Synthesis, Structure, and Electron Paramagnetic Resonance Study of

Article pubs.acs.org/JPCC

Synthesis, Structure, and Electron Paramagnetic Resonance Study of a Mixed Valent Metal−Organic Framework Containing Cu2 PaddleWheel Units Mantas Šimeṅ as,† Merten Kobalz,‡ Matthias Mendt,§ Pierre Eckold,∥ Harald Krautscheid,‡ Juras Banys,† ̅ and Andreas Pöppl*,§ †

Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius, Lithuania Faculty of Chemistry and Mineralogy, Universität Leipzig, Johannisallee 29, D-04103 Leipzig, Germany § Faculty of Physics and Earth Sciences, Universität Leipzig, Linnestrasse 5, D-04103 Leipzig, Germany ∥ Institute of Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany ‡

S Supporting Information *

ABSTRACT: We report synthesis and composite study of a novel metal−organic 3 framework (MOF) compound of chemical formula ∞ [Cu2ICu2II{H2O}2{(Me−trz− mba)2thio}2]Cl2, where (Me−trz−mba)2thio2− stands for 3,3-(5,5-(thiophene-2,5-diyl)bis(3-methyl-4H-1,2,4-triazole-5,4-diyl))dibenzoate. This coordination polymer was synthesized by solvothermal synthesis. The crystal structure was determined using single crystal X-ray diffraction. The main building block of this compound is a so-called Cu2 paddle-wheel (PW) unit, which contains two Cu2+ ions connected via four carboxylate groups. Magnetic properties of the investigated MOF were studied by continuous-wave electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies in a wide temperature range. Mononuclear Cu2+ ions were observed in the EPR spectra and characterized by spectral simulations. In addition, the obtained EPR data allowed us to detect and investigate three distinct magnetic interactions related to the Cu2+ pairs. At higher temperatures the fine structure pattern was observed in the EPR spectra and the spin−spin interaction tensor D was determined. The origin of this pattern was assigned to the thermally populated excited triplet states of the Cu2+ pairs. It was found that two Cu2+ ions within a single PW unit couple antiferromagnetically with the exchange coupling constant J = −258 cm−1. Moreover, the EPR spectra of dehydrated MOF samples show a broad, poorly resolved spectral feature, the origin of which is an exchange of the spin triplets between neighboring Cu2 PW units. By simulating the powder pattern of this interdinuclear exchange line, we estimated the exchange coupling between neighboring PW units (|J′| = 4.9 cm−1). It was also found that the interdinuclear exchange gradually disappears, if the dehydrated samples are allowed to interact with air, demonstrating that this exchange can be rather easily manipulated in the investigated MOF.

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selective gas adsorption,12,13 and separation,14 as well as chemical sensing,15 catalysis,16,17 or air purification.18 In addition to the technological applications, coordination polymers also draw more fundamental scientific attention due to the peculiar magnetic,19 ferroelectric,20 or optical21 phenomena observed in these materials. One of the most studied MOFs is the Cu3(btc)2(H2O)3· xH2O (btc = benzene 1,3,5-tricarboxylate) compound,22 which possesses specific magnetic properties.23 The magnetism in this material arises from a so-called Cu2 paddle-wheel (PW) unit, which consists of two Cu2+ ions (also called Cu2+ pairs) interconnected via four carboxylate groups in the syn−syn fashion. In general, the Cu2 PW unit is the main building block for a vast number of dinuclear compounds with general formula Cu2(OOCR)4L2.24,25 A classical electron paramagnetic

INTRODUCTION

Materials possessing highly porous structures are of high scientific and technological interest due to their broad applicability. For a long time, this class of materials has mainly consisted of inorganic (e.g., zeolite and mesoporous silica) and activated carbon-based solid compounds.1In the 1990s, a third type of porous material called porous coordination polymers or metal−organic frameworks (MOFs) was intensively developed, since it was recognized that a huge diversity of highly porous geometries can be obtained by combination of polydentate ligands with a variety of metals.2−5 In general, MOFs are highly porous ordered crystalline materials, which consist of two main building blocks: metal ions or secondary building units as connectors and organic molecules as linkers.6,7 Transition metal and lanthanide ions are the most popular connectors due to the potentially high coordination number, which together with the great diversity of organic linkers give rise to a vast number of different topologies. As a result of the porosity, MOFs are highly attractive for applications related to gas storage,8−11 © 2015 American Chemical Society

Received: December 18, 2014 Revised: January 30, 2015 Published: February 2, 2015 4898

