Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Switchable Ferro‑, Ferri‑, and Antiferromagnetic States in a Piezoand Hydrochromic Metal−Organic Framework Teng Gong, Peng Li, Qi Sui, Li-Jiao Zhou, Ning-Ning Yang, and En-Qing Gao* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China S Supporting Information *
hydrochromic and solvatomagnetic response,27 but the magnetic changes after radical formation are not remarkable. Here we report the multiple and remarkable responsive properties of a Mn(II) MOF derived from a viologen-based tetracarboxylate ligand. The MOF demonstrates viologen-based piezochromism and hydrochromism in MOFs. More interesting is the remarkable magnetic response. Water release/reuptake switches the magnetic states between ferro- and antiferromagnetic, while compression leads to a ferrimagnetic state. The MOF [MnL(H2O)2]·2.5H2O (1) was synthesized from Mn(ClO4)2 and [H4L]Cl2 [H4L = 1,1′-bis(3,5-dicarboxyphenyl)-4,4′-bipyridinium]. The powder X-ray diffraction (PXRD) pattern is in good accordance with that calculated from singlecrystal data (Figure S1). According to single-crystal structural analysis, the MOF exhibits a 2D coordination network based on dimanganese units. Each Mn(II) is octahedrally coordinated by six O atoms, of which four equatorial ones come from carboxylates and two axial ones from water molecules (Figure 1a). Two Mn(II) ions are bridged by two μ-O(carboxylate)
ABSTRACT: The Mn(II) metal−organic framework with a viologen-based tetracarboxylate ligand exhibits reversible optical (color) and magnetic changes concomitant with stimuli-induced electron transfer from carboxylate to viologen. Compression causes a magnetic transformation from ferro- to ferrimagnetic, while water release/reuptake switches the magnetic behavior between ferro- and antiferromagnetic.
R
esponsive molecular magnetic materials that undergo reversible changes in their magnetic properties under external stimuli such as light, pressure, and molecule sorption have attracted much attention because of their promising applications in molecular devices such as switches and sensors.1−5 Metal−organic frameworks (MOFs), a huge class of hybrid materials with almost infinite diversity, are promising candidates of responsive materials because they can exhibit a wide range of potentially interdependent properties arising from the inorganic and organic components.6−8 A core issue in the crossing field between molecular magnetism and responsive MOFs is how to effectively manipulate the magnetic properties of the framework through physical or chemical stimulation.9−11 The judicious strategy is to introduce into the MOFs appropriate stimuli-responsive motifs that have switchable magnetic states or can interfere in magnetic coupling. Viologen cations (V2+, 1,1′-disubstituted 4,4′-bipyridinium) are capable of forming radicals (V•+) through reversible electron transfer (ET) triggered by external stimuli such as light, heat, electricity, and chemical species.12 For these capabilities, viologens have attracted much interest in electrochemistry,13,14 photochemistry15,16 and solar energy conversion systems.17,18 The color change concomitant with radical formation has made viologen a nice building unit for the design of responsive materials.19 Electro- and photochromism are well established properties of organic or metal−organic viologen derivatives,20−22 and very recently we have for the first time demonstrated that viologen compounds can also be piezo- and hydrochromic in specific structures.23−25 Considering the formation of paramagnetic radicals, one can expect that introducing viologen into molecular magnetic systems could impart stimulus responsiveness in not only optical (color) but also magnetic properties. However, the magnetic effects still remain largely unexplored. We have recently reported two viologen-containing MOFs with dual-response (optical and magnetic) properties, one exhibiting photochromic and photomagnetic response26 and the other © XXXX American Chemical Society
Figure 1. (a) Coordination environments and a dinuclear unit in 1. Symmetry codes: A = 2 − x, 1 − y, −z; B = x, −1 + y, z; C = −1 + x, y, z; D = x, 1 + y, z; E = 1 + x, y, z. (b) 2D network with two ligands highlighted to show the connection between the units.
atoms to form a dinuclear unit, with Mn···Mn = 3.502 Å and Mn−O−Mn = 105.5°. Of the four carboxylates of L2−, one provides the μ-O bridge for the dinuclear unit, two are monodentate, and the fourth is uncoordinated. The dinuclear units are doubly connected into chains by isophthalate moieties, and the chains are interlinked into a thick 2D layer because of coordination of the other isophthalate moiety in each L2− (Figure 1b). The top view of the layer shows large rectangular windows, and the side view reveals 1D channels within the layer (Figure S2). The offset and interdigitating packing of the layers blocks Received: April 25, 2018
A
DOI: 10.1021/acs.inorgchem.8b01141 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry the windows; the 1D channels are occupied by the uncoordinated carboxylates from adjacent layers and disordered water. While many viologen-based compounds are photochromic,21 1 is not but exhibits a reversible color change upon dehydration/ rehydration (Figure 2a). The yellow sample turns green when
Figure 3. (a) XPS spectra of 1 and 1-D-G. (b) Interactions around the ligand (Table S2). The purple and black dashed lines are the ET pathways (between carboxylate and viologen) and the interactions with water molecules, respectively. Figure 2. (a) Photographs showing the color change of 1. UV−vis (b) and ESR spectra (c) before and after dehydration.
