes are octacarboxylate, trinuclear [M3(O2C)8(H2O)4] cluster, and pentanuclear

Figure 3. View of tetranuclear [Co4(OH)(RCO2)8] cluster SBUs (a), each bptc linked by four Co4 clusters (b), each Co4 cluster linked by eight bptc groups (c), 3D open anionic framework [Co2(bptc)(OH)(H2O)2]− along the a axis showing channels filled with organic [Me2NH2]+ cations (d), and the 3D (4,8)-connected scu net along the a axis (e) and along the b axis (f) in 1. For clarity, organic [Me2NH2]+ cations have been omitted in panels e and f. 306



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Figure 4. Magnetic susceptibility data for 1−5 per M4 unit presented as plots of χmT versus T under an applied dc 1000 Oe field (a) and field dependence of magnetization for 1−5 at 2 K (b).

Figure 5. FC and ZFC magnetization at 10 Oe for 3−5.

10-fold excess of K+ over Li+ and Na+, the exchange yield ratio of Li+, Na+, to K+ in the final products is 2.8:6.1:1. SEM examinations indicate that crystals of the pristine compound 1 possess a relatively clean surface. After the ionexchange step with alkali cations, surface roughness appears, but crystals retain their original shape as shown in Figure S4 of the Supporting Information. PXRD patterns of cationexchanged 1-M+ are almost identical with that of 1 (Figure S5, Supporting Information), indicating that anionic framework of 1 is retained. EDS analyses (Figure S6, Supporting Information) clearly exhibit signals of alkali metal ions appearing in cation-exchanged products. IR spectra (Figure S7, Supporting Information) display that the intensity of N−H stretching vibration peaks of [Me2NH2]+ at 3136 cm−1 in 1 is weakened or disappeared after ion-exchanged. Raman spectroscopy (Figure S8, Supporting Information) is used to gain further insight into the ion-exchange of 1. The bands at about 841 cm−1 and 1085 cm−1 were associated with the symmetric and asymmetric stretching NC2 vibration of [Me2NH2]+. After

ion-exchange, the intensity of these bands became weakened, suggesting that partial [Me2NH2]+ cations are exchanged. In addition, the symmetry of the bptc ligand is retained after ionexchange, indicated by remarkable bands at 1603 cm−1 and 1295 cm−1 assigned to aromatic-ring stretching vibration21 and the in-plane symmetric C−H bending vibration of the bptc group, respectively. All these experimental data obviously demonstrate that alkali metal cations have entered into the channels via exchange interaction. Magnetic Studies. Magnetic susceptibility was measured on crystalline samples of 1−5 in the temperature range 300−2 K at a field of 1000 Oe. For 1, the χmT value per Co4 unit is 10.12 cm3 K mol−1 at room temperature, which is higher than the expected value of 7.5 cm3 K mol−1 for four isolated highspin Co(II) ions with S = 3/2 and g = 2.00, owing to the significant orbital contribution of Co(II) in an octahedral environment. Upon cooling, the χmT value decreases gradually to reach a minimum of 1.92 cm3 K mol−1 at 2 K. Such decrease is a characteristic of dominant antiferromagnetic (AF) exchange 307



