Porous Coordination Polymers Based on {Mn6} Single-Molecule

Jun 3, 2016 - Two-dimensional robust porous coordination polymers based on {Mn6} single magnetic magnets show SMM behavior and selectivity for CO2 ove...
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Porous Coordination Polymers Based on {Mn6} Single-Molecule Magnets Xiang Jiang,† Cai-Ming Liu,‡ and Hui-Zhong Kou*,† †

Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China Beijing National Laboratory for Molecular Sciences, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

‡

S Supporting Information *

ABSTRACT: In this paper, three isostructural porous coordination polymers, namely, [Mn6(μ3-O)2(sao)6(DMF)4(L1)2/3]· 4DMF·2H2O·2CH3OH (1), [Mn6(μ3-O)2(sao)6(DMF)4(L2)2/3]·4DMF·2H2O·2CH3OH (2), and [Mn6(μ3-O)2(sao)6(DMF)4(L3)2/3]·4DMF·4H2O·2CH3OH (3) (DMF = dimethylformamide, H2sao = salicylaldoxime, H3L1 = benzene-1,3,5trisbenzoic acid, H3L2 = 4,4′,4″-s-triazine-2,4,6-triyltribenzoic acid, and H3L3 = 2,4,6-tris(4-carboxyphenoxy)-1,3,5-s-triazine), based on the oximato-bridged {Mn6} single-molecule magnet (SMM) and tricarboxylic acid ligands, were designed and synthesized. X-ray structural analysis shows that they possess a two-dimensional layered structure, where the {Mn6} moieties are linked by the corresponding (Lx)3− carboxylate ligands (x = 1, 2, 3) forming a huge honeycomb layer. These compounds not only show the SMM behavior as confirmed by alternative current susceptibility measurements but also show selectivity for CO2 over N2 at 273 K. On the basis of the magnetic fitting to the magnetic susceptibilities and the field dependence of magnetization for complexes 1−3, the spin ground states are S = 4. Compared with isolated {Mn6} SMMs with S = 4, the out-of-phase susceptibilities of 1−3 show obvious peaks only under the external direct-current field of 2 kOe. However, no peaks in χm″ are observed in the partially desolvated sample of compound 1.

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INTRODUCTION Isolated compounds exhibiting single-molecule magnet (SMM) or single-ion magnet (SIM) behavior have received wide attention due not only to their significance in fundamental physics but also to their potential applications in information storage and devices for future quantum computing.1−5 Considering that isolated SMMs/SIMs may serve as building blocks to form coordination polymers, SMMs/SIMs-based coordination polymers have become a new research focus in the field of moleculebased magnets. Initially, most efforts focused on new magnetic materials based on SMMs. For example, by using RCOO−, N3−, or cyano linker, compounds exhibited single-chain magnet (SCM) behavior; ferromagnetic and anti-ferromagnetic ordering were isolated.6−10 Over the past few years, a new strategy assembling SMMs/ SIMs into coordination polymers by steric ligands emerged.11−24 Since the magnetic exchange between SMM/SIM moieties is not effective in the polymers, the real isolation of the SMM/SIM nodes can be achieved in coordination polymers.24 Meanwhile, the dynamic magnetic property would be tuned via the selection of the steric ligands. It is noteworthy that porous coordination polymers (PCPs) or metal−organic frameworks (MOFs) might © XXXX American Chemical Society

further modulate the SMM behavior of nodes via adjusting coordinated guests or isolated guests in the porous structure.25 For example, by means of the porous nature of PCPs/MOFs, a series of cationic MOFs based on Dy3+-SIMs with anionexchange capabilities were reported.12 Very recently, some guestinduced different SMM-PCPs were designed, and the effective energy barrier of such MOFs can be switched in a single-crystalto-single-crystal manner.13,15 Related cases are still rare, and available SMM-PCPs mostly contain Co(II) or Dy(III) center.25 However, manganese-based SMMs play a dominant role in the history of SMM area and have been the most thoroughly studied. From this view, the construction of SMM-PCPs based on manganese seems more important, and the comparison of SMMPCP with isolated SMM is more meaningful. Besides, realizing that those known classical SMMs always contain carboxylate groups, it is possible to form SMM-PCPs by the replacement of the carboxylate groups by complicated multicarboxylic ligands. Among the manganese-based SMMs, {Mn6} species play an important role in the history of SMM.26 Until now, no Received: January 23, 2016