DOI: 10.1021/jp512629c J. Phys. Chem. C 2015, 119, 4898−4907

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The Journal of Physical Chemistry C resonance (EPR) study of such a compound is presented in a prosperous work of B. Bleaney and K. D. Bowers, in which the anomalous continuous-wave (CW) EPR signal of the copper(II) acetate monohydrate was explained by proposing that the two Cu2+ ions couple antiferromagnetically as a result of the superexchange interaction transmitted via diamagnetic carboxylate paths.26,27 Besides the intradinuclear interaction between cupric ions, the much weaker interdinuclear exchange was observed in the EPR spectra of polymeric chains of similar Cu2 dinuclear units.28 Generally, the CW EPR spectroscopy proved to be a very powerful tool to probe such magnetic interactions in dinuclear29−32 as well as MOF33−36compounds, containing pairs of Cu2+ ions. Here we report the synthesis, structural, and EPR study of a new MOF with a chemical formula 3∞[CuI2CuII2 {H2O}2{(Me− trz−mba)2thio}2]Cl2, where (Me−trz−mba)2thio2− stands for 3,3′-(5,5′-(thiophene-2,5-diyl)bis(3-methyl-4H-1,2,4-triazole5,4-diyl))dibenzoate. The structure of this MOF is based on the Cu2 PW secondary building unit. This compound was synthesized by solvothermal synthesis, and its structure was determined using single crystal X-ray diffraction. The magnetic properties of this material were investigated by X- and Q-band CW EPR spectroscopy in a broad temperature range from 10 to 293 K. Such experiments allowed us to study the antiferromagnetic coupling of the two cupric ions in a single PW unit, determine the exchange coupling constant of this interaction and characterize mononuclear Cu2+ ion species, which were also detected in the samples. Additionally, the interdinuclear exchange interaction emerged in sufficiently dehydrated samples. Using the exchange narrowing theory, we simulated the observed powder pattern of the interdinuclear exchange line and determined the exchange coupling constant of this interaction.

Table 1. Investigated Samples and Their Post-Synthesis Treatmenta sample

temperature (°C)

time (hours)

1 2 3 4 5 6 7 8 9

no activation 23 80 120 23 23, 80 23, 120 23, 100 120

no activation 24 24 24 72 24, 1 24, 1 24, 2 72

a

Temperature and time of sample activation in vacuum is presented in the second and the third columns, respectively.

CW EPR measurements were carried out with a Bruker EMX 10−40 spectrometer. For most of the room temperature measurements 20 mW microwave power, a modulation strength of 5 G and a modulation frequency of 100 kHz were used. To avoid saturation effects, the measurements at low temperature were performed at 2 mW microwave power. All simulations of spectra were carried out using EasySpin simulation software implemented under Matlab environment.37 Structural Characterization. The single crystal X-ray diffraction data was measured using a IPDS-2T (STOE) image plate detector system with laboratory Mo−Kα X-ray ragdiation (λ = 71.073 pm). The data set was processed with the STOE XAREA software.38 The structure was solved by direct methods and refined using SHELX-97.39 The non-hydrogen atoms of the framework were refined anisotropically. The coordinates of the hydrogen atoms of the framework were calculated for idealized positions. Due to the fact that no solvent molecules could be localized in the difference Fourier map, the SQUEEZE-routine of the PLATON software was applied.40 DIAMOND 3.2f41 was used to visualize the structures. The powder diffraction data were collected at room temperature on a STADI-P (STOE) diffractometer in Debye−Scherrer-Geometry with Cu−K α-radiation (λ = 154.060 pm). The measuring cell was a glass capillary (0.5 mm in diameter).

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EXPERIMENTAL SECTION Synthesis and Sample Preparation. The 3 I II ∞[Cu2Cu2 {H2O}2{(Me−trz−mba)2thio}2]Cl2 MOF samples were synthesized using solvothermal synthesis with H2O/ MeCN (water/acetonitrile, volume ratio 1:1) as a solvent. A Teflon beaker was loaded with 0.1 mmol (1.0 equiv) of the protonated ligand H2(Me−trz−mba)2thio, 0.4 mmol (4.0 equiv) CuCl2·2H2O and 5 mL of a mixture of water/ acetonitrile (v/v = 1:1). The vessel was sealed and the reaction mixture was heated up to 140 °C within 1 h. The temperature was kept on a constant level for 5 h, and then the autoclave was cooled to room temperature during a period of 59 h. After filtration, the crystalline product was washed with H2O/MeCN to get the target compound in 72% yield. In order to achieve a complete solvent exchange, selected samples were Soxhlet extracted with methanol for 7 days. After each day, a new batch of methanol was used. We investigated nine samples of 3∞[CuI2CuII2 {H2O}2{(Me− trz−mba)2thio}2]Cl2, which differ by their postsynthesis treatment. All samples and their treatment are listed in Table 1. Sample 1 was not activated, and samples 6, 7, and 8 were activated twice at different conditions in vacuum. All samples except 1 were Soxhlet extracted with methanol. EPR Measurements. The X-band (∼9.5 GHz) CW EPR measurements were performed using a Bruker ELEXYS E580 spectrometer. The temperature dependent EPR signal intensity was measured using a rectangular dual mode cavity, since it allowed for simultaneous measurement of a reference sample, which was kept at room temperature. The Q-band (∼34 GHz)

