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A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from Tc = 26 to 80 K Ie-Rang Jeon, Bogdan Negru, Richard P. Van Duyne, and T. David Harris J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b10382 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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Journal of the American Chemical Society

A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from Tc = 26 to 80 K Ie-Rang Jeon, Bogdan Negru, Richard P. Van Duyne and T. David Harris* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston IL 60208-3113

Supporting Information Placeholder ABSTRACT: The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6dihydroxy-1,4-benzoquinone (LH2) with FeII affords the solid (Me2NH2)2[Fe2L3]·6DMF·2H2O. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of FeIII centers with mixed-valence ligands of the form (L3)8– that result from a spontaneous electron-transfer from FeII to L2–. Upon removal of DMF and H2O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me2NH2)2[Fe2L3], and a fit to N2 adsorption data of this activated compound gives a BET surface area of 885(105) m2/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally-characterized extended solid that contains semiquinone radical ligands. The formation of extended solids that exhibit both permanent porosity and magnetic ordering represents an important synthetic challenge, as these materials may find use in applications including lightweight magnets, magnetic sensors, and the magnetic extraction of paramagnetic gases such as O2 and NO. 1 To date, most porous magnets have featured paramagnetic metal ions connected by inorganic bridging ligands such as oxide or cyanide. While the vast majority of these compounds display magnetic ordering well below 100 K,1d they have been shown to order at temperatures of up to 219 K. 2 Nevertheless, the chemistry of inorganic solids is limited by an inherent lack of ligand functionalization and structural diversity, both of which are critical to systematically vary and control magnetic behavior and porosity. In contrast, employment of organic bridging ligands to form metal-organic frameworks (MOFs) gives rise to a diverse and modular set of structures that can also exhibit high surface areas. However, the large metal-metal separations and thus weak magnetic interactions enforced by multi-atom organic bridging ligands generally leads to very low magnetic ordering temperatures, as ordering temperature is directly correlated to strength of exchange interactions. 3 In fact, the highest ordering temperature yet reported for a MOF with a well-defined surface area is only Tc = 32 K. 4 As an alternative, one can envision the use of organic radical ligands to link paramagnetic metal centers in an extended solid, 5 such that the ligand-based magnetic orbital will overlap with that of the metal to promote strong metal-radical direct exchange and, consequently, a high ordering temperature. Indeed, this approach has been extensively investigated using derivatives of nitroxide, 6 organonitrile, 7,8 perchlorotriphenylmethyl, 9 and triplet carbene radicals. 10 While magnetic solids that order even above room temperature have been synthesized with these ligands,8a they have not been shown to exhibit well-defined permanent porosity.

Derivatives of 2,5-dihydroxy-1,4-benzoquinone are particularly well-suited for the construction of microporous MOFs with strong magnetic coupling. These ligands readily undergo redox chemistry to stabilize both diamagnetic and paramagnetic, or semiquinone, isomers. Indeed, a number of mononuclear 11 and dinuclear 12 molecular complexes containing related radical ligands have been shown to exhibit extremely strong magnetic interactions, in some cases so strong that the spin ground state is isolated even at 300 K. 13,14 In addition, these ligands have been shown to generate extended solids of varying dimensionality with large estimated void volumes, and as such could potentially give rise to materials with permanent porosity. 15-17 Despite this potential, no structurally-characterized example of an extended solid that incorporates semiquinone radicals has been reported. In an attempt to synthesize a semiquinone radical-containing extended solid, we targeted a system involving spontaneous electron transfer from a metal ion to a diamagnetic ligand upon solid formation. Similar electron-transfer has been observed in a number of molecular transition metal complexes containing related dioxolene ligands, 18 and one-electron oxidation of dinuclear FeII2

Figure 1. X-ray crystal structure of [Fe2L3]2–, as viewed along the crystallographic c axis (upper) and b axis (lower), with selected Fe···Fe distances (Å). Orange, green, red, and gray spheres represent Fe, Cl, O, and C atoms, respectively.

