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Physical and Structural Characterization of Imidazolium Based Organic-Inorganic Hybrid: (CNH)[CoCl] 3

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Anna Piecha-Bisiorek, Alina Bienko, Ryszard Jakubas, Roman Boca, Marek Damian Weselski, Vasyl Kinzhybalo, Adam Pietraszko, Martyna Wojciechowska, Wojciech Medycki, and Danuta Kruk J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11924 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 10, 2016

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The Journal of Physical Chemistry

Physical and Structural Characterization of Imidazolium Based Organic-Inorganic Hybrid: (C3N2H5)2[CoCl4] Anna Piecha-Bisioreka*, Alina Bieńkoa, Ryszard Jakubasa, Roman Bočab, Marek Weselskia, Vasyl Kinzhybaloc, Adam Pietraszkoc, Martyna Wojciechowskaa, Wojciech Medyckid and Danuta Kruke

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Faculty of Chemistry, University of Wrocław, 14 F. Joliot - Curie, 50-383 Wrocław, Poland. b

Department of Chemistry, FPV, University of SS Cyril and Methodius, Trnava, Slovakia.

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Institute of Low Temperature and Structure Research, PAS, Okólna 2, 50-422 Wrocław, Poland.

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e

Institute of Molecular Physics, PAS, M. Smoluchowskiego 17, 60-179 Poznań, Poland.

Faculty of Mathematics and Computer Science, University of Warmia & Mazury, Słoneczna 54, 10-710 Olsztyn.

ACS Paragon Plus Environment

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ABSTRACT: (C3N2H5)2[CoCl4] (ICC) was characterized in a wide temperature range by single-crystal X-ray diffraction method. Differential scanning calorimetry revealed two structural phase transitions: continuous at 245.5 K (from phase I to II) and discontinuous one at 234/237K (cooling/heating) (II→III). ICC adopts monoclinic space groups C2/c and P21/c in phase (I) and (III) respectively. The intermediate phase (II) appears to be incommensurate modulated. Dynamic properties of polycrystalline ICC were studied by means of dielectric spectroscopy and proton magnetic resonance (1H NMR). The presence of low frequency dielectric relaxation process in phase III reflects libration motion of the imidazolium cations. The temperature dependence of the 1H spin–lattice relaxation time indicated two motional processes with similar activation energies that are by about an order of magnitude smaller than the activation energy obtained from dielectric studies. There are no abrupt changes in the 1H relaxation time at the phase transitions indicating that the dynamics of the imidazolium rings gradually varies with temperature, i.e. it does not change suddenly at the phase transition. Negative values of the Weiss constant and the intermolecular exchange parameter where obtained, confirming the presence of weak antiferromagnetic interaction between the nearest cobalt centers. Moreover, the magnitude of zero field splitting was determined. The AC susceptibility measurements show that a slow magnetic relaxation is induced by small external magnetic field.


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INTRODUCTION: Contemporary material science largely concentrates on material multifunctionality. Novel materials combining in the same crystal lattice various physical properties can be designed by the tools offered by molecular chemistry. Obtaining such material in the form of continuous lattice solids is very complicated or even impossible. However, it can be obtained by building up hybrid solids formed by two networks, such as anion and cation, where each network furnishes distinct properties to the solid. In consequence, the mutual interaction between the two individual networks can lead to developing materials of novel properties. A promising examples of such compounds are multiferroics which raise significant interest from technological point of view.1,2 To design a hybrid organic-inorganic system with expected electric/magnetic behavior it is necessary to use complexes of transition metal ions and organic, polar ligands. A2[MX4] type of compounds (β-K2SeO4 analogues), where A is an organic cation, M is a divalent transition metal ion (Zn, Mn, Co, Fe, Cd) and X is a halide ion (Cl, Br, I), belongs to the best known group of ferroic compounds3-7 important also from the application point of view.8-10 An essential feature of this group is that they undergo several solid-solid phase transitions (PTs), from normal to incommensurate (IC) and commensurate phases ending at ordered phases. It is postulated that magnetic properties of this family of compounds depend on the interatomic distances between [MX4]2- anions which are correlated with the properties of the organic cation. Recently, multiferroics properties were also found in some crystals of R2MX4-type which belong to the hybrid

perowskite

family

e.g.

ethylammonium

analogues:

(C2H5NH3)2[CuCl4]

and

(C2H5NH3)2[FeCl4] for which simultaneous coexistence of ferroelectric and dominant ferromagnetic properties were confirmed.11,12


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The crystal structure of (C3N2H5)2[CoCl4] (ICC) was previously reported by Z. Hang et al.13 ICC at 223 K adopts monoclinic space group, P21/c and contains discrete [CoCl4]2- anions and two imidazolium cations in the asymmetric part of the unit cell.13 The solid state synthesis, crystal structure of the room temperature phase (monoclinic C2/c) and thermal stability were reported by Adams et al.14 As the authors of both the papers, 13 and 14, limited themselves only to the structural characterization of the compound at selected temperatures, in the present work we take up the subject of physical properties of ICC. Our attention has been mainly focused on magnetic and electric studies, especially the expected interesting dielectric response in the vicinity of structural PTs. The expectations are due to the fact that the compound contains both the strongly polar imidazolium cation and tetrachlorocobaltate(II) anion (with 3d7 electronic structure) in the crystal structure. This paper reports the synthesis, crystal structure, magnetic and dielectric properties of bis(imidazolium) tetrachlorocobaltate(II): (C3N2H5)2[CoCl4] (ICC). The molecular motions of imidazolium cations have been studied by means of the proton magnetic resonance (1H NMR) relaxation technique in a wide temperature range. Structural description of the PTs between the room and the low temperature phase (I and III, respectively) is also presented.