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RESULTS AND DISCUSSION Crystal Structure of 1. Crystallization of 1 occurs in the orthorhombic space group I212121 (No. 24) with four formula units per unit cell, including a copper(II) PW coordinated by ((Me−trz−mba)2thio)2−-ligands and two water molecules on the apical positions. In addition, the linker is coordinating with nitrogen donor atoms of the triazole ring to four Cu+ ions. Furthermore, a disordered chloride ion was refined, which contributes to the charge-neutrality of the framework. A segment of the crystal structure of 1, focusing on the coordination environments of Cu1 and Cu3, is shown in Figure 1. Table 2 summarizes the selected bond lengths and angles of 1 (details of the crystal structure data are given in the Supporting Information, SI). With the exception of the sulfur atom in the thiophene ring, the ((Me−trz−mba)2thio)2−-ligand is coordinating to both copper ions using all available donor atoms. Each ligand is coordinating to two PW units and Cu+ ions. The distance between the Cu2+ ions in the PW unit was assigned to be 264.2(2) pm, and it is similar to other coordination polymers exhibiting similar structure motif.22,35 In 4899


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The Journal of Physical Chemistry C

Figure 1. Polyhedral presentation of the coordination environments of the Cu1 and Cu3 (left, 50% probability ellipsoids) and buildup of the threedimensional network of 1 (right). Symmetry codes: a: 0.5 − x, y,1 − z; b: − x, 0.5 + y, 1.5 − z; c: x, 1 − y, 1.5 − z; and d: − x, 0.5 − y, z.

Table 2. Selected Distances and Angles in 1 atoms 1−2

bond length (pm)

atoms 1−2−3

angle (deg)

Cu1···Cu1 Cu1O5 Cu1O1 Cu1O2a C17O1 C17O2 Cu3N3 Cu3N6b

264.2(1) 216.0(2) 196.1(1) 195.9(1) 127.1(1) 126.8(1) 205.3(1) 205.0(1)

O5Cu1O1

92.96(1)

O5Cu1O3

98.90(1)

N3Cu3N6b

105.27(2)

N3Cu3N3c

108.27(1)

Figure 3. Normalized experimental (black) and simulated (red) (a) Xband and (b) Q-band spectra of powder sample 2 at 10 K and room temperature. The M-symbol marks the line of mononuclear Cu2+ ions. The PW unit with collinear gz and Dz principal axes of Cu2+ pair g- and D-tensors are also indicated in (b).

a single PW unit, an axial symmetry along the CuCu is generated. Cu3 is distorted tetrahedral coordinated by nitrogen donor atoms of triazole rings provided by four ((Me−trz− mba)2thio)2−-ligand molecules. This coordination mode leads to the formation of a three-dimensional network exhibiting mgc-topology ({410.62.816}{45.6}2). Utilizing PLATON,40 the solvent accessible pore volume was determined to be 30% (sorption isotherms of N2 and CO2 are presented in the SI). Pore channels of approximately 420 × 420 pm2 along crystallographic a-axis are responsible for the porosity of this coordination polymer (see Figure 2). EPR of 2. The experimental X and Q-band CW EPR spectra of 2 measured at 10 K and at room temperature are presented in Figure 3. The X-band room temperature spectrum consists

of four lines at magnetic fields of about 50 mT, 300 mT, 500 mT, and 620 mT, while five lines are observed in the Q-band case at 500 mT, 700 mT, 950 mT, 1170 mT, and 1350 mT. Similar powder patterns, as recorded in the room temperature spectrum of 2, were detected for vast number of copper dinuclear systems, 42,43 especially copper carboxylates (Cu2(O2CR)4L2),28,31,44,45 as well as for Cu3(btc)2(H2O)3,33 [Zn1−xCux(bdc)(dabco)]0.534 and STAM-133 MOFs, which

Figure 2. Ball and stick (left) and space filling (right) projections of 1 containing a three-dimensional pore system. Pore channels are extended along crystallographic a-direction. 4900


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PW units: 0.320 cm−1 was found for Cu3(btc)2(H2O)3,33 0.340 cm−1 for [Zn1−xCux(bdc)(dabco)]0.534 and 0.337 cm−1 for STAM-I MOF.35 In addition, very similar values were also observed for many monomolecular Cu2 dinuclear compounds.28,44,45,49 The simulation of the Cu2+ pair signal does not explain the line marked by the M-symbol at about 300 mT (X-band) and 1170 mT (Q-band), which also prevails in the 10 K spectra (see Figure 3). This signal is the well-known anisotropic EPR powder pattern of the mononuclear Cu2+ ions possessing an S = (1/2) electron spin which interacts with the nuclear magnetic moments of the 63Cu and 65Cu isotopes (both having nuclear spin I(Cu) = (3/2)). Two axially symmetric patterns of two slightly distinct Cu2+ ion species (A and B) can be identified in the experimental X-band spectrum of 2. To further characterize these two species, spectral simulations were carried out using the spin Hamiltonian with included electron Zeeman and hyperfine interactions:

