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Oct 15, 2010 - Commun. 2004, 2114. (b) Bhatt, P. M.; Desiraju, G. R. ... (b) Alberola, A.; Clements, O. P.; Collis, R. J.; Cubbitt, L.;. Grant, C. M.;...
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DOI: 10.1021/cg100826c

Polymorphism and Magnetism of (cis-Cyclohexane1,4-diammonium)(Dicyclohexano[18]crown-6)2[Ni(dmit)2]2 Salts

2010, Vol. 10 4856–4860

Qiong Ye,*,†,‡ Tomoyuki Akutagawa,*,§ Shin-ichiro Noro,† Takayoshi Nakamura,*,† and Ren-Gen Xiong‡ †

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0020, Japan, ‡Ordered Matter Science Research Center, Southeast University, Nanjing 210096, P. R. China, and §Institute of Multidisciplinary Research for Advanced Materials (IMRAS), Tohoku University, 2-1-1 Katahira, Aobaku, Sendai 980-8577, Japan Received June 21, 2010; Revised Manuscript Received September 22, 2010

ABSTRACT: Supramolecular cationic structures of (CHDA2þ)(DCH[18]crown-6)2 were introduced into [Ni(dmit)2]- salts (where CHDA2þ, DCH[18]crown-6, and dmit2- represent cis-cyclohexane-1,4-diammonium, meso-dicyclohexano[18]crown-6, and 2-thioxo-1,3-dithiole-4,5-dithiolate, respectively). The crystal polymorphs of (CHDA2þ)(DCH[18]crown-6)2[Ni(dmit)2]2 (crystals 1 and 2) within the same crystallization batch were classified in the space groups P21/n (1) and P1 (2). The N-Hþ∼O hydrogen-bonding interactions between the axial- and equatorial-ammonium moieties of the CHDA2þ cation and the oxygen atoms of DCH[18]crown-6 yielded a sandwich-type 1:2 adduct of the supramolecular cation in 1 and 2. Although the overall cationic structures in 1 and 2 resembled one another, the [Ni(dmit)2]- anion arrangements were distinctively different from each other. The lateral [Ni(dmit)2]- dimer along the short axis of the anion was observed in 1, whereas two types of [Ni(dmit)2]π-dimer existed independently in 2. The weak intermolecular interactions between the [Ni(dmit)2]- anions in 1 was reflected by the temperature dependent magnetic susceptibility, which followed the Curie-Weiss model. For 2, two different magnetic exchange energies arising from the different strengths of the π-dimer units dominated the magnetic properties.

Introduction Crystal polymorphs have often been observed in organic crystals, where two or more crystalline phases with the same stoichiometry occur simultaneously.1 The molecular arrangements and/or molecular conformations in the polymorphs are different from each other, depending on the crystallization conditions. Crystal polymorphism is an interesting phenomenon from the viewpoint of structure-physical property relationships, the effects of crystallization forces on the molecular conformation, crystal engineering, and crystal structure prediction.1,2 The solubility and melting point of crystal polymorphs have sometimes affected their medical activity; therefore, control of the crystallization is essential in the pharmaceutical industry.2,3 Besides medicinal activity, crystal polymorphism can significantly change the electrical, optical, and magnetic properties of crystals. For instance, 5-methyl2-[(2-nitrophenyl)amino]-3-thiophene carbonitrile gave six different crystal forms with the torsional angle between the thiophene-ring and the nitroaniline-ring ranging from 21.7 to 104.7°.4c The structural flexibility of the molecules yielded different molecular conformations, and the color of the crystals varied from red to yellow. Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) salts showed quite rich polymorphism: eight different crystal phases with the diverse electrical conducting properties ranging from semiconductor, to metal, to superconductor.5,6 The structural flexibility of the two terminal ethylenedithio groups played an important role for the crystal polymorphism. Such crystal polymorphism has also been observed for (dimethyldiethylammoniumþ)[Ni(dmit)2]2 salts (dmit2- = 2-thioxo-1,3-dithiole-4,5-dithiolate) *To whom correspondence should be addressed. E-mail: akuta@ tagen.tohoku.ac.jp (T.A.); [email protected] (T.N.). pubs.acs.org/crystal

