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Cite this: Chem. Commun., 2014, 50, 1915 Received 30th October 2013, Accepted 17th December 2013
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Reversible crystal-to-amorphous-to-crystal phase transition and a large magnetocaloric effect in a spongelike metal organic framework material† Chong-Bin Tian,ab Rui-Ping Chen,a Chao He,a Wei-Jin Li,ab Qi Wei,ab Xu-Dong Zhanga and Shao-Wu Du*a
DOI: 10.1039/c3cc48325h www.rsc.org/chemcomm
Reversible crystal-to-amorphous-to-crystal phase transition accompanied by changes in magnetic and NLO properties was first observed in a rigid non-porous spongelike MOF material. The crystal phase exhibits a high magnetocaloric effect, while the amorphous phase has potential application as a magnetic DMF sensor.
The design and synthesis of phase change materials (PCMs) is one of the current hot issues in materials science due to their potential applications in molecular devices, such as sensors, data storage and switches.1 In this area, coordination chemistry plays a key role by providing remarkable examples of PCM in which the phase transition can be reversibly obtained through the external stimulus. Recently, single-crystal-to-single-crystal transformation (SCSC) through various experimental stimuli has been observed in some metal–organic frameworks (MOFs).1a,2 In some rare cases, however, the structure of the crystalline solid may collapse upon removal of solvent molecules, leading to an amorphous state. Remarkably, the original crystallinity can be restored in the presence of the initial solvent, resulting in the occurrence of crystal-to-amorphous-to-crystal (CAC) transition. Compounds capable of such a transformation are more often observed in flexible porous MOFs with guest solvent molecules than in rigid non-porous MOFs with coordinated solvent molecules,3,4 as the latter is more difficult to desolvate. Compared to the SCSC transition, the CAC transformation is specially appealing because it usually causes the rearrangement of the coordination sphere around the paramagnetic centre, which may influence its electronic structure and the magnetic exchange with neighboring atoms, and affect subsequently not only the magnetic properties,3,4 but also other physical functions (e.g. colour, and second-order nonlinear optical (NLO) and ferroelectric properties).3c,4b,5 As far as we know, there have a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian, Fuzhou 350002, P. R. China. E-mail: [email protected]; Fax: +86 591 83709470 b University of Chinese Academy of Sciences, Beijing 100039, P. R. China † Electronic supplementary information (ESI) available: Detailed synthesis, X-ray crystallographic data, TGA, IR, PXRD, SEM, EPR spectra and other physical measurements. CCDC 959274 (1). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc48325h
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been no reports on rigid non-porous MOFs which exhibit reversible CAC phase transition accompanied by changes in magnetic and NLO properties simultaneously. Molecular magnetic materials that exhibit a large magnetocaloric effect (MCE) are of great interest for their applications as magnetic refrigerants at low temperature.6 To construct materials with a high MCE, the isotropic GdIII ion, which has the maximum entropy (110 J kg 1 K 1), was an excellent choice.7 Despite the fact that some Gd-based materials show a large MCE, they exhibit inherent drawbacks for future investigation because of the cost and scarcity of Gd on earth. Hence, it is important and urgent to identify new materials to replace the present Gd-based materials. The isotropic transition-metal MnII ions with five unpaired 3d electrons were selected because they are cheap and sufficient in amount. However, to our knowledge, the MnII-based MOFs that exhibit a large MCE have never been reported so far. It is not surprising since the magnetic interactions between MnII ions are usually antiferromagnetic (AF) and often two orders of magnitude (100) larger than that of GdIII (10 2), thus leading to a small MCE. One effective method to obtain a large MCE is to reduce the magnitude of the magnetic interaction by controlling the bridging mode among MnII ions. In this communication, we present the first rigid non-porous spongelike MOF material, namely [Mn(Me-ip)(DMF)]n (1) (Me-ipH2 = 5-methylisophthalic acid), which exhibits reversible CAC phase transition accompanied by magnetic and NLO alterations during the progress of de-DMF/re-DMF. The solvated sample 1 exhibits a high MCE value of 42.41 J kg 1 K 1, which is also the first MnII-based MOF having such a large MCE. The desolvated sample 2 shows sensing ability towards DMF molecules, and has potential application as a magnetic DMF sensor. Compound 1 is a 3D non-porous coordination polymer consisting of infinite Mn–O–C–O chains. The coordination environment of the central MnII ion can be described as a distorted octahedron with four carboxylate oxygens (O1, O2B, O4A, O3A) from three bridging Me-ip2 anions in the equatorial plane, one carboxylate oxygen atom (O3C) from one Me-ip2 ligand and one oxygen (O5) from the DMF molecule at the axial position (Fig. S2a, ESI†). The Mn1–O3A bond distance is 2.699(1) Å, which is rather longer than that of the other five Mn–O bonds which are in the normal range of (2.099(2)–2.178(2) Å).