contain similar Cu2 PW units. The first interpretation of such spectrum was already given in 1952 by B. Bleaney and K.D. Bowers,26,27 who observed an atypical single crystal spectrum of Cu2+ acetate monohydrate, (Cu(CH3COO)2(H2O)2). They concluded that the two S = (1/2) spins of two interconnected Cu2+ ions (electronic configuration of a single Cu2+ ion is 3d9, S = (1/2)) couple antiferromagnetically into the S = 0 ground state (EPR silent), but the excited S = 1 triplet state (susceptible to EPR spectroscopy) is thermally populated at elevated temperatures. This coupling occurs because two unpaired electrons reside in the dx2−y2 orbitals of Cu2+, and thus they can be exchanged as a result of the superexchange interaction, which is mediated via diamagnetic carboxylate groups.43 The PW unit in the investigated MOF also contains two Cu2+ ions connected via four carboxylate groups (see Figure 1). Therefore, we propose that the origin of the observed room temperature powder pattern is the coupling of the two unpaired electron spins, resulting in the excited triplet S = 1 state. This proposal is verified by spectral simulations, which were performed for an anisotropic and isolated S = 1 spin system. The spin Hamiltonian used for the simulations includes the electron Zeeman interaction and the fine structure (fs) term:46

/ (Cu) = βeB0giS + SAi(Cu)I (Cu) i

Here the subscript i marks species A or B and A(Cu) denotes the i hyperfine tensor of copper nucleus. Note that it was not possible to properly simulate the parallel part of the Cu2+ signal in the Q-band spectrum, since the hyperfine interaction at this frequency was not properly resolved. The simulated low temperature spectra are also presented in Figure 3. Both Cu2+ species were assumed to have axially symmetric and collinear gand A(Cu)-tensors. For the species A, the parallel components of g- and A(Cu)- tensors are g∥,A = 2.337(2) and A(Cu) ∥,A = 160(2) × 10−4 cm−1. For species B, these parameters are slightly −4 different: g∥,B = 2.320(2) and A(Cu) cm−1. ∥,B = 174(2) × 10 The perpendicular components g⊥ of the g-tensors are 2.05(1) and 2.06(1) for the A and B species, respectively. Since the g⊥ spectral region is not sufficiently well resolved, the determination of A⊥ was obscure. The occurrence of the mononuclear Cu2+ ions in the investigated MOF is an unexpected result, since, according to the structural analysis data, all Cu2+ ions are coupled into pairs within the PW units. However, mononuclear Cu2+ ions were also observed in Cu 3 (btc) 2 (H 2 O) 3 , 33 [Zn 1−x Cu x (bdc)(dabco)]0.534 and STAM-I MOFs,35 where they were attributed to extra-framework impurities. In this work, we will not further investigate the origin and nature of these Cu2+ defect centers. Intradinuclear Exchange. To verify the proposal that the two electron spins within a single PW unit couple antiferromagnetically into the S = 0 ground state, the spectrum of 2 was recorded at different temperatures. The obtained experimental Q-band spectra are presented in Figure 4 (see SI for the X-band spectra). As already discussed, the 10 K spectrum consists only of the mononuclear Cu2+ signal, meaning that all electronic spins in the PW units are in their S = 0 ground state and therefore are EPR silent. However, the thermally excited S = 1 state starts to populate at 50 K, as the weak Bx2,y2 signal is observed. With further increase of temperature, pairs of Cu2+ ions continue to occupy the S = 1 state, resulting in higher signal intensity of its typical fs EPR spectrum. The intensity of the temperature dependent fs EPR spectrum of the Cu2+ pairs can be used to determine the energy splitting (isotropic exchange coupling) J between the S = 0 ground state and the S = 1 excited state (see inset in Figure 5 for the energy level diagram). This is carried out by taking into account that

/ (Cu 2) = βeB0gS + SDS = βeB0gS + DSz2 + E(Sx2 − Sy2) −

2D 3

(2)

(1)

