Electron Paramagnetic Resonance Study of Guest Molecule

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Electron Paramagnetic Resonance Study of Guest MoleculeInfluenced Magnetism in Kagome Metal−Organic Framework † Mantas Šimeṅ as,*,† Ryotaro Matsuda,‡,¶ Susumu Kitagawa,§ Andreas Pöppl,∥ and Juras ̅ Banys †

Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius, Lithuania Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ¶ Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan § Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ∥ Faculty of Physics and Earth Sciences, Universität Leipzig, Linnestrasse 5, D-04103 Leipzig, Germany ‡

S Supporting Information *

ABSTRACT: We present a continuous-wave electron paramagnetic resonance (EPR) study of Cu(ipa)(H2O) (ipa stands for isophthalate ligand) metal−organic framework (MOF) that is based on the kagome topology. The main structural unit of this compound is a copper paddle-wheel (PW) consisting of two Cu2+ ions interconnected via four carboxylate groups in the syn−syn fashion. X- and Q-band EPR measurements of this MOF allow us to probe distinct magnetic interactions of different magnitudes that originate from these dinuclear units. EPR experiments performed at different temperatures reveal that two Cu2+ ions in a single PW are antiferromagnetically coupled to the total spin S = 0 state with the exchange coupling constant J = −285 cm−1. The fine structure pattern observed in the spectra indicates the thermal population of the excited triplet S = 1 spin states of the Cu2+ pairs. Spectral simulations are used to determine the fine and hyperfine structure tensors of the PW units. At higher temperatures a collapse of the fine structure to a single EPR line is observed, indicating the interdinuclear exchange of the S = 1 spin states mediated via the isophthalate linkers. Comparison of the EPR results with the structural X-ray diffraction data reveals that the dehydration of the compound opens an additional spin triplet exchange path joining apical positions of the PWs. We estimated the effective exchange coupling constants in the as-synthesized and dehydrated forms of the compound by simulating the observed powder patterns. It is shown that the additional path is reversibly closed after the dehydrated sample is allowed to interact with air, demonstrating a peculiar relationship between the adsorbed guest molecules and magnetic properties of kagome MOFs even at room temperature.

(dabco)0.5,21 STAM-122 or 3∞[CuI2CuII2 {H2O}2L2Cl2] (L stands for 3,3′-(5,5′-(thiophene-2,5-diyl)bis(3-methyl-4H-1,2,4-triazole-5,4-diyl))dibenzoate).23 The PW units consist of two Cu2+ ions connected by four carboxylate groups in the syn−syn fashion which act as the superexchange paths for the unpaired electrons of the ions. Usually the exchange coupling is such that the antiferromagnetic ground state of the total electron spin S = 0 is formed, but at room temperature the thermally excited S = 1 state can be substantially populated.24,25 In addition to the intradinuclear exchange, a much weaker interdinuclear exchange of the S = 1 spins is frequently observed in MOFs containing rather short linkers which interconnect the PWs (e.g., 1,3,5-benzenetricarboxylic linker in Cu3(btc)2).20−23 In our previous continuous-wave (CW) electron paramagnetic resonance (EPR) study23 we have demonstrated that the guest water molecules reversibly close the interdinuclear spin exchange path in 3∞[CuI2CuII2 {H2O}2L2Cl2] MOF, providing

INTRODUCTION Metal−organic frameworks (MOFs) or coordination polymers are highly attractive and promising porous materials that can be potentially used to adsorb,1 separate,2 and store3 gases, sense chemicals,4 catalyze chemical reactions,5 or even deliver drugs.6 The broad applicability of these crystalline compounds stems from their structural diversity determined by the constituent units: metal ion centers (nodes) and organic linkers.7 Frequently, transition metal or lanthanide cations are chosen as nodes due to their high coordination number and the ability to adapt different coordination geometries.8 These metal centers and various organic linkers are responsible for peculiar physical properties of these materials.9−11 In some cases the adsorbed guest molecules may also induce or significantly modify MOF properties (the so-called Guest@MOF concept).12 A special interest is concentrated on the guestinfluenced magnetism, because such a phenomenon is attractive for gas sensing applications.13−16 A rich magnetic behavior is observed in MOFs that contain the so-called dinuclear Cu2+ paddle-wheels (PWs).17,18 Examples of such compounds are Cu3(btc)2,19,20 Cu(bdc)© 2016 American Chemical Society

