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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Magnetic Structure, Single-Crystal to Single-Crystal Transition, and Thermal Expansion Study of the (Edimim)[FeCl4] Halometalate Compound Palmerina González-Izquierdo,† Oscar Fabelo,*,‡ Garikoitz Beobide,*,§ Oriol Vallcorba,∥ Fabio Sce,† Jesús Rodríguez Fernández,† María Teresa Fernández-Díaz,‡ and Imanol de Pedro*,† †

CITIMAC, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain Institut Laue-Langevin, BP 156X, F-38042 Grenoble Cedex, France § Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain ∥ ALBA Synchrotron Light Source, Cerdanyola del Vallés, Barcelona, Spain ‡

S Supporting Information *

ABSTRACT: This contribution addresses standing questions about the nature and consequences of the ion self-assembly and magnetic structures, as well as the molecular motion of the crystalline structure as a function of the temperature, in halometalate materials based on imidazolium cation. We present the magnetic structure and magnetostructural correlations of 1ethyl-2,3-dimethylimidazolium tetrachloridoferrate, (Edimim)[FeCl4], resolved by neutron diffraction studies. Single-crystal, synchrotron powder X-ray diffraction and powder neutron diffraction techniques have been combined to follow the temperature evolution on its crystallographic structure from 2 K close to its melting point (340 K). In this sense, slightly above room temperature (307 K) (Edimim)[FeCl4] presents a single-crystal to single-crystal transition (SCSC), from phase I (space group P21/n) to phase II (P21/m), accompanied by a notable increase in the disorder of the imidazolium cation, as well as in the metal complex anion. The temperature evolution and solid-phase transitions of the presented compound were followed in detail by synchrotron X-ray powder diffraction (SXPD), which confirms the occurrence of another phase transition at 330 K, phase III (P21/m), the crystal structure of which was elucidated from the SXPD pattern. Moreover, this material presents an anisotropic thermal expansion with a switch from axial positive to negative thermal expansion coefficients as the temperature is raised above the first phase transition, which has been correlated with the molecular motion of the imidazolium-based molecules, producing not only a shortening of the counterion···counterion distances but also the occurrence of different quasi-isoenergetic crystal structures as a function of the temperature.

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INTRODUCTION

relatively low melting point (most of the members of this family are liquid at room temperature) or to the complexity of the thermodynamics of these systems.14 These materials exhibit a wide variety of interactions ranging from nonspecific and isotropic forces, weak (e.g., van der Waals (vdW), solvophobic, dispersion forces) and strong (Coulombic), to specific and anisotropic forces (e.g., hydrogen bonding, halogen bonding, dipole−dipole, magnetic dipole, electron pair donor/acceptor interactions), some of them influenced by temperature but not others.9,15 Therefore, further studies are needed to fully understand the influence of these parameters on the crystal structure.

Halometalate compounds, synthesized by a metal halide reacting with an organic salt, display interesting properties such as strong Lewis acidity,1,2 paramagnetism,3,4 and novel electrochemical or catalytic properties, depending on the identity and concentration of the metal ions.5−8 A subset of these materials can be presented as magnetic ionic liquids (MILs), defined as paramagnetic salts with a melting point below 100 °C, which can combine IL characteristics9 with additional functionalities such as thermochromism,10 magnetoelectrochromism,11 luminescence,12 and magnetoresistance13 that is determined by the enclosed paramagnetic ion. Currently, there is a lack of published research papers giving experimental and theoretical investigations into the effect of temperature on the solid phases of halometalates based on an imidazolium cation. This fact is probably related to their © XXXX American Chemical Society

Received: October 16, 2017

A

DOI: 10.1021/acs.inorgchem.7b02632 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry As a prelude to this work, we have carried out a magnetic and structural characterization of (Edimim)[FeCl4] at low temperature. Through magnetic susceptibility and heat capacity measurements it was found that this compound shows antiferromagnetic interactions and 3D magnetic ordering below 2.9 K.16 Furthermore, its crystal structure at 100 K was initially determined by SXPD, herein named phase I (P21/ n; a = 9.6703 Å, b = 14.3513 Å, c = 9.5744 Å, and β = 94.261°). At this temperature, its crystal structure can be described as a succession of anionic layers of tetrahedral [FeCl4]− complex entities and cationic layers of 1-ethyl-2,3-dimethylimidazolium extended in the ac plane and piled up along the b axis.16 Although the macroscopic magnetic measurements point to an antiferromagnetic order, neither the microscopic origin nor the magnetic structure were known. In this paper, we have completed this study by determining the magnetic structure and magnetostructural correlations by powder neutron diffraction studies. Moreover, single-crystal, synchrotron powder X-ray, and powder neutron diffraction were combined to follow the temperature evolution of the crystal structure up to the melting point (from 2 to 340 K). The occurrence of a single-crystal to single-crystal (SCSC) phase transition, the change in the behavior of the axial thermal expansion coefficients from positive to negative (during warming of the sample), the solid-phase transitions, and the phase diagram close to the melting point (340 K) should be of special interest for the materials science community.17,18 Moreover, the present work should help to understand the nature and consequences of the ion self-assembly and magnetic interactions of ionic liquids based on halometalate ions,19 as well as the molecular motion in crystalline solids of imidazolium-based compounds.20−23