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DOI: 10.1021/acs.inorgchem.6b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

diffraction (XRD) measurements were recorded on a Bruker D8 ADVANCE X-ray diffractmeter. Thermogravimetric analyses (TGA) were measured on Boyuan DTU-2A equipment with a heating rate of 10 °C/min. Magnetic susceptibility measurements were measured on a Quantum Design SQUID magnetometer. The experimental susceptibilities were corrected according to the Pascal table. Gas adsorptions were measured with ASAP-2020 M equipment. Before test, the desolvated samples 1d−3d were prepared under a vacuum at 423 K for 10 h. Ligands H3L1, H3L2, and H3L3 were prepared according to the literature methods.31−33 Compounds 1−3 were prepared as follows. In a single tube, a DMF solution (10 mL) of H3L1−3 (0.10 mmol) was layered with a {Mn6} solution preformed by mixing Mn(ClO4)2·6H2O (0.3 mmol), H2sao (0.3 mmol), and triethylamine (0.6 mmol) in CH3OH (10 mL) at room temperature. After approximately two weeks of slow diffusion, black crystals were obtained between two layers. Yield: 55% (1), 40% (2), and 35% (3) based on H3L1−3. Anal. Calcd for 1 (C86H108Mn6N14O30): C, 48.10; H, 5.07; N, 9.13%, found: C, 48.88; H, 5.03; N, 9.31%. For 2 (C84H106Mn6N16O30): C, 46.94; H, 4.97; N, 10.43%, found: C, 47.30; H, 4.93; N, 10.68%. For 3 (C84H110Mn6N16O34): C, 45.50; H, 5.00; N, 10.11%, found: C, 45.92; H, 4.57; N, 10.12%. IR, (1): 2930(w), 1649(s), 1590(m), 1540(m), 1471(m), 1439(m), 1390(s), 1284(s), 1199(s), 1030(s), 916(s), 756(s), 684(s), 456(s). (2): 2934(w), 1641(s), 1590(s), 1538(s), 11504(s), 1470(s), 1440(s), 1389(s), 1291(m), 1201(m), 1107(s), 1022(s), 915(s), 821(s), 753(s), 676(m), 651(s). (3): 3416(s), 2925(w), 1653(s), 1590(s), 1435(s), 1362(s), 1294(s), 1208(s), 1030(s), 925(s), 752(s), 679(s). Activated samples were prepared under an evacuated container at 423 K for 24 h. Final formulas of compounds 1d−3d were derived from elemental analyses data, namely, [Mn6(μ3O)2(sao)6(DMF)2(H2O)2(L1)2/3] (1d+H2O), [Mn6(μ3-O)2(sao)6(DMF)2(H2O)2(L2)2/3] (2d+H 2O), [Mn6(μ3-O)2(sao)6(DMF)2(H2O)2(L3)2/3] (3d+H2O). Anal. Calcd for 1d+H2O (C66H58Mn6N8O22): C, 48.38; H, 3.28; N, 6.11%, found: C, 48.19; H, 3.55; N, 6.81%. For 2d+H2O (C64H56Mn6N10O22): C, 46.68; H, 3.43; N, 8.51%, found: C, 46.803; H, 3.09; N, 7.75%. For 3d+H2O (C64H56Mn6N10O24): C, 45.79; H, 3.36; N, 8.34%, found: C, 45.46; H, 3.09; N, 7.63%. The additional water molecules is ascribed to the exposure of activated sample in the air. Single-crystal XRD data for compounds 1−3 were collected on a Rigaku R-AXIS RAPID IP diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) at 293(2) K. The structures were solved by direct methods using the SHELXS-2014 program and refined with full-matrix least-squares on F2 using the SHELXL-2014 program.34 All the non-hydrogen atoms were refined anisotropically, and hydrogen atoms were added geometrically on parent atoms. Coordinated DMF molecules are disordered, and therefore they are located and refined with restraints (DFIX, SADI, ISOR, and SIMU) to obtain satisfactory results. After the designation of all non-hydrogen atoms, there are almost no obvious residual peaks left. The discrete solvents in voids were determined on the basis of the TGA and elemental analyses results.

