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Article

Chloranilato-Based Layered Ferrimagnets with Solvent-Dependent Ordering Temperatures

by
Cristian Martínez-Hernández
,
Samia Benmansour
* and
Carlos J. Gómez-García
*
Instituto de Ciencia Molecular (ICMol), Departamento de Química Inorgánica, Universidad de Valencia, C/Catedrático José Beltrán 2, 46980 Paterna, Spain
*
Authors to whom correspondence should be addressed.
Magnetochemistry 2019, 5(2), 34; https://doi.org/10.3390/magnetochemistry5020034
Submission received: 2 May 2019 / Revised: 25 May 2019 / Accepted: 28 May 2019 / Published: 4 June 2019
(This article belongs to the Special Issue A Themed Issue of Functional Molecule-based Magnets: Dedicated to Professor Masahiro Yamashita on the Occasion of his 65th Birthday)
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Abstract

:
We report the synthesis and the characterization of six new heterometallic chloranilato-based ferrimagnets formulated as (NBu4)[MnCr(C6O4Cl2)3]·nG with n = 1 for G = C6H5Cl (1), C6H5I (3), and C6H5CH3 (4); n = 1.5 for G = C6H5Br (2) and n = 2 for G = C6H5CN (5) and C6H5NO2 (6); (C6O4Cl2)2− = 1,3-dichloro,2,5-dihydroxy-1,4-benzoquinone dianion. The six compounds are isostructural and show hexagonal honeycomb layers of the type [MnCr(C6O4Cl2)3]− alternating with layers containing the NBu4+ cations. The hexagons are formed by alternating Mn(II) and Cr(III) connected by bridging bis-bidentate chloranilato ligands. The benzene derivative solvent molecules are located in the hexagonal channels (formed by the eclipsed packing of the honeycomb layers) showing π-π interactions with the anilato rings. The six compounds behave as ferrimagnets with ordering temperatures in the range 9.8–11.2 K that can be finely tuned by the donor character of the benzene ring and by the number of solvent molecules inserted in the hexagonal channels. The larger the electron density on the aromatic ring and the larger the number of solvent molecules are, the higher Tc is. The only exception is provided by toluene, where the formation of H-bonds might be at the origin of weaker π-π interactions observed in this compound.

Graphical Abstract

1. Introduction

One of the main advantages of molecule-based magnets is the possibility to modulate or tune the properties of the magnets by simply changing or modifying the building blocks used to prepare them [1]. This strategy led, at the end of last century, to the synthesis of different series of molecule-based magnets whose ordering temperatures and coercive fields could be modified with ease. A typical example is provided by the series of cyano-bridged heterometallic compounds formulated as AxMy[M’(CN)6]z·nH2O, where A is a monovalent cation, and M and M’ are trivalent or divalent transition metal ions [2,3,4]. In this series, the magnetic exchange through the CN bridge can be modulated [5,6] by changing A, M, and M’ to obtain materials with interesting magnetic properties as photomagnetism [7,8,9], single molecule magnets [10,11], and even magnetic order above room temperature [12]. Another family of molecule-based magnets whose properties can be easily modified by changing the constituent metallic atoms is the series of oxalato-based two-dimensional (2D) magnets formulated as (A)[MIIMIII(C2O4)3] (A+ = monocation; MII = Mn, Fe, Co, Ni, Cu, …; MIII = Fe, Cr, …; C2O42− = oxalate dianion, Figure 1b) that show ferro-, ferri-, or canted antiferromagnetic ordering with Tc ranging from 6 K to 48 K depending on M(II) and M(III) [13,14,15,16,17,18,19,20,21,22].
A third and recent example is the series of anilato-based heterometallic 2D honeycomb magnets formulated as (A)[MIIMIII(C6O4X2)3]·G, where A+ is a monocation (see Table 1); M(II) and M(III) are transition metal ions as Mn(II), Fe(II), Cr(III), and Fe(III), G may be many different solvent molecules (see Table 1), and C6O4X22− is the 1,3-disubstituted-2,5-dihydroxy- 1,4-benzoquinone dianion (with X = H, Cl, Br, NO2, … Figure 1a), known as anilato-type ligands [23]. This family of magnets shows a honeycomb (6,3)-2D structure with the same topology as the oxalato ones and, as in the oxalato family, it is also possible to change the magnetic properties by simply changing the building blocks [23,24]. Albeit, there are three important differences between these two series; the first difference is observed in the sign of the magnetic coupling—it is always antiferromagnetic in the anilato-based compounds, whereas it may be ferro- or antiferromagnetic depending on the metal ions, for the oxalato series. This fact precludes the presence of magnetic ordering in the homo-metallic anilato-based lattices but not in the hetero-metallic ones, where long range ferrimagnetic is observed when the spin states of the metal ions are different [Mn(II)Cr(III) and Fe(II)Fe(III), see Table 1]. The second difference is the rigidity of the oxalato ligand (Figure 1b) in contrast to the anilato ligand that can be easily modified by changing the X group (X = H, F, Cl, Br, I, CH3, Cl/CN, NO2, …) [25]. This change has already allowed a tuning of the ordering temperatures in the series of compounds (NBu4)[MnCr(C6O4X2)3] (X = H, Cl, Br, and I) [23] The third important difference is the size; the hexagonal cavities of the honeycomb structure are twice as large in the anilato-based compounds and, when packed in an eclipsed way, originate hexagonal channels with BET areas of up to 1440 m2/g [26]. These hexagonal channels may be filled with solvent molecules (in contrast to the oxalato-based compounds) that can be easily removed, giving rise, in some cases, to important changes in the magnetic properties. Thus, in compound (NMe2H2)2[Fe2(C6O4Cl2)3]·2H2O·6DMF, the removal of the solvent molecules results in a decrease of the ordering temperature from 80 to 26 K [27]. In compound (Et(i-Pr)2NH)[MnCr(C6O4Br2)3]·H2O·0.5CHCl3, the removal of the solvent molecules changes the magnetic behavior (the solvated compound is a metamagnet with a critical field of 490 mT at 2 K, whereas the de-solvated phase is a ferrimagnet with Tc = 9 K) [28].
The possibility to change the magnetic properties (ordering temperatures, magnetic behavior, or critical and coercive fields) by simply changing the solvent molecules is, therefore, a very appealing strategy to modulate Tc in these series of 2D magnets. Furthermore, when using lanthanoids as metal ions, the solvent molecules also play a key structural role in these (6,3)-2D lattices [24,29,30,31,32,33,34,35,36,37,38].
In this context, we have recently initiated a detailed study of the role played by the solvent molecules located in the hexagonal channels in the structure and the magnetic properties of these 2D magnets formulated as (A)[MIIMIII(C6O4X2)3]·G. To perform this study, we have initially selected Mn(II) and Cr(III) as MII and MIII, since this couple of metal ions crystallizes more easily (Table 1). We have selected A = NBu4+ as the cation and chloranilato as the ligand (X = Cl), and we have focused on a series of benzene derivative solvent molecules (C6H5X with X = Cl, Br, I, CH3, CN, and NO2), since they seem to play a template role that facilitates the crystallization of these 2D lattices. With this idea in mind, we have prepared the series of compounds formulated as (NBu4)[MnCr(C6O4Cl2)3]·n C6H5X with n/X = 1/Cl (1), 1.5/Br (2), 1/I (3), 1/CH3 (4), 2/CN (5), and 2/NO2 (6). This series of compounds are solvates since they are isostructural and only differ in the solvent molecules. They present the structure of compound (NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2O (A) [39] and show a fine modulation of the ordering temperatures in the range 9.8–11.2 K depending on the electronic properties of the benzene derivative molecule and on the number of solvent molecules inserted in the channels.
Here, we present the chemical and the magnetic characterization of the six compounds and show that it is possible to fine-tune the ordering temperatures with a simple change of the solvent molecules.