EXPERIMENTAL SECTION All materials used in this work were of reagent grade purity and were used as commercially obtained. Crystals of ICC were synthesized in the same way how it was described by H. Zhang.13 The purity of the compound was confirmed by PXRD (Fig. S1-Supporting Information) and elemental analysis which gave the following mass percentages (values in bracket are theoretical): C: 21.02 ±0.1 (21.26); N: 16.84 ±0.1 (16.53); Cl: 41.55±0.1 (41.84), H: 3.02 ±0.05 (2.97).


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Differential scanning calorimetry (DSC) was recorded using Perkin Elmer DSC-7 in the temperature range 100–300 K. Thermogravimetric analysis (TGA) and Differential Thermal Analysis (DTA) measurements were performed on Setaram SETSYS 16/18 instrument in the temperature range 300 – 700 K with a ramp rate 2 K min-1. The scan was performed in flowing nitrogen (flow rate: 1 dm3/h). The complex electric permittivity (ε* = ε’ – iε’’) was measured between 100 and 300 K by Agilent 4284A Precision LCR Meter in the frequency range between 135 Hz and 2 MHz. The overall error does not exceed 5%. The sample was prepared in a form of pellets with diameter of 10 mm and thickness of 1.2 mm. Silver electrodes were deposited onto opposite disk faces. Before measurements the sample was blown with dry nitrogen for about 12 h. The dielectric measurements were carried out in a controlled atmosphere (N2). Magnetic measurements were performed using Quantum Design SQUID-based MPMSXL-5type magnetometer in the temperature range of 1.8 − 300 K. The measurements were performed at a magnetic field of B = 0.5 T. Corrections based on subtracting the sample–holder signal were made and the contribution of diamagnetism was estimated from the Pascal constans. The magnetization measurements were conducted at 2 K in the magnetic field from 0 to 5 T. 1

H NMR spin–lattice relaxation time (T1) measurements were carried out as a function of

temperature at 25 MHz using Bruker SXP 4/100 pulse spectrometer. Inversion recovery pulse sequence was used. The temperature of the sample was varied from 77 K to 330 K using a continuous gas flow helium cryostat (CF1200 Oxford Instruments Cryosystem) and controlled to the accuracy of 0.5 K. Single - exponential magnetization decays were observed in the whole temperature range. The estimated average error of the measured T1 values is 5%.


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The X-ray diffraction data were collected on Oxford Diffraction Xcalibur four-circle diffractometer equipped with Atlas CCD camera and cryocooler device. The data were collected using MoKα radiation, on heating from 100 to 300 K (between 229 and 242 K every 1 K). The data were corrected for absorption effects and were used for cell parameters and structure refinement. Additionally the temperature behavior of the modulation vector for the intermediate phase was studied.

RESULTS AND DISCUSSION Thermal properties. Figure 1 shows DSC traces obtained for cooling and heating scans (at 5 K min-1). The measurements below room temperature (RT) reveal the presence of two fully reversible structural PTs at: 245.5 K and 234/237 K (cooling/heating scans). It should be emphasized that the PTs temperatures depend strongly on the speed with which the measurements were performed. The results obtained for the speed of 20 and 10 K min-1 indicate that the PTs overlap and, in consequence, one cannot determine the temperatures of the individual transitions. The lower temperature PT (at 234/237 K) being of the first order type is associated with a quite small transition entropy (∆Str) which was estimated to be about 1 J mol-1K-1. The higher temperature PT (at 245.5 K) with the transition entropy ∆Str = 0.42 J mol-1K-1 shows a negligible temperature hysteresis. It should be noted that the thermal anomaly seen as a well-shaped peak is characteristic of a first-order transition. Nevertheless, given the fact that we did not see the temperature hysteresis, the PT at 245.5 K may be classified as a second order type. Thermal stability of the crystal studied by means of simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) indicates that ICC is stable up to about 440 K


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(see Fig. S2). Above this temperature continuous decomposition of the sample takes place leading to complete degradation observed at about 870 K.

Figure 1. DSC traces for ICC crystal upon cooling and heating runs (5 K min−1, m = 10.144 mg)

X-ray structural studies. Crystal structure of the low-temperature phase (III) was first described in 13 and then in 14 including the room-temperature phase (I). As the DSC measurements have revealed the existence of an intermediate phase (II), a detailed variable-temperature single-crystal X-ray diffraction studies were conducted in a wide temperature range (100-300 K) with strong emphasis on the PTs region (229-242 K). As it was already reported in

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the RT phase (I) is disordered (imidazolium cations rotate in-

plane) and crystallizes in C2/c space group, while the lowest-temperature phase (III), characterized by similar cell parameters, is ordered and crystallizes in P21/c space group (see Figure 2). The independent part of the RT phase structure consists of one disordered imidazolium cation and one half of [CoCl4]2– anion (it lays on 2-fold axis).