Received: September 29, 2016 Revised: November 4, 2016 Published: November 7, 2016 27462

DOI: 10.1021/acs.jpcc.6b09853 J. Phys. Chem. C 2016, 120, 27462−27467


The Journal of Physical Chemistry C

activation (see the Supporting Information for more details). The powder X-ray diffraction pattern of 1as was recorded to verify the structure of the compound (see the Supporting Information). EPR Spectroscopy. CW EPR experiments at X-band frequency (∼9.5 GHz) were performed with Bruker ELEXSYS E580 spectrometer. A Bruker EMX 10-40 spectrometer was used to measure Q-band (∼34 GHz) CW EPR spectra. For room temperature EPR experiments, a 1 mW microwave power was used. To avoid EPR signal saturation, measurements at low temperature were performed at 0.05 mW microwave power. The strength and frequency of the modulating field were correspondingly 6 G and 100 kHz. All simulations of EPR spectra were performed using EasySpin 5.1.1 simulation package36 implemented in Matlab environment.

an example of the Guest@MOF concept. However, due to the lack of structural information, the origin of this exchange path was not identified. In this study we give further insight on the interdinuclear exchange interaction and guest-induced magneto-structural correlations in MOFs by employing CW EPR spectroscopy to investigate kagome-type MOF of chemical formula Cu(ipa)(H2O) (ipa stands for isophthalate linker). The structure of a kagome MOF containing azidoisophthalate linker instead of the isophthalate is known in both the as-synthesized and dehydrated forms of the compound (Figure 1).26 This

RESULTS AND DISCUSSION The X- and Q-band CW EPR spectra of 1as recorded at 90 K are presented in Figure 2. The spectral lines denoted as Bz1, Bz2, Bx1,y1, and Bx2,y2 constitute a well-known fine-structure (fs) powder pattern of a system possessing an electron spin S = 1.37 Such EPR spectra are frequently detected for various MOFs20−23 and other dinuclear compounds30−34 containing Cu2+ PW units. The relation between the S = 1 spin states and these units was first recognized by B. Bleaney and K. D. Bowers in the famous EPR study of Cu2+ acetate monohydrate, Cu(CH3COO)2(H2O)2.24,25 They concluded that there is a magnetic superexchange coupling17 between the two S = 1/2 electron spins residing in the dx2−y2 orbitals of the Cu2+ ions (electronic configuration 3d9). This coupling is mediated via the diamagnetic carboxylate groups joining Cu2+ ions of the PW unit. It is antiferromagnetic and thus results in the ground state of the total spin S = 0, which is EPR silent. However, the strength of the coupling is such that the excited triplet S = 1 state (detectable by EPR) is thermally populated at elevated temperature. To verify that the observed EPR spectra of 1as originate from the triplet S = 1 states, we performed spectral simulations using the following spin Hamiltonian:37,38

Figure 1. Crystal structure of [Cu(aip)(H2O)] (where aip is azidoisophthalate) kagome MOF along the [001] direction (left). Schematic representation of the kagome-lattice composed of the Cu2+ PW units (right). Crystal structure taken from ref 26.

compound contains Cu2+ PW units connected via the azidoisophthalate linkers into the kagome-type sheet structure.26−28 The available structural information allows us to identify the influence of the guest molecules on the interdinuclear spin exchange behavior in Cu(ipa)(H2O). We observe a reversible closing of an additional spin exchange path by guest molecules which can be detected even at room temperature. Note that the intradinuclear exchange interaction in a kagome MOF was previously investigated by directly measuring the magnetic susceptibility of the as-synthesized compound.27 Some indications of the interdinuclear exchange were also recognized in the same study. However, methods measuring the total magnetic susceptibility usually detect the prevailing spin exchange, whereas the much weaker interactions are poorly resolved.19 In contrast, EPR spectroscopy proved to be the method of choice to study even the weakest magnetic interactions in MOFs20−23,29 and other dinuclear compounds24,30−35 containing Cu2+ PW units.

