Paramagnetic iron-containing ionic liquid crystals

Journal Pre-proof Paramagnetic iron-containing ionic liquid crystals

Xinjiao Wang, Martin Valldor, Eike T. Spielberg, Frank W. Heinemann, Karsten Meyer, Anja-Verena Mudring PII:

S0167-7322(19)33469-5

DOI:

https://doi.org/10.1016/j.molliq.2020.112583

Reference:

MOLLIQ 112583

To appear in:

Journal of Molecular Liquids

Received date:

20 June 2019

Revised date:

22 December 2019

Accepted date:

26 January 2020

Please cite this article as: X. Wang, M. Valldor, E.T. Spielberg, et al., Paramagnetic iron-containing ionic liquid crystals, Journal of Molecular Liquids(2018), https://doi.org/ 10.1016/j.molliq.2020.112583

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© 2018 Published by Elsevier.

Journal Pre-proof

Paramagnetic Iron-Containing Ionic Liquid Crystals

Xinjiao Wang,*a,b Martin Valldor,c Eike T. Spielberg,b,d Frank W. Heinemann,e Karsten Meyer,*e Anja-Verena Mudring*b,f a

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Department of Chemistry, College of Sciences, Northeastern University, Shenyang, Liaoning, 110819, P. R. China, E-mail: [email protected] b Former affiliation: Anorganische Chemie III - Materials Engineering and Characterization, Fakultät für Chemie and Biochemie, Ruhr-Universität Bochum, 44801 Bochum, Germany c IWF Dresden – Leibniz Institute for Solid State and Materials Research Dresden, Helmholtzstr. 20, 01069 Dresden, Germany d Universität Duisburg-Essen, Universitätsstr. 9, 45141 Essen, Germany e Department of Chemistry and Pharmacy, Inorganic Chemistry, University Erlangen-Nuremberg, Egerlandstr. 1, 91058 Erlangen, Germany E-mail: [email protected] f Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16C, 106 91 Stockholm, Sweden, E-mail: [email protected]

1

Journal Pre-proof Abstract. Iron-containing ionic liquid crystals of N,N’-dialkylimidazolium salts ([(CnH2n+1)2IM]3[Fe(CN)6], with n = 12 (1), 14 (2), 16 (3), 18 (4), IM = imidazolium) were prepared and their structures and properties were characterized. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM) as well as powder X-ray diffraction (PXRD) analysis reveal for all compounds thermotropic liquid crystalline behavior. The observed textures point to smectic A and smectic X phases. The crystal structure of the solvate [C12C12IM]3[Fe(CN)6]·2 DMSO (DMSO =

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dimethylsulfoxide) was examined by single crystal X-ray diffraction analysis which reveals two crystallographically independent [C12C12IM]+ cations with different conformation, U-shape and rod-shape, together with a nearly ideal octahedral

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[Fe(CN)6]3– anion and two DMSO solvent molecules. The crystal packing is characterized by alternating bilayers of almost linearly arranged imidazolium cations

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with the U-shaped cations being arranged in a head-to-head fashion and the cations´

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hydrophobic dodecyl chains being interdigitated. Magnetic measurements for compounds 1-4 show paramagnetic moments, eff, of ~1.7 B, which are close to the

(effs.o.=1.73 B).

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theoretically expected value for a d

5

low-spin Fe3+ spin-only S = 1/2 system

It was possible to obtain an Fe3+ containing ILC material that

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C) with high magnetic susceptibility by mixing compound 3 with

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170

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combines liquid crystalline behavior over a large mesophase window (from 33 oC to

[C16C16IM][FeCl4] (5), which contains the d

5

high-spin Fe3+ ion (S = 5/2). All

compounds are highly hydrophobic and stable in the atmosphere indicate a great potential of applications as functional materials.

2

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Introduction Over the last years, ionic liquid crystals (ILCs) have been receiving growing interest as they combine the characteristics and advantages of ionic liquids (ionic conductivity, thermal stability, low vapor pressures, etc.) and liquid crystals (anisotropy of physical properties), rendering them attractive materials for a number of potential applications, particularly as anisotropic ion-conductors, organized reaction media, or templates for synthesis of zeolites, mesoporous materials, and nanomaterials.1,2 Additionally, ILCs

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also possess a high promise as electrolytes in the application of dye-sensitized solar cells.3 Our recent studies focus on elucidating structure-property relationships and adjusting functional groups to control secondary interactions in these compounds in

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order to control the phase behavior and enlarge the mesophasic temperature window.4

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Recently, metal-containing ILCs have moved into focus because the incorporation of metal ions allows for the introduction of a series of additional properties, such as

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redox-activity, color, luminescence, or magnetism.5 In our previous work on

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metal-containing ILCs, we mostly focused on rare-earth metal containing compounds as they reliably show strong luminescence with high color purity originating from

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interconfigurational f-f transitions combined with a strong response to external magnetic fields due to the large number of unpaired electrons.6 Because of the unique

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properties of the 4f valence electrons rare-earth metal-based ILCs are generally superior to transition metal-containing complexes. However, 4f-based ILCs typically suffer from a considerable drawback – they are quite prone to hydrolysis which is a limitation for potential applications. In that respect transition-metal based complexes are generally more stable. Most of the reported transition metal-based ILCs contain tetrahalometallate anions, [MX4]2– with M = Co, Ni, Cu, Zn, Pd, Cd, X = Cl, Br.7 By incorporating on cluster anions such as [Mo6Cl14]2–we managed to obtain water-stable ILCs based on transition metals which also showed bright luminescence.8 With respect to magnetic ILCs based on transition metals such that contain hs-Fe3+ are particularly interesting. However, to the best of our knowledge, no iron-containing, 3