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Figure 2. Crystal packing (a) and symmetry elements (b) comparison of phases III and I (color key: green tetrahedra – [CoCl4]2– anions, blue balls – nitrogen). Nitrogen atoms positions are not shown in phase I due to rotational disorder. Symmetry elements that disappear on cooling to phase III are shown in red color.


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Figure 3. Modulation vector (a* and c* projections) (a) and magnitude (b) behaviour versus temperature.

The transformation from C-centered (phase I) to primitive cell (phase III) requires the appearance of additional reflections (h + k = 2n + 1) and doubling of the independent part. The low-temperature phase (III) is characterized by two ordered cation moieties and one anion moiety in the independent part of the crystal structure (Figure 2). The appearance of additional reflections that break the C-centering turned out to be due to the emergence of the incommensurately modulated intermediate phase II between the room- and low-temperature phases in a quite narrow temperature range of about 10 K. The modulated phase is characterized by weak satellites (only first-order satellites are observed). The modulation vector is very short


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and oriented in the [-3 0 5] direction. With increasing temperature the vector increases its length until the modulation disappears above 242 K (Figure 3(a) and (b) and supplementary animated gif).

Magnetic properties. The study of the magnetic susceptibility data for ICC has been performed within the temperature range of 1.80–300 K. The magnetic properties of the title complex are shown in Figure 4 in the form of molar susceptibility χM and product function χMT vs T. The χM values increase slowly on temperature lowering but in the low temperature range a rapid increase of the value of the molar susceptibility occurs.

Figure 4. Thermal dependence of χM (blue) and χMT (green) for ICC complex. The inset show thermal dependence of inverse magnetic susceptibility. At room temperature the χMT value is equal to 2.53 cm3 K mol-1 (4.51 µB) as expected for high–spin S = 3/2 tetrahedral complex of Co(II).15,16 The χMT value gradually decreases in a wide range of temperatures and rapidly decreases at the lowest temperatures to 1.01 cm3 mol-1 K (2.84 µB). A decrease of χMT in the low temperature range can be either due to D (axial) zero-field


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splitting effect of Co(II) ions and eventually due to intermolecular exchange interactions between next-nearest neighboring Co(II) ions in the crystal lattice.17 The tetrahedral crystal field around Co(II) e4t32 electronic configuration with 4A2 ground term and 4T2 as the first excited one situated at ∆ = 10Dq. The ligand field lower than cubic removes the orbital degeneracy of the 4T2 term, and the spin-orbit coupling splits the ground term into two Kramers doublets MS = ±3/2 and MS = ±1/2 separated by a gap  1 1  2 D = 8λ 2  −  ( Eq.1) ∆   ⊥ ∆  where: D refers to the axial zero field splitting parameter and λ = −ξ Co / 2S is the spin orbit

splitting parameter; ∆ ⊥ and ∆ are excitation energies due to the splitting of 4T2. For D < 0 the MS = ±3/2 state refers to the ground Kramers doublet. In the absence of the magnetic exchange the magnetic data have been fitted by assuming the anisotropic spin Hamiltonian in the form:

Hˆ a = D ( Sˆz2 − Sˆ 2 / 3)h −2 + g a µB BSˆa h −1 ( Eq. 2) where: the second contribution is the Zeeman interaction term in the direction defined by polar angles a ≡ (ϑi , ϕi ) . The D parameter and the magnetogyric factors gz and gx are related.18

D = λ ( g z − g x ) / 2 ( Eq. 3) where the spin-orbit splitting parameter for Co(II) is λ/hc = -172 cm-1. The matrix of the spin Hamiltonian has been diagonalized and the obtained eigenvalues inserted into the van Vleck formula for the susceptibility and the partition function for the magnetization, respectively. Three magnetic fields have been used in order to determine the van Vleck coefficients numerically in the vicinity of the reference field.


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A more straightforward procedure is to exploit analytical formulae for the magnetic susceptibility (see for instance18-20). However, they are approximate solutions containing the D parameter in the denominator of the perpendicular component owing to which the formula diverges for small D. More accurate formulae for the magnetization components are also available.18-20 The drawback of them, however, lies in the fact that they refer only to three principal components. Therefore for a highly anisotropic system, as the Co(II) complex, averaging the susceptibility/magnetization over a number of grids distributed uniformly over a sphere is needed. The minor improvements refer to the molecular-field correction zj (sensitive to the lowtemperature window) and the uncompensated temperature-independent magnetism χTIM (that affects the high-temperature data). χ corr =