energy one (287.8 eV) is due to carboxylate and the shoulder peak at 286.0 eV has a contribution from viologen.28 After dehydration, the shoulder peak is weakened and nearly merged into the main peak at lower energy (284.7 eV), and the carboxylate signal is slightly shifted toward higher energy. The changes confirm that carboxylate and viologen are respectively the electron donor and acceptor.29 1-D-G quickly returns yellow if contacted with water or moisture. The PXRD and UV−vis profiles of 1 are recovered. The color-switching processes can be repeated for at least five cycles, as monitored by UV−vis spectra (Figure S5). The recovery process is induced only by water uptake, no matter what atmosphere (air, O2, or N2). Other molecules such as methanol, ethanol, ether, trichloromethane, acetonitrile, and tetrahydrofuran cause no response. Moreover, heating is not the necessary input for the forward ET and merely serves to accelerate dehydration: the yellow-to-green change can occur slowly at room temperature in the dry air maintained by concentrated sulfuric acid. Therefore, the chromic phenomenon is a kind of hydrochromism involving radical formation/annihilation through forward/back ET induced by water release/ uptake.23,27,30 This is phenomenologically similar to, but mechanistically completely different from, the behaviors of some transition-metal compounds, where water release/uptake causes color changes through modulation of the ligand field. The MOF is also piezochromic. It turns green after compression under a hydraulic press (Figure 2a) and returns yellow in hours at ambient pressure. The pressure-induced green state (1-P-G) shows no significant differences from the pristine state in IR and PXRD profiles, suggesting that the molecular and crystal structures are maintained (Figure S6). The UV−vis and XPS spectra of 1-P-G are similar to those for 1-D-G, confirming a radical mechanism through pressure-induced ET (Figures S6 and S7). The processes have been repeated for five cycles without indications of fatigue (Figure S7). The donor−acceptor (D−A) interactions in 1 were inspected to justify the chromic behaviors (Figure 3b and Table S2). There are two types of possible pathways for ET: carboxylate O atoms approach the viologen rings either from above with NV···O = 3.27 Å or from the side to form C−HV···O hydogen bonds with HV··· O = 2.37−2.54 Å. Water cannot serve as donor to viologen for its high ionization energy,31 but it imposes a high impact on ET by interacting with the donor and acceptor.25 The water molecules
heated above 100 °C. Thermogravimetric analysis (Figure S3) revealed two overlapping weight-loss steps before 150 °C, suggesting the removal of both free and coordinated water (found, 14.5%; calcd, 13.1%). The steeper weight drop above ∼100 °C indicates the loss of coordinated water. The dehydration process was further examined by in situ temperature-variable IR spectroscopy and PXRD (Figures S1 and S4). The very broad ν(O−H) absorption around 3410 cm −1 decreases with increasing temperature. The fact that the higher frequency side of the band decreases more rapidly indicates that the guest water is released first. The new PXRD peaks observed at 100 °C indicate phase transformation due to the loss of coordinated water. The original phase disappears above 130 °C, and the new phase remains unchanged up to at least 200 °C. The MOF shows no color change below 100 °C, suggesting that the chromic phenomenon is concomitant with the phase transformation induced by the loss of coordinated water. UV−vis and electron-spin-resonance (ESR) spectroscopy was performed to monitor the chromic properties. The pristine yellow sample (1) shows strong UV absorption with weak and smooth tailing in the visible region. New visible absorption begins to appear upon heating at 110 °C, develops into strong bands at 465, 670, and 728 nm at higher temperature, and reaches saturation at 150 °C (Figure 2b). These multiple bands are characteristic of viologen radicals,23 which are generated through ET to the cationic viologen units. The X-band ESR spectrum of 1 is typical of Mn(II), showing a strong signal at g = 2.024 and weak hyperfine signals (Figure 2c). The dehydrated green state (1-D-G) shows a less intense signal at a similar position. The change could be due to radical formation and possible interactions between the paramagnetic centers. X-ray photoelectron spectroscopy (XPS) spectra of 1 and 1-DG were recorded (Figure 3a). The Mn 2p spectra are almost identical, so the metal ion remains at the II+ state. The O 1s peak is shifted toward higher energy (from 530.8 to 531.0 eV) upon going from 1 to 1-D-G, indicating that carboxylate O atoms are electron donors. Compared with 1, 1-D-G shows a new N 1s band at relatively low energy (400.7 eV), suggesting that the viologen moiety receives electrons after dehydration. The C 1s spectra show three peaks, of which the weakest and highestB
DOI: 10.1021/acs.inorgchem.8b01141 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