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Figure 1. DSC thermogram of (Edimim)[FeCl4], with the black line showing the first cooling cycle and second heating cycle (heating rate 5 K/min). The inset shows an enlarged area of the DSC curve between 310 and 340 K.

we accomplished the single-crystal structure determination from 100 to 310 K of phases I (I@100−304 K) and II (II@310 K). Above this temperature, the low quality of the data precludes a correct crystal structure resolution. In order to overcome this problem, SXPD data above 310 K have been used. Despite SXPD data showing a significant change in the unit cell parameters, the space group of phase II is retained in phase III. The crystal structure derived from Rietveld refinement allows us to attribute the weak II/III phase transition to a reorientation of the imidazolium molecules, prompted by thermal agitation near the melting point (see details hereafter). Finally, the DSC peak observed above 340 K corresponds to the melting of the sample, which is also in good agreement with the SXPD data described below. Note that the cooling branch of the DSC demonstrates that all phase changes (I/II/III/liquid) are reversible, while the shift to lower temperatures occurring for the liquid/III transition can be attributed to the supercooling of the sample. Neutron Diffraction Analysis and Magnetic Structure Determination. Neutron powder diffraction patterns of (Edimim)[FeCl4] at 300, 100, and 10 K were collected in the D2B diffractometer at ILL (λ = 1.5938 Å) (Figure S1a in the Supporting Information). As can be seen, no structural phase transition is detected in the neutron diffraction patterns below room temperature down to 10 K. The data at 100 K (Figure S1b in the Supporting Information) were indexed using the same space group (P21/n) and unit cell parameters (a = 9.654(1) Å, b = 14.361(1) Å, and c = 9.571(1) Å with β = 94.24(2)°) similar to those of the reported structure obtained from the SXPD data at 100 K.16 At 10 K, a slight shift of the Bragg reflections was detected due to thermal lattice contraction. The temperature evolution of the neutron diffraction patterns from 10 to 1.5 K was collected using a high-flux D1B diffractometer, working at λ = 2.52 Å. The data below 3 K display additional elastic intensity which is compatible with the magnetometry results (Neel temperature TN = 2.9 K).16 A comparative view of a pattern in the paramagnetic state (10 K) and a magnetically ordered state (2 K) reveals the presence of

EXPERIMENTAL SECTION

Sample Preparation. A polycrystalline sample of (Edimim)[FeCl4] was freshly obtained through the thermal treatment of a stoichiometric mixture of FeCl3 and (Edimim)Cl inside a glovebox, as previously described in ref 16. Single crystals suitable for X-ray diffraction were grown by the recrystallization of the compound (0.081 g, 0.25 mmol) in a mixture of 2-propanol (10 mL) and 1-heptanol (15 mL). Physical Measurements. Details of the thermal analysis, neutron diffraction experiments, single-crystal X-ray diffraction analysis, and variable-temperature synchrotron X-ray powder diffraction analysis are thoroughly described in the Supporting Information.

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RESULTS AND DISCUSSION Phase transitions of (Edimim)[FeCl4]. Previous DSC measurements on the present compound (heating rate, 10 K/ min; sample mass, 15 mg)16 showed two endothermic peaks corresponding to a solid to solid phase transition from crystalline phase I to phase II and to the sample melting point, respectively. The second peak features a marked broadening (ΔT = 23 K) that almost overlaps the former peak. Herein, DSC experimental parameters were fitted to improve the resolution of the thermogram. Precisely, when the heating rate and sample mass were lowered to 5 K/min and 5 mg, respectively (Figure 1), a weak endothermic peak (325 K) corresponding to a new solid to solid phase transition (phase II to III) is distinguished between the former I/II phase transition (307 K) and the latter melting point of the sample (347 K). While the crystal structure of phase I was determined at 100 K by SXPD in a previous work,16 the high-temperature structures (II and III) have been unknown to date. In this sense, herein B