χ mol + χ TIM ( Eq. 4) 1 − ( zj / N A µ0 µB2 ) χ mol

An advanced fitting procedure (see Figure 5) accounts simultaneously for the two data-sets: χ = f(T, B0 = 0.5 T) and Μ = f(B, T0 = 2 K). Powder average has been done by taking 210 points of polar coordinates distributed over one hemisphere. It converged to the following set of magnetic parameters: gz = 2.362, gx = 2.225, zj/hc = -0.154 cm-1, and χTIM = 4.37 × 10-9 m3 mol-1. The constrained D value calculated via eqn (3) amounts to D/hc = -12.0 cm-1. The quality of the fit is good as expressed by the discrepancy factors for the susceptibility R(χ) = 0.0047 and magnetization R(M) = 0.098 – Figure 6. Finally, the constraint of the D-parameter was released in order to check the stability of the solution; the fitting procedure converged to the same set of magnetic parameters. The value of the D-parameter spans an interval expected for tetrahedral Co(II) systems, D ≈ 10 cm-1 (for a perfect tetrahedron D = 0).19 However, the sign of D is


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difficult to predict as the magnetic data for Co(II) systems is only little sensitive to the sign of D.19 Second, the distortions of tetrahedron involve the radial shifts (bond lengths Co-X) as well as angular variations (bond angles X-Co-X) and these distortions could be mutually correlated. For instance, the [CoCl4]2- ion in Cs3CoCl5 possesses D2d symmetry and is slightly elongated with D = -4.5 cm-1. The same anion in Cs2CoCl4 has nearly C2v symmetry and is slightly compressed (in the direction of a square) with D > 0.21 In [CoCl2(PPh3)2] both angles Cl-Co-Cl = 117.3° and P-Co-P = 115.9° are larger than the tetrahedral value and then D/hc = -14.8 cm-1 applies.22

Figure 5. Magnetic parameters for ICC. Open symbols-experimental data, lines-fitted.

In the present case the X-ray structure analysis reveals two pairs of bond lengths Co-Cl(a) = 2.271 Å and Co-Cl(b) = 2.249 Å and the bond angles Cl(a)-Co-Cl(a) = 106.9° and Cl(b)-CoCl(b) = 115.6°. This means that for the shorter pair the bond angle increases from its tetrahedral value that matches the VSEPR (Valence Shell Electron Pair Repulsion) ideas. The molecular


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symmetry is C2v; the coordination polyhedron refers to a bisphenoid that on one side is oblate (flattened), while on the other prolate. The best fit parameter D/hc = -12 cm-1 implies that the ground state is the Ms = ±3/2 Kramers doublet whereas the first excited doublet refers to Ms = ±1/2. Therefore the magnetization possesses an easy axis as displayed by the 3D-diagram in Figure 6.

Figure 6. 3D-view of magnetic functions for ICC. (a) magnetization at B = 3.0 T and T = 2.0 K; (b) susceptibility at B= 0.1 T and T = 2.0 K.

As mentioned in the structural discussion,13,14 this complex can be viewed as a pseudo-chain, in which the cations and anions connected by medium strength N—H···Cl hydrogen bonds create a three-dimensional network. These contacts may create a magnetic exchange pathway. The effect of hydrogen bonds mediating the magnetic exchange interactions is well known in literature. The small magnitude of this interaction, zj/hc = −0.154 cm-1, is a result of the rather long Co–Co separation (6.4070(2) Å and 7.7560(23) Å). For S = 3/2 the magnetization per formula unit should saturate to the value of M1 = Mmol/NA = 3.0 µB.23 However, at the magnetic field B = 5 T and the temperature as low as T = 2.0 K the magnetization adopts a value of only M1 = 2.15 µB; this is a fingerprint of the sizable zero-field splitting, as confirmed by the data fitting.


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The AC susceptibility data was conducted at the applied field of BDC = 0.3 T for 12 frequencies f between 1 and 1500 Hz and varying temperature (Figure 7). This data shown that the out-ofphase components do not follow a simple trend indicates a rather complex behaviour. In this reason we add the theoretical description of AC measurement to Supporting Materials as a only rough estimates data.

Figure 7. Field dependence of the AC susceptibility components at T = 1.9 K for a set of frequencies of the AC field.

1

H spin-lattice relaxation studies

1

H spin-lattice relaxation time, T1, has been measured versus temperature at the proton resonance

frequency of ωH = 25 MHz revealing interesting features of the relaxation processes. Generally one should consider two contributions to the overall 1H spin-lattice relaxation rate (R1 = (T1)-1). The first one results from dipole-dipole interactions between protons of the imidazolium rings (denoted as the 1H-1H relaxation contribution), while the second relaxation channel is provided by the dipole-dipole couplings of protons with the unpaired electron spin of 59Co (denoted as the 1

H-59Co relaxation contribution), i.e.:


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R1 = R1H − H + R1H −Co

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( Eq. 5)