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Fig. 1
(a) View of the 3D framework and (b) the 1D chain of 1.
The adjacent Mn2+ centers are connected by two carboxylate groups (one in a syn–syn bridging mode, and the other in a m2–Z2 bridging mode) to form a 1D infinite chain running along the a axis (Fig. 1b). The Mn–Mn distance within the chain is 3.875 Å. With the help of the Me-ip2 ligands adopted a (k1–k1)–(k2–m2)–m4 bridging mode, each of the infinite chains are connected to four adjacent chains to form a 3D non-porous rigid framework (Fig. 1a and Fig. S2b, ESI†). TGA analysis reveals that the coordinated DMF molecules of 1 are released at 180 1C (Fig. S4, ESI†). The weight loss of 23.91% in the temperature range from 180 to 310 1C corresponds well to the weight of one DMF molecule in 1 (23.87%). The de-DMF sample 2 was obtained after heating 1 at 250 1C for 8 h under vacuum. The TGA curve of 2 remains unchanged up to 410 1C, indicating that all the coordinated DMF molecules have been removed from 1 (Fig. S4, ESI†). The complete removal of the coordinated DMF molecules was further confirmed by the IR spectrum of 2, in which the CQO stretching of DMF (1667 cm 1) disappeared (Fig. S5, ESI†). The release of DMF molecules is also indicated by the electron microscope pictures (Fig. S6, ESI†). Material 2 was amorphous as revealed by its powder X-ray diffraction (XRD) pattern which does not show any peaks. Interestingly, when 2 (15 mg) was immersed in DMF (2 mL) for 48 h, it recovered to 1 by re-absorption of DMF molecules, as confirmed by the powder XRD results. As shown in Fig. 2, such a DMF removal/ recovery process can be repeated several times, indicating a high degree of reversibility. This DMF-induced ‘shrinking-breathing’ dynamic effect suggests that a CAC phase transition takes place reversibly during the cycles of de-DMF and re-DMF.
Fig. 2
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PXRD patterns of solvated and desolvated samples.
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Fig. 3 (a) Plots of xmT vs. T for 1 and 2 at 5 kOe. (b) M–H curves for 1 and 2 at 2 K. (c) EPR spectra of 1 and 2 showing different shapes at 300 K. (d) The uptake–release of DMF between crystal phase (CP) and amorphous phase (AP) is totally reversible and can be monitored by the different shapes of EPR signals.