Here βe is the Bohr magneton, g denotes the Cu2+ pair g-tensor and D is a traceless fs tensor with its components parametrized by the D (axial) and E (orthorhombic) zero-field splitting (zfs) parameters. Other symbols have their usual meanings. For the orthorhombic case (D ≠ 0 and E ≠ 0), it is expected that the two allowed EPR transitions (ΔmS = ± 1, where mS refers to the magnetic spin quantum number) result in six resonance lines, since for each transition there are three principal directions. However, if a system has axial symmetry (D ≠ 0 and E = 0), then according to the standard notation46 the number of resonances is reduced to four: Bx1,y1, Bx2,y2, Bz1, and Bz2. We note that the 63,65Cu hyperfine splitting was not sufficiently well resolved in the experimental spectra and thus we do not include the hyperfine interaction into the spin Hamiltonian. The simulated room temperature spectra are also presented in Figure 3. For X and Q-band simulations the same set of spin Hamiltonian parameters has been used. Note that in the Qband spectrum the Bx2,y2 and Bz2 resonance fields are almost the same and in the low field region (∼500 mT) the so-called forbidden transition (ΔmS = ± 2) is detected. Simulations reveal that in the X-band spectrum the forbidden and the Bx1,y1 transitions are not observed, since the zfs parameter D and the X-band microwave frequency are of comparable magnitude. The spin Hamiltonian parameters used for simulations are gx = gy ≡ g⊥ = 2.072(2), gz ≡ g∥ = 2.383(2), D = −0.355(1) cm−1 and E ≈ 0. From our EPR experiments it is not possible to determine the sign of D, but it was chosen to be negative in accordance with recent theoretical ab initio47 and experimental high-field EPR48 findings for similar Cu2 PW systems. The obtained axially symmetric and collinear D and g-tensors confirm the results of structural analysis, which indicate that the PW unit has D4h symmetry (see Figure 3b). The absolute value of the axial zfs parameter D is in a good agreement with the previously reported values for MOF systems, containing Cu2 4901


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antiferromagnetically. These results are further justified by the SQUID magnetic susceptibility measurements, since the value of J = −278(2) cm−1 was obtained for 1 (see SI). The slight discrepancy between the SQUID and EPR measurements could arise due to the different treatment of the studied samples after the synthesis. In addition, a deviation between J values obtained from the magnetic susceptibility and EPR data was already observed for other similar systems.27,33 The determined value of J is in a good agreement with values found for other compounds containing Cu2 dinuclear units.23,27,33,35,50,51 Interdinuclear Exchange. In order to investigate what effect the activation temperature has on the studied MOF, we also performed EPR measurements of 3. The temperature dependent Q-band spectra of this sample are presented in Figure 6 (see SI for the X-band spectra). This dependence has a

Figure 4. Normalized Q-band spectra of powder sample 2 recorded at different temperatures. The M-symbol indicates the mononuclear Cu2+ line.

Figure 6. Normalized temperature dependent Q-band spectra of 3. Asterisk and M-symbol indicate interdinuclear exchange and mononuclear Cu2+ lines, respectively.

Figure 5. Temperature dependence of the normalized EPR intensity of the fs Cu2+ pair X-band signal Bx2,y2 of 2. Solid curve is the best fit to the Bleaney−Bowers equation. The zero-field energy level diagram for the S = 0 ground state and the excited S = 1 state is illustrated in the inset (the zfs of the S = 1 state is exaggerated).

rather similar trend as observed in spectra of 2: the excited S = 1 state of the Cu2+ pairs starts to populate at around 60 K and a further increase in temperature results in higher signal intensity of its fs EPR spectrum. From a first impression, it seems that the mononuclear Cu2+ signal dramatically increases with temperature as well, but this assumption clearly disagrees with the paramagnetic character of these species. This indicates that the signal at around 1100 mT (marked by the asterisk symbol in Figure 6) cannot be solely assigned to the paramagnetic mononuclear Cu2+ ions. The observed temperature dependence of this line suggests that the mononuclear line is superimposed by another signal, originating from the Cu2+ pairs, because the intensity of this line starts to increase at around the same temperature at which the resolved fs spectrum of the Cu2+ pairs emerges. This line has an approximate Lorentzian line shape with a line width Γpp ≈ 80 mT and its gvalue is approximately 2.17, which is in a perfect agreement with the average g-value (g = ((2g⊥ + g∥)/3) = 2.175(1)) of the S = 1 spectrum of the Cu2+ pairs. These observations suggest that the anisotropic Cu2+ pair signal collapses (is averaged) into a single line, which overlaps with the mononuclear Cu2+ signal.

the intensity of an EPR line is generally proportional to the magnetic susceptibility χ, which for dinuclear compounds follows the Bleaney−Bowers equation:26,43 χ = μ0

2βe2g 2 kBT

(3 + e−J / kBT )−1 −7

(3)

−1

where μ0 = 4π·10 T·m·A is the permeability of vacuum, g is an average of the principle values of the g-tensor, and J is the isotropic exchange coupling. Note that here we use the following form of the Heisenberg exchange Hamiltonian: / = −JS1S2 . For temperature dependent measurements the Bx2,y2 line was chosen in the X-band spectra (see SI) and its intensity was determined by double numerical integration. Afterward, the exchange coupling J was obtained by fitting the experimental data with eq 3 (see Figure 5). The reliable fit yields J = −258(5) cm−1. The negative sign of the exchange coupling confirms that Cu2+ ions in the PW units are coupled 4902