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and CoII2 complexes bridged by tetraoxolenes ligands has given radical-bridged FeIII212c and CoIII2 19 complexes through rearrangement of valence electrons. Herein, we report the synthesis and magnetic properties of a microporous layered solid comprised of FeIII centers bridged by a radical form of chloranilic acid (LH2 = 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone). Upon activation, the solid exhibits a surface area of 885(105) m2/g, and its magnetic ordering temperature changes from 80 to 26 K. To our knowledge, this material represents the first structurally characterized solid incorporating a paramagnetic semiquinone ligand. Reaction of Fe(BF4)2·6H2O with LH2 in DMF at 130 °C under a dinitrogen atmosphere afforded shiny black, hexagonal plateshaped crystals of (Me2NH2)2[Fe2L3]·2H2O·6DMF (1). Singlecrystal X-ray analysis of 1 reveals a trigonal structure featuring two-dimensional honeycomb-like layers comprised of Fe centers bridged by Ln−. Each Fe center resides in an octahedral coordination environment and is ligated by six O donor atoms from three different deprotonated Ln− ligands (see Figure 1, upper). The Fe center is situated on a site of crystallographic three-fold symmetry, with three Ln− ligands related in a propeller-like arrangement. The charge of the dianionic network is compensated by Me2NH2+ ions (see Figure S1), which are generated in situ through decomposition of DMF. 20 Within 1, the layers are eclipsed along the crystallographic c axis, with a H2O molecule positioned in between Fe centers, leading to the formation of onedimensional hexagonal channels. The channels are occupied by disordered DMF molecules, as was confirmed by microelemental analysis and thermogravimetric analysis (TGA) (see Figure S2). The structure of 1 features an intra-layer Fe···Fe distance along the diagonal of the hexagonal channel of 15.6614(6) Å, with an interlayer Fe···Fe distance of 8.7449(5) Å (see Figure 1, lower). Finally, an analogous reaction of Zn(NO3)2·6H2O with LH2 gave purple, hexagonal plate-shaped crystals of the isostructural compound (Me2NH2)2[Zn2L3]·2H2O·6DMF (2, see Figure S3). While compounds 1 and 2 feature isostructural dianionic networks, close inspection of bond lengths reveals several key differences. First, the mean C-C bond distance in 1 is 1.414(8) Å, 1.6% shorter than that of 1.436(7) Å found in 2. In addition, the C-O bond distance of 1.280(6) Å in 1 is 2.9% longer than that of 1.244(5) Å in 2. The distances in 2 are consistent with those previously reported in diamagnetic, dianionic tetraoxolenecontaining honeycomb-like networks,16 while 1 exhibits bond distances characteristic of a higher net C-C bond order and a lower net C-O bond order. Nevertheless, the mean C-C and C-O distances in 1 are still slightly longer and shorter, respectively, than those reported for semiquinone, trianionic tetraoxolene radicals in molecular complexes, which feature mean C-C and C-O distances of 1.353(3) and 1.293(3) Å, respectively.12c Additional-

Figure 2. Raman spectra collected for solid samples of 1 (blue), 1a (red), and 2 (green) following excitation at 405 nm.