Direct-current magnetic susceptibilities and magnetization measurements on 1 and 2 show dramatically different magnetic behavior. At 300 K, the xmT value per MnII unit is 4.33 cm3 mol 1 K for 1 and 4.18 cm3 mol 1 K for 2, respectively (Fig. 3a), which are close to the spin-only value (4.38 cm3 mol 1 K) expected for non-interacting MnII ions with S = 5/2. Upon cooling, for 1, the xmT remains constant to about 30 K, after which it decreases abruptly to a minimum value of 3.41 cm3 mol 1 K at 2 K, whereas for 2, the xmT value decreases smoothly and reaches a minimum value of 0.45 cm3 mol 1 K at 2 K. This behavior indicates a stronger antiferromagnetic coupling in 2 than in 1. The stronger AF in 2 can also be supported by the lager Weiss constant of 2 (y = 19.26 K) than that of 1 (y = 0.59 K). On the other hand, the magnetization (M) versus field (H) plot for 1 shows a linear increase in the low-field region and reaches a saturation value of 4.99 Nb at 8 T, while the magnetization of 2 increases linearly with the increase of field, and reaches 2.03 Nb at 8 T (Fig. 3b). This observation agrees well with the stronger antiferromagnetic interaction in 2. The magnetic properties of 1 and 2 are analyzed using the 1D infinite-chain proposed by Fisher to obtain the coupling constants (Fig. 3a).8 The resulting analysis of the xmT data gives g = 1.99, J = 0.065 cm 1 and R = 1 10 4 for 1, g = 1.98, J = 1.59 cm 1 and R = 6.8 10 3 for 2, respectively. According to Goodenough’s rules, a m2-O bridge (Mn–O–Mn = 106.061) can mediate weak ferromagnetic interaction,9 while a syn–syn carboxylate bridge generally induces antiferromagnetic exchange. Therefore, the very weak exchange constant in 1 is due to the counter-complementarity effect of one m2-O and one syn–syn exchange pathway. Upon removal of the coordinated DMF molecules, the magnitude of the magnetic interaction between adjacent MnII ions of 2 is two orders higher than that in 1. This may be ascribed to the change in the magnetic exchange pathway in 2. It should be noted that there exists a weak Mn–O bond length (Mn1–O3A = 2.699(1) Å) in 1. We suggest that this weak bond breaks after desolvation, generating a new geometry of the central MnII ion (distorted tetrahedral) and one more syn–syn carboxylate bridge (Scheme S1, ESI†). Therefore, a large coupling constant can be anticipated in 2 through two syn–syn carboxylate bridges.
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Fig. 4 DSm obtained from the magnetization data of 1 at various fields and temperatures.
Isothermal magnetization curves are obtained in the range H = 0–8 T and T = 2–7 K to investigate the MCE of 1 and 2 (Fig. S8, Ð ESI†). Based on the Maxwell relation Sm(T)DH = [qM(T,H)/qT]H dH, the magnetic entropy changes DSm of 1 and 2 are calculated from the experimental magnetization data. For 1, the maximum value of DSm is 42.41 J kg 1 K 1 for DH = 8 T at 2 K (Fig. 4), which is smaller than that of 48.63 J kg 1 K 1 calculated for one isolated MnII spin using the equation DSm = nRln(2S + 1) = Rln6 = 1.79R. Among the reported MOF materials, the purely GdIII-based10 and mixed GdIII–MnII-based11 MOF materials with large MCE values have been studied. Nevertheless, the MnII-based MOFs with large MCE values have never been reported. Thus, compound 1 represents the first MnII-based MOF