2. Results and Discussion

2.1. Syntheses of the Complexes

The six compounds were synthesized by carefully layering solutions containing the precursor [Cr(C6O4Cl2)3]3− anion and Mn(II) ions with the corresponding benzene derivative solvents. In all cases, an intermediate layer was needed to slow down the crystallization process and prevent the formation of amorphous or low quality crystalline materials. All the attempts to obtain good quality single crystals failed, although the X-ray powder diffractograms showed that they are all isostructural to the closely related compound (NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2O (A) [39].

2.2. FT-IR Spectra

As expected, the six compounds showed very similar IR spectra (Figure 2). The only differences corresponded to the bands associated with the solvent molecules. Table 2 lists the main bands and their assignments. The FT-IR spectra in all cases confirmed the presence of the corresponding solvent molecules (C6H5Cl in 1, C6H5Br in 2, C6H5I in 3, C6H5CH3 in 4, C6H5CN in 5, and C6H5NO2 in 6), in agreement with the chemical and the thermo-gravimetric analysis.

2.3. Thermogravimetric Analysis

The thermogravimetric analysis of compounds 1–6 (Figure 3) showed an initial weight loss with a plateau in the range ca. 200–290 °C depending on the sample (Table 3). This weight loss corresponded to the release of the benzene derivative solvent molecules. The experimental weight loss (Table 3) indicated that compounds 1, 3, and 4 contained one solvent molecule (C6H5Cl in 1, C6H5I in 3, and C6H5CH3 in 4), whereas compound 2 contained 1.5 C6H5Br molecules and compounds 5 and 6 contained two solvent molecules (C6H5CN in 5 and C6H5NO2 in 6). These values are in agreement with the elemental analysis in all cases (see experimental section). We can, therefore, conclude that the used solvents (C6H5Cl, C6H5Br, C6H5I, C6H5CH3, C6H5CN, and C6H5NO2) entered in the structures of compounds 1–6 (as observed in the IR spectra), and that compounds 1, 3, and 4 contain one solvent molecule, compound 2 contains ca. 1.5, and compounds 5 and 6 contain around two solvent molecules per formula. At higher temperatures (around 350 °C), all compounds showed an abrupt weight loss corresponding to the decomposition and the release of the chloranilato ligand. As can be seen in the derivative plot, compound 4 needed a higher temperature (around 300 °C) to release the toluene molecule, suggesting that this molecule has a stronger interaction with the 2D lattice (probably due to the formation of H-bonds, as already observed in other anilato-based lattices) [34].