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The Journal of Physical Chemistry C Such a line was already observed in Cu 3 (btc) 2 , 33 [Zn1−xCux(bdc)(dabco)]0.534 and STAM-135 MOFs as well as in many dinuclear complexes.28,42,52−54 Kozlevčar et al.28 have investigated the origin of this line by measuring the magnetic properties and EPR spectra of isolated and polymeric dinuclear Cu2+ complexes. Such a signal was found only for the polymeric compounds, indicating that its origin lies in the interdinuclear exchange interaction between adjacent PW units, in which Cu2+ pairs are in the excited S = 1 triplet state. This phenomenon is sometimes called the exchange narrowing and it is caused by the stochastic spin triplet exchange between the interconnected PW units.29,30,55,56 This is consistent with the observation that the collapsed line intensity decreases as the temperature is lowered, since some of the Cu2+ pairs relax into the S = 0 ground state. The occurrence of the interdinuclear exchange line in the spectrum of 3 and the lack of such a line in the spectrum of sample 2 is an unexpected result for which there seems to be two possible explanations: either the activation at 80 °C destroys the framework, and as a result interdinuclear exchange paths are formed or the dehydration of the sample opens the existing exchange paths. To investigate which scenario is more plausible, a new sample was prepared and activated at 120 °C for 24 h under vacuum and subsequently sealed in a quartz tube (4). The X-band room temperature spectrum of this material is presented in Figure 7, indicating the superposition of the

significantly less water is left in 3 compared to 2 after the activation (see SI). This is an important result, because it shows that in this material the opening and closing of the interdinuclear exchange paths is a reversible, dehydrationinduced process. In addition to the interdinuclear exchange line, the room temperature Q-band spectrum of 3 shows a well resolved anisotropic fine structure spectrum (see Figure 6). This observation raises the question whether the dehydration of the sample was incomplete and thus some PW units are not participating in the interdinuclear exchange or the exchange frequency is just too small to average out the whole fs spectrum (situation observed e.g. in refs 29 and 30). To answer this question, samples activated under different conditions were prepared (samples 5, 6, 7, 8, and 9) and measured at Q-band frequency. The room temperature spectra of these samples are presented in Figure 8a (together with the spectra of 1, 2, and

Figure 8. Investigated samples activated at different conditions: (a) Normalized Q-band room temperature spectra, (b) intensity ratio between the interdinuclear exchange and resolved fine structure signals vs the D parameter. Collapsed line is marked by the asterisk in (a).

3). It can be seen that higher temperatures and/or a longer duration of the activation process increase the intensity of the interdinuclear exchange signal. For example, the spectrum of 9 shows a higher intensity of the interdinuclear exchange line than observed for 3. This suggests that 3 was not completely dehydrated and therefore the observed spectrum is a superposition of the resolved anisotropic and fully collapsed fine structures. In addition, the absolute value of the axial zfs parameter D also increases with increasing time and/or temperature of the activation. Its absolute value ranges from 0.338(1) cm−1 (1) to 0.367(2) cm−1 (3 and 9) (see Table 3 for relevant magnetic parameters of Cu2+ pairs). One possible explanation for change

Figure 7. X-band spectra of 4 just after the activation (top) and after 1, 4, and 14 days of keeping the sample at ambient conditions. The asterisk symbol indicates the superposition of the mononuclear Cu2+ and the interdinuclear exchange lines.

mononuclear Cu2+ and the collapsed fs signal at around 300 mT as the dominant spectral feature (denoted by the asterisk symbol). Afterward, the top of the sample tube was cleaved to allow contact with air. The X-band spectra after 1, 4, and 14 days of keeping the sample at ambient conditions are also shown in Figure 7. It can be seen that after allowing the sample to interact with air, the interdinuclear exchange signal shrinks significantly as the intensity of the anisotropic resolved fs signal increases, and the final spectrum becomes indistinguishable from the prior-activation spectrum. This observation demonstrates that a permanent framework destruction did not occur while heating the MOF up to 120 °C. In addition, the PXRD pattern of 3 shows just minor differences in some reflection positions, indicating only slight changes in the framework, while the overall MOF structure is preserved (see SI). Therefore, the second scenario, which proposes that the dehydration of the framework opens the existing exchange paths, seems to be more plausible. This is further justified by the differential thermal analysis (DTA) measurements which indicate that

Table 3. Relevant Magnetic Parameters of Cu2+ Pairs in Different Samplesa

a

4903

sample

−D (cm−1)

−J (cm−1)

1 2 3 5 6 7 8 9

0.338(1) 0.355(1) 0.367(2) 0.358(2) 0.357(1) 0.360(2) 0.365(2) 0.367(2)

278(2)b 258(5)c

|J′| (cm−1)

4.9(7)