Published on Web 10/15/2010

as two different crystal phases, in which [Ni(dmit)2] forms π-dimer and spanning-overlap arrangements.7 We have investigated supramolecular cationic structures, such as organic ammonium-crown ether assemblies combined with the [Ni(dmit)2]- anion having an open-shell electronic structure of S = 1/2 spin, that form magnetic crystals. Various supramolecular cations with structural diversity and flexibility can be obtained by design of the hydrogen-bonding interactions in combinations of organic ammonium and crown ethers, which have realized a wide range of assembly structures with [Ni(dmit)2]- anions in the crystals.8-11 The intermolecular interaction modes between [Ni(dmit)2]- anions in the assemblies ranged from π-stack, lateral S∼S contacts along the short- and long-axes of the anion, and orthogonal π-overlap, to ladder and square lattice arrangements.10 In addition, organic ammonium, such as anilinium and adamantylammonium, have rotational freedom along the C-N axis of the cations in the solids.11 When a fluorine substituent was introduced at the meta-position of anilinium, a large dielectric response was observed, which resulted from dipole inversion upon the flip-flop motion of the aryl group; (m-fluoroanilinium)(dibenzo[18]crown-6)[Ni(dmit)2]- exhibited a ferroelectric-paraelectric phase transition at 346 K.11d The spherical-shaped adamantylammonium had smooth rotation in the crystal with a rotational barrier of ca. 10 kJ mol-1, which is much smaller than those of π-planar anilinium rotators (ca. 100 kJ mol-1).11 The adamantane crystal showed a phase transition from the ordered crystal to a disordered plastic crystalline state at 108 K.12 A plastic crystalline phase of cyclohexane has also been reported. Herein, we introduce structurally flexible cis1,4-cyclohexanediammonium to form organic ammoniumcrown ether assemblies in [Ni(dmit)2]- salts. We have already r 2010 American Chemical Society

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Scheme 1. Molecular Structures of CHDA2þ, DCH[18]crown-6, and [Ni(dmit)2]-

Table 1. Crystal Data, Data Collection, and Reduction Parameters of 1 and 2 at 100 K 1

Experimental Section Preparation of (CHDA2þ)(BF4-)2. A 42% aqueous solution of HBF4 (2 mL) was added dropwise to cis-cyclohexane-1,4-diamine (700 mg) in CH3OH (20 mL) over a period of 20 min and stirred for a further 30 min at room temperature. The solvent was then removed under vacuum. A white precipitate was recrystallized from CHCl3-hexane (1:1). Calcd for C6H14N2B2F8: C, 24.87; H, 5.56; N, 9.67. Found: C, 24.76; H, 5.61; N, 9.43. Preparation of Crystals 1 and 2. (n-Bu4N)[Ni(dmit)2] was prepared according to the literature.13 Single crystals of 1 and 2 were prepared by the standard diffusion method in an H-shaped cell (50 mL). An acetonitrile solution of (n-Bu4N)[Ni(dmit)2] (20 mg in 25 mL) and an acetonitrile solution of (CHDA2þ)(BF4-)2 (40 mg in 25 mL) and DCH[18]crown-6 (ca. 200 mg) were introduced into the left and right sides of the H-shaped cell, respectively. The solution was left to stand for approximately one week at room temperature. Single crystals of 1 and 2 were grown in the same crystallization batch. The yields of crystals 1 and 2 were ca. 55 and 15%, respectively, with respect to (n-Bu4N)[Ni(dmit)2]. The crystallization was carried out in three different H-shaped cells, in which crystals 1 and 2 were obtained in 50 ( 5 and 10 ( 5% yield, respectively. The crystal morphologies of 1 and 2 were blackcolored plate and rhombic crystals, respectively, and were separated by hand-picking for physical measurements. The stoichiometries of both compounds were determined by X-ray diffraction and elemental analyses. Crystal 1: Calcd for C29H44O6S10N1Ni: C, 39.49; H, 5.03; N, 1.59. Found: C, 39.25; H, 4.95; N, 1.60. Crystal 2: Calcd for C29H44O6S10N1Ni: C, 39.49; H, 5.03; N, 1.59. Found: C, 39.54; H, 5.23; N, 1.68. Crystal Structure Determination. Crystallographic data (Table 1) were collected using a Rigaku RAXIS-RAPID diffractometer with Mo KR (λ = 0.71073 A˚) radiation from a graphite monochromator (T = 100 K). Structure refinements were made using the full-matrix least-squares method on F2. Calculations were performed using the Crystal Structure and SHELX software packages.14 Parameters were refined using anisotropic temperature factors except for the hydrogen atoms; the hydrogen atoms of the disordered cyclohexane ring were removed from the refinements. Calculation of Transfer Integrals. The transfer integrals (t) between the [Ni(dmit)2]- anions were calculated within the tightbinding approximation using the extended H€ uckel molecular orbital method. The lowest unoccupied molecular orbital (LUMO) of