The 1H-1H relaxation rate is described by the formula:24,26

 τc  τc R1H − H = CHH  J (ωH ) + 4 J ( 2ωH )  = CHH  +4 ( Eq. 6) 2 2 2 2 ω τ ω τ 1 + 1 + 4 H c H c   where the quantities J(ω) are referred to as spectral density functions being Fourier transform of time correlation functions describing the stochastic fluctuations of the dipole-dipole interaction. When the correlation function is exponential the corresponding spectral densities are of Lorentzian form, as assumed in eq. (6). The parameter τc is a characteristic correlation time reflecting the time scale of the modulations of the 1H-1H dipole-dipole interactions, i.e. the time scale of the motion of the imidazolium rings. Assuming that the imidazolium dynamics is a thermally activated process following the Arrhenius law one can write:

 Ea   ( Eq. 7)  RT 

τ c = τ 0 exp 

where EA is an activation energy, while τ0 denotes a high-temperature limit of the correlation time, τc. CHH in Eq. 6 is referred to as an effective relaxation dipole-dipole constant; for dipolarly interacting protons it is equal to:

C HH

2  µ 0 γ H2 h   =  3 3  4π rHH 

2

,

where γH is proton gyromagnetic ratio, rHH is an effective inter-spin distance which accounts for the fact that for imidazolium rings several pairs of spins (protons) contribute to the effective relaxation constant. The 1H-59Co relaxation rate is described by the expression:24-26   τ eff ,1 τ eff ,2 R1H −Co = CHCo 3 J (ω H ) + 7 J (ω S )  = CHCo 3 + 7 ( Eq. 8)  2 2 1 + 4ω S2τ eff2 ,2   1 + ω Hτ eff ,1


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This formula requires some explanation. As 59Co atoms possess unpaired electrons the R1H-Co relaxation rate depends also on the spectral density J(ωS) taken at the electron frequency ωS which is approximately 650 times larger (the ratio between the electron and proton gyromagnetic factors) than ωT. The spectral densities taken at ωH and ωS are associated with different correlation times, τeff,1 and τeff,2, respectively, defined as: τeff,i-1 = τc-1 + Ri,e,, where R1,e and R2,e are electron spin-lattice and electron spin-spin relaxation rates, respectively.24-28 The electron spin has its own relaxation mechanism independent of the presence of the neighboring nuclei, caused by zero field splitting interactions.24-28 From the perspective of the nuclear (proton) relaxation the electronic relaxation plays the role of an additional contribution to the effective modulations of the nuclear spin – electron spin dipole-dipole coupling. One should note that in the expression for τeff,i-1 it has been assumed that the direct modulations of the 1H-59Co dipoledipole coupling are characterized by the same correlation time τc as the 1H-1H interaction. The CHCo relaxation constant is defined as:

C HCo

 µ γ γ h 2 = S ( S + 1)  0 H3 S  15  4π rHCo 

2

where again rHCo is an effective inter-spin distance, γs is the electron gyromagnetic ratio, while S is the electron spin quantum number; for Co(II) S=3/2. As γs/ γH ≅ 650 the relaxation constants CHCo and CHH become comparable for rHCo ≅ 8rHH, otherwise if rHCo is smaller, the R1H-Co contribution prevails. As for ICC it is justified to interpret the experimental results assuming that the observed relaxation is mostly due to the 1H-59Co dipole-dipole interactions. Moreover, taking into account that correlation times in solids are rather long one can also assume that the contribution to the relaxation rate R1 ≅ R1H-Co (R1H-H has already been neglected) associated with J(ωS) can be neglected as well


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 τc  τc > 1 Thus the final expression for the 1H spin-lattice relaxation rate can be simplified to: R1 (ωH ) = K

τ eff ,1 1 + ωH2 τ eff2 ,1

( Eq. 9)

The electron spin-lattice relaxation time is also temperature dependent. Nevertheless, one can expect that the correlation time τc is shorter than T1,e so one can neglect the electron spin relaxation effect and set τeff,1 = τc. Under this assumption the relaxation data can be reproduced in terms of two motional processes: R1 (ω H ) = K1

τ c ,1 τ c ,2 + K2 2 2 1 + ω Hτ c ,1 1 + ω H2 τ c2,2

( Eq.10)

with the parameters K1 = 1.1·1012 Hz2, Ea,1 = 3.8 kJ/mol and K2 = 0.5·1012 Hz2, Ea,2 = 2.7 kJ/mol as shown in Figure 8. Some discrepancies between the experimental results and the theoretical predictions are seen in the range of 1000/T: (7-9) and above 11. The discrepancies in the range of (7-9) (i.e. 110 K140 K) reflect the simplifications of our description – we have assumed only two motional processes modeling the entire temperature dependence of the relaxation time following, in addition, the very simple Arrhenius law. Possible effects of the electron spin relaxation have been neglected as well. In principle one could get a better agreement with the experimental results including a third process, however, we found this somewhat speculative. As far as the low temperature discrepancies are concerned we demonstrate in Figure 8 that electron spin relaxation can lead to a shortening of the nuclear spinlattice relaxation time by contributing to the effective correlation time of the fluctuations of the


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1

H-59Co dipole-dipole coupling. To show this effect we just introduced one time constant for the

electron spin-lattice relaxation time not considering its hypothetic temperature dependence.

