Synthesis and Magnetic Characterization of Fe(III)-Based 9

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Synthesis and Magnetic Characterization of Fe(III)-Based 9‑Metallacrown‑3 Complexes Which Exhibit Magnetorefrigerant Properties Chun Y. Chow,† Régis Guillot,‡ Eric Rivière,‡ Jeff W. Kampf,† Talal Mallah,*,‡ and Vincent L. Pecoraro*,† †

Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, United States Institut de Chemie Moléculaire et des Matériaux d’Orsay, CNRS, Université de Paris Sud 11, 91405 Orsay Cedex, France

‡

S Supporting Information *

ABSTRACT: The structural characterization and magnetic properties of three related 9-metallacrown-3 (9-MC-3) structures are reported. Each of these iron complexes is shown to exhibit significant magnetic refrigerant properties. Fe III (acetate) 3 [9-MC Fe I I I N(shi) -3](MeOH) 3 ·MeOH·7H 2 O (1-OAc) and FeIII(benzoate)3[9-MCFeIIIN(shi)-3](MeOH)3·MeOH·4H2O (1-OBz) are structurally analogous tetranuclear iron(III) clusters which exhibit drastically different magnetic properties, due to differences in intermolecular and intramolecular π interactions which affect superexchange. 1-OAc displays a magnetocaloric effect with a maximum entropy change of −ΔSm = 15.4 J kg−1 K−1 at T = 3 K and an applied field change of μoΔH = 7 T, whereas 1-OBz exhibits a maximum −ΔSm = 7.4 J kg−1 K−1 at T = 7 K and μoΔH = 7 T and displays an inverse magnetocaloric effect at lower temperatures and field changes. 1-OAc has −ΔSm values comparable to those of other Fe-based MCE materials and displays a significant MCE at lower applied fields, with −ΔSm = 11.2 J kg−1 K−1 at 3 K and μoΔH = 3 T. The tetranuclear core of 1 may be linked with isophthalate to form an octanuclear FeIII2(isophthalate)3[9MCFeIIIN(shi)-3]2 dimer (2) that crystallizes in a honeycomb packing arrangement and exhibits solvation-dependent magnetic properties. The MCE for this molecule ranges from −ΔSm = 9.9 J kg−1 K−1 at T = 5 K and μoΔH = 7 T, when the pores of the material are highly occupied with solvent, to −ΔSm = 5.4 J kg−1 K−1, when the system is fully desolvated.

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INTRODUCTION The control of ligand-field and metal−metal interactions in coordination chemistry has physical implications for the development of new magnetic and luminescent materials. To optimize properties, it is desirable to predict the relative arrangement of the metal ions, their geometry, and their stoichiometry within the molecular polynuclear complexes, in order to control the electronic and magnetic exchange interactions. Remarkably, such control still poses a unique challenge to the inorganic community. Due to the ability to control the first and second coordination sphere, as well as metal−metal interactions, the metallacrown (MC) strategy has led to advances in areas such as molecular magnetism1−5 and luminescent complexes for biological imaging.6−8 In fact, the first Mn−Ln single-molecule magnet (SMM) was a metallacrown5 and, subsequently, both d−f and d orbital based SMMs1−4,9 and mononuclear SMMs have appeared.10−12 Because the classical energy barrier for reversal of magnetization in Ising type complexes is given by E = ST2|D|, where ST is the total spin and D the axial zero field splitting parameter, significant effort has been expended to obtain ferromagnetically coupled systems with large magnetic anisotropy in order to optimize the behavior of SMMs. The issue here is to generate a robust high-spin ground state that is © XXXX American Chemical Society

well separated from the excited states and has large uniaxial anisotropy, in order to increase the classical energy barrier and also to minimize quantum tunneling. Another major application for polynuclear transition-metal complexes that has emerged is the magnetocaloric effect (MCE), which requires polynuclear complexes with metal ions bearing large spin values. However, unlike SMMs, these materials require weak exchange coupling and no magnetic anisotropy in order to achieve high low-lying spin density states and obtain large magnetic entropy.13 Continuous-cooling adiabatic demagnetization refrigerators with potential applications in small-scale laboratory and spaceborne environments have advanced significantly recently.14,15 Consequently, developing molecular MCE materials has become an area of focus in magnetochemistry.14,16−18 In contrast to SMMs, molecular magnetic refrigerants often have many closed spaced energy levels that exhibit rapid relaxation. The ideal system would be a molecule that has low magnetic anisotropy, weak (preferably ferromagnetic) magnetic exchange, and many energy levels that can be populated within a small change in temperature and/or field. Therefore, the three most suitable metal ions for MCE materials are high-spin MnII, Received: June 20, 2016

A

DOI: 10.1021/acs.inorgchem.6b01404 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry FeIII, and GdIII ions.13,19 The MnII ion has an isotropic S = 5/2 configuration and has been used to develop materials with large MCE;20 however, its tendency to oxidize to MnIII, a highly anisotropic ion, often causes problems with synthesis and stability. Due to an S = 7/2 spin state and intrinsically weak magnetic coupling, GdIII has been the most common metal ion used to synthesize high-performance MCE materials.13,21 FeIII ions that possess a chemically stable isotropic S = 5/2 spin state may be a good compromise if transition-metal ions are to be used, particularly given that molecules can be designed using the metallacrown strategy to afford control of the magnetic coupling. The Fe4 species FeIII(acetate)3[9-MCFeIIIN(shi)-3](MeOH)3· MeOH·7H2O, herein denoted 1-OAc, was already prepared in 1989.22 We re-examined its magnetic properties and found that it does not exhibit slow magnetic relaxation. The magnetic data were analyzed and showed an S = 5 ground state due to exchange coupling competition between the central and the peripheral FeIII ions on one hand and the peripheral ions on the other hand. Since the complex exhibited weak magnetoanisotropy, we felt that it was a target of interest for MCE studies. To expand on this model, modulation of the magnetic coupling was achieved through substitution of the carboxylate bridging ligand. The acetate bridge in 1-OAc was replaced with benzoate to form the compound Fe III (benzoate) 3 [9MCFeIIIN(shi)-3](MeOH)3·MeOH·4H2O (1-OBz). To further examine intramolecular coupling, two FeIII[9-MCFeIIIN(shi)-3] subunits were connected by three isophthalate bridging ligands to form the dimeric complex Fe III 2 (isophthalate) 3 [9MCFeIIIN(shi)-3]2 (2) containing eight FeIII metal ions with weak coupling expected between the two MC units linked by isophthalate. This octanuclear species generates a large number of low-lying spin states and, thus, a potentially large magnetic entropy for the molecule. The intra- and intermolecular interactions of these MCs were investigated through structural and magnetic analysis.