2.4. Structures of Compounds 1–6

Compounds 1–6 are isostructural to compound (NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2O (A), [39] as shown by their X-ray powder diffractograms (Figure 4). The unit cell parameters of compounds 1–6 (Table 4), calculated from their X-ray powder diffractograms based on the structure of A with X’Pert HighScore Plus software, [45] further confirmed the isostructurality. Interestingly, compounds 1–6 did not show any correlation between the size and the number of the solvent molecules and the unit cell parameters, suggesting that the solvent molecules are located in the hexagonal channels rather than in the interlayer space and, therefore, they do not modify the structure.
Compound A (and compounds 1–6) presents a layered structure where anionic layers of formula [MnCr(C6O4X2)3]− with X = Br (A) and Cl (1–6) alternate with cationic layers of NBu4+ cations (Figure 5a). The interlayer distance is 8.7 Å. The anionic layers show the classical honeycomb structure with Mn(II) and Cr(III) ions alternating in the vertex of the hexagons and the anilato ligands in the sides (Figure 5b). The benzene derivative solvent molecules are located in the hexagonal channels formed by the eclipsed packing of the honeycomb layers (Figure 5c) and show strong π-π interactions with the anilato rings with an interplane angle of 6.95°, a centroid–centroid distance of 3.870 Å, and a shift distance of 1.425 Å (Figure 5d). The Br···Br distances between the Br atom of the C6H5Br molecule and the closest bromanilato ligands are 3.82 and 3.90 Å, only slightly above the sum of the van der Waals radii (3.72 Å).

2.5. Magnetic Properties

As expected, compounds 1–6 show quite similar magnetic properties, although, as we note below, there are slight differences in the ordering temperatures and the coercive fields depending on the solvent inserted in the hexagonal channels. All compounds show χmT values at room temperature in the range 6.25–6.35 cm3 K mol−1 (Table 5 and Figure 6a)—very close to the calculated spin only values for S = 3/2 Cr(III) and S = 5/2 high spin Mn(II) ions [χm is the molar magnetic susceptibility per Mn(II)Cr(III) couple]. These χmT values show a continuous smooth decrease on cooling the samples and reach minimum values at around 20 K, followed by a sharp increase at around 12 K (inset Figure 6a). This behavior indicates the presence of a ferrimagnetic Mn-Cr coupling in all the samples, as observed in all the previously characterized [MnCr(C6O4X2)3]− lattices (Table 1). At ca. 10 K, all the samples show a maximum in χmT, followed by an abrupt decrease due to the saturation effects of χm at low temperatures (Figure 6b). The sharp increase observed at ca. 10–12 K suggests the onset of a long range ferrimagnetic ordering, in agreement with the sharp increase observed in the thermal variation of χm at ca. 10–11 K in all compounds (inset in Figure 6b).
The isothermal magnetization cycles at 2 K of all the samples provide a further confirmation of the long range ferrimagnetic ordering (Figure 7). Thus, these measurements show a sharp increase of the magnetization at low fields and hysteresis cycles for all compounds with coercive fields in the range 16.2–56.2 mT (Figure 7b and Table 5). The magnetization values at 5 T in all cases are close to 2.1–2.2 μB (Figure 7a and Table 5), which is the expected value for a ferrimagnetic coupling between the S = 3/2 and 5/2 of the Cr(III) and the Mn(II) ions of the lattice. Moreover, at high fields, the magnetization shows a linear smooth increase, further confirming the ferrimagnetic coupling in compounds 1–6.
A further confirmation of the long range ferrimagnetic order and a more precise determination of the ordering temperatures was obtained with magnetic susceptibility measurements in the presence of an alternating magnetic field (AC measurements). These measurements show, in all cases, a sharp peak in the in-phase (χ’m) and in the out-of-phase (χ”m) signals that does not change with the frequency (Figure 8), confirming the presence of a long range order at low temperatures in all cases. The ordering temperatures, determined as the temperature where χ”m become non-zero, are all in the range 9.8–11.2 K (Table 5). These values are similar to those observed in most of the reported [MnCr(C6O4X2)3]− lattices (Table 1).
In order to compare the ordering temperatures (Tc) of compounds 1–6, we have plotted the thermal variation of χ’m and χ”m at a fixed frequency (110 Hz) for all compounds (Figure 9). We can see that, even if the differences in some cases are very small, the order of Tc is: C6H5NO2 (6) > C6H5I (3) ≈ C6H5CN (5) > C6H5Br (2) > C6H5Cl (1) > C6H5CH3 (4) (Table 5).
Although we do not have details of the crystal structure, we can assume that, in all cases, the solvent molecules (between one and two per hexagonal cavity) must interact via strong π–π stacking with the anilato rings. This assumption is supported by the similar unit cell parameters determined from the X-ray powder diffractograms (Table 4). If the solvent molecules were located out of the hexagonal cavities (i.e., in the interlayer space), then the a and the c parameters and the unit cell volume (that are determined by the interlayer space) would be quite different in compounds 1–6, in contrast with the experimental data. Since Tc depends on the magnetic coupling through the anilato ring, and this coupling depends on the electron density of the anilato rings, [23] we can presume that the differences in Tc reflect the differences in the electron density of the anilato rings (since the six compounds contain the same anilato-type ligand). This modulation of Tc with the electron density on the anilato ring was also observed in the closely related series (NBu4)[MnCr(C6O4X2)3] with X = H, Cl, Br and I [23]. On one hand, the sequence observed for the halobenzene derivatives (C6H5I > C6H5Br > C6H5Cl) agrees with the idea that the aromatic ring in C6H5Cl had less electron density and, accordingly, donates less electron density to the anilato ring, resulting in a weaker magnetic coupling and a lower Tc. On the other hand, the higher values observed for the C6H5CN and C6H5NO2 derivatives may be attributed to the fact that there are two aromatic molecules per hexagon in these two compounds. The only compound that do not follow the expected trend is the C6H5CH3 derivative (4). Since the -CH3 group is electron donating, it should present a higher Tc than the three halobenzene derivatives. A possible reason to explain this anomaly might be the formation of H-bonds between the -CH3 group and the oxygen atoms or the chlorine atoms of the chloranilato ligand. The formation of such H-bonds has already been observed in other related Ln(III)-containing anilato-based lattices [34]. The higher temperature needed in the thermogravimetric measurements to release the toluene molecule in this compound agrees with this idea. These H-bonds are expected to shift the aromatic ring of the toluene molecule from its ideal position, reducing the π–π stacking with the anilato ring and, accordingly, the electron density on the anilato ring and Tc.
The idea that the solvent molecules play an important role in Tc is further supported by the fact that the de-solvated compound (NBu4)[MnCr(C6O4Cl2)3] (B) [23] shows an ordering temperature of 5.5 K (Table 1), well below the observed ones in compounds 1–6. Although compound B has a slightly different structure (the honeycomb layers are alternated, and the hexagonal rings are completely planar), the lower value of Tc in the de-solvated compound suggests that the presence of the solvent molecules increases the electron density in the anilato rings and, accordingly, the magnetic coupling and the ordering temperatures. In fact, preliminary measurements performed on compound 6 after heating the sample at 400 K under vacuum to remove the PhNO2 molecules show a slight decrease in Tc (and an important decrease of the coercive field), further supporting the idea that the solvent molecules are responsible for the fine tuning of Tc.
Despite compounds 1–6 show very close unit cell parameters (Table 4), we cannot discard that, besides the electronic effect, the solvent molecules exert a very tiny structural effect. Although with more important structural changes, this structural effect has already been observed in compounds (NMe2H2)2[Fe2(C6O4Cl2)3]·2H2O·6DMF [27] and (Et(i-Pr)2NH)[MnCr(C6O4Br2)3]·H2O·0.5CHCl3 [28].