/ = βe BgS + S

A iIi + S DS

i = 1,2

= βe BgS + S

∑ i = 1,2

A iIi + DSz 2 + E(Sx 2 − Sy 2) −

2D 3 (1)

EXPERIMENTAL DETAILS Sample Preparation. A crystalline powder sample of Cu(ipa)(H2O) was prepared as follows. A methanol (200 mL) solution of H2ipa (3.30 g, 20.0 mmol) and pyridine (3.2 mL, 40.0 mmol) was refluxed at 80 °C. A methanol (200 mL) solution of Cu(NO3)2·3H2O (4.85 g, 20 mmol) and pyridine (1.6 mL, 20 mmol) was dropped into the refluxed solution for over 25 min and stirred with relaxing for 2 h. The mother liquor was decanted and the light blue solid was isolated by a centrifugal separator and then washed with ethanol three times to remove unreacted materials. Cu(ipa)(H2O) MOF (1as) (2.65 g) was obtained as light blue powder after drying in vacuum for 2 h at room temperature. The activated form (1act) of the compound was obtained by evacuating Cu(ipa)(H2O) MOF at 120 °C for about 12 h in an EPR tube that was sealed afterward. Thermogravimetric (TG) analysis was performed to confirm removal of water molecules from the sample upon

Here the first term describes the electron Zeeman interaction characterized by the g-tensor of the Cu2+ pair. B and βe denote the external magnetic field and the Bohr magneton, respectively. The second term in the equation represents the hyperfine (hf) interaction between the total electron spin S and nuclear spin I of copper (63Cu and 65Cu isotopes have the same nuclear spin I = 3/2). Here the sum runs over two equivalent copper nuclei in the PW unit, and the hf tensors A1 and A2 can be expressed as A1(2) = ACu/2, where ACu is the hf tensor of a single copper nucleus.32,38 The third term describes the fs of the spectra, which is characterized by the traceless fs tensor D with its components parametrized by the D (axial) and E (orthorhombic) zero-field splitting (zfs) parameters. Six allowed (ΔmS = ±1, where mS is the magnetic spin quantum number) EPR lines are expected in the powder pattern for the orthorhombic case (D ≠ 0 and E ≠ 0), whereas for the axial 27463