DOI: 10.1021/acs.inorgchem.7b02632 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) Neutron diffraction profiles of (Edimim)[FeCl4] at 10 (red) and 1.5 K (blue) obtained in D1B. The asterisks show the magnetic contributions. (b) Observed (red points) and calculated (black solid line) powder diffraction patterns of (Edimim)[FeCl4] at 1.5 K obtained with the D1B instrument (λ = 2.52 Å). Positions of the Bragg reflections are represented by vertical green bars. The observed−calculated difference patterns are depicted as a blue line at the bottom of the figure.

the first refinements for the magnetic moment component along the b axis, its value was fixed at zero, giving rise to a collinear antiferromagnetic behavior with the magnetic moments contained in the ac plane (see Figure 3). The refined

four new sharp Bragg peaks (marked with asterisks in Figure 2a), which are related to the three-dimensional magnetic ordering, and a weak diffuse scattering superimposed around 2θ = 15°. This increase in the background at low temperature has also been detected in (Dimim)[FeCl4] (Dimim = 1,3dimethylimidazolium),14 where polarized neutron diffraction was used to discard the magnetic origin of this background. The rise in the background was attributed to an increase in the incoherent scattering signal. A similar scenario is expected for the present compound. Rietveld refinements below the magnetic phase transition show that the P21/n symmetry of the crystal structure is also maintained down to 1.5 K. A Rietveld refinement was performed using the atomic coordinates and anisotropic temperature factors of the nuclear part obtained from the SXPD calculations at 100 K.16 The additional elastic intensities can be indexed with the propagation vector k = (−0.5, 0, 0.5), indicating that there is a 2-fold increase of the magnetic unit cell along the a axis and c axis with respect to the nuclear cell. Using the transformation from P21/n symmetry to the standard group P21/c (a′ = −a − c, b′ = b, c′ = a), the propagation vector will be modified to (0.5, 0, 0). The occurrence of a propagation vector k ≠ (0, 0, 0) involves a strictly antiferromagnetic structure due to the time-inversion symmetry. It should be noted that the indexed propagation vector differs from the previous vector obtained for MILs based on the 1,3dimethylimidazolium cation, Dimim[FeX4] (X = Cl, Br), which, until now, always presents a k = (0, 0, 0) propagation vector (corresponding to the same magnetic and nuclear cell).14,24 In order to determine the magnetic structure, the irreducible representations (irreps) compatible with the propagation vector (k = (−0.5, 0, 0.5)) were investigated using Bertaut’s symmetry analysis method.25 This approach allows us to determine the symmetry constraints between the different magnetic moments of the Fe3+ atom within the magnetic unit cell. The total magnetic representation of the propagation vector group can be decomposed into four irreducible representations26 (see details in the Supporting Information). In all cases there are three degrees of freedom for each Fe3+ site. The best magnetic model is described by irrep Γ3 (see Figure 2b and details of magnetic model calculations in the Supporting Information). It corresponds to antiferromagnetic layers extended into the ac plane, which are antiferromagnetically coupled along the b axis direction. The model allows a possible tilting along the b axis. However, the neutron diffraction data do not account for enough accuracy to refine it. Therefore, in order to avoid overparametrization and due to the negligible value obtained in

Figure 3. Magnetic structure of (Edimim)[FeCl4] at 1.5 K. Color code: yellow, iron; green, chloride; blue, magnetic spin.

magnetic structure is compatible with the Pa21/c (14.80) magnetic space group, described with the unit cell according to the standard setting, a = 19.075(1) Å, b = 14.260(1) Å, and c = 14.043(1) Å with β = 136.84(1)° (relation of new setting to old setting: basis, {(0,0,−1),(0,−1,0),(−1/2,0,1/2)}; origin, (0.500,0.000,0.000)), and is in complete agreement with the macroscopic magnetic susceptibility analysis.16 The best fitting of the D1B data at 1.5 K for the iron lattice gives Rtetha = 76(1)° and a magnetic moment of 4.11(1) μB, a value which is slightly lower than the expected value of 5 μB for Fe3+. However, this value is in agreement with previous results of similar tetrahalide ferrate compounds in which the spin density estimated by DFT calculations is not strictly localized on the iron ions but is partially delocalized onto the nearest halide atoms of the metal complex ion.27 According to the magnetic model obtained from neutron diffraction data, the main magnetic couplings of (Edimim)[FeCl4] are via direct superexchange anion−anion interactions (Fe−Cl···Cl−Fe) between an iron ion and its first shell of neighboring iron ions (Figure 4a). There are two intraplane interactions, J⊥ (J1 and J2), connecting the iron atoms, giving rise to chains which are connected to each other and forming the ac plane with Cl···Cl distances of 3.834(1) Å for J1 and 3.948(1) Å for J2 at 10 K. The atoms involved in this exchange coupling display a mixture of superexchange angles, (i) Fe− Cl···Cl of 146.5 and 139.6° and (ii) Cl···Cl−Fe of 108.9 and C

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Figure 4. Schematic view of the possible intraplane (a) and interplane (b) magnetic exchange pathways for (Edimim)[FeCl4] via a Fe−Cl···Cl−Fe bridge. The Edimim counterions have been omitted for the sake of clarity.