Measurement method. bSQUID. cEPR. DOI: 10.1021/jp512629c J. Phys. Chem. C 2015, 119, 4898−4907

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is a set of Euler angles for which the collapse occurs, and Γpp,0 is a residual line width, considered as a constant. Unfortunately, we did not manage to grow sufficiently large single crystals of the investigated MOF and by measuring the collapse in the single crystal EPR spectra to directly determine | J′|. Nevertheless, we obtained an approximate magnitude of |J′| by simulating the powder pattern of the interdinuclear exchange line observed in the Q-band spectrum of 3 (see Figure 9). In the simulation, it was assumed that the observed

in D is the following: during the activation some of the axially coordinating H2O molecules are removed from the PW units and the distance between two Cu2+ ions is reduced. Therefore, the dipole−dipole coupling component Ddip of the D parameter becomes bigger, resulting in an overall increase of the absolute value of D. Assuming this proposal is correct and the spin− orbit part of D does not change if water is removed, then according to the point-dipole approximation (Ddip = −β2e (g2∥ + g2⊥/2)/r3)48 the observed increment in |D| would require a reduction of CuCu distance r of roughly 0.1 Å. A similar situation was observed for [Cu(Cl3CCOO)2(3−Xpy)]2 complexes with X = Cl, Br, H, CN, or CH3, since a significant decrease in |D| was found for compounds in which CN or CH3 was bonded to the axially coordinating pyridine (py).57 It was concluded that |D| decreased due to the distortion of the metal geometry from the square pyramidal (Cl, Br, and H) toward the trigonal-bipyramidal (CN and CH3). Moreover, the significant broadening of the fs signal for samples activated at higher temperatures indicates a possible distribution of the D values, suggesting that some PW units have two, one or none axially coordinating H2O molecules. In Figure 8b, the intensity ratio between the interdinuclear exchange and the resolved anisotropic fs spectrum is plotted against the D parameter for all investigated samples. A correlation between these two quantities can be observed the increment of |D| is accompanied by the increase in the intensity of the interdinuclear exchange line. However, it is not completely clear if this is a direct correlation, i.e., if the exchange paths are opened by the removal of water from the PW units. It might be that some water species coordinating to the linker are eliminated, and thus the interdinuclear exchange becomes possible. To get a deeper understanding about this exchange process, spectral simulations of the collapsed line were performed. Simulation of the Interdinuclear Exchange Line. In refs 29, 30, and 32, the interdinuclear exchange coupling constant was determined from the single crystal EPR spectra using a method based on the exchange narrowing theory.55,56 The main idea behind this method is that due to the stochastic interactions of the dinuclear units (migration of the triplet excitons through the crystal lattice), the fs spectrum collapses into a single Lorentzian line for some orientations of a single crystal with respect to the external magnetic field. The collapse range ΔBcollapse is given by the following: ΔBcollapse =

hνex gβe

Figure 9. Simulation of the room temperature Q-band spectrum of 3 with included interdinuclear exchange line: (a) mononuclear Cu2+ signal, (b) resolved Cu2+ pair fine structure, (c) collapsed fine structure, (d) sum of all, and (e) experimental spectrum.

spectrum is a superposition of the resolved Cu2+ pair fs spectrum and the interdinuclear exchange line (collapsed fs spectrum). This assumption is based on the observation that the intensity ratio between the resolved fs spectrum and the interdinuclear exchange signal depends on the sample activation temperature (see Figure 8), meaning that some PW units do not participate in the exchange. Otherwise, it would mean that the exchange frequency (collapse range) depends on the sample activation temperature or water content in the sample (see Figure 7), which is not a likely scenario. In addition, to obtain the best agreement with the experimental pattern, we also included the simulated mononuclear Cu2+ line in the final spectrum (for simulation, the same set of spin Hamiltonian parameters was used as for 2). The simulation of the interdinuclear exchange line was performed in the following way. First, the angular dependent single crystal spectra were simulated using the g- and D-tensors as determined from the resolved fs powder pattern. Then the splitting ΔBcalc(θ) between the two resonances was calculated for all orientations θ between the g∥ principal value and the external magnetic field. Afterward, the exchange frequency νex (collapse range) was chosen and for all orientations θ, for which ΔBcalc(θ) < ΔBcollapse, the two resonance lines were averaged into a single line with a line width given by eq 6. In the final step, the powder average of the determined lines was calculated. In this way, the parameters νex and Γpp,0 were varied until the best agreement with the experimental spectrum was achieved (see Figure 9). A more detailed description of the simulation is given in the SI. The determined parameter values are νex = (205 ± 30) GHz and Γpp,0 = 13 ± 5 mT. The absolute value of interdinuclear exchange coupling constant can be calculated using eq 5 to be |J′| = 4.9 ± 0.7 cm−1 (this method does not allow to obtain the sign of the interaction). The obtained value of Γpp,0 is in a good agreement with the values determined for similar dinuclear systems.30,32 Similar values of the interdinuclear exchange coupling constant were observed in the chains of Cu2 dinuclear