3. Experimental Section

3.1. Starting Materials

Chloranilic acid (H2C6O4Cl2), MnCl2·4H2O, and all the used solvents (C6H5Cl, C6H5Br, C6H5I, C6H5CH3, C6H5CN, and C6H5NO2) are commercially available and were used as received without further purification. The precursor salt (NBu4)3[Cr(C6O4Cl2)3] was prepared as reported in the literature [23].

3.2. Synthesis of (NBu4)[MnCr(C6O4Cl2)3]·C6H5Cl (1)

Compound 1 was prepared by carefully layering, at room temperature, a solution of (NBu4)3[Cr(C6O4Cl2)3] (70 mg, 0.05 mmol) in acetonitrile (10 mL) on top of a solution of MnCl2·4H2O (40 mg, 0.2 mmol) in 4 mL of MeOH and 6 mL of chlorobenzene. An intermediate layer with a mixture of methanol:chlorobenzene (8:1) was used in order to slow down the diffusion. The solution was allowed to stand for two weeks to obtain a dark powder, which was filtered and air-dried. FT-IR (ν/cm−1, KBr pellet): 3440 (vs), 2960 (m), 2930 (w), 2873 (w), 1608 (m), 1497 (vs), 1360 (vs), 1306 (w), 1008 (w), 858 (s), 743 (m), 700 (w), 685 (w), 628 (s), 578 (m), 513 (s), 465 (m).
Anal. Calcd. (%) for C40H41Cl7CrMnNO12: C, 44.37; N, 1.29; H, 3.82. Found (%): C, 43.74; N, 1.21; H, 3.99. Elemental ratio estimated by electron probe microanalysis (EPMA): found: Mn:Cr:Cl = 11.5:11.4:77.1 (1.0:1.0:6.8). Calc. for C40H41Cl7CrMnNO12: Mn:Cr:Cl = 1:1:7.

3.3. Synthesis of (NBu4)[MnCr(C6O4Cl2)3]·1.5C6H5Br (2)

Compound 2 was prepared as 1 but using bromobenzene instead of chlorobenzene. The solution was allowed to stand for two weeks to obtain a dark powder, which was filtered and air-dried. FT-IR (ν/cm−1, KBr pellets): 3420 (vs), 2961 (m), 2930 (w), 2872 (w), 1608 (m), 1497 (vs), 1361 (vs), 1305 (w), 1066 (w), 1006 (m), 856 (s), 736 (m), 683 (w), 670 (w), 626 (s), 576 (m), 513 (s), 463 (m).
Anal. Calcd. (%) for C43H43.5Br1.1Cl6CrMnNO12: C, 42.83; N, 1.16; H, 3.63. Found (%): C, 42.82; N, 1.05; H, 3.45. Elemental ratio estimated by electron probe microanalysis (EPMA): found: Mn:Cr:Cl:Br = 9.9:10.2:63.4:16.4 (1.0:1.0:6.4:1.6). Calc. for C43H43.5Br1.5Cl6CrMnNO12: Mn:Cr:Cl:Br = 1:1:6:1.5.