Journal Pre-proof neither Fe2+ nor Fe3+ containing ILCs have been reported. Compounds like [C12C1IM][FeBr4] (C12C1IM = 1-methyl-3-dodecylimidazolium) do unlike simple [C12C1im]X with X = Cl, Br, I not adopt a mesophase; as confirmed by our research group.9

Previous investigations of iron containing ionic liquids (ILs) based on

tetrahalido-iron(III) also confirmed that these systems do not adopt a mesophase, despite their interesting magnetic properties.10 As the tetrahalidoferrate ILs are also prone to hydrolysis, we chose to study a collection of hexacyanoferrate containing

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imidazolium ILCs. For the cation we chose a set of 1,3-dialkylimidazolium ions, with [(CnH2n+1)2IM]+ with n = 12, 14, 16, 18 as it is known that imidazolium salts with 12 or more carbon containing alkyl groups are likely to form thermotropic liquid crystals

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with lamellar mesophases.4,5,11

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Indeed, all of the prepared compounds, [(CnH2n+1)2IM]3[Fe(CN)6], with n = 12 (1), 14 (2), 16 (3), 18 (4); IM = imidazolium, are paramagnetic and exhibit liquid

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crystalline behavior. Moreover, they are highly hydrophobic and air-stable; thus, rendering these ILCs suitable for applications as functional materials. In addition,

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[C16C16IM]3[FeCl4] (5) and mixtures of 3 and 5 in the molar ratio 1 : 1 (Mix 1) and 2 : 1 (Mix 2) were prepared. Although compound 5 itself does not show a liquid

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crystalline phase, it does possess a high magnetic susceptibility due to the presence of

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high-spin Fe3+. By mixing compounds 3 and 5, it was possible to obtain an Fe3+ containing ILC material that combined liquid crystalline behavior over a large mesophase window (from 33 oC to 170 oC) with high magnetic susceptibility.

Results and discussion Synthesis

Scheme 1. Schematic representation of synthesis of [(CnH2n+1)2IM]3[Fe(CN)6] studied in this work. Conditions: room temperature, in water, 24h. 4

Journal Pre-proof Compounds with the general formula of [(CnH2n+1)2IM]3[Fe(CN)6] ([(CnH2n+1)2IM]+ = N, N’-dialkyl imidazolium cations with alkyl chains CnH2n+1, with n = 12, 14, 16, 18) were synthesized via a metathesis reaction from the corresponding imidazolium chloride or bromide with K3Fe(CN)6 in water as shown in Scheme 1. After stirring at room temperature for 24 hours, yellow precipitates formed, which were purified by washing thoroughly with water and dried under vacuum for 12 hours at 90 oC. Afterwards, the products were collected (yield ~ 85 %) and characterized by FT-IR

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vibrational as well as UV-Vis electronic absorption spectroscopy. The IR vibrational spectra of compounds 1-4 are shown in Figure 1. As expected, all spectra are very similar and the discussion of [C12C12IM]3[Fe(CN)6] (1) is representative for all

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compounds. Most characteristic, the IR spectrum of 1 (Figure 1) shows a sharp band

cm–1),12

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centered at 2103 cm-1, similar to (Bu4N)3[Fe(CN)6] (Bu = n-butyl, in CH2Cl2 at 2100 which corresponds to the CN stretching mode of the complex anion

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[Fe(CN)6]3–.13 Vibrations characteristic for long chain 1,3-dialkylimidazolium cations can also be observed in the IR spectra.4b The absorption bands centered at 3139, 3087,

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1160, 657, and 614 cm–1 are attributed to C–H vibrations of the aromatic imidazolium ring in the [(CnH2n+1)2IM]3+ moiety, and bands at 2957, 2917, 1465, 849, 791, and 721

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cm–1 are assigned to the C–H vibrations (stretching and bending) of the alkyl chains.

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Finally, a characteristic absorption observed at 1563 cm–1 is assigned to the C–C stretching vibration of the imidazolium ring.

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FT-IR vibrational spectra of compounds 1, 2, 3, and 4, recorded in KBr

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Figure 1.

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UV-Vis absorption spectra of compounds 1-4 were measured in CH2Cl2 at a

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concentration of 0.4 mmol/L. For comparison, an aqueous solution of K3[Fe(CN)6] with same concentration was prepared.

As it shows in Figure 2, K3[Fe(CN)6]

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exhibits four absorption bands centered at 260, 303, and 319nm as well as a relatively

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broad feature at 421 nm with an unresolved shoulder on the high-energy side of the spectrum. These ligand-field centered bands are typical characters for the low-spin Fe3+ ion in [Fe(CN)6]3–.14 The general appearance of the electronic absorption spectra of the imidazolium hexacyanidoferrate salts are quite similar, showing the four absorption bands of the [Fe(CN)6]3– anion at 265, 306, 325, and 424 nm. The 2T2g → 2

A1g transition characteristic for d5 low-spin Fe3+ is observed at 265 nm and bands

centered at 306 nm and 325 nm are assigned to the 2T2g → 2Eg transition. The broad feature at 424 nm with its unresolved shoulder at ca. 400 nm is attributed to 2T2g → 2

T1g, 2A2g transitions.15 The d-d transitions for compounds 1-4 with respect to the

addition of imidazolium cation into the [Fe(CN)6]3– anion results in the red-shift of 6

Journal Pre-proof K3[Fe(CN)6] which suggest strong interaction between the imidazolium cations and

UV-Vis electronic absorption spectra of compounds 1, 2, 3, and 4,

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Figure 2.