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Figure 1. (top) The common MC ligand salicylhydroxamic acid (H3shi). (bottom) The typical [M-N-O] unit repeated three times (shown in bold) to form a 9-MC-3 ringed structure. group C2/c, a = 18.4496 Å, b = 18.4496 Å, c = 31.7599 Å, α = 90°, β = 90°, γ = 120°, V = 9362.32 Å3. General Procedure for Fe III 2 (isophthalate) 3 [9-MC Fe II I N(shi) 3]2(EtOH)6·xH2O·yEtOH (2a, x = 24, y = 0; 2b, x = 4, y = 2; 2c, x = 4, y = 1; 2d, x = 1, y = 0). H3shi (229.7 mg, 1.5 mmol), FeCl3·6H2O (540.6 mg, 2.0 mmol), and isophthalic acid (124.6 mg, 0.75 mmol) were dissolved in 120 mL of ethanol and 8 mL of H2O. Ammonium bicarbonate (474.5 mg, 6 mmol) was added to the solution, and the mixture was stirred for 1.5 h. A dark precipitate formed and was filtered off. The solution was allowed to slowly evaporate over 3−4 weeks to yield small dark crystals. The sample was filtered, washed with 60 mL of a 1/1 solution of ethanol, and H2O and allowed to airdry. Single-crystal unit cell: a = 33.2164(9) Å, b = 33.2164(9) Å, c = 61.4234(16) Å, α = 90° ,β = 90°, γ = 120°, V = 58691(3) Å3. The crystallization of this complex is very sensitive to crystallization conditions. As such, three samples (2a−c) were prepared with varying levels of solvation in the open channels. Sample 2a was crushed to a fine powder and then dried in vacuo for 3 days to remove the residual solvents (herein referred to as 2d). 2a: yield 214.4 mg (35.5%). Anal. Calcd for Fe8C66H96N6O60: C, 33.30; H, 4.07; N, 3.53. Found: C, 33.59; H, 4.06; N, 3.54. 2b: yield 147.5 mg (27.7%). Anal. Calcd for Fe8C70H70N6O42: C, 39.77; H, 3.34; N, 3.98. Found: C, 39.47; H, 3.25; N, 3.91. 2c: yield 149.2 mg (28.6%). Anal. Calcd for Fe8C68H62N6O41: C, 39.53; H, 3.02; N, 4.07. Found: C, 39.53; H, 2.81; N, 4.10. 2d: Anal. Calcd for Fe8C66H50N6O37: C, 40.32; H, 2.56; N, 4.27. Found: C, 40.81; H, 2.59; N, 4.30. X-ray Crystallography. Crystal data for the compound 1-OBz were collected at 85(2) K on an AFC10K Saturn 944+ CCD-based Xray diffractometer equipped with a Micromax007HF Cu-target microfocus rotating anode (λ = 1.54187 Å), operated at 1200 W power (40 kV, 30 mA). The data were processed with CrystalClear 2.023 and corrected for absorption. The structure was solved and refined with the SHELXTL (version 6.12) software package24 All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were placed in their idealized positions. Experimental parameters and crystallographic data are provided in Table S1 in the Supporting Information. X-ray diffraction data for compound 2 was collected by using a Kappa X8 APPEX II Bruker diffractometer with graphite-monochromated Mo Kα radiation. The crystal was mounted on a CryoLoop (Hampton Research) with Paratone-N (Hampton Research) as cryoprotectant and then flash-frozen under a nitrogen-gas stream at 100 K. The temperature of the crystal was maintained at the selected

EXPERIMENTAL SECTION

All reagents were purchased from commercial sources and were used without further purification. Elemental analysis was performed by Atlantic Microlabs Inc. All reactions were carried under aerobic conditions. Synthesis. FeIII(acetate)3[9-MCFeIIIN(shi)-3](MeOH)3·MeOH·7H2O (1OAc). The synthesis follows a modified literature procedure.22 H3shi (153.1 mg, 1.0 mmol) was dissolved in 10 mL of methanol (Figure 1). To this was added a solution of Fe(acetate)2 (260.9 mg, 1.5 mg) in 10 mL of methanol dropwise, and the mixture was stirred for 1 h. The solution was filtered and allowed to slowly evaporate for ∼3 weeks, which yielded dark cube-shaped crystals. The sample was filtered, washed with 20 mL of methanol, and air-dried. Yield: 134.1 mg (36.4%), Anal. Calcd for Fe4C31H51N3O26: C, 33.69; H, 4.65; N, 3.80. Found: C, 33.47; H, 4.60; N, 3.78. Single-crystal unit cell: monoclinic, space group C2/c, a = 21.55 Å, b = 21.55 Å, c = 21.55 Å, α = 90°, β = 90°, γ = 90°, V = 10014.4 Å3. FeIII(benzoate)3[9-MCFeIIIN(shi)-3](MeOH)3·MeOH·4H2O (1-OBz). H3shi (153.1 mg, 1.0 mmol) and FeCl3·6H2O (360.4 mg, 1.33 mmol) were dissolved in 20 mL of methanol. Sodium benzoate (432.3 mg, 3.0 mmol) was added to the solution, and the mixture was stirred for 1.5 h. The solution was filtered and allowed to slowly evaporate for ∼3 weeks, which yielded dark crystals. The sample was filtered, washed with 20 mL of methanol, and air-dried. Yield: 191.8 mg (45.9%). Anal. Calcd for Fe4C46H51N3O23: C, 44.65; H, 4.15; N, 3.40. Found: C, 44.86; H, 4.12; N, 3.40. Single-crystal unit cell: monoclinic, space B