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ly, the Fe-O distance of 2.020(3) Å is consistent with high-spin FeIII ions in similar coordination environments. 21 Taken together, these structural observations suggest a configuration for the network in 1 of [FeIII2(L3)8–]2–, a form which presumably results from a spontaneous electron-transfer from FeII to L2–, while the network in 2 can be described as [ZnII2(L2–)3]2–. To our knowledge, 1 represents the first structurally characterized example of any extended solid incorporating a semiquinone radical bridging ligand. Upon soaking in THF and subsequent activation at 120 ºC under reduced pressure, 1 can be desolvated to give (Me2NH2)2[Fe2L3] (1a), where complete removal of solvent molecules was confirmed by IR spectroscopy, TGA, and elemental analysis (see Figures S4 and S5 and Experimental Section). The X-ray powder diffraction pattern of 1a exhibits a significantly different profile than that of 1 (see Figure S6). A simulation of this pattern using MOF-FIT 2.0 22 reveals a slight structural distortion of the hexagonal channels upon desolvation, accompanied by a contraction of interlayer Fe···Fe separation of 9.3% (see Figure S7). N2 adsorption data collected at 77 K for 1a gave a BrunauerEmmett-Teller (BET) surface area of 885(105) m2/g, confirming the presence of permanent microporosity (see Figure S8). This value is the second highest reported for a porous magnet, eclipsed only by a value of 1050 m2/g reported for a lactate-bridged CoII solid.4 Finally, the structural distortion is fully reversible, as evidenced by powder X-ray diffraction, with 1a converting back to 1 upon soaking in DMF (see Figure S6). Similar “breathing” behavior has been previously observed in a number of MOFs.22,23 In order to confirm the presence of FeIII in the Fe2L3 solid, Mössbauer spectra were collected for 1 and 1a (see Figure S9). At 80 K, the spectrum for 1 exhibits a sharp quadrupole doublet, and a fit to the data give an isomer shift of δ = 0.576(1) mm/s and a quadrupole splitting of ∆EQ = 1.059(2) mm/s. These parameters can be unambiguously assigned to a high-spin FeIII center.16e,21 At 5 K, the spectrum for 1 exhibits a major sextet and a second minor doublet assigned to a small amount of FeII-containing impurity. This sextet indicates slowing down of the magnetic relaxation and suggests that 1 undergoes spontaneous magnetization at 5 K. Similarly, the spectrum collected for 1a at 80 K exhibits a doublet that can be fit to give parameters of δ = 0.595(7) mm/s and a slightly larger quadrupole splitting of ∆EQ = 1.103(9) mm/s, indicating presence of a high-spin FeIII center. The slight increase in quadrupole splitting of 1a compared to 1 likely stems from the local distortion of the Fe coordination sphere upon activation. The oxidation state of the bridging ligand was further probed by Raman spectra collected for samples of 1, 1a, and 2 at ambient temperature (see Figure 2). Raman bands observed at 1492, 1497, and 1617 cm–1 for 1, 1a, and 2, respectively, are assigned to the νCO stretching vibration of the bridging ligand. Based on reported literature values for related dioxolene ligands, 24 the bridging ligand in 2 can be unambiguously assigned as L2–. In contrast, the νCO stretching vibration in 1 and 1a appears slightly higher in energy than the typically observed frequency of 1425-1466 cm–1 for the related tetraoxolene semiquinone bridging ligands in dinuclear complexes,14a,25 which is consistent with structural observations discussed above. Moreover, the presence of only one CO stretch in 1 suggests that the two electrons transferred from FeII in [Fe2L3]2− are delocalized over the three ligands on the Raman timescale. Additionally, the strong bands at 1364, 1364, and 1360 cm–1 for 1, 1a, and 2 are assigned to the intraligand CC vibrations, and the features at 500-600 cm–1 are tentatively assigned to νMO stretching modes that are coupled to the CC stretch. 26 To investigate the magnetic behavior of 1 and 1a, variabletemperature dc magnetic susceptibility data were collected, and the resulting plots of χM and χMT vs T for both compounds are shown in Figures S10-S12. In the case of 1, χMT = 16.6 cm3·K/mol at 300 K, which is higher than the value of 9.50 cm3·K/mol expected for two magnetically isolated FeIII centers


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Figure 3. Variable-temperature field-cooled magnetization data for 1 (blue) and 1a (red), collected under an applied dc field of 10 Oe. Inset: Variable-field magnetization data for 1 at 60 K (blue) and 1a at 10 K (red).