Table 1. Crystallographic Data and Structure Refinement Details of (Edimim)[FeCl4] at Different Diffraction Temperatures I@100 K empirical formula formula wt cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z ρ (g cm−3) ρ (g cm−3)a μ (mm−1) no. of rflns collected no. of unique data/params R goodness of fit (S)b R1c/wR2d (I > 2σ(I)) R1c/wR2d (all data)

I@250 K

I@293 K

monoclinic P21/n 9.5889(2) 14.3617(2) 9.6727(2) 94.215(2) 1328.45(4) 4 1.641

monoclinic P21/n 9.6914(2) 14.5300(3) 9.7789(2) 94.212(2) 1373.31(5) 4 1.561

1.906 10920 3000/156 0.0328 1.070 0.0301/0.0572 0.0396/0.0621

1.844 11306 3111/127 0.0431 1.103 0.0424/0.0778 0.0718/0.0919

C7H13Cl4FeN2 322.84 monoclinic P21/n 9.7694(5) 14.6321(8) 9.8343(6) 93.969(5) 1402.41(14) 4 1.529 1.52(2) 1.805 11338 3152/127 0.0548 1.054 0.0612/0.1295 0.1185/0.1656

I@304 K

II@310 K

monoclinic P21/n 9.8177(12) 14.6786(17) 9.8489(14) 93.258(13) 1417.0(3) 4 1.513

monoclinic P21/m 6.8741(3) 14.6758(6) 7.1003(3) 90.430(5) 716.28(5) 2 1.497

1.787 11262 3240/170 0.1000 1.013 0.0829/0.2135 0.2101/0.2972

1.767 5169 1544/73 0.1112 1.086 0.0863/0.2868 0.1112/0.3247

Experimental density measured at 20 °C. bS = [∑w(Fo2 − Fc2)2/(Nobs − Nparam)]1/2. cR1 = ∑||Fo| − |Fc||/∑|Fo|. dwR2 = [∑w(Fo2)2 − Fc2)2/ ∑w(Fo2)2]1/2; w = 1/[σ2(Fo2) + (aP)2 + bP], where P = (max(Fo2,0) + 2Fc2)/3 with a = 0.0176 (I@100 K), 0.0241 (I @250 K), 0.0465 (I @293 K), 0.1040 (I @304 K), 0.1911 (II @310 K) and b = 0.7167 (I @100 K), 0.4427 (I @250 K), 1.6427 (I @293 K), 0.2582 (II @310 K). a

79.3° with τ values of 123.8 and 160.2° for J1 and J2, respectively. According to the refined magnetic structure, J1 and J2 show antiferromagnetic and ferromagnetic behavior, respectively. If we compare the distances, the stronger magnetic exchange constant between them should be J1, which is correlated with shorter Cl···Cl lengths.28 In the case of the angles, remarkably, J1 in Dimim[FeX4] (X = Cl, Br) compounds links the iron atoms, forming linear ferromagnetic chains along the [1,0,1] direction.14,24 Both compounds based on the Dimim cation also show a value of X···X−Fe angle near 90° (between 88 and 91° for Cl and Br, respectively), as observed in J2 for the present compound. Therefore, an increase in the Cl···Cl−Fe angle up to 108.9° affects this orthogonal angle enough to force changes in the sense of this magnetic interaction (for ferromagnetic to antiferromagnetic coupling). Finally, the J3 magnetic coupling, or interplane superexchange interaction, gives rise to zigzag chains, creating a ladder arrangement which is extended along the b axis direction