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Inorganic Chemistry value (100 K) by means of a 700 series Cryostream cooling device to within an accuracy of ±1 K. The data were corrected for Lorentz− polarization and absorption effects. The structures were solved by direct methods using SHELXS-9725 and refined against F2 by fullmatrix least-squares techniques using SHELXL-201424 with anisotropic displacement parameters for all non-hydrogen atoms. Hydrogen atoms were located on a difference Fourier map and introduced into the calculations as a riding model with isotropic thermal parameters. All calculations were performed by using the Crystal Structure crystallographic software package WINGX.26 Powder X-ray Diffraction (PXRD). Powder X-ray diffraction data for air-dried samples of 1-OAc, 1-OBz, and 2a−d were collected at room temperature using a Bruker D8 Advance diffractometer with Cu Kα raciation (1.5406 Å, 40 kV, 40 mA). Powder diffraction patterns were collected at room temperature from 3 to 50° (2θ) using a step size of 0.05° and a scan time of 0.5 s/step. In order to obtain PXRD data of fresh (solvated) samples of 2, crystals were collected from the mother liquor and quickly submerged in mineral oil to minimize solvent loss. The samples were then mounted on a CryoLoop, and PXRD data were collected using a Rigaku R-Axis Spider diffractometer with an image plate detector and graphite-monochromated Cu Kα radiation (1.5406 Å, 50 kV, 40 mA) source. Images were collected for 7 min while rotating the sample about the φ axis and oscillated in ω to reduce preferred orientation. Images were integrated from 3 to 50° with a 0.02° step size. Thermogravimetric Analysis. Thermogravimetric analyses were performed on a TA Instruments Q50 TGA device. The temperature was ramped from 25 to 600 °C at a rate of 20 °C/min under a flow of 40% O2/60% N2 gas. Magnetic Measurements. Variable-temperature susceptibility and variable-field magnetization measurements on polycrystalline samples mulled in eicosane were performed on a Quantum Design MPMS SQUID magnetometer. Variable-temperature dc susceptibility measurements were performed at 2000 Oe from 300 to 2 K. Isothermal magnetization measurements were performed at 2−30 K from 0 to 7 T. dc susceptibilities were corrected for the sample holder and eicosane and for diamagnetism of constituent atoms using Pascal’s constants.

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RESULTS AND DISCUSSION Synthesis and Structural Considerations. The first ironbased MC, 1-OAc, was reported by Lah and co-workers in 1989 (Figure 2, top).22 The magnetic coupling in this system could be described by a two-J model, with analysis in this manner indicating a high-spin (S = 5) ground state. The synthetic simplicity of the complex combined with its high-spin ground state made it a viable candidate to study how its magnetic interactions related to the MCE. Replacement of the acetate bridge in 1-OAc with benzoate moieties resulted in the complex 1-OBz (Figure 2, bottom). Finally, we hypothesized that generating intramolecular coupling between two FeIII[9MCFeIIIN(shi)-3] subunits could lead to a higher density of lowlying excited spin states and to greater magnetic entropy change. The dimeric complex Fe III 2 (isophthalate) 3 [9MCFeIIIN(shi)-3]2 (2) was synthesized by bridging two 9-MC-3 units via isophthalate bridges (Figure 3). The synthesis and structural description of 1-OAc has been previously reported.22 A modified synthetic procedure was employed, where Fe(acetate)2 was reacted in air with H3shi in methanol to yield dark blocklike crystals which have the same unit cell dimensions as the reported structure; the bulk sample was confirmed to be pure by PXRD (Figure S1 in the Supporting Information), suggesting that even after it was airdried the sample retained crystallinity. The crystal packing suggests that the individual molecules are well isolated (Figure

Figure 2. Crystal structures of 1-OAc (top) and 1-OBz (bottom). Color scheme: yellow, FeIII; red, O; blue, N; gray, C. Hydrogens and lattice solvents are omitted.

S2 in the Supporting Information); the closest intermolecular Fe−Fe distance is 7.595 Å with negligible π−π interactions. The reaction of FeCl3·6H2O, H3shi, and sodium benzoate in methanol yielded dark black crystals of 1-OBz. Sodium benzoate was employed as both a base and a source of carboxylate bridging anions. The PXRD pattern showed that the sample retained crystallinity upon exposure to air (Figure S3 in the Supporting Information), and as such, the crystal structure can be reliably used to explain its magnetic properties. The resulting structure is analogous to 1-OAc, with the more bulky benzoate groups replacing the acetate bridges. As shown in Figure S4 in the Supporting Information, the overlaid structures of 1-OBz and 1-OAc reveal similar overall molecular geometries. The molecular structure of 1-OBz displays a more puckered arrangement of the shi3− ligands. The more electron withdrawing nature of the benzoate group leads to a larger central Fe to carboxylate oxygen distance in 1-OBz in comparison to 1-OAc with values of 2.015 and 2.005 Å, respectively (Table 1). The larger average central Fe−Ocarb bond length in Fe4OBz is accompanied by a concomitant decrease in average ring−FeIII bond length (Table 1). C