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[Fe(CN)6]3– pointing to the formation of ion pairs in solution.16

recorded in CH2Cl2, and – for comparison – the spectrum of K3Fe(CN)6, recorded in

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H2O.

Single Crystal X-Ray Structure Determination of [C12C12im]3[Fe(CN)6]·2 DMSO (1 · 2 DMSO)

Yellowish colored crystalline plates of [C12C12IM]3[Fe(CN)6]·2 DMSO suitable for single crystal X-ray structure analysis were obtained by cooling a solution of [C12C12IM]3[Fe(CN)6] in DMSO to 4 °C. Both the molecular structure and crystal packing of this compound are displayed in Figure 3. Selected interatomic distances and angles are reported in the Figure caption. The DMSO solvate of compound 1, [C12C12IM]3[Fe(CN)6]·2 DMSO, crystallized 7

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in the triclinic space group P 1 (No.2) (Table 1) with 1.5 crystallographically independent [C12C12IM]+ cations, one half of a [Fe(CN)6]3– anion, and one DMSO molecule in its asymmetric unit; thus totaling two DMSO solvates per structure formula. One of the imidazolium cations as well as the hexacyanoferrate(III) anion reside on centers of inversion. The [Fe(CN)6]3– anion has a nearly ideal octahedral geometry with a maximum angle deviation of 3.4° from its ideal value of 90°. The doubly dodecyl derivatized cation [C12C12IM]+ is composed of two long hydrocarbon

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chains and an imidazolium head core. Interestingly, the two independent cations in the asymmetric unit show different conformations: One moiety exhibits a U-shaped conformation, with the two alkyl chains being oriented perpendicular to the

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imidazolium head core plane, and the other one is described by a rod-shape, with the

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two alkyl chains being stretched outward along the imidazolium core plane. Both U and rod-shape conformations of the imidazolium and triazolium cations have

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previously been observed for the related [N(CN)2]– or [C(CN)3]– (U-shaped)17 and

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[BF4]– (rod-shaped)4d compounds as well as for other salts.18-20 The rod-shaped imidazolium cation and one of the aliphatic C12 arms of the U-shaped cation were

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found to be disordered, with two refined alternative orientations in each case. All observed orientations of the alkyl chains follow an all-staggered pattern.

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The crystal packing of (1)·2 DMSO is characterized by alternating bilayers of almost linearly arranged imidazolium cations. The U-shaped cations are arranged in a head-to-head fashion, building the non-charged part of the crystal structure. Perpendicular to these bilayers, stacks of polar anions and imidazolium head cores alternate with the nonpolar stacks of interdigitating aliphatic chains (Figure 3b, c, and d). The [Fe(CN)6]3– anion is situated between the head cores of two rod-shaped imidazolium cations. Finally, and as expected, the polar DMSO solvate molecules occupy positions within the polar region of the crystal packing. A number of weak C-H···N type hydrogen bridges within the polar domains interconnect the [Fe(CN)6]3– anions with the imidazolium cations and the DMSO solvent molecule. The nitrogen 8

Journal Pre-proof atoms of the CN groups act as acceptors in these weak hydrogen bonds with H···N distances ranging from 2.18 to 2.51 Å and the corresponding N···C distances ranging from 3.00(3) to 3.45(3) Å.

In summary, the most characteristic features of the

crystal structure of (1)·2 DMSO are distinctly separated hydrophilic and hydrophobic structure parts. The latter consist of the long alkyl-tails of the imidazolium cations which interact with each other via van-der-Waals forces. The hydrophilic structure parts contain the charged imidazolium headgroups of the cations and the

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hexacyanidoferrate(III) anions that are bound together predominately by Coulombic forces. In addition, this region accommodates the solvent molecules being bound by hydrogen bonds. Upon desolvation it can be anticipated that the DMSO molecules are

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lost without destruction of the layered structure. As the Coulombic forces are rather strong compared to the van-der-Walls forces it can be anticipated that upon heating

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the layered structure will be preserved and that the long alkyl-tails will gain some

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freedom leading to the formation of layered, smectic mesophases as observed by

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polarizing optical microscopy for compounds 1-4.

a)

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c)

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b)

d)

Figure 3. a) Molecular structure of [C12C12IM]3[Fe(CN)6]·2 DMSO (50% probability 10

Journal Pre-proof ellipsoids), hydrogen atoms and disorder omitted for clarity; selected distances [Å] and angles [°]: Fe(1) –C(43) 1.933(16), C(43)–N(5) 1.15(2), N(1)–C(1) 1.33(2), N(1)–C(3) 1.37(2), N(2)–C(1) 1.324, N(2)–C(2) 1.37(2). C(43)–Fe(1)–C(44) 89.3(7), C(1)–N(1)–C(3) 108.6(13), N(2)–C(1)–N(1) 108.5(14), N(1)–C(4)–C(5) 111.7(18); b) packing diagram of [C12C12IM]3[Fe(CN)6] · 2 DMSO; view along the crystallographic a axis; c) packing diagram of [C12C12IM]3[Fe(CN)6]·2 DMSO; view along the crystallographic b axis; d) packing diagram of [C12C12IM]3[Fe(CN)6]·2 DMSO; view

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along the crystallographic c axis.