{Mn6}-based PCPs have been reported. Recently, Brechin et al. reported a two-dimensional (2D) framework built from {Mn6} salicylaldoxime SMM and 1,3,5-benzene-tricarboxylic acid ligand.27 Since the 1,3,5-benzene-tricarboxylic acid ligand is not large enough, such 2D coordination polymer did not show any porous channels after the stacking of layers. It is widely recognized that extending the organic ligand may induce a porous structure.28−30 In this vein, by using longer organic linkers, three 2D robust PCPs based on {Mn6} salicylaldoxime SMMs, namely, [Mn6(μ3-O)2(sao)6(DMF)4(L1)2/3]·4DMF· 2H2 O·2CH 3 OH (1), [Mn6 (μ3-O) 2(sao)6(DMF) 4 (L 2) 2/3 ]· 4DMF·2H 2 O·2CH 3 OH (2), and [Mn 6 (μ 3 -O) 2 (sao) 6 (DMF)4(L3)2/3]·4DMF·4H2O·2CH3OH (3) (DMF = dimethylformamide, H2sao = salicylaldoxime, H3L1 = benzene-1,3,5trisbenzoic acid, H3L2 = 4,4′,4″-s-triazine-2,4,6-triyltribenzoic acid, and H3L3 = 2,4,6-tris(4-carboxyphenoxy)-1,3,5-s-triazine) were synthesized and characterized (Scheme 1). These Scheme 1. Molecular Structure of H3L1−3

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RESULTS AND DISCUSSION Crystallographic parameters for 1−3 are summarized in Table 1. Compounds 1−3 are isomorphous and consist of pre-designed {Mn6}2+ fragment and triple carboxylic ligands (Figure 1a and Figures S1−S3, Supporting Information). The bond valence sum (BVS) calculation demonstrates that all Mn atoms are trivalent (Table S1). Each {Mn6}2+ unit consists of two [Mn3(μ3-O)(sao)3]+ triangles linked by two oximato oxygen atoms. The deprotonated ligand (L1−3)3− binds three {Mn6}2+ fragments to form a neat 2D 63 network that possesses honeycomb cavities of ca. 43−50 Å size (Figure 1b). Although the single layer in those compounds contains large diameter of honeycomb-like cavity, interpenetration is not observed. This phenomenon comes from two reasons. On one hand, the neat layer structure in these compounds is flat and shows almost no undulation, which forbids the interpenetration.35 On the other

compounds not only show the SMM behavior but also exhibit selectivity for CO2 over N2 at 273 K. Interestingly, the dynamic magnetic properties of 1−3 are different from that of isolated {Mn6} SMMs with S = 4. Furthermore, the magnetic property of partially desolvated compound 1 (1d) was investigated.

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EXPERIMENTAL SECTION

FT-IR spectra were recorded on a RAYLEIGH WQF-510A infrared spectrometer (KBr pellets). Microelemental analyses (C, H, N) were measured on Vario ELIII elemental analyzer. The powder X-ray B

DOI: 10.1021/acs.inorgchem.6b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Parameters of Compounds 1−3 chemical formula formula weight crystal system space group a (Å) b (Å) c (Å) volume (Å3) Z, Dc (Mg·m−3) absorption coefficient (mm−1) F(000) θ min, max (deg) reflections collected independent reflections data/restraints/parameters GOFa final R indices [I > 2σ(I)] R indices (all data) a