(Figure 4b). Cl···Cl distances have been shown to be 4.209(2) Å and Fe−Cl···Cl and Cl···Cl−Fe angles to be 153.1 and 169.5°, with a τ value of 60.57°. π−d interactions of the aromatic rings with the [FeCl4]− metal complexes were also detected (see the single-crystal analysis below), which should display weak indirect superexchange anion−anion interactions through the imidazolium cation (Fe−Cl···Im···Cl− Fe). Single-Crystal to Single-Crystal (SCSC) Phase Transition. Single-crystal X-ray diffraction (SCXRD) measurements were initially used to probe the structural evolution of phase I upon heating from 100 to 304 K (I@100−304 K). Further heating of the sample prompted an SCSC transformation from I to II, implying a contraction in the crystallographic a and c axes and a switch to the P21/m space group (II@310 K). The changes in the molecular conformation and packing arrangement after the phase transition are not severe enough to lead to crystal breakage, and as a result a study on both phases by SCXRD was feasible. The relatively weak interactions (hydrogen bonding, Couloumbic interactions, and vdW forces) D

DOI: 10.1021/acs.inorgchem.7b02632 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Views of the crystal packing for I@100 K, I@304 K, and II@310 K along the [001] and [101] crystallographic directions. C, Cl, Fe, and N atoms are shown in gray, green, purple, and blue, respectively. The second component of the disordered species is depicted by single-color sticks (pale pink, pink and red for I@100 K, I@304 K and I@310 K, respectively). Hydrogen atoms are omitted for clarity.

Figure 6. Thermal evolution of (a) mean equivalent isotropic displacement parameters for the metal center (red circles), chloride ligands (blue squares), and organic cation (black tilted squares) and (b) mean [FeCl4]− metal···metal distance differences.

As stated in the Introduction, the crystal structure of phase I was previously elucidated from SXRD data collected at 100 K using synchrotron radiation. Herein, collected single-crystal data provide a more accurate resolution that allows envisaging the disorder of the molecular building units, their thermal evolution, and their relationship within I/II SCSC transformation. I@100 K shows a positional disorder for the imidazolium counterion that was modeled using two fragments (refined ratio 0.923(2):0.077(2)). This disorder can be depicted by the flipping of the imidazolium cation ring (Figure 5). At intermediate temperatures, I@250 K and I@293 K, the thermal agitation does not allow the disorder of the organic cation to be resolved, as it accounts for a small fraction (ca. 8%). At a higher temperature, I@304 K, thermal agitation prompts a marked increase in the structural disorder that involves both the organic counterion and the [FeCl4]− metal complex entity. It should be noted that at 304 K the disorder was modeled using the same disorder parameter for the metal complex and for the counterion, giving rise to a ratio of 0.801(6):0.199(6) for each part. Further increase in the temperature accomplishes the SCSC transition from the lowtemperature P21/n (I@100−304 K) to the high-temperature P21/m (II@310 K) phase. The disorder of the latter structure is ascribed to the symmetry plane and inversion center that entail the splitting of two chloride ligands (Cl3 and Cl4, as labeled in the CIF files and the next figures) and the imidazolium cation into two symmetry-related positions with an occupation factor

between the organic and inorganic moieties enable the molecular motion of the cations and anions under mild thermal energy input, resulting in an increase of the disorder degree, in phase I, and in a reversible SCSC transformation giving rise to the phase II behavior, which is in agreement with previously reported examples of SCSC phase transitions.29,30 Crystallographic data and details of the structure determination and refinement at 100, 250, 293, 305, and 310 K are summarized in Table 1. CIF files are available as detailed at the end of the paper. As the temperature is raised, thermal agitation degrades the diffraction data quality and, accordingly, figures of merit are worsened. Data acquired at temperatures greater than 312 K were unsuitable to accomplish a structural elucidation. Therefore, the structural analysis has been completed close to the melting point (340 K) by variable-temperature SPXRD analysis (see below). The most relevant bond distances and angles are gathered for I and II along the paper. In the studied temperature range of the SCXRD, 100−310 K, intramolecular bond distances and angles are consistent with the data found in the Cambridge Structural Database for analogous entities.31 Crystal phases I and II can both be described by the successive assembly of anionic layers of tetrahedral [FeCl4]− complex entities and cationic layers of 1-ethyl-2,3-dimethylimidazolium (Edimim) molecules which pile up through the [010] crystallographic direction (Figure 5). The 3D network is mainly sustained by an intricate network of weak dispersive and electrostatic interactions, which will be further described below. E