Table 1. Crystallographic data, data collection, and refinement details of

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[C12C12IM]3[Fe(CN)6] · 2 DMSO

[C12C12IM]3[Fe(CN)6]·2DMSO

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Compound Empirical formula

1585.37

Crystal size [mm]

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Crystal system

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Temperature [K]

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Mol. Weight

C91H171FeN12O2S2

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Space group

0.23×0.17×0.04 150(2) triclinic P 1 (No. 2)

a [Å]

9.6202 (18)

b [Å]

11.602 (2)

c [Å]

22.219 (4)

 [°]

90.820 (4)

 [°]

102.385 (4)

 [°]

94.229 (4)

V [Å3]

2414.5 (8)

Z

1

 [g cm-3] (calc.)

1.090

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0.249

F (000)

875

Tmin; Tmax

0.602; 0.746

2 interval [ °]

5.1 ≤ 2 ≤ 54.2

Collected reflections

37979

Independent reflections;

10605

Rint

0.0482 7152

(F0 ≥ 4 (F))

716

wR2 (all data)

0.1766

R1 [I ≥ 2 (I)]

0.0608

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No. refined parameters

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Observed reflections

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GooF on F2

1.017

max/min

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0.803 / –0.724

Thermotropic LC Behavior.

The thermotropic LC behaviour of compounds 1-4

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were investigated by thermogravimetry (TG), differential scanning calorimetry (DSC),

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polarizing optical microscopy (POM), and variable-temperature powder X-ray diffraction (PXRD). TG measurements show that the decomposition of compounds 1-4 starts at 200°C, and ends at 260 °C. DSC measurements were carried out to acquire the phase transition temperatures and reveal the mesophase window. The samples were heated up to 200 °C for the first heating cycle to avoid thermal decomposition, which allows for a reliable determination of the reversible phase transitions in the subsequent cooling cycle. Attempts to determine the clearing point of the compounds in a subsequent heating cycle up to 260 °C was failed. The samples start to decompose before the melting process completed. The corresponding transition temperatures and enthalpies, beginning from the second heating cycle of the 12

Journal Pre-proof DSC, are recorded and listed in Table 2. Upon heating for [C12C12IM]3[FeCN6] (1) in the second heating cycle from -45 °C, a solid-mesophase transition at -7.7 °C, with a ΔH value of 21.0 kJ/mol, was observed. A bright focal conical texture can be seen from its POM image at 127 °C (Figure S1), which is consistent with the formation of some sort of highter order smectic X (SmX) phase, an as-yet unidentified higher-order smectic phase.21 The second phase transition occurred at 145.1 °C, with a ΔH value of 26.2 kJ/mol. A typical focal conical texture was observed at 170 °C by POM, pointed

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a smectic A (SmA) phase was formed. The compound transforms into an isotropic liquid at 186.0 °C. [C14C14IM]3[FeCN6] (2) shows the most complex behavior of all samples with the second heating cycle showing five endothermic transitions at 0.2,

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21.6, 134.3, 171.9, and 211.0 °C (onset temperatures). The first transition at 0.2 °C,

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with a ΔH value of 9.0 kJ/mol, is connected to a transition from one solid crystalline phase to another solid phase. The second transition at 21.6 °C, which is associated

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with a ΔH value of 16.45 kJ/mol, corresponds to the transition from the crystalline to a mesophase. This exhibits a brighter oily-streak texture under POM, was assigned to

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the formation of a SmX phase, which will be further proved by PXRD below. The phase transition at 134.3 °C (ΔH = 12.0 kJ/mol) corresponds to the transition from the

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SmX phase to a SmA phase, and the subsequent transition at 171.9 °C (ΔH = 4.9

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kJ/mol) is assigned to a transition from the SmA phase to a second SmA’ phase. Lamellar oily-streak textures characteristic for SmA phases were observed by POM (Figure 4b). Finally, the transition at 221.0 °C (ΔH =7.7 kJ/mol) corresponds to the transition from the mesophase to the isotropic phase. However, the sample already starts to decompose at 210 °C and the decomposition is completed at 260 °C. For 3 and 4 the transformation of the crystalline phase to a SmX phase at 29.0 °C and 39.1 °C, respectively, was observed followed by the transformation of a SmA mesophase at 133.1 °C and 131.0 °C before melting at 204.2 °C and 217.4 °C. However, it has to be noted that all melting processes are accompanied by the onset of thermal decomposition. The mesophase ranges are appreciably wide for all 13

Journal Pre-proof compounds (over 150 °C). In general, the clearing temperatures of these compounds increase slightly with growing alkyl chain length while the transition temperatures to the SmA phase show the reverse trend. This results in an overall increase of the crystalline temperature window with increasing alkyl chain length.

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Table 2. Phase transition temperatures (ºC) of compounds [CnCnIM]3[Fe(CN)6] (n = 12, 14, 16, 18), determined by DSC at 5 ºC /min (onset data, second heating cycle, Cr = crystalline phase, SmA = smectic A phase, SmX = smectic X phase, Iso = isotropic phase), ΔH (kJ/mol) and decomposition temperatures (Td, ºC), determined by TG at 5 ºC /min (onset data). Compounds

Phase transition behavior

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n = 12 (1)

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n = 14 (2)

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n = 16 (3)

by DSC at the starting point (onset) of decomposition.