1

2

3

C43H54Mn3N7O15 1073.75 trigonal R3̅ 39.510(6) 39.510(6) 15.795(3) 21 352(7) 18, 1.503 0.861 10 008 3.022−26.496 60 992 9828 9828/127/500 1.033 R1 = 0.0950, wR2 = 0.2647 R1 = 0.2039, wR2 = 0.3214

C42H53Mn3N8O15 1074.74 trigonal R3̅ 39.196(6) 39.196(6) 15.796(3) 21 017(7) 18, 1.528 0.875 10 008 3.029−27.474 63 976 10 667 10 667/97/475 1.072 R1 = 0.0821, wR2 = 0.2568 R1 = 0.1318, wR2 = 0.2993

C42H55Mn3N8O17 1108.76 trigonal R3̅ 39.356(6) 39.356(6) 15.783(3) 21 171(7) 18, 1.565 0.875 10 332 3.027−24.991 84 626 8284 8284/196/502 1.000 R1 = 0.1043, wR2 = 0.3003 R1 = 0.2071, wR2 = 0.3516

GOF = [∑w(Fo2 − Fc2)2/(nobc − nparam)]1/2; R1 = ∥F0| − |Fc∥/∑|F0|, wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

Figure 1. (a). Molecular structure of compound 1, symmetry codes A: 1 − x, −y, 2 − z; B: 1 − y, x − y, z; C: 1 − x + y, 1 − x, z. (b) 2D 63 network; all hydrogen atoms and discrete solvents were omitted. (c) Accessible surfaces; the blue color represents the porous surface.

coordination polymers based on {Mn6} SMMs have been reported,24 compounds 1−3 are the first examples with a porous structure. Stability and Gas Adsorption. Activated solids of 1−3 (1d−3d) were obtained by heating the samples under vacuum at 150 °C. In 1−3, the Mn(3) atoms exhibit octahedral coordination geometry with two DMF molecules at axial positions.

hand, steric {Mn6} building block hampers the interpenetration of layers. Because of the ···ABCABC··· packing mode, the larger honeycomb cavity was overlapped by equivalent sheets, giving rise to a microporous channel (Figure 1c). The channel is occupied by coordinated molecules and discrete solvents (Figure S4, Supporting Information). Finally, the potential porosity is ∼27.3% (for 1), 27.0% (for 2), and 29.5% (for 3).36 Although several C

DOI: 10.1021/acs.inorgchem.6b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry The TGA data indicate that compounds 1−3 roughly lost approximately one coordinated DMF molecule on the Mn(3) atom and all discrete guests to form 1d−3d, resulting in a tetragonal pyramid geometry on Mn(3). This phenomenon can be explained by the coordination behavior of Mn3+ that favors six- and five-coordination geometry. Hence, after the loss of ca. one coordinated DMF, the framework is still stable. The square planner geometry of Mn3+ has not been observed until now. However, the microelemental analyses result shows that the desolvated samples tend to absorb water yielding [Mn6(μ3O)2(sao)6(DMF)2(H2O)2(Lx)2/3] when exposed to the air. This indicates that the five-coordination of Mn(3) is not so stable as six-coordination. The XRD patterns of 1d−3d remain unchanged when the samples were heated at 423 K, indicating a stable framework (Figures S11−S13, Supporting Information). The stability of 1−3 is due to the π···π stacking interactions between adjacent layers. The center-to-center distance of adjacent benzene−benzene or triazine−triazine planes is 3.63, 3.57, and 3.36 Å for 1−3, respectively (Figure S14, Supporting Information).37−39 The porosity of 1d−3d was confirmed by N2 adsorption at 77 K. From the adsorption isotherms, the BET for 1d−3d is 60.9, 77.5, and 122.7 m2·g−1 for 1−3, respectively (Figure S15, Supporting Information). Although the BET surface areas are small, partial coordinatively unsaturated metal centers and narrow pore size spur us to assess their CO2 adsorption performance. The CO2 isotherms (Figure 2) show that the CO2 uptakes at 1 atm are

Figure 3. Variable-temperature magnetic susceptibilities of 1−3. Solid lines represent the fitting results.