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found in other ILs with 1-ethyl-3-methylimidazolium cation.33,34 The value of the C2−N1−C6−C7 torsion angle at 100 K points to 83(1)°, which decreases to 76(2)° at 310 K. In contrast to the aforementioned cell expansion, Fe−Cl bond distances decrease progressively as the temperature is raised (mean Fe−Cl distances 2.198(6), 2.190(6), 2.187(5), 2.187(3), and 2.177(9) Å at 100, 250, 293, 304, and 310 K, respectively). This unexpected behavior is closely related to the detriment of noncovalent interactions as thermal agitation is increased, which results in a strengthening of the intramolecular Fe−Cl bond. To illustrate the previous statement, we can take a look at the set of interactions established by [FeCl4]− units of the anionic layer at 100 K (Figure 8). They self-assemble into one-

of 0.5 for each part. Therefore, the I/II SCSC transition can be related to the enhancement of the disorder of the cation and anion with heating, causing the atom-position displacements and affecting their integral arrangement in the solid state; thus, some crystallographic symmetry elements were transformed, resulting in the observed thermally induced space group transition. The disorder degree on the crystal structure seems to be frozen at temperatures below 293 K, as increasing it requires further flipping of the imidazolium rings. However, at greater temperatures, the thermal agitation seems to be enough to overcome the steric hindrance related to the ring flipping and the disorder exhibits a sudden increase in the 304−310 K temperature range. Such a trend becomes clear on comparison of the evolution of the equivalent isotropic displacement parameters for the metal center, chloride ligands, and imidazolium cation (Figure 6a). They all increase monotonically within the 100−293 K temperature range and thereafter they exhibit a sudden increase. Furthermore, the thermal agitation and ring flipping imply a net cell expansion, increasing the specific volume by ca. 8% at 310 K (0.668 cm3 g−1) in comparison to I@100 K (0.620 cm3 g−1). Further details of the thermal expansion will be thoroughly studied below in an analysis of variable-temperature SXPD data. The increase in temperature has a notable effect not only on the isotropic thermal parameters but also on the anion−anion distances (see Figure 6b). As a representative case, considering the metal center of [FeCl4]− unit as a node, the anionic layer can be described as a 4-connected square-planar net (Figure 7). The mean metal···metal distance increases continuously from 6.86 Å at 100 K to 6.99 Å at 310 K.

Figure 8. Noncovalent interactions around [FeCl4]− entity. Color codes: gray, carbon; blue, nitrogen; white, hydrogen. The halide···π interactions (red), hydrogen bonding (green), and electrostatic interactions (gray) are shown by dashed lines.

dimensional supramolecular chains by means of the weak electrostatic interactions occurring between the vortex of the tetrahedron (Cll) and the centroid of the face enclosed by three chloride ligands (Cl2, Cl3, Cl4) of a neighboring iron(III) complex (Cl1···centroid, 3.647 Å; angle, 83.9°). Apart from the fact that the cohesiveness of the supramolecular architecture is further strengthened by means of Car−H hydrogen bonding (C4−H4···Cl3, 2.913 Å; C5−H5···Cl4, 2.904 Å) and weak halide···π interactions (Cl1···C4, 3.412 Å; Cl2···C4, 3.534 Å) which are similar to those described for other imidazolium ILs.35 At 310 K, these interactions exhibit a mean lengthening that is more significant for the aforementioned [FeCl4]−··· [FeCl4]− (Cl2···Cl1,Cl3,Cl4-centroid, 4.460 Å) and halide····π (Cl1···C4, 3.673 Å; Cl2···C5, 3.805 Å) interactions than for the hydrogen bonding (C4−H4···Cl3, 3.102 Å; C5−H5···Cl4, 2.822 Å). Variable-Temperature Synchrotron Powder Diffraction (SXPD). SXPD has been used to investigate in detail the thermal expansion process of (Edimim)[FeCl4] from 100 to 340 K under ambient-pressure conditions. Furthermore, the crystal structures at representative temperatures of 300 and 320 K were refined by starting from the previously reported singlecrystal structure model of phases I and II. Finally, the crystal structure of phase III@335 K (P21/m) (a = 7.0636(2) Å, b = 14.6120(3) Å, c = 7.0702(2) Å with β = 90.975(1)°) has been elucidated from the SXPD pattern. The experimental, calculated, and difference powder profiles can be found in the Figure S3c in the Supporting Information and the atomic coordinates in the corresponding CIF files. Final structural parameters and figures of merit are given in Table S2 in the

Figure 7. View of the anion layer, [FeCl4]−, along the [010] direction in phases I@100 K (blue) and II@310 K (red).