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a) Observed

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n = 18 (4)

a)

14

Td 256.1

262.3

263.1 261.5

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Figure 4.

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b) POM images of [C14C14IM]3[FeCN6] (2) at a) 103 °C and b) 180 °C.

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The mesomorphic structures of these compounds were further examined by powder

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X-ray diffraction (Table 3, Figure 5). Compounds [C12C12IM]3[Fe(CN)6] (1) and [C14C14IM]3[FeCN6] (2) in the mesophase exhibit three sharp peaks at the lower angel

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region and two faint diffused scattering at ~2 ϴ = 7.1o and 20o. For compound 1, the strong sharp peak suggest a well-defined lamellar structure corresponding with a

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repeating layer distance of 27.12 Å at 25 oC. The faint halo indicates liquidlike alkyl

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chains with averaged domain of ~12.5 and 4.5 Å. The mesomorphic layer distance of 27.12 Å is 4.9 Å longer than the 22.2 Å observed in the crystalline state. This large

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increase in the layer distance from crystalline phase to smectic phase suggests a structural change from a highly interdigitated bilayer stacking to a partially or none interdigitated bilayer structure. In the same mesophase domain for compound 1, the layer spacing d001 is 27.84 Å at 100 oC, which suggested that the mesophase dspacing increases slightly with increasing temperature. A brighter focal conic texture can be seen at 127 oC under POM. However, according to the literatures,22,23 the d-spacing of SmA phase decreases steadily with increasing temperature, which attributed to a greater alkyl chain mobility upon increasing the temperature. Thus, this higher-order smectic phase observed for compound 1 was ascribed as SmX phase, an as-yet unidentified higher-order smectic phase. Upon further heating the sample of 1, 15

Journal Pre-proof the layer spacing d001 decreases slightly to 27.03 Å at 160 oC, which indicates a SmX to SmA phase transition consistent with the DSC and POM measurements. A focal conic texture, a typical texture of SmA phase, was observed at 170 oC under POM. For compound 2, similar mesophase structures can be obtained from the PXRD data conbined with DSC and POM measurements. For compound 3 and 4, we only can obtain the crystal layer spacing d001, 30.77 Å for 3 and 34.01 Å for 4. Whereas, the mesophase layer spacing d001 cannot be seen in the 2ϴ range of 2-40o. This results

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combined with the POM observation indicates the peak ascribed to the smectic layered structures is in the range lower than 2o, and thus the layer spacing d001 is larger than 44.2 Å. The brighter focal conic textures of 3 and 4 under POM were

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ascribed to SmX phase (Figure S3 and Figure S4). Upon further heating, typical focal

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conic textures of SmA phase were obtained.

Table 3. XRD Data of compounds [CnCnIM]3[Fe(CN)6] (n = 12, 14, 16, 18). (Cr:

Compounds

Temperature /°C

d-Spacing/Å

Phase

[C12C12IM]3[Fe(CN)6]

25

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27.12

SmX

100

27.84

SmX

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crystal phase; SmX: smectic X phase; SmA: smectic A phase)

160

27.03

SmA

25

30.62

SmX

100

31.51

SmX

160

30.77

SmA

[C16C16IM]3[FeCN6]

25

34.01

Cr

[C18C18IM]3[FeCN6]

25

36.45

Cr

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[C14C14IM]3[FeCN6]

16

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Intensity

160 oC

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100 oC

10

15 20 2-theta

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5

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25 oC

25

30

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and SmA phase (160 oC).

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Figure 5. PXRD pattern for [C12C12IM]3[Fe(CN)6] (1) at SmX phase (25 oC, 100 oC)

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Magnetic properties.

Figure 6. Reciprocal magnetic susceptibility of compounds 1, 2, 3, and 4 as functions

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of temperature. All measured data are marked with individual markers and the

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corresponding Curie-Weiss fits are drawn with full lines. The inset shows the field dependent magnetization of compound 1 at 2 K. (1 = [C12C12IM]3[Fe(CN)6] , 2 =

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[C14C14IM]3[Fe(CN)6], 3 = [C16C16IM]3[Fe(CN)6], 4 = [C18C18IM]3[Fe(CN)6])

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Compounds 1, 2, 3, and 4 exhibit a very similar magnetic behavior that was fitted with Curie-Weiss approximations. The calculated paramagnetic moments, µeff, are 1.75 (1), 1.72 (2), 1.76 (3), and 1.68 (4) B; and thus, are close to the theoretically expected value for a spin-only S = 1/2 system (µs.o. = 1.73 B). The low temperature magnetization of compound 1 at 2 K displays the saturation moment in the vicinity of approx. 0.95 B (inset Figure 6). This observation confirms the low-spin t2g5eg0 electron configuration of the Fe(III) ion in [Fe(CN)6]3–, since full polarization of an S = 1/2 ion results in 1 B.