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Inorganic Chemistry

ring FeIII atoms of adjacent molecules are a relatively short 4.800 Å. The dimeric molecules pack in a honeycomb arrangement, with large ∼15 Å solvent channels (Figure S8 in the Supporting Information). Due to this large size, the lattice electron density was too diffuse and required the use of the SQUEEZE routine of PLATON28 to remove the diffraction contribution from these solvents. Furthermore, there is significant solvent loss upon exposure to air, which can be observed by comparing the PXRD patterns of a fresh sample extracted from the mother liquor and immersed in mineral oil with an air-dried sample (Figure S9 in the Supporting Information). The PXRD pattern of the fresh sample retains crystallinity and matches the simulated powder pattern from the crystal structure. However, exposure to air will lead to loss of solvent and crystallinity, with the dried product having a large diffraction peak at an angle (2θ) of ∼8°. All air-dried samples of 2 show this peak at approximately the same angle, regardless of lattice solvent composition. The solvent content in the dimeric Fe8 complex was found to be quite sensitive to crystallization conditions such as temperature, humidity, and crystallization time. Three separate batches of 2 were synthesized and yielded different levels of solvation and will be referred to as 2a−c (ordered in decreasing levels of solvation). An aliquot of 2a was crushed and dried in vacuo at 50 °C for 3 days to remove solvent retained in the lattice and will be referred to as 2d. The molecular formula and molecular weight (MW) of these samples were determined through a combination of elemental analysis, TGA, and PXRD data (see the Supporting Information). For consistency, the MWs of 1-OAc and 1-OBz were also determined by the same method. A summary of the MWs for all compounds is presented in Table S2 in the Supporting Information and will be the values used to treat the magnetic data. dc Magnetic Properties. The room-temperature χMT product for both 1-OAc and 1-OBz is around 10 cm3 K mol−1, much smaller than the expected value of 17.5 cm3 K mol−1 for four noninteracting FeIII (S = 5/2, g = 2), suggesting that significant antiferromagnetic exchange coupling is present. For 1-OAc, the χMT product gradually decreases from 9.67 cm3 K mol−1 at 300 K down to 8.29 cm3 K mol−1 at 150 K (Figure 4), which is followed by an upswing to a maximum value of 14.35 cm3 K mol−1 at 5.5 K and then a decrease to 13.96 cm3 K mol−1 at 2 K. The behavior of 1-OBz is similar, but with a maximum for χMT at ∼30 K instead of 5.5 K for 1-OAc and a much lower value of 1.2 cm3 K mol−1 at 2 K (Figure 4). The presence of a minimum in the χ M T curve is the signature of an antiferromagnetic coupling scheme which leads to an irregular spin state pattern that would be expected for dominant interactions between the central and peripheral FeIII ions. For Fe4 this coupling scheme leads to a ground state S = (3 × 5/2) − 5/2 = 5. For 1-OAc, the χMT value of 14.35 cm3 K mol−1 at 5.5 K corresponds to an S = 5 spin state (with g = 1.96), which is in line with the simple coupling scheme suggested. This

Figure 3. Crystal structure of 2. Color scheme: yellow, FeIII; red, O; blue, N; gray, C. Hydrogens and lattice solvents are omitted.

The crystal packing of the 1-OBz molecules appear to be dictated by π−π interactions between adjacent benzoate groups on neighboring molecules. Most notably, two 1-OBz molecules facing tail to head are arranged such that the benzoate bridges are clasped together to form an intermolecular dimer, resulting in a short central FeIII−central FeIII intermolecular distance of 6.390 Å (Figure S5 in the Supporting Information). The interacting benzoates are situated 4.623 Å apart at a 60° angle and can be classified as a repulsive face-to-face π stacking.27 The 1-OBz intermolecular dimers subsequently assemble in a hexagonal packing arrangement (Figure S6 in the Supporting Information), leading to further intermolecular interactions. In this arrangement, the phenyl moiety of a shi3− ligand is involved in edge-to-face π interactions with two benzoate groups on an adjacent 1-OBz dimer. The cumulative effect of many interactions has a large effect on the low-temperature magnetic properties (vide infra). Compound 2 was synthesized through the reaction of H3shi, FeCl3·6H2O, and isophthalic acid in an ethanol/water solution, with ammonium bicarbonate as a base. This molecule can be simply described as two 9-MC-3 units connected through bifunctional isophthalate bridging ligands, forming an intramolecular dimer (Figure 3). The central FeIII atoms of the 9MC-3 units are 6.959 Å apart, which is actually longer than the central Fe−central Fe distance in the 1-OBz intermolecular dimer (6.389 Å). Each dimeric molecule in 2 is involved in extensive π interactions with neighboring molecules, which are oriented in a parallel manner (Figure S7 in the Supporting Information). In addition, the FeIII−FeIII distances between the Table 1. Selected Bond Distancesa 1-OBz 1-OAc 2 a

Fec−Fer distance (Å)

Fer−Fer distance (Å)

av Fec bond length (Å)

av Fer bond length (Å)

av Fec−Ocarb distance (Å)

av Fec−Oox distance (Å)

3.416 3.415 3.386

4.841 4.850 4.839

2.015 2.002 1.983

2.016 2.020 1.976

2.015 2.005 1.971

2.023 1.999 1.993

Definitions: Fec, central Fe; Fer, ring Fe; Ocarb, carboxylate oxygen; Oox, oxime oxygen. D