and two L3–• ligands, indicative of strong magnetic coupling between paramagnetic centers. Upon lowering the temperature, χMT gradually increases and then climbs abruptly below 120 K, reflecting the increase in the magnetic correlation length within the network. Additionally, magnetic susceptibility data collected on cooling the sample with or without an applied dc field exhibit a divergence that is indicative of spontaneous magnetization. The corresponding plot of variable-temperature magnetization shows the spontaneous magnetization to occur below T = 100 K (see Figure 3). The plot of χMT vs T for 1a reveals a similar profile, albeit in a much lower temperature regime. Here, the value of χMT at 300 K of 9.37 cm3·K/mol is close to that expected for two magnetically isolated FeIII centers and two L3–• ligands, suggesting minimal interactions at 300 K. The field-dependence of χM is consistent with that observed for 1, suggesting that the magnetic ground state is preserved upon activation (see Figure S12). However, spontaneous magnetization occurs only below T = 30 K (see Figure 3). The decrease in characteristic temperature associated with 1a compared to 1 may stem from a decrease in coupling strength between FeIII centers and L3–• ligands due to the distortion of the framework and/or the creation of defects upon activation. Solvent-induced switching of magnetic ordering has been previously observed in both inorganic and metal-organic solids. 27 Finally, low-field remnant magnetization experiments suggest the magnetic structure of both 1 and 1a at the characteristic temperature to approximate a dimensionality of two (see Figure S13). To precisely determine the characteristic temperatures of 1 and 1a, variable-temperature ac susceptibility data under zeroapplied field were collected at selected temperatures (see Figure S14 and S15). The data for 1 show a slightly frequency-dependent peak in both in-phase (χM') and out-of-phase (χM'') susceptibility, and give a characteristic temperature of Tc = 80 K. The frequency dependence can be quantified by the Mydosh parameter, in this case φ = 0.023, which is consistent with glassy magnetic behavior (see Supporting Information). 28 Such glassiness can result from factors such as crystallographic disorder and spin frustration arising from magnetic topology. The presence of two types of spin carrier in 1, in conjunction with electron delocalization in the hexagonal layer, likely results in a complicated network of magnetic interactions and possibly some degree of frustration. In contrast, the plot of χM' vs T for 1a exhibits a sharp, frequencyindependent peak with a maximum at 26 K, indicating that 1a undergoes long-range magnetic ordering at Tc = 26 K. Finally, in order to probe the presence of magnetic hysteresis, variable-field magnetization data were collected for 1 and 1a at

selected temperatures under applied fields of up to 70 kOe (see Figure 3 inset and S16). Coercive fields of HC = 2630, 790, 150, and 40 Oe were observed for 1 at 1.8, 10, 40, and 60 K, respectively, with field-sweep rates of 4, 0.8, 0.8, and 0.8 Oe/s. The data at 1.8 K reach a value of M = 8.2 µB at 70 kOe, close to the value of 8 µB expected for antiferromagnetic coupling between two FeIII ions and two L3–• ligands. In the case of 1a, coercive fields of HC = 4650, 20, and 10 Oe were observed at 1.8, 10, and 20 K, respectively, with field-sweep rates of 4, 0.8, and 0.2 Oe/s. The higher HC values observed for 1a likely reflect the larger magnetic anisotropy engendered by structural distortion at the FeIII center. Additionally, the absence of an inflection point in M vs H at temperatures close to the characteristic temperature for both compounds suggests that the interlayer interaction is negligible or ferromagnetic (see Figure S17). Taken together, these data demonstrate that 1 and 1a behave as magnets that involve dominant intra-layer antiferromagnetic interactions between adjacent spins. The foregoing results demonstrate the incorporation of semiquinone radical ligands into an extended solid and highlight the ability of these ligands to generate a two-dimensional magnet with Tc = 80 K. Moreover, this solid exhibits permanent porosity, with its activated phase undergoing a slight structural distortion and associated decrease in magnetic ordering temperature to Tc = 26 K. Work is underway to further probe the magnetism and electrical conductivity of these solids, and to synthesize related solids that contain high-magnetic anisotropy metal ions.