material that exhibits a large MCE value. For 2, the maximum value of DSm is 5.28 kg 1 K 1 at 3 K in the magnetic field range 0–8 T (Fig. S9, ESI†). This value is far smaller than the theoretical value of 63.93 J kg 1 K 1 obtained according to the above equation. Since the lower the MW/NMn value, the larger the MCE, a large MCE value should be anticipated in 2 due to the lower MW/NMn of 2 compared with 1.7c However, it is noticeable that a larger MCE is found in 1 than that in 2, which indicates that the much larger antiferromagnetion interactions can also reduce the entropy change in 2. Moreover, we also calculated the specific heat capacity (C) to verify the validity of the DSm calculated from the experimental magnetization data. As shown in Fig. S10 (ESI†), the DSm values obtained from the C data are in agreement with those calculated from the magnetization data. The change in the coordination geometry of the Mn(II) ion leads to a clearly different shape in room-temperature EPR spectra of 1 and 2 (Fig. 3c and Fig. S11, ESI†). Compound 1 exhibits a broad EPR signal, whereas 2 shows a sharp EPR signal. Besides, 2 cannot be recovered in other common organic solvents, such as CH3OH, CH3CH2OH, CH3CN, CH2Cl2 and DMAC, but can be recovered in the mixed solvents of DMF with the above solvents in the ratio of 1 : 1 (Fig. S12, ESI†). This provides the opportunity to monitor the DMF molecules by measuring the EPR signals (Fig. 3d). Hence, 2 has potential application as a magnetic DMF sensor. The space group Pna21 of 1 belongs to the 10 polar point groups, which are associated with a second harmonic generation response (SHG) and ferroelectric behavior (Fig. S13, ESI†). Indeed, the powder sample of 1 exhibits a strong SHG response (>KDP), whereas 2 shows
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a weak SHG response (oKDP). This means that 1 and 2 have different responses to the laser. The P–E plots for the powder pellet samples of 1 and 2 have a banana shape, which could not support the presence of ferroelectric behavior,12 but indicates that they both have the same response to the AC electric field. In conclusion, we have reported a non-porous spongelike MOF material, which is the first rigid MOF that exhibits reversible CAC transition accompanied with changes in magnetic and NLO properties during the de-DMF–re-DMF process. The crystal phase material 1 shows a high MCE value of 42.41 J kg 1 K 1, which is also the first MnII-based MOF material with a large MCE value. In addition, the amorphous phase material 2 displays sensing ability towards DMF molecules and has potential use as a magnetic DMF sensor. This work suggests that the synthesis of the MnII-based MOF material not only provides a new way to obtain PCMs, but also offers a successful route to design and obtain high MCE materials. We thank the National Basic Research Program of China (973 Program 2012CB821702), the National Natural Science Foundation of China (21233009 and 21173221) and the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences for financial support.