Figure 8 Experimental 1H spin-lattice relaxation times for ICC at 25 MHz – open circles. Theoretical temperature dependence of the relaxation time resulted from a superposition of two motional processes - red solid line; contributions of the individual processes – solid lines numbered according to the description in the text (K1 = 1.1·1012 Hz2, Ea,1 = 3.8 kJ mol-1 and K2 = 0.5·1012 Hz2, Ea,2 = 2.7 kJ mol-1). Effects of the electron spin relaxation for T1,e = 5.5·10-8 s dashed red line and T1,e = 3.0·10-8 s - dotted red line. Dashed vertical line indicates the PT temperature.

Dielectric properties Figure 9(a) shows temperature dependence of the real (ε’) part of the complex dielectric permittivity measured between 2.6 kHz -2 MHz for the polycrystalline sample, in the vicinity of the low temperature PTs. Two subtle anomalies on the ε’ vs. temperature curve are clearly visible. PT from phase I to II is illustrated only as a negligible change of the slope, while the transition from phase II to III is accompanied by an insignificant leap, with a dielectric increment (∆ε’) being of the order of


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0.15. Additionally over the phase III a clear relaxation process in the kilohertz frequency region is observed (see Figure 9(b)).

Figure 9 Temperature dependence of the (a) real and (b) imaginary parts of complex dielectric permittivity for ICC at low temperatures

Dielectric response in the temperature range of 100 to 200 K can be described by the ColeCole relation:

ε * (ω ) = ε ∞ +

εo − ε∞ 1−α 1 + ( iωτ c )

( Eq.11)

where: ε0 and ε∞ are respectively the low and high frequency limits of the dielectric permittivity, ω is angular frequency, τc is the macroscopic dielectric relaxation time. The Cole–Cole diagrams at selected temperatures are presented in Figure 10.


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Figure 10 Cole-Cole plots of ε’’ vs. ε’ at four selected temperatures showing relaxation nature of the dielectric dispersion in ICC.

The Cole-Cole plots in the range 110-170 K deviate from semi-circles and the parameter α varies between 0.30 and 0.42 which indicates that in this temperature region we deal with a polydispersive relaxation process. In the systems, for which the dipole–dipole interactions are relatively small one can assume that the macroscopic relaxation time is equivalent to the microscopic one, thus the energy barrier Ea was estimated on the basis of the Arrhenius relation for τc. In the wide temperature region (110-170 K) Ea was estimated to be constant ca. 26 ± 2 kJ mol-1. It should be emphasized that temperature and frequency behavior of ε” (T, ν) resembles the dielectric responses for materials characterized by a glassy state for which increment of ∆ε” (see Figure 9 (b)) increases significantly with increase of frequency (ν). The general conclusion of the dielectric studies is that, the dielectric relaxation process observed in the temperature range of 100-200 K is characterized by a small dielectric increment (∆ε ≈ 0.2-0.8) which rather excludes reorientational motion of strongly dipolar imidazolium


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cations but postulates librational motion of the imidazole ring. This is in agreement with singlecrystals X-ray measurements in phase III.13,14

Conclusions Calorimetric measurements of ICC disclosed two structural PTs; continuous at 245.5 K and discontinuous at 234/237 (cooling/heating). The later transition is accompanied by a small transition entropy magnitude (∆S ≈ 1 J mol-1K-1) which is typical for compounds exhibiting both ‘order-disorder’ and ‘displacive’ PT mechanism. Single-crystal X-ray diffraction and calorimetric studies proposed the following sequence of PTs:

The phases I and III were characterized in

13,14

but the intermediate, IC modulated phase II is

described for the first time in the present work. Modulation is characterized by small magnitude vector and presence of only first order satellite reflections. In the RT phase I the imidazolium cations appeared to be dynamically disordered.14 The change in the motional state of cations was confirmed by X-ray diffraction, calorimetric, 1H NMR and dielectric studies. The LT phase seems to be ordered (X-Ray at 223 K

13

) with respect to the in-plane reorientational motion of

cations, however in this phase one can not exclude small-angle librational motion of cations. Such a possibility is strongly confirmed by the presence of the low frequency dielectric relaxation process. The temperature dependence of the 1H spin –lattice relaxation time indicates two motional processes with similar (rather low) activation energies. The identified processes might be treated as components of a ‘two-step’ dynamics of the imidazolium rings. In the first step the faster, anisotropic dynamics leads to a partial averaging of the dipole-dipole coupling


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between protons, and then, in the next step this remaining part of the dipolar interactions is averaged out due to a slower, isotropic motion. The fast, anisotropic dynamics can be attributed to localized librations of the cations, why the slower, isotropic motion can be associated with a jump-like (finite angle) rotation of the cations. The ratio between the dipolar constants K1 : K2 ≅ 2:1 reflects the relative contribution of these two processes. We would not like to speculate at this stage about the mechanism of the cationic dynamics, however, it should be stressed that the activation energies of the dynamical processes revealed by NMR relaxation studies are by about an order of magnitude smaller than of those observed in the dielectric studies. This might indicate that the dynamics revealed by NMR is attributed to a single imidazolium cation, while the slow relaxation process observed in dielectric studies stems from a collective dynamics of the crystal lattice. As there are no abrupt changes in the relaxation time at the PT temperatures one can conclude that the PTs do not lead to a sudden change of the imidazolium dynamics, i.e the time scale of the motion changes gradually with temperature in the whole range. The direct-current (dc) magnetic susceptibility and magnetization data confirm weak intermolecular antiferromagnetic interaction between nearest Co(II) ion in the crystal lattice and a large-magnetic anisotropy. ICC complex showed a frequency-dependent out-of-phase signal in alternating current (ac) susceptibility under a dc bias of 0.3 T, indicating the existence of three relaxation processes.