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Inorganic Chemistry

low-lying excited states.33 Assuming antiferromagnetic coupling, MCE materials based on the FeIII4 9-MC-3 metallacrown topology can theoretically be optimized by having a J2/J1 ratio of 0.333 (vertical black dash-dotted line, Figure S10), at the spin-frustrated state where the S = 5 state and doubly degenerate S = 4 state are of equal energy (vide infra). It must be stated that the absolute value of the energy between the spin states is dependent on the strength of the coupling; thus, ideally, J1 and J2 should be small. The susceptibility data for 1-OBz gave values of J1 = −24.9 cm−1, J2 = −4.5 cm−1, g = 1.98, and zJ = −0.69 cm−1 (Figure 4). For this compound, the data cannot be fit without considering intermolecular interaction; however, including D does not affect the quality of the fit. The comparable J values for 1-OBz and 1OAc can be attributed to the structural similarities, as shown Figure S4 in the Supporting Information. The phenyl group in benzoate is a more electron withdrawing substituent than the methyl substituent in acetate. It is reasonable that the weaker electron density in the carboxylate bridge for 1-OBz is manifested in its slightly smaller J1 exchange interaction. The J2/J1 ratio for 1-OBz is ∼0.18, which is indicative of an S = 5 ground state. This ratio is less than that of 1-OAc (0.23) and is further from the ideal value of 0.333 (Figure S10 in the Supporting Information). Consequently, the −ΔSm values for 1-OBz are lower than those of 1-OAc, as described in the next section. It should be noted that the coupling parameters obtained for 1-OBz lead to an S = 5 ground state for the molecular complex. The χM = f(T) plot (Figure S11 in the Supporting Information) presents a maximum at T = 7 K consistent with a collective behavior and an antiferromagnetic phase transition below that temperature where, when the magnetic properties are considered, the molecular nature of the compound fades away.34 For 1-OBz, the intermolecular face-to-face and edge-toface π interactions appear to have led to significant antiferromagnetic exchange, since the χMT product is very close to 0 at low temperature. It has been previously reported that π interactions can effectively mediate magnetic exchange. A 2007 study found that organic biphenylalenyl biradicaloid molecules have a large −0.29 eV (−2339 cm−1) intermolecular exchange interaction facilitated by π−π stacking.35 Although this extremely large magnetic exchange was due to the spin delocalization of organic radicals, there have also been recent reports of π interactions affecting magnetic properties in superparamagnetic36 and photomagnetic coordination complexes.37 The temperature-dependent susceptibility for 2a−d presented in Figure 5 shows similar χMT data at temperatures above 200 K. For the most solvated sample, 2a, the general profile of the susceptibility is similar to that of the Fe4 monomer complexes and reaches a maximum value of 18.81 cm3 K mol−1 at 15 K. Particularly, the temperature of the minimum of the χMT curve is almost the same as that for 1OAc, confirming that the Fe4 molecular spin state energy spectrum is conserved in compound 2. The second most solvated sample, 2b, has a maximum at 20 K, but at a much lower value of 14.63 cm3 K mol−1. Compounds 2c,d do not exhibit maxima, and the χMT product decreases with decreasing temperature. This overall behavior is consistent with an increase of the intermolecular antiferromagnetic exchange coupling upon removal of the solvent. Due to the large spin multiplicity, it was not possible to fit a model with eight Fe(III) ions of compound 2a. Therefore, we

Figure 4. χMT vs T for 1-OAc and 1-OBz. Experimental data are shown as open symbols and the fits (see text) as solid lines. Inset: coupling scheme for the Fe4 systems.

means that the antiferromagnetic exchange coupling between the central and the peripheral FeIII ions (J1 in Figure 4) dominates J2 and that intermolecular interactions are very weak. The same coupling scheme holds for 1-OBz, but because the maximum of χMT is at higher temperature and its value at 2 K is close to 0, the antiferromagnetic couplings between the peripheral ions and/or between the molecules are larger than those for 1-OAc. It is also worth noting that the temperatures of the minima of the χMT curves are very close, indicating that the J1 values are very close for the two complexes. On the basis of this qualitative analysis, it is possible to attempt a fit of the data using the coupling scheme depicted in the inset of Figure 4 that leads to the following spin Hamiltonian, which includes zero-field splitting within the ground state: H = −J1(S1̂ ·S2̂ + S1̂ ·S3̂ + S1̂ ·S4̂ ) − J2 (S2̂ ·S3̂ + S3̂ ·S4̂ ⎛ S(S + 1) ⎞ ⎟ + S2̂ ·S4̂ ) + D⎜Sz2 − ⎝ ⎠ 3

(1)

where J1 is the ring Fe−central Fe exchange, J2 is the ring Fe− ring Fe exchange, Ŝi values are the spin operators of the FeIII ions, and D is the zero-field splitting parameter associated with the S = 5 ground state. The eigenvalues of eq 1 can be determined through the Kambe method,29 and a theoretical expression for χMT vs T was derived from the van Vleck equation and eigenvalues of eq 1. A fit for the entire temperature range of the χMT data for 1OAc gave values of J1 = −28.0 cm−1, J2 = −6.4 cm−1, g = 1.97, and D = −0.3 cm−1 (Figure 4). It is also possible to obtain a good fit when intermolecular interactions are taken into account. However, the D value obtained in the above fit is in the same range as those for other structurally similar Fe4 complexes.30−32 The larger absolute value for J1 in comparison to J2 is not surprising, since ring Fe−central Fe exchange interactions may go through the oxime bridge (single atom) as well as the carboxylate and oximate bridge, whereas the ring Fe−ring Fe interactions are mediated only through a two-atom oximate connection. As such, the energy diagram of the spin states as a function of J2/J1 can be obtained using the eigenvalues obtained from eq 1 (Figure S10 in the Supporting Information). The experimentally determined value of J2/J1 is 0.23, indicating that the S = 4 first excited state is around 23 cm−1 above the S = 5 ground state (vertical green dashed line, Figure S10). In principle, good MCE materials have multiple E


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Inorganic Chemistry

per monomer subunit) at 7 T for 2a−d, respectively (Figure 7). We have tried to simulate the magnetization data of 2a by only

Figure 5. χMT vs T for 2a−d. The best fit for 2a is shown by the solid black line. Figure 7. M/NμB (per Fe8) vs field for 2a−d.