DOI: 10.1021/acs.jpcc.6b09853 J. Phys. Chem. C 2016, 120, 27462−27467


The Journal of Physical Chemistry C

2.368(2), A1(2),zz ≡ A∥ = 70(1) × 10−4 cm−1 ,and D = −0.3337(2) cm−1. The orthorhombic zfs parameter E was set to zero, and the perpendicular components of the hf tensors were not resolved (also set to zero in the simulation). Our EPR experiments allowed us to determine only the magnitude of the axial zfs parameter D. Its negative sign was chosen according to the ab initio39 and high-field EPR40 studies. For the simulation the g, A1(2), and D tensors were taken to be collinear. The determined components of the g and D tensors are in a good agreement with a vast number of previous EPR studies of MOFs20−23,41 and other dinuclear compounds17,30−34 containing Cu2+ PW units. Almost identical values of A∥ were observed for the Cu2(O2CCHCHCH3)4(DMF)232 and Cu2(β-alanine)4Cl231 compounds. The determined value of A∥ translates to ACu,∥ = 140(2) × 10−4 cm−1, which is the parallel hf tensor component of a single copper nucleus. Such values of g∥ and ACu,∥ indicate a copper ion in a square pyramidal environment.42 Our EPR findings agree with the XRD study,26 which shows that the PW unit has a D4h point group symmetry and each Cu2+ ion has a square pyramidal coordination geometry. They also confirm that the origin of the observed fs of the spectra is the Cu2+ PWs. Note that our simulations do not reproduce the experimentally detected line at about 300 mT (X-band) and 1170 mT (Q-band) (marked by the M-symbol in Figure 2). This line is a typical anisotropic EPR powder pattern of the paramagnetic mononuclear Cu2+ ions (electron spin S = 1/2). The splittings occurring at the lower magnetic field of this signal arise from the hf interaction between the electron and copper nuclear spins. The observation of this signal is unexpected, because XRD analysis26 indicates that all cupric ions belong to the PW units and thus are coupled to the S = 0 or S = 1 spin states. However, such signals of mononuclear copper were previously detected in the majority of MOF compounds containing copper PWs.20,22,23,41 Our recent single crystal EPR study of 3∞[CuI2CuII2 {H2O}2L2Cl2] MOF revealed that the signal of mononuclear copper originates from a small amount of broken PW units with one missing Cu2+ ion.29 Thus, we assign this EPR line in 1as to such defective structural motifs. A more detailed analysis of the mononuclear copper signal is presented in the Supporting Information. The observed EPR spectra of 1as were further investigated by performing measurements at temperatures from 10 to 300 K. The obtained Q-band spectra are presented in Figure 3a. Xband spectra of 1as are presented in the Supporting Information. As expected, the low temperature (below 50 K) spectra consist of the mononuclear Cu2+ line only, because all PW units are in the EPR silent S = 0 ground state. As the temperature is increased, the fs of the spectrum appears and becomes more intense indicating thermal population of the excited S = 1 spin states. Surprisingly, an additional signal at g = 2.180(3) (indicated by the arrow in Figure 3a) emerges above 90 K and becomes the dominant spectral feature near the room temperature. Such behavior suggests that this line is related to the dinuclear PW units, because for paramagnetic mononuclear Cu2+ species the EPR intensity should decrease with increasing temperature. This line was also observed in other MOFs containing Cu2+ PWs such as Cu3(btc)2,20 [Zn1−xCux(bdc) (dabco)]0.5,21 STAM-1,22 and 3∞[CuI2CuII2 {H2O}2L2Cl2]23 as well as in many dinuclear complexes.30,34,43−45 The origin of this signal was revealed by Kozlevčar et al., who investigated the isolated and polymeric (chain-like) dinuclear copper complexes.34 It was recognized that this line appears only for the

Figure 2. Experimental (a) X- and (b) Q-band CW EPR spectra of 1as recorded at 90 K and corresponding spectral simulations. The inset in (b) emphasizes the ΔmS = ±2 transition. The M-symbols indicate mononuclear Cu2+ lines.

symmetry (D ≠ 0 and E = 0) the expected number of the resonances is four: Bx1,y1, Bx2,y2, Bz1, and Bz2.37 The simulated X- and Q-band spectra are also presented in Figure 2. Note that simulations at both frequencies were performed using the same spin Hamiltonian parameters. Simulations revealed that the resonances Bx2,y2 and Bz2 almost coincide in the Q-band spectrum, whereas the Bx1,y1 transition is not observed at the X-band frequency, because the axial zfs parameter D and the microwave frequency have similar magnitudes. At the low-field part of the Q-band spectrum the so-called forbidden transition (ΔmS = ±2) is observed37 (see inset in Figure 2b). It consists of seven well resolved lines originating from the hf interaction of electrons with two copper nuclei (2nI + 1 = 7, where n = 2 and I = 3/2). The observed splitting allowed us to determine the axial component of the hf tensors A1 and A2. The best simulation was obtained using the following spin Hamiltonian parameters: gxx = gyy ≡ g⊥ = 2.067(2), gzz ≡ g∥ = 27464

DOI: 10.1021/acs.jpcc.6b09853 J. Phys. Chem. C 2016, 120, 27462−27467


The Journal of Physical Chemistry C

as-synthesized form. The absence of the crystal structure of the activated MOF prevented us from drawing definite conclusions about the relationship between the structural changes and the occurrence of the interdinuclear exchange. However, we speculated that the interdinuclear spin exchange path forms after water is removed from the apical sites of the Cu2+ PWs. In contrast, the crystal structures of the as-synthesized and activated kagome MOFs are known.26 This allows us to study the influence of the adsorbed guest molecules on the interdinuclear spin exchange in more detail. The structural arrangements of the PW units in the as-synthesized and activated kagome MOFs are presented in Figure 4a,b,