ASSOCIATED CONTENT This material is available free of charge via the Internet at http://pubs.acs.org. Details of thermal (TGA-DTA) and magnetic properties, PXRD are presented in Supporting Information.


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AUTHOR INFORMATION

Corresponding Author *e-mail: [email protected] Notes The authors declare no competing financial interest ACKNOWLEDGMENT The work was partially supported by National Science Centre, Poland (DEC2012/05/B/ST3/03190). Grant agencies (Slovakia: VEGA 1/0522/14, VEGA 1/0534/16, APVV14-0078) are acknowledged for the financial support of theoretical analysis of magnetic data. REFERENCES (1) Spaldin, N. A.; Fiebig, M. The Renaissance of Magnetoelectric Multiferroics. Science 2005, 309, 391-392. (2) Eerenstein, W.; Mathur, N.; Scott, J. Multiferroic and Magnetoelectric Materials. Nature

2006, 442, 759-765. (3) Albrecht, A. S.; Landee, C. P.; Slanic, Z.; Turnbull, M. M. New Square S=1/2 Heisenberg Antiferromagnetic Lattices: Pyridinium Tetrahalocuprates and Bispyrazinecopper(II) Tetrafluoroborate. Mol. Cryst. Liq. Cryst. 1997, 305, 333-340. (4) Gesi, K.; Ozawa, K. Effect of Hydrostatic Pressure on the Phase Transitions in [N(CH3)4]2MnCl4. J. Phys. Soc. Jpn. 1984, 53, 627. (5) Gesi, K. Effect of Hydrostatic Pressure on the Lock-in Transitions in Tetramethylammonium Tetrahalogenometallic Compounds, [N(CH3)4]2XY4. Ferroelectrics, 1986, 66, 269-286. (6) Cummins, H. Z. Experimental Studies of Structurally Incommensurate Crystal Phases. Phys. Rep. 1990, 185, 211-409. (7) Kubinec, P.; Birks, E.; Schranz, W.; Fuith, A. Ultrasonic Study of Normal-IncommensurateCommensurate Phase Transitions in [N(CH3)4]2MnCl4. Phys. Rev. 1994, B49, 6515-6523. (8) Coronado, E.; Day, P. Magnetic Molecular Conductors. Chem. Rev. 2004, 104, 5419-5448. (9) Zhang, W.; Xiong, R-G. Ferroelectric Metal–Organic Frameworks. Chem. Rev. 2011, 112, 1163-1195. (10) Hang, T.; Zhang, W.; Ye, H.-Y.; Xiong, R.-G. Metal-Organic Complex Ferroelectrics. Chem. Soc. Rev. 2011, 40, 3577-3598.


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(11) Kundys, B.; Lappas, A.; Viret, M.; Kapustianyk, V.; Rudyk, V.; Semak, S.; Simon, Ch.; Bakaimi, I. Multiferroicity and Hydrogen-Bond Ordering in (C2H5NH3)2CuCl4 Featuring Dominant Ferromagnetic Interactions. Phys. Rev. 2010, B81, 224434-1-224434-6. (12) Beattig, P.; Oguchi, T. Theoretical Investigation of the Crystal Structure and Electronic and Dielectric Properties of the Potential Multiferroic (C2H5NH3)2FeCl4. Jpn. J. Appl. Phys. 2010, 49, 080206-080206-3. (13) Zhang, H.; Fang, L.; Yuan, R. Bis(imidazolium) Tetrachlorocobaltate(II). Acta Cryst. 2005, E61, m677-m678. (14) Adams, C. J.; Kurawa, M. A.; Lusi, M.; Orpen, A. G. Solid State Synthesis of Coordination Compounds from Basic Metal Salts, CrystEngComm. 2008, 10, 1790-1795. (15) Figgis, B. N.; Lewis, J.; Mabbs, F. E. The Magnetic Properties of Some d3-Complexes. J. Chem. Soc. 1961, 3138-3145. (16) Hunoor, R. S.; Patil, B. R.; Badiger, D. S.; Vadavi, R. S.; Gudasi, K. B.; Chandrashekhar, V. M.; Muchchandi, I. S. Spectroscopic, Magnetic and Thermal Studies of Co(II), Ni(II), Cu(II) and Zn(II) Complexes of 3-Acetylcoumarin-Isonicotinoylhydrazone and Their Antimicrobial and Anti-Tubercular Activity Evaluation. Spectrochimica Acta, Part A. 2010, 77, 838-844. (17) Griffith, J. S. The Calculation of Spin-Orbit Coupling Energies, Trans. Faraday Soc. 1960, 56, 193-205. (18) Boča, R. Magnetic Parameters and Magnetic Functions in Mononuclear Complexes Beyond the Spin-Hamiltonian Formalism. Struct. Bonding. 2006, 117, 1-260. (19) Boča R. Zero-Field Splitting in Metal Complexes, Coord. Chem. Rev. 2004, 248, 757-815. (20) Boča, R. Theoretical Foundations of Molecular Magnetism; Elsevier, Amsterdam, 1999. (21) Van Stapele, R. P.; Beljers, H. G.; Bongers, P. F.; Zijlstra, H. Ground State of Divalent Co Ions in Cs3CoCl5 and Cs3CoBr5, J. Chem. Phys. 1966, 44, 3719-3725. (22) Krzystek, J.; Zvyagin, S. A.; Ożarowski, A.; Fiedler, A. T.; Brunold, T. C.; Telser, J. Definitive Spectroscopic Determination of Zero-Field Splitting in High-Spin Cobalt(II). J. Am. Chem. Soc. 2004, 126, 2148-2155. (23) Kahn, O. Molecular Magnetism. VCH: New York, 1993. (24) Abragam, A. The Principles of Nuclear Magnetis. Oxford: Oxford University Press, 1961. (25) Slichter, C. P. Principles of Magnetic Resonance. Springer-Verlag, Berlin, 1990.