Mixed ILCs 18

Journal Pre-proof As expected, and due to the low-spin electron configuration of the Fe3+ in [Fe(CN)6]3–, compounds 1 to 4 exhibit a relatively low magnetic susceptibility. In order to enhance the magnetic susceptibility of the material, we decided to mix [C16C16IM]3[Fe(CN)6] (3) with the corresponding [FeCl4]– salt, namely [C16C16IM][FeCl4] (5), with an Fe3+ center in the high-spin state. Compound 5 was obtained by stirring [C16C16IM]Cl with an equimolar amount of FeCl3•6H2O at room temperature for 0.5 h and subsequent removal of water. The UV-Vis electronic absorption spectrum of 5 shows absorption

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maxima centered at 529, 620, 686, and 711 nm (ESI), corresponding to the 6A1g → 4

A1g, 4Eg (529 nm), the 6A1g → 4T2g (620 nm), the 6A1g → 2T2g (686 nm), and the

6

A1g → 4T1g (711 nm) transitions of the FeCl4– anion.18 DSC measurements of

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compound 5 show no indications for the formation of a liquid crystalline phase but rather the direct transformation from the crystalline solid to an isotropic liquid at 51.9

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C. In an effort to test if it is possible to form solutions of mesogenic 3 with

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non-mesogenic 5, with retention of mesogenic behavior but enhanced magnetic response, mixtures of 3 and 5 were prepared in a molar ratio of 1:1 (Mix 1) and 2:1

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(Mix 2) by dissolving the appropriate compound amounts in a small volume of dichloromethane, then removing the solvent and drying the mixtures under vacuum.

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Subsequently, the thermal behavior of compound 5 as well as Mix 1 and Mix 2 were

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investigated by DSC and POM. The transition temperatures and enthalpies of compound 5 and both mixtures are reported beginning from the second heating cycle of the DSC studies, and are provided in Table 4. For Mix 1, two endothermic transitions are observed in the thermograms. The first transition at 44.8 °C, with a ΔH value of 65.6 kJ/mol, corresponds to the transition from the crystalline phase to a liquid crystalline phase. The second transition at 144.9 °C, with a ΔH value of 5.2 kJ/mol, corresponds to the transition from liquid crystalline phase to an isotropic liquid phase. This phase transition was monitored by POM. Figure 7 shows the POM image of Mix 1 at 100.5 °C with a conical fan texture, which was ascribed to the formation of a SmA phase. The melting point of Mix 1 is lower than that of the pure 19

Journal Pre-proof [FeCl4]– salt (48.8 °C), and the clearing temperature of this mixture is lower than that of the pure [Fe(CN)6]3– salt (204.2 °C). The larger decrease of the clearing point temperature, compared to that of the melting point, indicates that the presence of mixed anions apparently has a greater influence on the clearing than on the melting process. Mix 2 with a higher molar ratio of [FeCl4]– than [Fe(CN)6]3– exhibits an even lower melting and higher clearing point than Mix 1, hence, enlarged mesophasic window, with transition temperatures being 33.5 °C and 170.3 °C. Both mixtures

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show the formation of a liquid crystalline SmA phase. Remarkably, the mesophase windows are over 100 oC wide. As expected, the magnetic measurements confirm a higher magnetic susceptibility of Mix 1 and Mix 2 compared to the pure [Fe(CN)6]3–

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salt (Figure 8). By scaling the magnetic moments according to the mix ratio, the

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expected moments are calculated to 3.90 and 3.18 B for Mix 1 and Mix 2,

values (4.06 and 3.25 B).

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respectively, which is in reasonable agreement with the experimentally determined

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Compounds

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Table 4. Phase-transition temperatures (ºC) of compound [C16C16IM][FeCl4] (5) and the mixtures of [C16C16IM]3[Fe(CN)6] (3) and [C16C16IM][FeCl4] (5) with a molar ratio 1:1 and 2:1 determined by DSC at 5 ºC /min. Onset data, second heating cycle, Cr = crystalline phase, SmA = smectic A phase, Iso = isotropic phase, ΔH in kJ/mol.

[C16C16im][FeCl4] (5)

Phase transition behavior Cr

3:5=1:1

Cr

3:5=2:1

Cr

48.8 (69.5) Iso

44.8 (65.6)

144.9 (5.2) Iso

SmA 33.5 (55.1)

170.3 (4.9) SmA

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Iso

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a)

b) Figure 7. POM images for the mixture of 3 and 5 a) Mix 1 (1:1) at 100.5 °C and b) Mix 2 (2:1) at 100.2 °C.

21

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Figure 8.

Reciprocal magnetic susceptibility of compounds 3, 5 (3 =

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[C16C16IM]3[Fe(CN)6], 5 = [C16C16IM][FeCl4]) and Mix 1 (3:5 = 1:1), Mix 2 (3:5 = All measured data are

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2:1) as functions of temperature in a static field of B = 0.1 T.

marked with individual markers and the corresponding Curie-Weiss fits in the upper

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Conclusion

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right inset are drawn with full lines.