well-known that the magnetic interactions through μ-O can be either ferro- or antiferromagnetic, sensitive to the Mn−O distance and the Mn−O−Mn angle, although no clear magnetostructural correlation has been established.33 1 represents a unique system where water release/uptake causes subtle changes in the bridging parameters and thereby switches the interactions between ferro- and antiferromagnetic. Quite differently, compression causes a magnetic change from ferro- to ferrimagnetic. As shown in Figure 4, the χT−T plot of 1P-G, with obvious contributions from radical spins, shows a broad minimum around 60 K. Because the structure is retained after compression, the coupling between Mn(II) ions should remain ferromagnetic. The ferrimagnetic behavior can be due to the uncompensated antiferromagnetic interactions between Mn(II) and the radical. The fact that the magnetization (4.78 Nβ) of 1-P-G at 2 K and 5 T is lower than that for 1 (Figure S9) also supports a ferrimagnetic ground state. To our knowledge, this is the first time to demonstrate the pressure-induced magnetic response associated with ET of viologen. To summarize, we have described a multimode responsive Mn(II) MOF bearing the ET-active viologen chromophore. It shows reversible hydro- and piezochromism due to stimuliinduced ET from carboxylate to viologen. Most notably, three distinct magnetic states can be achieved in the chromic processes. The pristine ferromagnetic state and an antiferromagnetic state can be switched to and fro through water removal/reuptake, while compression leads to a ferrimagnetic state. This work demonstrates that incorporating stimuli-responsive viologen chromophores into magnetic MOFs can provide new perspectives for the design of multifunctional magnetic materials for potential use in switching and sensing devices.
in 1 interact with viologen through Nv···Ow (3.39−3.83 Å) and Hv···Ow (2.29−2.33 Å) and also with carboxylate through O− Hw···Ocarboxylate bonds. The solvation interactions reduce the donor/acceptor strength of carboxylate/viologen and stabilize the V2+···COO− state more than V•+···COO•, so water brings in an additional barrier for ET. The nonphotochromic nature of 1 suggests that photoexcitation, which works on individual chromophores, cannot overcome the impeding effects. However, the effects can be overcome by compression, which works on the whole framework and proactively reduces D−A contacts. This explains the occurrence of piezochromism. On the other hand, the occurrence of ET upon dehydration can be due to withdrawal of the impeding effects and perhaps also because the D−A contacts can be closer after dehydration. Water reuptake rebuilds the solvation interactions in favor of the nonradical state, so backET occurs. To probe the magnetic effects of the hydro- and piezochromic ET processes, temperature-dependent magnetic measurements were performed with 1, 1-D-G, and 1-P-G (Figure 4). The χT
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Figure 4. Plots of χT versus T for 1, 1-D-G, and 1-P-G at 1 kOe.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01141.
value for 1 at 300 K is 4.41 emu K mol−1, comparable to the value (4.375 emu K mol−1) expected for high-spin Mn(II). As the temperature is lowered, χT increases to a maximum at 3.0 K. The Curie−Weiss law is followed above 20 K with C = 4.39 emu K mol−1 and θ = 7.35 K. The behaviors suggest ferromagnetic exchange via the double μ-O bridges within the dinuclear unit. Fitting the data to the van Vleck equation based on H = −JS1S232 leads to J = 1.65 cm−1 and g = 2.01. The magnetization isotherm at 2 K (Figure S9) confirms ferromagnetic interactions: the magnetization approaches saturation more rapidly than expected for isolated Mn(II) ions and reaches 5.10 Nβ at 5 T. Remarkably, dehydration reverses the magnetic coupling from ferro- to antiferromagnetic. The high-temperature χT product (4.8−4.9 emu K mol−1) of 1-D-G is higher than that of 1, indicative of the contribution from the radical spins. At lower temperature, the product decreases continuously. The fit to the Curie−Weiss law gave a negative Weiss constant (θ = −2.11 K with C = 4.85 emu K mol−1). The isothermal magnetization at 2 K increases slowly, and the value (3.36 Nβ) at 5 T is far below the saturation value expected for Mn(II) (Figure S9). All of these results suggest an antiferromagnetic behavior. Considering the ready solid-state recovery of the pristine structure upon rehydration, the framework should not undergo dramatic changes after dehydration, but the carboxylate groups may undergo some changes in coordination modes and bond parameters to compensate for the loss of water, as implied by the changes of the ν(COO) bands in IR spectra (Figure S4). It is
Experiments, tables, and additional graphics (PDF) Accession Codes
CCDC 1835456 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
En-Qing Gao: 0000-0002-5631-2391 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21471057 and 21773070) and China Postdoctoral Science Foundation (Grant 2018M632060). C
DOI: 10.1021/acs.inorgchem.8b01141 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.8b01141 Inorg. Chem. XXXX, XXX, XXX−XXX