3.4. Synthesis of (NBu4)[MnCr(C6O4Cl2)3]·C6H5I (3)

Compound 3 was prepared as 1 but using iodobenzene instead of chlorobenzene. The solution was allowed to stand for two weeks to obtain a dark powder, which was filtered and air-dried. FT-IR (ν/cm−1, KBr pellets): 3443 (vs), 2961 (m), 2926 (w), 2872 (w), 1606 (m), 1496 (vs), 1363 (vs), 1306 (w), 1011 (m), 858 (s), 730 (m), 683 (w), 666 (w), 627 (s), 577 (m), 513 (s), 458 (s).
Anal. Calcd. (%) for C40H41ICl6CrMnNO12: C, 40.91; N, 1.19; H, 3.52. Found (%): C, 39.40; N, 0.99; H, 3.99.

3.5. Synthesis of (NBu4)[MnCr(C6O4Cl2)3]·C6H5CH3 (4)

Compound 4 was prepared as 1 but using toluene instead of chlorobenzene. The solution was allowed to stand for two weeks to obtain a dark powder, which was filtered and air-dried. FT-IR (ν/cm−1, KBr pellets): 3440 (vs), 2961 (m), 2930 (w), 2872 (w), 1608 (m), 1497 (vs), 1361 (vs), 1305 (w), 1008 (m), 856 (s), 730 (m), 696 (w), 670 (w), 630 (s), 576 (m), 513 (s), 463 (m).
Anal. Calcd. (%) for C41H44Cl6CrMnNO12: C, 46.35; N, 1.32; H, 4.17. Found (%): C, 45.13; N, 1.12; H, 4.10.

3.6. Synthesis of (NBu4)[MnCr(C6O4Cl2)3]·2C6H5CN (5)

Compound 5 was prepared as 1 but using benzonitrile instead of chlorobenzene. The solution was allowed to stand for two weeks to obtain a dark powder, which was filtered and air-dried. FT-IR (ν/cm−1, KBr pellets): 3443 (vs), 2963 (m), 2930 (w), 2873 (w), 1605 (m), 1497 (vs), 1360 (vs), 1306 (w), 1005 (m), 858 (s), 755 (m), 733 (w), 686 (w), 627 (s), 577 (m), 547 (w), 513 (s), 461 (m).
Anal. Calcd. (%) for C44.5H44.5Cl6CrMnN2.5O12: C, 49.00; N, 3.57; H, 3.94. Found (%): C, 47.00; N, 3.16; H, 3.01.

3.7. Synthesis of (NBu4)[MnCr(C6O4Cl2)3]·2C6H5NO2 (6)

Compound 6 was prepared as 1 but using nitrobenzene instead of chlorobenzene. The solution was allowed to stand for two weeks to obtain a dark powder, which was filtered and air-dried. FT-IR (ν/cm−1, KBr pellets): 3443 (vs), 2961 (m), 2934 (w), 2872 (w), 1606 (m), 1496 (vs), 1360 (vs), 1306 (w), 1005 (m), 858 (s), 791 (w), 736 (w), 705 (m), 683 (w), 666 (w), 625 (s), 577 (m), 513 (s), 463 (m).
Anal. Calcd. (%) for C43H43.5Cl6CrMnN2.5O15: C, 45.42; N, 3.45; H, 3.81. Found (%): C, 44.56; N, 3.21; H, 3.42.

3.8. Magnetic Measurements

Magnetic measurements were performed with a Quantum Design MPMS-XL-5 SQUID magnetometer (San Diego, CA, USA) with an applied magnetic field of 0.1 T (0.5 T for compound 4) in the 2–300 K temperature range on polycrystalline samples of all the compounds with masses of 7.514, 3.824, 9.056, 1.002, 5.932, and 4.998 mg for compounds 1–6, respectively. The isothermal magnetization hysteresis measurements were done with fields from −5 to 5 Tesla at 2 K. AC susceptibility measurements were performed on the same samples with an AC field of 0.395 mT in the temperature range 2–14 K and in the frequency range 10–1000 Hz. Susceptibility data were corrected for the sample holder and for the diamagnetic contribution of the salts using Pascal’s constants [46].

3.9. X-ray Powder Diffraction

The X-ray powder diffractograms were collected on polycrystalline samples of compounds 1–6 using a 0.5 mm glass capillary that was mounted and aligned on an Empyrean PANalytical powder diffractometer using CuKα radiation (λ = 1.54177 Å). A total of six scans were collected at room temperature in the 2θ range 5–40° and merged in a single diffractogram.

3.10. Physical Properties

FT-IR spectra were performed on KBr pellets and collected with a Bruker Equinox 55 spectrophotometer. C, H, and N analyses were performed with a Thermo Electron CHNS Flash 2000 analyser and with a Carlo Erba mod. EA1108 CHNS analyzer. The Mn:Cr:Cl:X ratios (X = Cl in 1 and Br in 2) of the bulk samples were estimated by electron probe microanalysis (EPMA) performed in a Philips SEM XL30 equipped with an EDAX DX-4 microprobe. Thermogravimetric (TG) measurements were performed in Pt crucibles with a TA instruments TGA 550 thermobalance equipped with an autosampler. The TG measurements were done in the 30–700 °C temperature range at 10 °C/min under a N2 flux of 60 mL/min.