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In this work, we have prepared a series of functional iron-containing ionic liquid crystals (ILCs) based on N,N’-dialkylimidazolium [CnCnIM] salts of the paramagnetic [Fe(CN)6]3– complex anion. The molecular structure of the bis-dodecyl-substituted [C12C12IM]3[Fe(CN)6] · 2 DMSO was determined by X-ray structural determination and reveals an explanation for the formation of layered, smectic mesophases. The molecular structure and the crystal packing show that the complex metal ions are located between hydrophobic bilayers composed of U- and rod-shaped imidazolium cations, with highly interdigitated alkyl chains. Very likely, this structural arrangement is largely retained even without the presence of the solvate DMSO molecules. All N,N’-dialkylimidazolium salts ([(CnH2n+1)2IM]3[Fe(CN)6], n = 12 (1), 14 (2), 16 (3), 18 (4) feature smectic liquid crystalline phases (SmX and SmA) in a 22

Journal Pre-proof remarkably wide mesophase temperature range (over 150 °C for 2, 3 and 4). Their thermal decomposition starts around their clearing temperature at approx. 200 to 215 °C. Magnetic measurements for 1, 2, 3, and 4 show paramagnetic moments, eff, of ~1.7 B, which are close to the theoretically expected spin-only value for a d5 low-spin Fe3+ ion. Preparation of mixtures of compounds 3 and 5 allowed for the design of paramagnetic ILC materials that combined liquid crystalline and magnetic behavior over a large mesophase range. With higher ratio of 3, the mesophase window

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is larger but the magnetic susceptibility is lower. Thus, mixtures of 3 and 5 lead to the expected summation of both material properties. With this approach, we finally achieved to obtain Fe3+-containing ILCs that show both, an excellent wide mesophase

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range and good magnetic susceptibility.

Experimental Section

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All N,N’-disubstituted imidazolium halides were prepared according to the literature procedures.4d

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Preparation of [(CnCnIM)3[Fe(CN)6] (n = 12 , 14, 16, 18) In order to obtain [(C12C12IM)3[Fe(CN)6], an aqueous solution (10 mL) of

Immediately upon addition, a yellow precipitate is observed. Completion of

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20 mL.

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K3[Fe(CN)6] (0.52 g, 1.58 mmol) was added to [C12C12IM]Cl (0.8 g, 1.82 mmol) in

the metathesis reaction is reached by stirring the reaction mixture for 24 h. After this time, the yellow precipitate was filtered off, washed with water thoroughly, and dried in vacuo at 90 oC for 12 hours.

Compounds 2-4 were prepared analogously by following a very similar procedure but starting from the respective dialkylimidazolium halides.

[(C12C12IM)3[Fe(CN)6] (1) was obtained as a yellow powder in 85.4% yield. IR (cm–1): 3139, ν(NCH) of NC(H); 3087, νas ring HCCH; 2103, ν(CN); 1563, 23

Journal Pre-proof ν(CH2N) ring in plane; 1465, δ(HCH) of CH2(N); 1160, ν(CH2N) ring in plane; 657, ν(CH2N), ν(CH2N) of ring out of plane. Anal. Calcd for C87H159N12Fe: C, 73.12%; H, 11.21%; N, 11%.76. Found: C, 73.18%; H, 13.89%; N, 11.80%.

[(C14C14IM)3[Fe(CN)6] (2). IR (cm–1): 3138, ν(NCH) of NC(H); 3087, νas ring HCCH; 2103, ν(CN); 1564, ν(CH2N) of ring in plane; 1465, δ(HCH) of CH2(N);

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1161,ν(CH2N) of ring in plane; 657, ν(CH2N), ν(CH2N) of ring out of plane. Anal. Calcd for C99H183N12Fe: C, 74.44%; H, 11.55%; N, 10.52%. Found: C,

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74.13%; H, 11.60%; N, 10.22%.

[(C16C16IM)3[Fe(CN)6] (3). IR (cm–1): 3137, ν(NCH) of NC(H); 3085, νas ring HCCH;

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2105, ν(CN); 1564, ν(CH2N) of ring in plane; 1465, δ(HCH) of CH2(N); 1159,

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ν(CH2N) of ring in plane; 656, ν(CH2N), ν(CH2N) of ring out of plane. Anal. Calcd for C111H207N12Fe: C, 75.50%; H, 11.82%; N, 9.52%. Found: C,

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75.70%; H, 11.55%; N, 9.05%.

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[(C18C18IM)3[Fe(CN)6] (4). IR (cm–1): 3138, ν(NCH) of NC(H); 3084, νas ring HCCH;

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2103, ν(CN); 1562, ν(CH2N) of ring in plane; 1467, δ(HCH) of CH2(N); 1159, ν(CH2N) of ring in plane; 657, ν(CH2N), ν(CH2N) of ring out of plane. Anal. Calcd for C123H231N12Fe: C, 76.38%; H, 12.04%; N, 8.69%. Found: C, 76.69%; H, 12.14%; N, 8.40%.

Preparation of [C16C16IM][FeCl4] (5): FeCl3·6H2O (391 mg, 1.446 mmol) in 7 mL THF was added to a solution of [C16C16IM]Cl (0.8 g, 1.446 mmol) in 7 mL THF. The reaction mixture was stirred for 1 h at 40 oC, after filtering-off the brown-yellow product was dried under vacuum overnight at 60 oC. Yield (0.93 g, 90%). 24

Journal Pre-proof Raman spectrum: 333 cm-1 (FeCl4–). UV-Vis (nm): 529 nm, 620 nm, 686 nm. Anal. Calcd for C35H69N2FeCl4: C, 58.74%; H, 9.72%; N, 3.91%. Found: C, 58.51%; H, 9.99%; N, 3.88%.

Preparation of mixtures of [(C16C16IM)3[Fe(CN)6] (3) and [C16C16IM][FeCl4] (5): The mixtures were prepared by mixing 3 and 5 with 1:1 and 2:1 equivalences in 5 mL dichloromethane.