With regard to the influence of thermal agitation in the intramolecular bonding, it must be pointed out that in all cases the imidazole ring of the Edimim cation remains planar, with the C−C and C−N bond lengths lying in the expected range of other imidazolium compounds.24,32 Moreover, the conformational equilibrium of the imidazolium cation has a gauche (nonplanar) conformation with respect to the NCC angle of the ethyl group, in good agreement with the conformation F

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Inorganic Chemistry

Å3, which denotes 2.04% of the total at 100 K. Therefore, in this range of temperature, all cell parameters display a positive thermal expansion with an almost linear enhancement. Moreover, the β angle remains fairly constant with minimal deviation below 250 K and afterward it shows a rapid decrease (0.3°) up to 300 K (see inset of Figure 10a). After the phase transition at 307 K, the cell dimensions follow a different behavior (Figure 10b). In phase II from 310 to 330 K, the b axis decreases exponentially 0.04 Å, whereas the a parameter increases 0.20 Å and the c axis remains almost constant up to 322 K and then suddenly drops 0.02 Å around 330 K. The rise in the volume is 16.0 Å3, which denotes 2.3% of the total at 305 K. Finally, in phase III between 330 and 340 K, the variation of the cell parameters behaves again almost linearly with an enlargement in the volume of ca. 4 Å3, the b axis decreasing by 0.01 Å, and the a and c axes rising by 0.02 Å, with a β growth of ca. 0.07° from 300 to 340 K (see inset of Figure 10b). The present SXPD data allow us to calculate the shifts in length of the principal axis (X) as a function of the temperature of different phases (see Figure S6 in the Supporting Information). The direction of the principal axis (X1, X2, and X3) as a function of the crystallographic axes is described in Table S4 in the Supporting Information across the temperature range provided. In this context, X1 is inclined approximately 15° to the crystallographic a axis; X2 is approximately equal to the b axis and X3 is inclined by 10° to the crystallographic c axis for all phases. The principal thermal expansion (TE) coefficients, calculated as α = (1/l)(δl/δt)P (Table S4) were estimated via linear fits using the PASCal program.36 A plot of the indicatrices using the normalized components of the principal axes projected onto the crystallographic axes is given in Figure S7 in the Supporting Information. For phase I, from 100 to 300 K, the TE values for the principal axis are αX1 = 76(1), αX2 = 98(1), and αX3 = 108(2) 10−6 K−1 and the coefficient of the volume thermal expansion, αV, is [229(3)] × 10−6 K−1. Remarkably, the replacement of the anion (FeCl4− for Cl−) in the same imidazolium cation causes strong variations in the lengths of the principal orthogonal axis TE values calculated from 100 to 350 K. Whereas the αV value remains almost constant for (Edimim)Cl ([184(3)] × 10−6 K−1), the substitution of the metal complex anion for the halide leads to a shift in lengths from positive to negative along the b axis (αX2 = −[12.8(2)] × 10−6 K−1; axial negative thermal

Supporting Information. Structure solutions from powder diffraction data at III@335 K show that (Edimim)[FeCl4] is isostructural with phase II@320 K but is differentiated from it in the rotation of the organic cations (see Figure 9 and Figure

Figure 9. View of the crystal packing for III@335 K along the a axis. C, Cl, Fe, and N atoms are shown in gray, green, light blue, and dark blue, respectively. The component of the disordered species is depicted as red sticks for III@335 K. Hydrogen atoms are omitted for clarity.

S4 in the Supporting Information). Moreover, the mean Fe−Cl distance increases to 2.32 Å. This elongation is also observed in the [FeCl4]−···[FeCl4]− distances, with 5.703(16) Å being the shortest distance (Cl2···Cl1,Cl3,Cl4-centroid). SXPD has been analyzed to investigate in detail the thermal expansion of (Edimim)[FeCl4] from 100 to 340 K under ambient-pressure conditions. We carried out a fit of the cell parameters (Table S3 in the Supporting Information) from the experimental data (see Figure S5 in the Supporting Information) using the FullProf Suite26 and starting from the experimental coordinates obtained by the Rietveld analysis of SXPD at 100 K.16 The thermal expansion parameters of phase I from 100 to 300 K are anisotropic, with the b lattice increasing by 0.1050 Å and the a and c axes growing by 0.0633 and 0.0616 Å, respectively (Figure 10a). The increase in the volume is 27.1

Figure 10. Refined a (red), b (blue), and caxes (green) obtained from the SXPD patterns versus temperature (a) between 100 and 300 K for phase I and (b) between 310 and 340 K for phases II and III. The insets show the variations of the monoclinic angle. G