4. Conclusions

The series of six compounds formulated as (NBu4)[MnCr(C6O4Cl2)3]·nG with n = 1 for G = C6H5Cl (1), C6H5I (3), C6H5CH3 (4), n = 1.5 for C6H5Br (2), and n = 2 for G = C6H5CN (5), and C6H5NO2 (6) show that it is possible to prepare a complete series of isostructural 2D ferrimagnets by simply changing the solvent molecules. This series presents the classical honeycomb 2D lattice with an eclipsed packing of the layers giving rise to hexagonal channels and shows long range ferrimagnetic order with Tc ranging from 9.8 to 11.2 K. Interestingly, this fine modulation of Tc seems to be related to the electronic character and the number of solvent molecules that enter in the hexagonal channels of these 2D ferrimagnets. We have also prepared the corresponding bromanilato series with the same solvent molecules. Preliminary magnetic measurements show that they are also ferrimagnets with very similar Tc and with a very close sequence of ordering temperatures, further supporting the above results. Work is in progress to complete these series with other aromatic (and not aromatic) solvents and for other anilato ligands and even metal ions. Finally, attempts to remove and/or exchange the different solvent molecules are under investigation. Preliminary results suggest that it is indeed possible to exchange and even to remove the solvents molecules and, as expected, the exchanged and the evacuated compounds slightly modify their ordering temperatures.

Author Contributions

Conceptualization, S.B.; Data curation, C.M.-H., S.B. and C.J.G.-G.; Formal analysis, C.M.-H., S.B. and C.J.G.-G.; Funding acquisition, C.J.G.-G.; Investigation, C.M.-H., S.B. and C.J.G.-G.; Methodology, C.M.-H. and S.B.; Project administration, C.J.G.-G.; Supervision, S.B.; Visualization, S.B.; Writing—original draft, C.J.G.-G.; Writing—review & editing, S.B.

Funding

This research was funded by the Spanish MINECO (project CTQ2017-87201-P AEI/FEDER, UE). C.M.-H. and a pre-doctoral grant from the University of Valencia.