The mixtures were allowed to stir for 30 min before the solvent

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was evaporated to dryness to yield a green-yellow solid.

Thermal phase behavior characterization: Differential scanning calorimetry (DSC)

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was performed with a computer-controlled Phoenix DSC 204 F1 thermal analyser

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(Netzsch, Selb, D) with argon as the protection gas. Under an inert gas atmosphere, the samples were placed in aluminum pans, which were cold-sealed. All

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measurements were carried out with a thermal ramp of 5 °C /min. Experimental data are displayed in such a way that exothermic peaks occur at negative heat flow and

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endothermic peaks at positive heat flow. Given temperatures correspond to the peak of the respective thermal process. Optical analyses were carried out by hot-stage

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polarized optical microscopy (POM) with an Axio Imager A1 microscope (Carl Zeiss

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MicroImaging GmbH, Göttingen, D) equipped with a hot stage, THMS600 (Linkam Scientific Instruments Ltd, Surrey, UK), and temperature controller, Linkam TMS 94 (Linkam Scientific Instruments Ltd, Surrey, UK). The images were recorded as movies with a digital camera after initial heating during then cooling stage.

For the

measurement, the sample was placed under inert gas atmosphere between two hermetically sealed cover slips. Decomposition temperatures were determined on a TG 449 F3 Jupiter (Netzsch, Selb, Germany). Measurements were carried out in aluminum oxide crucibles with a heating rate of 5 °C/min and nitrogen as purge gas. Powder X-ray diffraction (PXRD) analysis: X-ray diffraction (XRD) measurements were performed by use of nickel-filtered Cu-Ka (λ = 1.542 Å) radiation 25

Journal Pre-proof monochromatized with a Empyrean powder diffractometer (PANalytical B.V., Netherlands). Elemental Analysis: Elemental analysis was conducted with a vario EL C, H, N, S analyzer of the company Elmentar (Hanau, Germany). X-ray Crystal Structure Determination Details: CCDC-1558484 for [C12C12IM]3[Fe(CN)6] · 2DMSO contain the supplementary crystallographic data for this paper.

These data can be obtained free of charge via

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http://www.ccdc.cam.ac.uk/data_request/cif (or from Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: ++44-1223-336-033; e-mail: [email protected]).

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Yellow plates of [C12C12IM]3[Fe(CN)6] · 2DMSO were obtained by cooling a DMSO

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solution to 4 °C. Suitable single crystals were embedded in protective perfluoropolyalkylether oil and transferred to the cold nitrogen gas stream of the diffractometer.

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Intensity data were collected using MoK radiation ( = 0.71073 Å) at 150 K on a Bruker Kappa APEX 2 IμS Duo diffractometer equipped with QUAZAR focusing

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Montel optics. Data were corrected for Lorentz and polarization effects, semiempirical absorption corrections were performed on the basis of multiple scans using

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SADABS. The structures were solved by direct methods and refined by full-matrix

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least-squares procedures on F2 using SHELXTL NT 6.12. The compound crystallizes with two molecules of DMSO per formula unit. The [Fe(CN)6]– anion is situated on a crystallographic inversion center.

One of the

imidazolium cations is situated on a general crystallographic site, while the other is situated on an inversion center. Both of the cations are disordered in their aliphatic chains.

Two alternative sites were refined in both cases that are occupied by 40(4)

and 60(4)% for the affected atoms C5 – C14 and C5A – C14A and by exact 50% (resulting from its situation on a crystallographic inversion center) for the atoms C31 – C42 and C31A – C42A. SAME, SIMU, and ISOR restraints were applied in the refinement of the disordered structure parts. 26

Journal Pre-proof Treatment of hydrogen atoms: All hydrogen atoms are geometrically positioned; their isotropic displacement parameters are tied to those of their corresponding carrier atoms by a factor of 1.2 or 1.5. Magnetism: A well-defined amount of each sample powder was placed in a polycarbonate ampoule under argon. To prevent any displacement of the powder, the powder was held in place by an airtight lid. Magnetic susceptibility measurements and magnetization up to 7 T were performed in a SQUID MPMSXL (Quantum Design,

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San Diego, USA) in the temperature range of 2-300 K. As the paramagnetic signal proved relatively weak at higher temperatures, most of the used data were collected at low temperatures.

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UV-Vis spectroscopy: UV-Visible absorption spectra were measured at room

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temperature on sample solutions using an Agilent Cary 5000 spectrometer. IR spectroscopy: Infrared spectroscopy (IR) was conducted with a Bruker Alpha-P

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ATR-spectrometer (Karlsruhe, Germany) in attenuated total reflection configuration.

Germany).

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Acknowledgements

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Data evaluation was carried out with the program OPUS (Bruker, Karlsruhe,

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AVM would like to thank the European Research Council and the Ruhr-Universiät Bochum for supporting supplies, chemicals as well as salaries. X.W. would like to thank the Fundamental Research Funds for the Central Universities (N170504018).

K.M.

acknowledges

generous

support

from

the

Friedrich-Alexander-University Erlangen-Nürnberg. We thank Dr. Carola Vogel for her help with the X-ray crystallographic analyses.

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Highlights Four iron-containing ILCs with the paramagnetic [Fe(CN)6]3– anion were prepared. All compounds feature smectic liquid crystalline phases (SmA and SmX).

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Four compounds show paramagnetic moments,eff, of ~1.7 B.

31