above 50 K were fitted with the Curie−Weiss law, giving a Curie constant of 19.07, 19.44, 18.69, and 18.01 cm3 K mol−1 and a Weiss temperature of −22.30, −41.88, −34.13, and −23.97 K respectively, which supports the presence of strong antiferromagnetic nature within the {Mn6} clusters (Figure S23, Supporting Information). As the temperature decreases, the χmT for 1−3 decreases regularly and reaches a minimum of 9.09, 8.15, and 8.62 cm3 K mol−1, respectively, at 18 K. Afterward, the χmT increases and attains to a maximum of 10.66, 9.03, and 9.49 cm3 K mol−1, respectively, at 6 K before decreasing again. Similar magnetic behavior has been frequently observed in oximato-bridged {Mn6} compounds that possess a spin ground state of S = 4.27,43−50 Different from compound 1, the χmT of 1d decreases in the whole temperature. This discrepancy is most probably due to the difference in interlayer magnetic interaction. For compound 1, interlayer magnetic interaction is weak. However, it becomes stronger upon desolvation. The direct-current (dc) magnetic susceptibilities of 1−3 and 1d can be fitted by using MagPack software51 based on the isotropic exchange spin Hamiltonian for {Mn6} cluster (Figure S24, Supporting Information). The fitting results correspond to a spin ground state of S = 4 (Table 2). Furthermore, the signs of J1 and J2 are consistent with the empirical rule that J is negative with the Mn−N−O−Mn torsion angle below 31° valid for {Mn6}.45 The field dependence of magnetization (Figure S25, Supporting Information) shows that the magnetization value for 1−3 and 1d at 50 kOe is ∼7.3, 6.8, 6.5, and 6.1 Nβ, respectively, which is smaller than the theoretical values of 8 Nβ for S = 4 owing to the magnetic anisotropy. Assuming the spin ground state of S = 4, the magnetization data are analyzed by using the program ANISOFT 2.0,52 giving the best fitting results collected in Table 2 The theoretical anisotropy energy barrier ΔE values calculated from ΔE = |D|S2 are in the range of 20.8−24.6 cm−1. Alternating current (ac) susceptibility measurements were performed to investigate the slow relaxation of magnetization. No peak is observed in the out-of-phase signal under zero external magnetic field (Figure S26, Supporting Information). When the external dc field of 2 kOe was applied, the out-of-phase susceptibilities (χm″) of 1−3 show obvious peaks (Figure 4). The effective energy barrier Ueff is 29.5, 27.6, and 29.2 cm−1, and the flipping rate τ0 is 1.50 × 10−10, 1.02 × 10−9, and 4.57 × 10−10 s for 1−3, respectively, based on the Arrhenius equation τ = τ0 exp(Ueff/kT) (Figure S28, Supporting Information). The Ueff values of complexes 1−3 are slightly larger than that (17−23 cm−1)27,43−50 of the reported {Mn6} complexes with S = 4. The parameter Φ = (ΔTp/Tp)/Δ(log f) is calculated to

Figure 2. CO2 and N2 adsorption isotherms measured at 273 and 293 K for compounds 1d−3d.

14 cm3·g−1 (for 1d), 16 cm3·g−1 (for 2d), and 25 cm3·g−1 (for 3d) at 273 K and 12 cm3·g−1 (for 1d), 14 cm3·g−1 (for 2d), and 21 cm3·g−1 (for 3d) at 293 K. The isosteric heats of adsorption (Qst) for 1d−3d were calculated using the adsorption data collected at 273 and 293 K by using the virial II model (Figure S19, Supporting Information). At zero coverage, the Qst is 42, 47, and 27 kJ·mol−1 for 1d−3d, respectively, higher than many PCPs.40 At 273 K, the uptake of N2 is lower than that of CO2, indicating that 1d−3d feature the selective adsorption of CO2 rather than N2 (Figures S20−S22, Supporting Information). Such behavior may be attributed to the existence of narrow porous channels and the large dipole moment of the triazine ring along with partial coordinatively unsaturated metal centers in activated compounds, which could facilitate dipole−quadruple interactions between framework and CO2.41,42 Magnetic Properties. As shown in Figure 3, the χmT at 300 K is 17.26, 17.15, 16.68, and 16.62 cm3 K mol−1 per Mn6 for compounds 1−3 and 1d, respectively, close to the calculated spin-only value of 18.0 cm3 K mol−1. The χm−1 versus T data D