Figure 3. Q-band CW EPR spectra of (a) 1as and (b) 1act recorded at different temperatures. The arrow in (a) indicates the EPR line originating from the collapse of the fs caused by the interdinuclear spin exchange. The topmost spectra are simulations of this line.

chain-like complexes (interconnected PW units), and thus it was assigned to the interdinuclear exchange of the spin triplets (S = 1 states), which causes the collapse of the fs into a single line.34,46 This is in agreement with the observed behavior of 1as where the interdinuclear line intensity grows with temperature, because more adjacent Cu2+ PW units are in the S = 1 spin state and therefore are able to participate in the triplet exchange. As the temperature is lowered, the intensity decreases because some of the PW units start to relax to the S = 0 state interrupting this process. According to the XRD data,26 this exchange in 1as must occur via the isophthalate linkers joining the neighboring PW units. Note that in Cu3(btc)220 and STAM-122 MOFs the interdinuclear spin exchange paths are the 1,3,5-benzenetricarboxylic linkers. The spectra measured at different temperatures were used to obtain the isotropic exchange coupling constant J of the antiferromagnetically coupled Cu2+ ions within a single PW unit (see the Supporting Information for more details). The determined value of J = −285(7) cm−1 is in a good agreement with vast number of studies on similar Cu2+ PWs,18,20,22,23,25 verifying the antiferromagnetic character of the exchange interaction. The exchange coupling J was previously investigated in kagome MOF using a magnetic susceptibility magnetometer.27 The reported value is −350 cm−1, which slightly differs from the value obtained from our EPR measurements. The discrepancy between the values of J measured using EPR and direct methods such as SQUID was also previously recognized for other similar systems.20,23,25 To investigate how the structural changes occurring upon the dehydration influence the magnetic properties of kagome MOF, we performed temperature-dependent EPR measurements of 1act. The obtained Q-band spectra are presented in Figure 3b (measurements at X-band frequency can be found in the Supporting Information). The spectra show a more intense interdinuclear exchange line compared to the spectra of 1as. In addition, the fs does not appear in the spectra of 1act above 200 K, whereas it is clearly resolved in the spectra of 1as. This indicates a more pronounced spin triplet exchange between the neighboring PWs in 1act. A similar influence of the sample activation on the EPR spectra was observed in our previous 3 study of ∞ [Cu2I Cu2II{H2O}2L2Cl2] MOF containing Cu2+ 23 PWs. The activation of this MOF caused the occurrence of the interdinuclear spin exchange, which was not observed in the

Figure 4. Arrangement of the Cu2+ PW units in the (a) as-synthesized and (b) activated kagome MOFs. Arrows indicate interdinuclear spin exchange paths J1 and J2. Crystal structure taken from ref 26.

respectively. In the as-synthesized compound the apical positions of the PW units are coordinated by water, whereas the PWs are interconnected only via the isophthalate linkers that mediate the interdinuclear exchange interaction. We denote this spin exchange path as J1 (interdinuclear (midpoint) distance is 9.2 Å). The activation removes water molecules allowing PW units to connect via the short Cu−O bonds (bond length is 2.48 Å), which likely act as an additional spin exchange path J2 (interdinuclear distance 5.2 Å). These results support our previous prediction of such paths in the activated form of 23 3 I II ∞[Cu2Cu2 {H2O}2L2Cl2] MOF. The same exchange path was previously reported for the connected aliphatic dinuclear copper complexes.34 Note that the activation of another wellinvestigated Cu3(btc)2 MOF did not change the EPR spectra, indicating the absence of the J2 exchange path, which is in agreement with the XRD data.47 The interdinuclear exchange coupling can be estimated using a method based on the exchange narrowing theory.33,46,48 For a precise determination of this coupling, one needs measurements of single crystal samples. In our previous study of 23 3 I II we successfully applied this ∞[Cu2Cu2 {H2O}2L2Cl2] MOF method also for powder samples. However, from powder measurements one cannot differentiate between the differently oriented PW units and easily separate influence of different exchange paths. Thus, one can only estimate the absolute value of the effective exchange coupling constant J′. In this study we used this procedure as described in detail in ref 23 to simulate the Q-band spectra of 1as and 1act recorded at 270 K and obtained J′ (Figure 3). The determined values are 1.4(4) and 2.3(3) cm−1, respectively, indicating that the interdinuclear exchange is indeed more pronounced in the activated form. Note that these values are similar to the previously obtained in −1 23 3 I II ∞[Cu2Cu2 {H2O}2L2Cl2] (|J′| = 4.9 cm ) and [Cu(trans−2− −1 33 butenoate)4]n (5.9 cm ) compounds. We performed additional EPR measurements to investigate whether the opening/closing of the spin exchange path J2 in 27465