Notes and references 1 (a) E. Coronado and G. Minguez Espallargas, Chem. Soc. Rev., 2013, 42, 1525; (b) M. Wriedt, A. A. Yakovenko, G. J. Haider, A. V. Prosvirin, K. R. Dunbar and H.-C. Zhou, J. Am. Chem. Soc., 2013, 135, 4040; (c) M. Wuttig and N. Yamada, Nat. Mater., 2007, 6, 824. 2 G. K. Kole and J. J. Vittal, Chem. Soc. Rev., 2013, 42, 1755. 3 (a) K. Uemura, S. Kitagawa, K. Fukui and K. Saito, J. Am. Chem. Soc., 2004, 126, 3817; (b) S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334; (c) D. Maspoch, D. Ruiz-Molina, K. Wurst, N. Domingo, M. Cavallini, F. Biscarini, J. Tejada, C. Rovira and J. Veciana, Nat. Mater., 2003, 2, 190. 4 (a) S. Mohapatra, B. Rajeswaran, A. Chakraborty, A. Sundaresan and T. K. Maji, Chem. Mater., 2013, 25, 1673; (b) J. Ferrando-Soria, R. RuizGarcia, J. Cano, S.-E. Stiriba, J. Vallejo, I. Castro, M. Julve, F. Lloret, P. Amoros, J. Pasan, C. Ruiz-Perez, Y. Journaux and E. Pardo, Chem.–Eur. J., 2012, 18, 1608; (c) R. M. P. Colodrero, K. E. Papathanasiou, N. Stavgianoudaki, P. Olivera-Pastor, E. R. Losilla, M. A. G. Aranda, L. Leon-Reina, J. Sanz, I. Sobrados, D. Choquesillo-Lazarte, J. M. Garcia-Ruiz, P. Atienzar, F. Rey, K. D. Demadis and A. Cabeza, Chem. Mater., 2012, 24, 3780; (d) N. Roques, D. Maspoch, I. Imaz, A. Datcu, J.-P. Sutter, C. Rovira and J. Veciana, Chem. Commun., 2008, 3160. 5 (a) K. S. Lim, D. W. Ryu, W. R. Lee, E. K. Koh, H. C. Kim and C. S. Hong, Chem.–Eur. J., 2012, 18, 11541; (b) I. Chung, C. D. Malliakas, J. I. Jang, C. G. Canlas, D. P. Weliky and M. G. Kanatzidis, J. Am. Chem. Soc., 2007, 129, 14996. 6 (a) R. Sessoli, Angew. Chem., Int. Ed., 2012, 51, 43; (b) M. Evangelisti, F. Luis, L. J. de Jongh and M. Affronte, J. Mater. Chem., 2006, 16, 2534. 7 (a) M. Evangelisti, O. Roubeau, E. Palacios, A. Camon, T. N. Hooper, E. K. Brechin and J. J. Alonso, Angew. Chem., Int. Ed., 2011, 50, 6606; (b) J. W. Sharples, Y.-Z. Zheng, F. Tuna, E. J. L. McInnes and D. Collison, Chem. Commun., 2011, 47, 7650; (c) F.-S. Guo, J.-D. Leng, J.-L. Liu, Z.-S. Meng and M.-L. Tong, Inorg. Chem., 2012, 51, 405; (d) L.-X. Chang, G. Xiong, L. Wang, P. Cheng and B. Zhao, Chem. Commun., 2013, 49, 1055. 8 M. E. Fisher, Am. J. Physiol, 1964, 32, 343. 9 (a) J. B. Goodenough, Magnetism and the Chemical Bond, Wiley, New York, 1963; (b) Z.-M. Wang, B. Zhang, H. Fujiwara, H. Kobayashi and M. Kurmoo, Chem. Commun., 2004, 416. 10 (a) R. Sibille, T. Mazet, B. Malaman and M. Francois, Chem.–Eur. J., 2012, 18, 12970; (b) G. Lorusso, M. A. Palacios, G. S. Nichol, E. K. Brechin, O. Roubeau and M. Evangelisti, Chem. Commun., 2012, 48, 7592. 11 F.-S. Guo, Y.-C. Chen, J.-L. Liu, J.-D. Leng, Z.-S. Meng, P. Vrabel, M. Orendac and M.-L. Tong, Chem. Commun., 2012, 48, 12219. 12 J. F. Scott, J. Phys.: Condens. Matter, 2008, 20, 021001.
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Cite this: Chem. Commun., 2014, 50, 1915 Received 30th October 2013, Accepted 17th December 2013
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Reversible crystal-to-amorphous-to-crystal phase transition and a large magnetocaloric effect in a spongelike metal organic framework material† Chong-Bin Tian,ab Rui-Ping Chen,a Chao He,a Wei-Jin Li,ab Qi Wei,ab Xu-Dong Zhanga and Shao-Wu Du*a
DOI: 10.1039/c3cc48325h www.rsc.org/chemcomm
Reversible crystal-to-amorphous-to-crystal phase transition accompanied by changes in magnetic and NLO properties was first observed in a rigid non-porous spongelike MOF material. The crystal phase exhibits a high magnetocaloric effect, while the amorphous phase has potential application as a magnetic DMF sensor.