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Information), which are in agreement with antiferromagetic coupling within the Co4 cluster and ferromagnetic exchange interaction within the Ni4 cluster. In order to investigate the influence of heterometallic substitution on magnetic behavior, Co−Ni mixed complexes 2−4 were synthesized. The color of crystals 1−5 varies regularly with purple, red, gray, greenish, and green, which is consistent with the different ratios of Co to Ni. As can be seen from Figure 4, magnetic susceptibility and field dependence of magnetization also show a regular change in 1−5. Similar to 1, red 2 with a large ratio of Co to Ni is also characteristic of antiferromagnet. For gray 3 with an approximately equivalent ratio of Co to Ni, the χmT value 8.22 cm3 K mol−1 at 300 K is higher than the spin-only value 5.75 cm3 K mol−1 for two Co(II) (S = 3/2 and g = 2) and two Ni(II) (S = 1 and g = 2.00) due to the orbital contribution of octahedral Co(II). As the temperature decreases, the value of χmT reduces smoothly to a minimum value of 6.13 cm3 K mol−1 at about 50 K, then rises abruptly to a maximum of 9.61 cm3 K mol−1 at 14 K, and finally drops rapidly to 3.72 cm3 K mol−1 at 2 K (Figure 4a). The minimum at 50 K indicates a ferrimagnetic-like behavior, which is supported by the fact that maximum χmT of low temperature is higher than that of room temperature.23Although the ratio of Co to Ni in 3 is close to one, there are a large number of possible cases involving arrangements of Co and Ni atoms. The following are several extreme cases: (1) each butterfly-like M4(OH)2 cluster contains a single type of metal atoms, and Co4(OH)2 and Ni4(OH)2 clusters are closely equal; (2) there are two Co and two Ni atoms within a M4(OH)2 cluster; (3) CoNi3 and Co3Ni clusters are closely equal; (4) Co and Ni are statistically occupied at M sites. The combination of the above four cases are also possible. The magnetization value of 5.95 Nβ at 5 T is larger than the arithmetical average of 1 and 5 but smaller than the expected ferromagnetic value of 10 Nβ for the Co2Ni2 unit (assuming SCo = 3/2, SNi =1, and gCo = gNi = 2). This eliminates the case of each M4(OH)2 cluster containing only a single type of metal atoms and indicates that magnetic exchange interactions within Ni−Co mixed crystals are different from homometallic 1 and 5. Similar to 3, greenish 4 also shows ferrimagnetic-like behavior indicated by the χmT versus T plot. The saturation magnetization value of 6.74 Nβ for 4 at 5 T is higher than that of 3 but lower than that of 5. FC magnetization of 4 is fielddependent (Figure S12a, Supporting Information), which is compatible with the ferrimagnetism.23 Furthermore, significant divergence between ZFC and FC was observed at 17.1 and 18 K for 3 and 4, respectively (Figure 5). A hysteresis loop was also observed at 2 K with a coercive field of 262 and 64 Oe, and a remnant magnetization of 0.14 and 0.13 Nβ for 3 and 4, respectively (Figure S13, Supporting Information). Thermal Properties. TGA of 1−5 in air atmosphere at a heating rate of 10 °C min−1 were performed on polycrystalline samples. As shown in Figure S14 of the Supporting Information, isostructural 1−5 have similar thermal stabilities, and thus, only 1 is discussed herein. For 1, the initial weight loss of about 7.3% (calcd 6.6%) in the temperature range of 70−255 °C corresponds to the removal of coordination water molecules. The second weight loss of 61.9% in the temperature range of 260−860 °C is in agreement with the removal of bptc groups and [Me 2 NH 2 ] + cations, accompanied by the decomposition of the framework. The final residues of 30.2% (calcd 29.6%) for 1 and 28.1% (calcd 27.5%) for 5 are close to