employed a two-J model of the Fe4 subunit, with intermolecular interactions, similar to the fit for 1-OBz. The fit resulted in values of J1 = −23.4 cm−1, J2 = −6.0 cm−1, g = 1.95, and zJ = −0.33 cm−1 (Figure 5), consistent with values obtained for 1OAc and 1-OBz. We were unable to obtain adequate fits of compounds 2b,c. The J2/J1 ratio for 2a is 0.26, which is closer to the ideal value of 0.333 in comparison to 1-OAc (0.23). However, intercluster or intermolecular interactions are significant in 2a and will affect the MCE properties. Magnetization data for 1-OAc and 1-OBz were collected at 2 K in fields from 0 to 7 T and plotted as M/NμB vs μoH in Figure 6, where N is Avogadro’s number and μB is the Bohr

considering one Fe4 unit and neglecting the possible interaction between the Fe4 units, but no reasonable fit could be obtained because the experimental magnetization values at saturation are too low. Thus, we conclude that the presence of appreciable antiferromagnetic coupling between the S = 5 ground states of the Fe4 clusters within the Fe8 molecule leads to a S = 0 ground state, but with the low-lying excited states with larger spin values being close to the ground state. Because of the very large spin multiplicity generated by the eight Fe(III) ions, it is not possible to carry out any simulation for this compound. The lower magnetization values per monomer subunit are smaller than those for 1-OAc and 1-OBz, indicating that not only intracluster interactions through the isophthalate but also intermolecular interactions are prevalent even at high fields and low temperatures. Interestingly, even though it is not possible to reach the magnetization values that correspond to the polarization of all the magnetic moments in the case of 2b−d, the curves do not have the S shape reminiscent of a phase transition. Such behavior is due to the absence of an ordered antiferromagnetic coupling throughout the lattice. Magnetocaloric Effect. In order to determine the magnetocaloric effect, magnetization experiments at fields between 0 and 7 T were performed from 2 to 20 K for 1OAc and 2 to 32 K for 1-OBz. The Maxwell relationship was applied to the data to estimate the magnetic entropy change, −ΔSm. The magnetization curves and magnetic entropy changes as a function of temperature for 1-OAc and 1-OBz are plotted in Figure 8. For 1-OAc, at all magnetic fields, −ΔSm increases with decreasing temperature and reaches a maximum of 15.4 J kg−1 K−1 at 3 K and μoΔH = 7 T (Figure 8). The MCE of 1-OBz is significantly reduced in comparison to that of 1-OAc, with a maximum −ΔSm = 7.4 J kg−1 K−1 at T = 7 K and μoΔH = 7 T (Figure 8). This behavior can be explained by the stronger antiferromagnetic interactions for 1-OBz, whereas molecules of 1-OAc are magnetically isolated (described above). Interestingly, at lower temperatures and fields, 1-OBz exhibits an inverse MCE, where −ΔSm assumes a negative value. This can be interpreted as a decrease in temperature upon magnetization of the sample and can be observed when the antiferromagnetic interactions are relatively strong with respect to the temperatures and fields employed,38 as is the case in the present compound. Characterizations of the MCE for 2a−d can be seen in Figures S13−S16 in the Supporting Information, respectively.

Figure 6. M/NμB (per Fe4) vs field for 1-OAc and 1-OBz.

magneton. For 1-OAc, the magnetization sharply rises at low fields and nearly saturates to a value of 9.61, which is consistent with an S = 5 system. It is possible to simulate the magnetization data with the same parameters extracted from the fit of χMT data (Figure S12 in the Supporting Information), which confirms the nature of the ground spin state. The magnetization curve for 1-OBz has an S shape with an inflection at ∼3 T, consistent with the collective behavior assumed from the susceptibility data (Figure S11 in the Supporting Information). The magnetization data suggest that intermolecular antiferromagnetic coupling is overcome by a field of 3 T. The magnetization reaches a value of 9.90 at 7 T: consistent with an S = 5 ground state for the molecular species as expected. The magnetization data at 2 K for the dimeric compounds steadily increase with increasing field, reaching values of 17.30 (8.65 per monomer subunit), 13.81 (6.91 per monomer subunit), 11.24 (5.62 per monomer subunit), and 8.35 (4.18 F


Article

Inorganic Chemistry

Figure 8. M/NμB vs field at temperatures between 2 and 20 K (top left) and the temperature-dependent magnetic entropy change (top right) for 1OAc. M/NμB vs field at temperatures between 2 and 20 K (bottom left) and the temperature-dependent magnetic entropy change (bottom right) for 1-OBz.

Figure 9. Temperature dependence of the magnetic entropy change normalized to R (left) and normalized to the number of FeIII ions (right) for 1OAc, 1-OBz, and 2a at μoΔH = 7 T.

The most solvated sample, 2a, exhibits the largest MCE, with −ΔSm = 9.9 J kg−1 K−1 at T = 5 K and μoΔH = 7 T. Samples which contain less channel solvent display weaker MCE, as 2b,c have −ΔSm = 9.1 J kg−1 K−1 (4 K and 7 T) and −ΔSm = 7.4 J kg−1 K−1 (4 K and 7 T), respectively. The nearly completely desolvated sample, 2d, exhibits the smallest −ΔSm value, −5.4 J kg−1 K−1. As is the case for 1-OBz, this trend may be explained by more prominent antiferromagnetic intermolecular interactions for the more desolvated compound 2 samples. Since the magnetic interactions in the Fe4 subunits are expected to be antiferromagnetic and similar to those of 1-OAc and 1-OBz, and the cluster−cluster interactions through the isophthalate bridges in compounds 2a−d should not change with respect to the level of solvation, it can be reasoned that the loss of solvent leads to stronger antiferromagnetic intermolecular interactions. This may be due to the collapse of the

channels and closer cluster−cluster contact. Furthermore, while we could not ascertain the strength or sign of the exchange between Fe4 subunits through the isophthalate bridges, it is likely non-negligible, since the susceptibility data for complexes 2a−d begin to diverge at 200 K. It is not believed that this is due to intermolecular interactions, since they do not manifest in 1-OBz until much lower temperatures (below 100 K). Of the three compounds reported here, compound 2a has the largest normalized maximum −ΔSm value of 2.9R (T = 5 K and μoΔH = 7 T, Figure 9, left). In comparison, 1-OAc has a maximum −ΔSm = 2.1R at 3 K and 7 T, which is close to the available entropy content for a strongly coupled S = 5 system, i.e., R ln(2S + 1) = 2.39R. On the other hand, 1-OBz has a smaller −ΔSm value per mole throughout the investigated temperature range, with a maximum −ΔSm = 1.1R (7 K and 7 T). The larger −ΔSm/R values for 1-OAc in comparison to 1G


Article

Inorganic Chemistry Table 2. Magnetic Entropy Change (−ΔS) for FeIII Molecular MCE Compoundsa compound III