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(26) Kruk, D. Theory of Evolution and Relaxation of Multi-Spin Systems. Application to Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR). Arima Publishing UK, 2007. (27) Kowalewski, J.; Kruk, D.; Parigi, G. NMR Relaxation in Solution of Paramagnetic Complexes: Recent Theoretical Progress for S >= 1, Adv. Inorg. Chem. 2005, 57, 41-104. (28) Kruk, D.; Kowalewski, J. Nuclear Spin Relaxation in Solution of Paramagnetic Complexes with Large Transient Zero-Field Splitting. Mol. Phys. 2003, 101, 2861-2874.


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Table of Contents Graphic

SYNOPSIS Two reversible phase transitions: continuous at 245.5 K and discontinuous at 234/237 K have been disclosed in bis(imidazolium) tetrachlorocobaltate(II). The molecular motions of imidazolium cations have been studied by means of the 1H NMR relaxation technique and dielectric spectroscopy. The study of magnetic susceptibility suggested that, the Co(II) ion possess large single ion anisotropy and has shown the tail of an out-of-phase signal in alternating current (AC) susceptibility measurement, indicative of slow relaxation of magnetization.


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Figure 1. DSC traces for ICC crystal upon cooling and heating runs (5 K min−1, m = 10.144 mg) 292x425mm (300 x 300 DPI)


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Figure 2. Crystal packing (a) and symmetry elements (b) comparison of phases III and I (color key: green tetrahedra – [CoCl4]2– anions, blue balls – nitrogen). Nitrogen atoms positions are not shown in phase I due to rotational disorder. Symmetry elements that disappear on cooling to phase III are shown in red color. 140x227mm (300 x 300 DPI)



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Figure 3. Modulation vector (a* and c* projections) (a) and magnitude (b) behaviour versus temperature. 110x152mm (300 x 300 DPI)


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Figure 4. Thermal dependence of χM (blue) and χMT (green) for ICC complex. The inset show thermal dependence of inverse magnetic susceptibility. 265x179mm (300 x 300 DPI)



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Figure 5. Magnetic parameters for ICC. Open symbols-experimental data, lines-fitted. 276x183mm (300 x 300 DPI)


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Figure 6. 3D-view of magnetic functions for ICC. (a) magnetization at B = 3.0 T and T = 2.0 K; (b) susceptibility at B= 0.1 T and T = 2.0 K. 192x93mm (300 x 300 DPI)



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Figure 7. Field dependence of the AC susceptibility components at T = 1.9 K for a set of frequencies of the AC field. 279x152mm (300 x 300 DPI)


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Figure 8 Experimental 1H spin-lattice relaxation times for ICC at 25 MHz – open circles. Theoretical temperature dependence of the relaxation time resulted from a superposition of two motional processes red solid line; contributions of the individual processes – solid lines numbered according to the description in the text (K1 = 1.1∙1012 Hz2, Ea,1 = 3.8 kJ mol-1 and K2 = 0.5∙1012 Hz2, Ea,2 = 2.7 kJ mol-1). Effects of the electron spin relaxation for T1,e = 5.5∙10-8 s - dashed red line and T1,e = 3.0∙10-8 s - dotted red line. Dashed vertical line indicates the PT temperature. 188x142mm (300 x 300 DPI)



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Figure 9 Temperature dependence of the (a) real and (b) imaginary parts of complex dielectric permittivity for ICC at low temperatures 292x204mm (300 x 300 DPI)


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Figure 10 Cole-Cole plots of ε’’ vs. ε’ at four selected temperatures showing relaxation nature of the dielectric dispersion in ICC. 292x204mm (300 x 300 DPI)


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