(4)

where νex is the exchange frequency, which is related to the interdinuclear exchange coupling constant J′ by the following equation: νex =

2 |J ′| h

(5)

In addition, the peak-to-peak line width Γpp of the collapsed line is as follows: Γpp =

gβe(ΔBcalc (Ω))2 hνex

+ Γpp,0

(6)

where ΔBcalc(Ω) is the calculated splitting between the collapsing lines in the absence of the exchange narrowing, Ω 4904


Article

The Journal of Physical Chemistry C units of formula [Cu(trans-2-butenoate)4]n (J′ = 5.9 cm−1, bridges between apical positions)31 and Cu2(syn,syn-η1:η1:μMeCOO)4(anti,anti-η1:η1:μ-MeCOO)(Et3NH)]n (J′ = −25 cm−1, bridges between basal positions).58 In these compounds, the exchange pathways are much shorter (up to 4 bonds) compared with the pathway that connects two PW units in the investigated MOF (19 bonds), making it a rather poor candidate for the electron exchange path. Moreover, if the exchange is mediated through the linker, then it is unclear why the interdinuclear exchange does not occur in 2. Therefore, it seems that the exchange among the triplets of the Cu2+ pairs is mediated via apical positions of the PW units, since the distance between two Cu2+ ions from the neighboring units is only 4.99 Å. The removal of the axially coordinating water may create a weak interaction between two PWs, permitting the exchange. For example, it was found28 that two dinuclear copper units form a stacked dimer unit via CuO coordination with Cu O distance of 2.22 Å, where Cu and O atoms are from different units. However, the CuO distance of 4.23 Å, determined for the investigated MOF, is too big to form such a bond. Thus, the observation of the interdinuclear exchange interaction across such an extended distance seems to be unique and the precise origin of the interdinuclear exchange path in the investigated MOF still remains unclear. It should be also mentioned that few approximations were involved in the simulation of the interdinuclear exchange line. First, only one interdinuclear exchange path with the exchange coupling constant J′ was considered. Second, for simplicity, all Cu2 PW units were treated as magnetically equivalent for all directions of the applied magnetic field, which clearly contradicts the observation of two distinct orientations of the PW units in the crystal structure.

a dehydration-induced process. To our knowledge, the synthesized MOF is the first material in which this phenomenon can be rather easily manipulateda property important for study and application of molecular magnets.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of the protonated ligand H2(Me−trz−mba)2thio, CO2 and N2 sorption data, additional EPR spectra, DTA data, SQUID magnetization measurements and more detailed simulation of the interdinuclear exchange line. CCDC 1046250 containing the supplementary crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/ data_request/cif. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 341 9732608. Fax: +49 341 9732649. E-mail: [email protected] (A.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the German Research Foundation DFG within the framework of its priority program SPP 1362 (”Poröse metallorganische Gerüstverbindungen”) is gratefully acknowledged. M.Š. thanks German Academic Exchange Service (DAAD) for the financial support. The authors thank Dr. J. Möllmer and M.Sc. M. Lange from the “Institut für Nichtklassische Chemie e.V. (INC)” for CO2 and N2 sorption measurements.

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CONCLUSIONS The investigated MOF ∞ 3 [Cu 2I Cu 2II {H 2 O} 2 {(Me−trz− mba)2thio}2]Cl2 was synthesized using solvothermal synthesis, and its structure was determined by single crystal X-ray diffraction. The network contains a well-known copper paddlewheel motif as a secondary building unit, formed by four carboxylate groups of the ligand and water molecules on the axial coordination sites. The temperature dependent X- and Qband CW EPR spectra reveal that Cu2+ ions in the investigated MOF are present in two different magnetic states. The first are the paramagnetic mononuclear Cu2+ defect species, which were characterized by spectral simulations. The second are Cu2+ pairs in the paddle wheel units, which are coupled antiferromagnetically, meaning that at low temperatures, all such pairs are in the EPR silent S = 0 ground state. At temperatures above 50 K, their excited S = 1 triplet state starts to populate, resulting in a well resolved fs CW spectrum. Simulation of the room temperature fs pattern allowed us to obtain the g- and D-tensors, characterizing the pairs of Cu2+. By following the temperature dependence of the resolved fs spectrum, it was possible to determine the isotropic exchange coupling constant J. Surprisingly, the activation of the MOF at higher temperatures opens additional spin exchange paths between excited spin states of the Cu2+ pairs. This exchange results in the collapse of the formally resolved fs spectrum into a single line. By simulating the powder pattern of the exchange narrowed line, we managed to determine the absolute value of the interdinuclear exchange coupling constant J′. After the interaction between the activated MOF and air, these paths are closed again, demonstrating that the interdinuclear exchange is

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REFERENCES

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