Fe 4(acetate)3[9-MC-3] (1-OAc) FeIII4(benzoate)3[9-MC-3] (1-OBz) FeIII8(isophthalate)3[9-MC-3]2 (2a) FeIII14O6(bta)6(OMe)18Cl6 FeIII14O6(ta)6(OMe)18Cl6 FeIII17O16(OH)12(pyr)12Br4b [Fe2III(L)2](ClO4) (Cl)·4CH3OH·2H2O [FeIII2(L)2](BF4)2·2H2O [FeIII2(L)2] (NO3)2·3CH3OH·2H2O [FeIII2(L)2](Cl)2·2CH3OH·4H2O a

−ΔSm (J kg−1 K−1)

−ΔSm/R

T (K)

μoΔH(T)

ref

15.4 11.2 7.4 9.9 15.3 20.3 8.9 22.9 27.7 24.1 26.5

2.1 1.5 1.1 2.9 4.3 5.4 3.3 3.1 3.4 3.1 3.3

3 3 7 5 6 6 2.7 3 3 3 3

7 3 7 7 7 7 7 7 7 7 7

this work this work this work 40 39 42 41 41 41 41

Unless otherwise noted, −ΔSm values were obtained from magnetization data. bValues for −ΔSm obtained from heat capacity measurements.

weights, this means that the [Fe2III(L)2] compounds exhibit lower −ΔSm values. On the other hand, in many multimetallic compounds such as 1-OBz, 2, related Fe 14 compounds, 39,40 and a Fe 17 compound (Table 2),42 complex intermolecular interactions can lead to convoluted spin state structures and thus complex magnetic behavior. The sum of the strong antiferromagnetic couplings leads to a dimished MCE.

OBz can be explained by the more ideal J2/J1 ratio and lack of antiferromagnetic interactions for the former (Figure S10 in the Supporting Information). Hence, modulation of the bridging ligands may provide a route toward improving the MCE properties in this 9-MC-3 system. On a per FeIII ion basis, 1-OAc has the highest −ΔSm value for the entire temperature range of 2−20 K (Figure 9, right). This, combined with the fact that it has the lowest molecular weight, means that 1-OAc has the largest maximum gravimetric entropy change (15.4 J kg−1 K−1, Table 2). When the temperature is lowered, the −ΔSm/(mol of FeIII) for 1-OBz increases down to 8 K, after which a sharp downturn occurs. This can be attributed to the antiferromagnetic phase transition at 7 K (Figure S11 in the Supporting Information). Although within each Fe4 cluster 2a has a more ideal J2/J1 ratio in comparison 1-OAc, the former still has lower −ΔSm/(mol FeIII) values throughout the entire temperature range (Figure 9, right). This suggests that intracluster or intermolecular interactions in 2a play an important role in dictating the −ΔSm values. This conclusion is compatible with the fact that the magnetization data of 2a could not be fit when the exchange coupling between the Fe4 units through the isophthalate ligands is neglected (see above). The weak (probably antiferromagnetic) coupling between the ground SFe4 = 5 Fe4 units generate 11 low-lying spin states (SFe8 from 0 to 10) that are responsible for the larger entropy change (normalized to R) of Fe8 in comparison to the pure Fe4 complexes 1-OAc and 1-OBz. The magnetic entropy change, −ΔSm, for 1-OAc is comparable to that of other FeIII complexes found in the literature (Table 2).39,40 Among the transition-metal-based compounds with the highest MCE, many are low-nuclearity clusters, such as the [Fe2III(L)2] compounds41 in Table 2. This is primarily due to the greater paramagnetic nature and high metal to ligand ratio of these compounds. In this respect, the well-isolated, ferrimagnetically coupled 1-OAc exhibits magnetic behavior that is readily understood, similar to the case for the aforementioned paramagnetic complexes. Although the metal to ligand weight ratio for 1-OAc (0.25) is larger than that for the [Fe2III(L)2] compounds (0.11−0.12), the stronger magnetic interactions for 1-OAc lead to less available entropy. For the [Fe2III(L)2] compounds, there are only weak magnetic interactions (with Weiss constants (θ) ranging from −0.72 to −1.67 K),41 leading to a total available entropy of 2 × R ln 6 = 3.58R. However, for 1-OAc, the available magnetic entropy is lower, at 2.39R. In combination with their lower molecular

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CONCLUSIONS

The MC topology allows for the control of magnetic coupling which affects physical phenomena such as the magnetocaloric effect. Here, we have structurally and magnetically characterized three FeIII-containing complexes containing the 9-MC-3 topology. The estimated −ΔSm = 15.4 J kg−1 K−1 for 1-OAc has been explained by a simple two-J magnetic coupling model, and this compound exhibits weak intermolecular interaction. In comparison, antiferromagnetic ordering occurs in 1-OBz, likely mediated by the π interactions between the benzoate groups. This subsequently leads to a weaker MCE in 1-OBz, in comparison to 1-OAc. For compounds 2a−d, the solvation level of the solvent channels had a drastic effect on the MCE properties. Greater solvation led to a decrease in intermolecular interactions and thus greater MCE values. The most solvated sample, compound 2a, had an MCE of −ΔSm = 9.9 J kg−1 K−1 at T = 5 K and μoΔH = 7 T, whereas the least solvated sample, 2d, exhibited the smallest maximum −ΔSm value of −5.4 J kg−1 K−1 at T = 5 K and μoΔH = 7 T. The evidence suggests that not only is there intermolecular coupling between Fe8 molecules in the lattice but also there is also superechange through the isophthalate linking groups. This coupling resulted in a high density of magnetic states above the ground state, which led to compound 2a having a larger −ΔSm/R value than 1-OAc (2.9 vs 2.1). Future work will include modifying the coupling such that we can achieve the ideal J2/J1 ratio of 0.333. We have shown that aromatic ligands which mediate intermolecular π−π interactions can lead to strong antiferromagnetic interactions which quench the MCE. This suggests that using ligands that are more aliphatic in composition may be advantageous by mitigating unwanted exchange coupling and reducing the molecular weight. However, it should be noted that ligands which are not bulky enough can also lead to strong intermolecular metal−metal exchange, as is the case for the MnII-based glycolate salts.20 H