The design and synthesis of phase change materials (PCMs) is one of the current hot issues in materials science due to their potential applications in molecular devices, such as sensors, data storage and switches.1 In this area, coordination chemistry plays a key role by providing remarkable examples of PCM in which the phase transition can be reversibly obtained through the external stimulus. Recently, single-crystal-to-single-crystal transformation (SCSC) through various experimental stimuli has been observed in some metal–organic frameworks (MOFs).1a,2 In some rare cases, however, the structure of the crystalline solid may collapse upon removal of solvent molecules, leading to an amorphous state. Remarkably, the original crystallinity can be restored in the presence of the initial solvent, resulting in the occurrence of crystal-to-amorphous-to-crystal (CAC) transition. Compounds capable of such a transformation are more often observed in flexible porous MOFs with guest solvent molecules than in rigid non-porous MOFs with coordinated solvent molecules,3,4 as the latter is more difficult to desolvate. Compared to the SCSC transition, the CAC transformation is specially appealing because it usually causes the rearrangement of the coordination sphere around the paramagnetic centre, which may influence its electronic structure and the magnetic exchange with neighboring atoms, and affect subsequently not only the magnetic properties,3,4 but also other physical functions (e.g. colour, and second-order nonlinear optical (NLO) and ferroelectric properties).3c,4b,5 As far as we know, there have a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian, Fuzhou 350002, P. R. China. E-mail: [email protected]; Fax: +86 591 83709470 b University of Chinese Academy of Sciences, Beijing 100039, P. R. China † Electronic supplementary information (ESI) available: Detailed synthesis, X-ray crystallographic data, TGA, IR, PXRD, SEM, EPR spectra and other physical measurements. CCDC 959274 (1). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc48325h
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been no reports on rigid non-porous MOFs which exhibit reversible CAC phase transition accompanied by changes in magnetic and NLO properties simultaneously. Molecular magnetic materials that exhibit a large magnetocaloric effect (MCE) are of great interest for their applications as magnetic refrigerants at low temperature.6 To construct materials with a high MCE, the isotropic GdIII ion, which has the maximum entropy (110 J kg 1 K 1), was an excellent choice.7 Despite the fact that some Gd-based materials show a large MCE, they exhibit inherent drawbacks for future investigation because of the cost and scarcity of Gd on earth. Hence, it is important and urgent to identify new materials to replace the present Gd-based materials. The isotropic transition-metal MnII ions with five unpaired 3d electrons were selected because they are cheap and sufficient in amount. However, to our knowledge, the MnII-based MOFs that exhibit a large MCE have never been reported so far. It is not surprising since the magnetic interactions between MnII ions are usually antiferromagnetic (AF) and often two orders of magnitude (100) larger than that of GdIII (10 2), thus leading to a small MCE. One effective method to obtain a large MCE is to reduce the magnitude of the magnetic interaction by controlling the bridging mode among MnII ions. In this communication, we present the first rigid non-porous spongelike MOF material, namely [Mn(Me-ip)(DMF)]n (1) (Me-ipH2 = 5-methylisophthalic acid), which exhibits reversible CAC phase transition accompanied by magnetic and NLO alterations during the progress of de-DMF/re-DMF. The solvated sample 1 exhibits a high MCE value of 42.41 J kg 1 K 1, which is also the first MnII-based MOF having such a large MCE. The desolvated sample 2 shows sensing ability towards DMF molecules, and has potential application as a magnetic DMF sensor. Compound 1 is a 3D non-porous coordination polymer consisting of infinite Mn–O–C–O chains. The coordination environment of the central MnII ion can be described as a distorted octahedron with four carboxylate oxygens (O1, O2B, O4A, O3A) from three bridging Me-ip2 anions in the equatorial plane, one carboxylate oxygen atom (O3C) from one Me-ip2 ligand and one oxygen (O5) from the DMF molecule at the axial position (Fig. S2a, ESI†). The Mn1–O3A bond distance is 2.699(1) Å, which is rather longer than that of the other five Mn–O bonds which are in the normal range of (2.099(2)–2.178(2) Å).
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Fig. 1
(a) View of the 3D framework and (b) the 1D chain of 1.