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Inorganic Chemistry Table 2. Magnetic Data of Compounds 1−3 and 1d compounds

S

J1/J2/J3 (cm−1)

g

D (cm−1)

ΔE = |D|S2 (cm−1)

Ueff (cm−1)a

1 2 3 1d

4 4 4 4

−5.2/−5.3/+11.1 −4.4/−2.5/+2.4 −5.3/−5.1/+9.5 −3.4/−2.6/+3.8

1.98 2.05 1.97 1.97

−1.14 −1.15 −1.24 n.a.b

20.8 21.2 24.6 n.a.c

29.5 27.6 29.2 n.a.c

Data from ac susceptibility. bNo satisfactory fitting results due to the presence of contribution of low-lying excited spin states.27 cNot available. a

measure the shift of the peak temperature (Tp) of χm″, giving a value of 0.14, 0.16, and 0.15, which is within the range expected for super-paramagnets (0.1 ≤ Φ ≤ 0.3).26,46 This suggests that the slow magnetic relaxation is consistent with SMM behavior. However, isolated {Mn6} SMMs exhibited distinct out-of-phase peak without external dc field.26 Such dynamic magnetic property usually originates from the fast quantum tunnelling mechanism (QTM), and the applied dc field should suppress the quantum tunnelling process.53 Considering that complexes 1−3 do not show apparent distinctions from the {Mn6} SMMs in the coordination modes, the magnetic coupling, and the spin ground state of the Mn6 moieties, other possibilities might be responsible. The marked difference between complexes 1−3 and the {Mn6} SMMs is the rigid structure of the former. The regular and flat layers in compounds 1−3 promote lattice vibrations, triggering the electron−phonon coupling to eliminate the magnetic relaxation. Under the applied dc field, the coupling is inhibited, and the dynamic magnetic property similar to isolated {Mn6} SMMs appears. As for the 1,3,5-benzene-tricarboxylatebridged 2D Mn6 framework, the uneven layers are not rigid enough to induce electron−phonon coupling.27 More examples are needed to confirm this assumption. As for the 1d, no peaks appear no matter zero-field or 2 kOe fields were employed at 997 Hz (Figure S29, Supporting Information). However, the strongly frequency-dependent in both in-phase and out-of-phase components of 1d indicate that it still features SMM property of slow magnetic relaxation. Compared to 1, the difference demonstrated solvent molecules within the pores plays an important role in dynamic magnetism.13,15 Hence, SMM/SIM-based PCPs can provide a platform for tuning dynamic magnetic behavior.

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CONCLUSIONS In conclusion, two-dimensional porous coordination polymers based on {Mn6} single molecule magnets have been designed and synthesized, which show SMM behavior and selectivity for CO2 over N2. The combination of two different areas of SMMs and porous coordination polymers will provide new cue to tune the dynamic magnetic behavior by assembly and guest.

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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.6b00179. Additional figures for structure description of compound 1−3, gas adsorption curves, XRD patterns, TGA data, M−H magnetic data, and ac magnetic data. (PDF) CCDC 1054947 (1). (CIF) CCDC 1054948 (2). (CIF) CCDC 1054949 (3). (CIF)

Figure 4. Temperature dependence of the out-of-phase component of the ac magnetic susceptibility for compound 1 (a), 2 (b), and 3 (c), Hdc = 2000 Oe. E

DOI: 10.1021/acs.inorgchem.6b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

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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 973 Program (2013CB933403) and the National Natural Science Foundation of China (Project Nos. 91222104 and 21571113).

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DOI: 10.1021/acs.inorgchem.6b00179 Inorg. Chem. XXXX, XXX, XXX−XXX