ASSOCIATED CONTENT Supporting Information Experimental details, crystallographic data, TGA, IR spectra, Mössbauer spectra, powder X-ray patterns, N2 adsorption isotherm, additional magnetic data, and crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was funded by the National Science Foundation through Grants DMR-1351959 (TDH), CHE-1152547 (RPVD), and CHE-1506683 (RPVD), the Institute for Sustainability and Energy at Northwestern, and Northwestern University. Purchase of the SQUID magnetometer was supported in part by the International Institute of Nanotechnology. We thank Prof. J. T. Hupp, Prof. O. K. Farha, and C. Audu for assistance with TGA, and L. Sun for assistance using MOF-FIT 2.0.

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(a) Larionova, J.; Chavan, S. A. ; Ya khmi, J. V. ; Frøy stein, A. G.; Sletten, J.; So urisseau, C.; Kahn, O. Inorg. Chem. 19 97, 36, 6374. (b) Larionova, J.; Kahn, O.; G olhen, S.; O uahab, L.; Cle ́r ac, R. J. Am. Chem. Soc. 1 999, 12 1, 3349 . (c) Tanase, S.; Tu na, F. ; Guio nneau, P.; Maris, T. ; Rombaut, G.; Mathon ière, C.; Andruh, M.; Kah n, O.; Sutter, J-P. Inorg. Chem. 2003 , 42, 1 625. (d) Oh koshi, S. ; Arai, K.; Sato, Y. ; Hashimoto , K. Nat. Ma ter. 2004 , 3, 85 7. (e) Kurmoo, M.; Kumagai, H.; Chapman, K. W.; Kepert, C. J. Chem. Commun. 2005, 3012. (f) Nelson , K. J. ; Giles, I. D.; Troff, S. A.; Arif, A. M. ; Miller, J. S. In org. Chem. 200 6, 45, 8922. (g) Yanai, N. ; Kane ko, W.; Y oneda, K.; Ohba, M.; K itagawa, S. J. Am . Chem. Soc. 2007, 129, 3 496. (h) Kane ko, W.; Ohba, M. ; Kitagawa, S. J. Am. Chem. Soc. 20 07, 129 , 1370 6. (i) M ilon, J.; Daniel, M.-C.; Kaiba, A. ; Guio nneau, P.; Brandes, S.; Su tter, J.-P. J. Am. Ch em. Soc. 2007, 12 9, 138 72. (j) Lopez, N.; Zhao, H.; Prosv irin, A. V.; Chouai, A. ; Shatru k, M.; Dun bar, K. R. Chem. Commun. 2 007, 4 611. ( k) Cheng, X.-N.; Zhang, W.-X.; Chen, X.-M. J. Am. Chem. Soc. 2007 , 129, 15738. ( l) Oh kosh i, S.; Ts unobuchi, Y.; Ta kahash i, H.; H ozumi, T .; Shiro, M.; Hash imoto, K. J. Am. Chem. Soc. 200 7, 129, 3084. (m) Duan, Z.; Zhang, Y.; Z hang, B. ; Zhu, D . J. Am. Chem. Soc. 2009, 131, 69 34. (n) Zhang, W.-X.; X ue, W.; Chen, X.-M. Inor g. Chem. 2 011, 50 , 309. (o) Wriedt, M.; Ya ko ven ko, A. A. ; Halder, G. J.; Prosv irin, A. V. ; Dunbar, K. R. J. Am. Chem. Soc. 2013 , 135, 4 040. (p) M oto kawa, N.; Matsu naga, S.; Ta kaish i, S.; M iy asaka, H.; Yamashita, M. ; Dunbar, K. R. J. Am. Chem. Soc. 2010 , 132, 1 1943. N ishio, M.; Miy asaka, H. Ino rg. Chem. 2014, 53, 471 6.

28 Note that a fit to data based on the Arrhenius law (ν = ν0exp(E/kBTf)) provides an activation energy of E/kB = 6442 K. This unphy sical value further proves that 1 show s glassy magnet behavior and not a s imple thermally activated energy barrier blocking.


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A 2D Semiquinone Radical-Containing ... - ACS Publications

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