The adjacent Mn2+ centers are connected by two carboxylate groups (one in a syn–syn bridging mode, and the other in a m2–Z2 bridging mode) to form a 1D infinite chain running along the a axis (Fig. 1b). The Mn–Mn distance within the chain is 3.875 Å. With the help of the Me-ip2 ligands adopted a (k1–k1)–(k2–m2)–m4 bridging mode, each of the infinite chains are connected to four adjacent chains to form a 3D non-porous rigid framework (Fig. 1a and Fig. S2b, ESI†). TGA analysis reveals that the coordinated DMF molecules of 1 are released at 180 1C (Fig. S4, ESI†). The weight loss of 23.91% in the temperature range from 180 to 310 1C corresponds well to the weight of one DMF molecule in 1 (23.87%). The de-DMF sample 2 was obtained after heating 1 at 250 1C for 8 h under vacuum. The TGA curve of 2 remains unchanged up to 410 1C, indicating that all the coordinated DMF molecules have been removed from 1 (Fig. S4, ESI†). The complete removal of the coordinated DMF molecules was further confirmed by the IR spectrum of 2, in which the CQO stretching of DMF (1667 cm 1) disappeared (Fig. S5, ESI†). The release of DMF molecules is also indicated by the electron microscope pictures (Fig. S6, ESI†). Material 2 was amorphous as revealed by its powder X-ray diffraction (XRD) pattern which does not show any peaks. Interestingly, when 2 (15 mg) was immersed in DMF (2 mL) for 48 h, it recovered to 1 by re-absorption of DMF molecules, as confirmed by the powder XRD results. As shown in Fig. 2, such a DMF removal/ recovery process can be repeated several times, indicating a high degree of reversibility. This DMF-induced ‘shrinking-breathing’ dynamic effect suggests that a CAC phase transition takes place reversibly during the cycles of de-DMF and re-DMF.
Fig. 2
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PXRD patterns of solvated and desolvated samples.
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Fig. 3 (a) Plots of xmT vs. T for 1 and 2 at 5 kOe. (b) M–H curves for 1 and 2 at 2 K. (c) EPR spectra of 1 and 2 showing different shapes at 300 K. (d) The uptake–release of DMF between crystal phase (CP) and amorphous phase (AP) is totally reversible and can be monitored by the different shapes of EPR signals.
Direct-current magnetic susceptibilities and magnetization measurements on 1 and 2 show dramatically different magnetic behavior. At 300 K, the xmT value per MnII unit is 4.33 cm3 mol 1 K for 1 and 4.18 cm3 mol 1 K for 2, respectively (Fig. 3a), which are close to the spin-only value (4.38 cm3 mol 1 K) expected for non-interacting MnII ions with S = 5/2. Upon cooling, for 1, the xmT remains constant to about 30 K, after which it decreases abruptly to a minimum value of 3.41 cm3 mol 1 K at 2 K, whereas for 2, the xmT value decreases smoothly and reaches a minimum value of 0.45 cm3 mol 1 K at 2 K. This behavior indicates a stronger antiferromagnetic coupling in 2 than in 1. The stronger AF in 2 can also be supported by the lager Weiss constant of 2 (y = 19.26 K) than that of 1 (y = 0.59 K). On the other hand, the magnetization (M) versus field (H) plot for 1 shows a linear increase in the low-field region and reaches a saturation value of 4.99 Nb at 8 T, while the magnetization of 2 increases linearly with the increase of field, and reaches 2.03 Nb at 8 T (Fig. 3b). This observation agrees well with the stronger antiferromagnetic interaction in 2. The magnetic properties of 1 and 2 are analyzed using the 1D infinite-chain proposed by Fisher to obtain the coupling constants (Fig. 3a).8 The resulting analysis of the xmT data gives g = 1.99, J = 0.065 cm 1 and R = 1 10 4 for 1, g = 1.98, J = 1.59 cm 1 and R = 6.8 10 3 for 2, respectively. According to Goodenough’s rules, a m2-O bridge (Mn–O–Mn = 106.061) can mediate weak ferromagnetic interaction,9 while a syn–syn carboxylate bridge generally induces antiferromagnetic exchange. Therefore, the very weak exchange constant in 1 is due to the counter-complementarity effect of one m2-O and one syn–syn exchange pathway. Upon removal of the coordinated DMF molecules, the magnitude of the magnetic interaction between adjacent MnII ions of 2 is two orders higher than that in 1. This may be ascribed to the change in the magnetic exchange pathway in 2. It should be noted that there exists a weak Mn–O bond length (Mn1–O3A = 2.699(1) Å) in 1. We suggest that this weak bond breaks after desolvation, generating a new geometry of the central MnII ion (distorted tetrahedral) and one more syn–syn carboxylate bridge (Scheme S1, ESI†). Therefore, a large coupling constant can be anticipated in 2 through two syn–syn carboxylate bridges.