interactions as well as the effect of spin−orbit coupling in Co(II) ions. The χmT−T curve of 1 is similar to that of the helical double-layered compound [Co2(OH)(3,4-PBC)3].8b Between 300 and 7 K, the susceptibility obeys the Curie− Weiss law with a Curie constant of C = 9.749(32) cm3 K mol−1 and Weiss constant of θ = −48.51(52) K (Figure S9a, Supporting Information). The C value corresponds to g = 2.28, which is consistent with the result of the following Heisenberg model fitting (Figure S10a, Supporting Information). The large negative value of the Weiss constant indicates a dominating antiferromagnetic coupling within the butterfly-like tetranuclear cluster. Owing to the depopulating of higher Kramers doublets caused by the splitting of the 4T1 g ground triplet under the combined action of spin−orbit coupling and noncubic crystalfield term,17b,22 single-ion anisotropy of Co(II) ions in an octahedral field also contributes to a negative Weiss constant. No magnetic hysteresis loop was observed. The magnetization of 1 at 2 K increases slowly and linearly with the applied field, which is characteristic of antiferromagnet. The field-dependent magnetization at the highest field 5 T is 1.87 Nβ (Figure 4b), far below the saturation value of 12 Nβ expected for four spinonly Co(II) species. For 5, the χmT value at 300 K per Ni4 unit is 4.15 cm3 K mol−1, which is close to the value expected, 4.0 cm3 K mol−1, for four spin-only Ni(II) ions with S = 1 and g = 2.00. Upon cooling, the χmT value increases to a maximum of 7.47 cm3 K mol−1 at 11.4 K and then finally goes down to a value of 5.21 cm3 K mol−1 at 2 K, indicating dominant ferromagnetic coupling within the Ni4 cluster at high temperature. The magnetic data in the range of 20−300 K obey the Curie−Weiss law with C = 4.031(8) cm3 K mol−1 and θ = 5.70(26) K (Figure S9b, Supporting Information). Compared with 1, magnetizations of 5 at 2 K rapidly increase and reach a saturation value of 7.67 Nβ at 5 T, which is close to the theoretical value of 8 Nβ for a ferromagnetic Ni4 unit. Field-cooled (FC) and zerofield-cooled (ZFC) magnetizations were performed under an applied field of 10 Oe, and the bifurcation at 15.6 K indicates the onset of long-range magnetic ordering (Figure 5). A small hysteresis loop is observed at 2 K with the remnant magnetization of 0.437 Nβ and the coercive field of 265 Oe for 5 (Figure S11, Supporting Information). The plots of χmT− T show that magnetization is field-dependent at low temperatures (Figure S12b, Supporting Information). Because the Ni(II) ion does not show strong single-ion anisotropy, spontaneous magnetization and magnetic hysteresis in 5 indicated that there exist weak ferromagnetic exchange interactions between adjacent Ni4 clusters. Neglecting single-ion anisotropy or spin−orbit coupling of metal ions, simulation of the temperature dependence of χmT values for 1 and 5 have been attempted with a simplified Heisenberg model. In this model, isotropic intracluster magnetic interactions and two coupling constants are adopted: Jb stands for inner exchange interaction of butterfly’s body− body, while Jw is the outer exchange interaction of wing−body (Scheme S1, Supporting Information).

Ĥ ex = − 2Jw (S1S2 + S2S3 + S3S4 + S4S1) − 2JbS1S3 =2Jw S13S24 − 2JbS1S3 The best nonlinear least-squares fittings of the theoretical and experimental data gave Jb = −3.80(1) cm−1, Jw = −2.15(5) cm−1, and g = 2.28(1) for 1, and Jb = 1.72(2) cm−1, Jw = 1.74(5) cm−1, and g = 1.98(9) for 5 (Figure S10, Supporting 308



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the percentage of Co3O4 and NiO, respectively. For 2−4, the final residuals are a mixture of Co3O4 and NiO.

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CONCLUSIONS A series of isostructural 3D anionic MOFs [Me2NH2][M2(bptc)(μ3-OH)(H2O)2] with (4,8)-connected scu topology are constructed by a tetranuclear butterfly-like [M4(OH)2] cluster and organic linker bptc. Interestingly, [Me2NH2]+ cations that filled in channels in 1 can be selectively exchanged by alkali metal cations, which is confirmed by FAAS, SEM-EDS, IR, Raman spectra, and PXRD. When the Co ions are gradually replaced by Ni ions from 1 to 5, the magnetic behaviors change from antiferromagnet, ferrimagnet-like, to ferromagnet. This is attributable to different magnetic anisotropy of Co and Ni ions as well as competing antiferromagnetic and ferromagnetic interactions within the butterfly-like tetranuclear cluster. This work may open a new avenue for preparation of anionic metalcarboxylate frameworks with adjustable magnetic behaviors.

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ASSOCIATED CONTENT

S Supporting Information *

IR spectra, Raman spectra, PXRD patterns, TGA curves, EDS, FAAS results, and table of selected bonds for 1−5. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 357 2051402. E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program 2012CB821701) IRT1156 and National Science Fund for Distinguished Young Scholars (20925101).

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Magnetic Modulation and Cation-Exchange in a Series of Isostructural

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