Article

Inorganic Chemistry

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(10) Happ, P.; Plenk, C.; Rentschler, E. 12-MC-4 metallacrowns as versatile tools for SMM research. Coord. Chem. Rev. 2015, 289−290, 238. (11) Cao, F.; Wang, S.; Li, D.; Zeng, S.; Niu, M.; Song, Y.; Dou, J. Family of Mixed 3d−4f Dimeric 14-Metallacrown-5 Compounds: Syntheses, Structures, and Magnetic Properties. Inorg. Chem. 2013, 52, 10747. (12) Ostrowska, M.; Fritsky, I. O.; Gumienna-Kontecka, E.; Pavlishchuk, A. V. Metallacrown-based compounds: Applications in catalysis, luminescence, molecular magnetism, and adsorption. Coord. Chem. Rev. 2016, DOI: 10.1016/j.ccr.2016.04.017. (13) Evangelisti, M.; Brechin, E. K. Recipes for enhanced molecular cooling. Dalton Trans. 2010, 39, 4672. (14) Pobell, F. In Matter and Methods at Low Temperatures; Springer: Berlin, Heidelberg, 2007; p 203. (15) Shirron, P.; Wegel, D.; DiPirro, M. A Continuous Adiabatic Demagnetization Refrigerator for Cooling to 10 mK and Below. AIP Conf. Proc. 2005, 850, 1573. (16) Evangelisti, M.; Luis, F.; de Jongh, L. J.; Affronte, M. Magnetothermal properties of molecule-based materials. J. Mater. Chem. 2006, 16, 2534. (17) Evangelisti, M. In Molecular Magnets; Bartolomé, J., Luis, F., Fernández, J. F., Eds.; Springer: Berlin, Heidelberg, 2014; p 365. (18) Gajewski, M.; Pełka, R.; Fitta, M.; Miyazaki, Y.; Nakazawa, Y.; Bałanda, M.; Reczyński, M.; Nowicka, B.; Sieklucka, B. Magnetocaloric effect of high-spin cluster with Ni9W6 core. J. Magn. Magn. Mater. 2016, 414, 25. (19) Tang, X.; Zhong, Q.; Xu, J.; Li, H.; Xu, S.; Cui, X.; Wei, B.; Ma, Y.; Yuan, R. Co(II)4Gd(III)6 phosphonate grid and cage as molecular refrigerants. Inorg. Chim. Acta 2016, 442, 195. (20) Chen, Y.-C.; Guo, F.-S.; Liu, J.-L.; Leng, J.-D.; Vrábel, P.; Orendác,̌ M.; Prokleška, J.; Sechovský, V.; Tong, M.-L. Switching of the Magnetocaloric Effect of MnII Glycolate by Water Molecules. Chem. - Eur. J. 2014, 20, 3029. (21) Sharples, J. W.; Collison, D. Coordination compounds and the magnetocaloric effect. Polyhedron 2013, 54, 91. (22) Lah, M. S.; Kirk, M. L.; Hatfield, W.; Pecoraro, V. L. The Tetranuclear Cluster FeIII[FeIII(salicylhydroximato) (MeOH) (acetate)]3 is an Analogue of M3+(9-crown-3). J. Chem. Soc., Chem. Commun. 1989, 1606. (23) CrystalClear 2.0; Rigaku Corp., Tokyo, Japan. (24) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112. (25) Sheldrick, G. M. SHELXTL (version 6.12), Program for Crystal Structure Solution; University of Göttingen, Göttingen, Germany, 1997. . (26) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837. (27) Hunter, C. A.; Sanders, J. K. M. The Nature of pi-pi Interactions. J. Am. Chem. Soc. 1990, 112, 5525. (28) Spek, A. L. PLATON; Utrecht University; Utrecht, The Netherlands, 2001. (29) Kambe, K. J. Phys. Soc. Jpn. 1950, 5, 48. (30) Barra, A. L.; Caneschi, A.; Cornia, A.; de Biani, F. F.; Gatteschi, D.; Sangregorio, C.; Sessoli, R.; Sorace, L. Single-molecule magnet behavior of a tetranuclear iron(III) complex. The origin of slow magnetic relaxation in iron(III) clusters. J. Am. Chem. Soc. 1999, 121, 5302. (31) Moragues-Canovas, M.; Riviere, P.; Ricard, L.; Paulsen, C.; Wernsdorfer, W.; Rajaraman, G.; Brechin, E. K.; Mallah, T. Resonant quantum tunneling in a new tetranuclear iron(III)-based singlemolecule magnet. Adv. Mater. 2004, 16, 1101. (32) Accorsi, S.; Barra, A. L.; Caneschi, A.; Chastanet, G.; Cornia, A.; Fabretti, A. C.; Gatteschi, D.; Mortalo, C.; Olivieri, E.; Parenti, F.; Rosa, P.; Sessoli, R.; Sorace, L.; Wernsdorfer, W.; Zobbi, L. Tuning anisotropy barriers in a family of tetrairon(III) single-molecule magnets with an S = 5 ground state. J. Am. Chem. Soc. 2006, 128, 4742. (33) Sessoli, R. Chilling with Magnetic Molecules. Angew. Chem., Int. Ed. 2012, 51, 43.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01404.

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Supplementary tables, figures, and X-ray crystallographic parameters, as detailed in the text (PDF) Crystallographic data for 1-OBz (CIF) Crystallographic data for 2 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail for T.M.: [email protected]. *E-mail for V.L.P.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the National Science Foundation under grant CHE-1361779. The SQUID facility at the University of Michigan is funded by the National Science Foundation, NSF-MRI grant CHE-1040008. We thank Marco Evangelisti for useful comments.

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REFERENCES

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Synthesis and Magnetic Characterization of Fe(III)-Based 9

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