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Fig. 4 DSm obtained from the magnetization data of 1 at various fields and temperatures.
Isothermal magnetization curves are obtained in the range H = 0–8 T and T = 2–7 K to investigate the MCE of 1 and 2 (Fig. S8, Ð ESI†). Based on the Maxwell relation Sm(T)DH = [qM(T,H)/qT]H dH, the magnetic entropy changes DSm of 1 and 2 are calculated from the experimental magnetization data. For 1, the maximum value of DSm is 42.41 J kg 1 K 1 for DH = 8 T at 2 K (Fig. 4), which is smaller than that of 48.63 J kg 1 K 1 calculated for one isolated MnII spin using the equation DSm = nRln(2S + 1) = Rln6 = 1.79R. Among the reported MOF materials, the purely GdIII-based10 and mixed GdIII–MnII-based11 MOF materials with large MCE values have been studied. Nevertheless, the MnII-based MOFs with large MCE values have never been reported. Thus, compound 1 represents the first MnII-based MOF material that exhibits a large MCE value. For 2, the maximum value of DSm is 5.28 kg 1 K 1 at 3 K in the magnetic field range 0–8 T (Fig. S9, ESI†). This value is far smaller than the theoretical value of 63.93 J kg 1 K 1 obtained according to the above equation. Since the lower the MW/NMn value, the larger the MCE, a large MCE value should be anticipated in 2 due to the lower MW/NMn of 2 compared with 1.7c However, it is noticeable that a larger MCE is found in 1 than that in 2, which indicates that the much larger antiferromagnetion interactions can also reduce the entropy change in 2. Moreover, we also calculated the specific heat capacity (C) to verify the validity of the DSm calculated from the experimental magnetization data. As shown in Fig. S10 (ESI†), the DSm values obtained from the C data are in agreement with those calculated from the magnetization data. The change in the coordination geometry of the Mn(II) ion leads to a clearly different shape in room-temperature EPR spectra of 1 and 2 (Fig. 3c and Fig. S11, ESI†). Compound 1 exhibits a broad EPR signal, whereas 2 shows a sharp EPR signal. Besides, 2 cannot be recovered in other common organic solvents, such as CH3OH, CH3CH2OH, CH3CN, CH2Cl2 and DMAC, but can be recovered in the mixed solvents of DMF with the above solvents in the ratio of 1 : 1 (Fig. S12, ESI†). This provides the opportunity to monitor the DMF molecules by measuring the EPR signals (Fig. 3d). Hence, 2 has potential application as a magnetic DMF sensor. The space group Pna21 of 1 belongs to the 10 polar point groups, which are associated with a second harmonic generation response (SHG) and ferroelectric behavior (Fig. S13, ESI†). Indeed, the powder sample of 1 exhibits a strong SHG response (>KDP), whereas 2 shows
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a weak SHG response (oKDP). This means that 1 and 2 have different responses to the laser. The P–E plots for the powder pellet samples of 1 and 2 have a banana shape, which could not support the presence of ferroelectric behavior,12 but indicates that they both have the same response to the AC electric field. In conclusion, we have reported a non-porous spongelike MOF material, which is the first rigid MOF that exhibits reversible CAC transition accompanied with changes in magnetic and NLO properties during the de-DMF–re-DMF process. The crystal phase material 1 shows a high MCE value of 42.41 J kg 1 K 1, which is also the first MnII-based MOF material with a large MCE value. In addition, the amorphous phase material 2 displays sensing ability towards DMF molecules and has potential use as a magnetic DMF sensor. This work suggests that the synthesis of the MnII-based MOF material not only provides a new way to obtain PCMs, but also offers a successful route to design and obtain high MCE materials. We thank the National Basic Research Program of China (973 Program 2012CB821702), the National Natural Science Foundation of China (21233009 and 21173221) and the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences for financial support.
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