Acknowledgments

Thanks for the funding from the Spanish MINECO (project CTQ2017-87201-P AEI/FEDER, UE). C.M.-H. and the University of Valencia for a pre-doctoral grant.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Magnetochemistry 05 00034 g001 550
Figure 1. (a) The anilato family of ligands (C6O4X2)2− showing the typical bis-bidentate coordination mode also shown by the oxalato ligand (b).
Figure 1. (a) The anilato family of ligands (C6O4X2)2− showing the typical bis-bidentate coordination mode also shown by the oxalato ligand (b).
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Magnetochemistry 05 00034 g002 550
Figure 2. (a) Absorbance FT-IR spectra of compounds 1–6 in the range 4000–400 cm−1 (a) and 1800–400 cm−1 (b).
Figure 2. (a) Absorbance FT-IR spectra of compounds 1–6 in the range 4000–400 cm−1 (a) and 1800–400 cm−1 (b).
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Magnetochemistry 05 00034 g003 550
Figure 3. Thermogravimetric measurements and the corresponding derivative curves in the temperature range 30–700 °C for compounds 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), and 6 (f).
Figure 3. Thermogravimetric measurements and the corresponding derivative curves in the temperature range 30–700 °C for compounds 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), and 6 (f).
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Magnetochemistry 05 00034 g004 550
Figure 4. Simulated X-ray powder diffractogram of compound (NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2O (A) and experimental diffractograms of compounds 1–6.
Figure 4. Simulated X-ray powder diffractogram of compound (NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2O (A) and experimental diffractograms of compounds 1–6.
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Magnetochemistry 05 00034 g005 550
Figure 5. Structure of (NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2O (A). (a) Side view of the alternating cationic (in red) and anionic (in yellow) layers. The solvent molecules are depicted in blue. (b) Top view of the honeycomb layer [same color code as in (a)]. (c) Perspective view of the hexagonal channels [same color code as in (a)]. (d) View of one hexagon showing the π–π interactions between the aromatic ring and one anilato ring (in purple). Color code: Mn = orange, Cr = dark green, C = grey, O = red, N = dark blue, and Br = brown. The H atoms were omitted for clarity.
Figure 5. Structure of (NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2O (A). (a) Side view of the alternating cationic (in red) and anionic (in yellow) layers. The solvent molecules are depicted in blue. (b) Top view of the honeycomb layer [same color code as in (a)]. (c) Perspective view of the hexagonal channels [same color code as in (a)]. (d) View of one hexagon showing the π–π interactions between the aromatic ring and one anilato ring (in purple). Color code: Mn = orange, Cr = dark green, C = grey, O = red, N = dark blue, and Br = brown. The H atoms were omitted for clarity.
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Figure 6. (a) Thermal variation of the χmT product (a) and χm (b) for compounds 1–6. Insets show zooms of the low temperature regions.
Figure 6. (a) Thermal variation of the χmT product (a) and χm (b) for compounds 1–6. Insets show zooms of the low temperature regions.
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Figure 7. (a) Isothermal magnetization cycles at 2 K for compounds 1–6. (b) The low field region.
Figure 7. (a) Isothermal magnetization cycles at 2 K for compounds 1–6. (b) The low field region.
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Figure 8. Thermal variation of the in-phase (χ’m, left scales, filled symbols) and the out-of-phase (χ”m, right scales, empty symbols) of compounds 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), and 6 (f) at different frequencies.
Figure 8. Thermal variation of the in-phase (χ’m, left scales, filled symbols) and the out-of-phase (χ”m, right scales, empty symbols) of compounds 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), and 6 (f) at different frequencies.
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Figure 9. (a) Thermal variation of χ’m (a) and χ”m (b) at 110 Hz for compounds 1–6.
Figure 9. (a) Thermal variation of χ’m (a) and χ”m (b) at 110 Hz for compounds 1–6.
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Table
Table 1. Magnetic properties of all the structurally characterized hetero-metallic layered compounds of the type (A)[MIIMIII(C6O4X2)3]·(solvent).
Table 1. Magnetic properties of all the structurally characterized hetero-metallic layered compounds of the type (A)[MIIMIII(C6O4X2)3]·(solvent).
CCDCFormulaPackingSpace GroupTc (K)Hcoer (mT) aRef.
MIRFEA[(H3O)(phz)3][MnCr(C6O4Cl2)3(H2O)] beclipsedP35.519.4[23]
MIRFIE[(H3O)(phz)3][MnCr(C6O4Br2)3]·2H2O·2CH3COCH3eclipsedP-31m6.334.0[23]
MIRFOK[(H3O)(phz)3][MnFe(C6O4Br2)3]·H2OeclipsedP-31m--[23]
MIRFUQ(NBu4)[MnCr(C6O4Cl2)3]alternatedC2/c5.511.8[23]
HOWHAE[Fe(sal2-trien)][MnCr(C6O4Cl2)3]·0.5CH2Cl2·CH3OH·0.5H2O·5CH3CNalternatedC222110.035[40]
HOWHEI[Fe(4-OH-sal2-trien)][MnCr(C6O4Cl2)3]·GalternatedP612210.487[40]