DOI: 10.1021/acs.jpcc.6b09853 J. Phys. Chem. C 2016, 120, 27462−27467


The Journal of Physical Chemistry C

especially because kagome MOFs were found to selectivity adsorb CO molecules at the apical positions of the PW units.26

kagome MOF is reversible by allowing 1act to interact with air. The X-band spectra after 1, 9, and 14 days of keeping the sample at ambient conditions are presented in Figure 5. A


S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09853. TG analysis data, PXRD pattern, additional EPR spectra, and determination of J (PDF)


Corresponding Author

*M. Šimėnas. E-mail: mantas.simenas@ff.vu.lt. Phone: +370 5 2234537. Fax: +370 5 2234537. ORCID

Mantas Šimėnas: 0000-0002-2733-2270 Notes

Figure 5. Room temperature X-band EPR spectra of activated kagome MOF upon water adsorption over 14 days. The spectrum of 1as is presented for comparison.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft (DFG) within the priority program SPP 1601 and STSM Grant from the COST Action MP1308-TO-BE is gratefully acknowledged. This work is partially supported by the PRESTO and ACCEL project of the Japan Science and Technology Agency (JST), and JSPS KAKENHI Grant-in-Aid for Young Scientists (A) (Grant No. 16H06032).

transformation from the spectrum of 1act to 1as can be observed over time period of several days, demonstrating that the guest molecules reversibly close the J2 spin exchange paths. This shows that the magnetic properties of kagome MOF can be partially controlled by the gas adsorption even at room temperature.

CONCLUSIONS We used CW EPR spectroscopy to study the magnetic behavior of Cu(ipa)(H2O) kagome MOF containing dinuclear Cu2+ PW units. The obtained temperature-dependent X- and Q-band EPR spectra revealed that the two Cu2+ ions within a single PW unit are coupled into the antiferromagnetic total spin S = 0 ground state. The EPR-active S = 1 state thermally populates at temperatures above 50 K. The magnetic coupling constant characterizing this electron exchange interaction was determined to be J = −285 cm−1. The simulation of the observed fs and hf patterns of the S = 1 spin states allowed us to determine the g, A, and D tensors, characterizing the copper pairs. In addition to the exchange interaction between the Cu2+ ions within a single PW unit, the interdinuclear exchange of the S = 1 spins was also detected in the EPR spectra as a single broad line resulting from the collapse of the fs. A comparison with the available XRD data revealed that this spin exchange occurs via the isophthalate linkers joining the adjacent PWs. Surprisingly, the EPR spectra of the activated MOF indicated a significantly more pronounced interdinuclear spin exchange interaction. By simulating powder patterns, we estimated the effective interdinuclear exchange coupling constants, and the obtained values are 1.4(4) and 2.3(3) cm−1 for the as-synthesized and activated forms of MOF. Analysis of the structural data of the activated compound shows that the activation procedure removes the water molecules from the apical sites of the PWs. This allows the neighboring units to come closer and form an additional Cu−O−Cu exchange path for the excited S = 1 spin states. We demonstrated that the readsorption of water closes this path, showing that the magnetic properties of kagome MOFs can be controlled by the guest molecules even at room temperature. This feature could be potentially interesting for applications related to sensing of molecules,


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DOI: 10.1021/acs.jpcc.6b09853 J. Phys. Chem. C 2016, 120, 27462−27467


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