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Inorganic Chemistry expansion) and a decrease in αX1 = [7.03(2)] × 10−6 K−1 with an increase in αX2 = [187(2)] × 10−6 K−1 TE coefficients.37 After the first solid phase transition (phases II and III), the orientation of principal axes is nearly the same but the thermal variation shows a more complex behavior with an induction of an axial negative thermal expansion. Therefore, the values obtained for αX2 are −[213(30)] × 10−6 and −[51(18)] × 10−6 K−1 and for αX3 are −[190(18)] × 10−6 and [126(18)] × 10−6 K−1 for phases II and III, respectively. This axial negative thermal expansion is not evident in the αV value, which varies almost linearly, ca. [864(13)] × 10−6 K−1 between 307 and 340 K. A possible mechanism of thermal expansion for this material at the molecular level can be inferred by comparing the different crystal structures between 100 and 340 K. As obtained from the equilibrium positions of the atoms derived from X-ray diffraction, the N−C and C−C imidazolium and Fe−Cl bond distances decrease progressively as the temperature is raised to 304 K due to vibrational effects. Moreover, the disorder degree for the crystal structure seems to be frozen at temperatures below 293 K, but at higher temperatures, the disorder exhibits a sudden increase in the 304−340 K temperature range, which substantially changes the 3D molecular conformation of the imidazolium cation and [FeCl4]− anion. The thermal agitation and ring flipping imply a cell expansion, which is notably larger than the thermal expansion presented by other organoinorganic compounds.38 This molecular movement requires having more accessible space; therefore, the distance between the adjacent molecules is increased in order to “allow” these flipping and vibration movements. The cell parameters increase as a consequence of this effect, with the largest variation located along the a axis. In order to estimate the change in the conformation of the (Edimim) counterion as a function of the temperature, we have determined the angle variation of the imidazolium plane between phases I and II and between phases II and III. As the cell parameters between phases I and II are not directly comparable, we have applied the transformation matrix [a′ = c − a, b′ = b, c′ = −a − c] to phase I. On the basis of this calculation, the angle between two equivalent (Edimim) molecules in phases I and II, respectively, is 21.74(15)° while the angle for the same molecules in phases II and III, is notably larger with a value of 164.4(4)° due to the flip of the molecule. This variation can explain the occurrence of a biaxial negative TE after the last phase transition. Consequently, the molecules can then approach each other along the b and c directions, leading to a small contraction along these axes.

and III preceding the melting point. We have confirmed that small changes in the molecular conformation and packing arrangement lead to a reversible SCSC transformation. Finally, a relationship between the phase transition and the induction of an anisotropic axial TE has been discussed. These unique properties of (Edimim)[FeCl4] result from an interplay of several effects, such as an intricate network of weak interactions (hydrogen bonding, CH−π, or vdW interactions) and the ability of the counterion to become disordered, resulting in a flexible framework. This flexibility allows the occurrence of quasi-isoenergetic crystal structures, which can be transformed into each other by slight changes in temperature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02632. Details on the thermal analysis, neutron diffraction experiments, single-crystal X-ray diffraction analysis, and variable-temperature synchrotron X-ray powder diffraction analysis (PDF) Accession Codes

CCDC 1579534−1579540 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for O.F.: [email protected]. *E-mail for G.B.: [email protected] *E-mail for I.d.P.: [email protected] ORCID

Oscar Fabelo: 0000-0001-6452-8830 Garikoitz Beobide: 0000-0002-6262-6506 Imanol de Pedro: 0000-0002-5581-2220 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Spanish Ministerio de Ciencia e Innovación (Proyects MAT2014-55049-C2-R and MAT201675883-C2-1-P), Universidad del Paiś Vasco/Euskal Herriko Unibertsitatea (PPG17/37)), and Becas Iberoamericas Jóvenes Profesores Investigadores, Santander Universidades, is acknowledged. The authors also gratefully acknowledge technical and human support provided by SGIKer (UPV/EHU, MINECO, GV/EJ, ERDF, and ESF). The paper is (partly) based on the results of experiments carried out at the ALBA Synchrotron Light Source in Barcelona and Institute Laue-Langevin (ILL) of Grenoble.

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CONCLUSIONS We have completed the magnetic study of (Edimim)[FeCl4] with neutron powder diffraction measurements, which allowed us to determine the magnetostructural correlations found in this compound. In this sense we have identified in its antiferromagnetic structure the important effect of the orthogonal angle Fe−Cl−Cl in the superexchange magnetic pathways of a halometalate based on an imidazolium cation and tetrachloridoferrate anion, which is considered essential to switch the magnetic coupling (from ferromagnetic to antiferromagnetic exchange coupling). Moreover, we have completed the phase diagram in the solid phase of (Edimim)[FeCl 4], which is a well-known representative of the tetrahaloferrate family. From a thermal and structural analysis, we have been able to detect a new phase transition (II/III) and to elucidate for the first time the crystal structure of phases II

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REFERENCES

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