HOWHIM[Fe(sal2-epe)][MnCr(C6O4Br2)3]·4CH3CNalternatedP21/c10.210[40]
HOWHOS[Fe(5-Cl-sal2-trien)][MnCr(C6O4Br2)3]·CH2Cl2·CH3OH·4H2O·1.5CH3CNalternatedP21/c9.866
MUMKUC[Fe(acac2-trien)][MnCr(C6O4Cl2)3]·2CH3CNalternatedC2/c10.865[41]
MUMLAJ[Fe(acac2-trien)][MnCr(C6O4Br2)3]·2CH3CNalternatedC2/c11.177[41]
MUMLEN[Ga(acac2-trien)][MnCr(C6O4Br2)3]·2CH3CNalternatedC2/c11.672[41]
SEPLAD(Me2NH2)[MnCr(C6O4Br2)3]·2H2OalternatedP-31c7.990[28]
SEPLEH(Et2NH2)[MnCr(C6O4Br2)3]alternatedP-31c8.9100[28]
SEQCID(Et3NH)[MnCr(C6O4Cl2)3]alternatedP-31c8.0150[28]
SEPROX(Et(i-Pr)2NH)[MnCr(C6O4Br2)3]alternatedP-31c9.04 c[28]
1910770(NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2OeclipsedP219.533[39]
QEFPOJ[(H3O)(phz)3][FeFe(C6O4Cl2)3]·12H2O declipsedP-31m2.41.0[42]
QEFPID[(H3O)(phz)3][FeFe(C6O4Br2)3]·12H2O declipsedP-31m2.11.0[42]
NIHJEW01[C(N2H3)3][FeFe(C6O4(CN)Cl)3]·29H2O declipsedP34.06[43]
1909314(NBu4)[MnCr(C6O4Cl2)3(C6H5CHO)]·C6H6 beclipsedP217.07.6[44]
1909315(NBu4)[MnCr(C6O4Br2)3(C6H5CHO)]·C6H6 beclipsedP216.710[44]
1909316(NBu4)[MnCr(C6O4Cl2)3(C6H5CHO)]·C6H5CHO beclipsedP216.85.0[44]
1909317(NBu4)[MnCr(C6O4Br2)3(C6H5CHO)]·C6H5CHO beclipsedP216.720[44]
a at 2 K; b the H2O or C6H5CHO molecule is coordinated to the Mn ion; c A solvated phase of this compound is metamagnetic with a critical field at 2 K of 490 mT. d These compounds are homo-metallic but show two different oxidation states.
Table
Table 2. Main IR bands (cm−1) and their assignments for compounds 1–6.
Table 2. Main IR bands (cm−1) and their assignments for compounds 1–6.
Band1 (C6H5Cl)2 (C6H5Br)3 (C6H5I)4 (C6H5CN)5 (C6H5CH3)6 (C6H5NO2)
ν(C-H)2959
2929
2873		2959
2929
2873		2960
2928
2873		2962
2931
2873		2960
2928
2873		2963
2931
2873
ν(C ≡ N) 1---2227--
ν(C=O)160916061607160716071606
ν(N-O)as 1-----1520
ν(C=C) 115061506 *15051507 *1505 *1505
ν(C=C) + ν(C-O)149514961496149714961495
δ(C-H)1383138213831383 *1383 *1383
ν(C-C) + ν(C-O)136013621361136013591361
ν(N-O)s 1-----1344 *
ν(C-N) 1-----877
δ(C-X)860858858858858857
ν(C-Cl) 1740-----
δ(C-H) 1700
684		737
682		730
685		734
687		730
693		705
683
ρ(C-X)577576576577576577
* shoulder; 1 Bands corresponding to the solvent molecules.
Table
Table 3. Experimental and calculated weigh losses (%) for compounds 1–6 at the first plateau at around 250–300 °C.
Table 3. Experimental and calculated weigh losses (%) for compounds 1–6 at the first plateau at around 250–300 °C.
CompoundTemperature
Range (°C)		Experimental
Weight Loss (%)		SolventCalculated
Weight Loss
(NBu4)[MnCr(C6O4Cl2)3]·C6H5Cl (1)30–20010.81 C6H5Cl10.4
(NBu4)[MnCr(C6O4Cl2)3]·1.5C6H5Br (2)30–25022.01.5 C6H5Br19.5
(NBu4)[MnCr(C6O4Cl2)3]·C6H5I (3)30–20016.61 C6H5I17.4
(NBu4)[MnCr(C6O4Cl2)3]·C6H5CH3 (4)30–2909.71 C6H5CH38.7
(NBu4)[MnCr(C6O4Cl2)3]·2C6H5CN (5)30–25016.82 C6H5CN17.5
(NBu4)[MnCr(C6O4Cl2)3]·2C6H5NO2 (6)30–22020.72 C6H5NO220.2
Table
Table 4. Unit cell parameters of compounds 1–6 determined from their X-ray powder diffractograms at room temperature.
Table 4. Unit cell parameters of compounds 1–6 determined from their X-ray powder diffractograms at room temperature.
Compounda (Å)b (Å)c (Å)(°)Volume (Å3)
(NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2O (A) a9.9557(5)23.6054(10)12.2129(6)105.187(5)2769.9(2)
(NBu4)[MnCr(C6O4Br2)3]·C6H5Br·0.5H2O (A) b10.104(4)23.70(1)12.363(5)106.78(4)2834.17
(NBu4)[MnCr(C6O4Cl2)3]·C6H5Cl (1)9.799(8)23.76(3)12.08(1)104.71(8)2719.83
(NBu4)[MnCr(C6O4Cl2)3]·1.5C6H5Br (2)9.89(1)23.66(4)12.06(2)103.7(1)2740.18
(NBu4)[MnCr(C6O4Cl2)3]·C6H5I (3)9.71(4)23.6(1)12.23(6)104.3(5)2720.50
(NBu4)[MnCr(C6O4Cl2)3]·C6H5CH3 (4)9.822(7)23.88(3)12.09(1)104.95(8)2739.65
(NBu4)[MnCr(C6O4Cl2)3]·2C6H5CN (5)9.887(9)23.55(3)12.08(1)104.5(1)2723.03
(NBu4)[MnCr(C6O4Cl2)3]·2C6H5NO2 (6)9.58(4)23.7(1)11.98(6)105.8(4)2611.89
a Single crystal X-ray data at 120 K [39]. b Determined from X-ray powder data at room temperature.
Table
Table 5. Magnetic properties of compounds 1–6.
Table 5. Magnetic properties of compounds 1–6.
CompoundχmT @ 300 K (cm3 K mol−1)Tc (K)M @ 5 T (μB)Hc (mT)
(NBu4)[MnCr(C6O4Cl2)3]·C6H5Cl (1)6.2910.42.2043.7
(NBu4)[MnCr(C6O4Cl2)3]·1.5C6H5Br (2)6.2210.72.1544.7
(NBu4)[MnCr(C6O4Cl2)3]·C6H5I (3)6.2611.02.1130.2
(NBu4)[MnCr(C6O4Cl2)3]· C6H5CH3 (4)6.269.82.2016.2
(NBu4)[MnCr(C6O4Cl2)3]·2C6H5CN (5)6.2710.82.2019.3
(NBu4)[MnCr(C6O4Cl2)3]·2C6H5NO2 (6)6.3011.22.1256.2

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Martínez-Hernández, C.; Benmansour, S.; Gómez-García, C.J. Chloranilato-Based Layered Ferrimagnets with Solvent-Dependent Ordering Temperatures. Magnetochemistry 2019, 5, 34. https://doi.org/10.3390/magnetochemistry5020034

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Martínez-Hernández C, Benmansour S, Gómez-García CJ. Chloranilato-Based Layered Ferrimagnets with Solvent-Dependent Ordering Temperatures. Magnetochemistry. 2019; 5(2):34. https://doi.org/10.3390/magnetochemistry5020034

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Martínez-Hernández, Cristian, Samia Benmansour, and Carlos J. Gómez-García. 2019. "Chloranilato-Based Layered Ferrimagnets with Solvent-Dependent Ordering Temperatures" Magnetochemistry 5, no. 2: 34. https://doi.org/10.3390/magnetochemistry5020034

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