2

)

C58H88O12S20N2Ni2 formula C58H88O12S20N2Ni2 MW 1763.94 1763.94 P1 (#2) space group P21/n (#14) a, A˚ 14.698(5) 12.4995(4) b, A˚ 23.631(9) 13.8659(4) c, A˚ 24.770(7) 24.1563(7) R, deg 90.00 78.6284(7) β, deg 16.752(17) 75.5395(10) γ, deg 90.00 74.4881(10) 7682(4) 3868.1(2) V, A˚3 Z 4 2 T, K 100(1) 100(1) -1 1.525 1.514 Dcalcd, g cm 10.897 10.821 μ, cm-1 reflns measured 59823 61009 independent reflns 17057 17509 0.113 0.028 Rint 0.0740 0.0597 R1 a 0.1499 0.1037 wR2(F2)a GOF 1.075 0.729 P P P P a R= Fo| - |Fc / |Fo| and Rw={ [w(|Fo|2 - |Fc|2)2]/ w(Fo2)2}1/2. )

reported the crystal structures and magnetic properties of (transcyclohexane-1,4-diammonium)([18]crown-6)2[Ni(dmit)2]2,11b in which the (trans-cyclohexane-1,4-diammonium)([18]crown-6)2 supramolecule had a rather rigid structure, due to a symmetrical sandwich structure with an equatorial conformation. In the present study, the supramolecular cation of cis-cyclohexane1,4-diammonium (CHDA 2þ)-dicyclohexano[18]crown-6 (DCH[18]crown-6) formed novel assembly structures in [Ni(dmit)2]- salts, and the crystal polymorphs of (CHDA2þ)(DCH[18]crown-6)2[Ni(dmit)2]2 (crystals 1 and 2) exhibited completely different magnetic properties. The overall symmetry of the (cis-CHDA2þ)(crown ether)2 supramolecular structure is lower than that of (trans-CHDA2þ)(crown ether)2 due to the low symmetry of cis-CHDA2þ(Scheme 1). The supramolecular cationic unit with lower symmetry provided freedom of the molecular arrangements in the crystals, which caused the crystal polymorph.

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the [Ni(dmit)2]- molecule was used as the basis function.15 Semiempirical parameters for Slater-type atomic orbitals were obtained from the literature.15 The t values between each pair of molecules were assumed to be proportional to the overlap integral (S) via the equation t = -10S eV. Magnetic Susceptibility. The dependence of the magnetic susceptibility and the magnetization magnetic field on temperature were measured using a Quantum Design MPMS-XL5 SQUID magnetometer. The applied magnetic field was 1 T for all temperaturedependence measurements. The black-colored plate (1) and rhombic crystals (2) were separated by hand-picking for the SQUID measurements. Dielectric Measurements. The temperature dependence of the dielectric constants was measured using the two-probe AC impedance method at a frequency of 100  103 Hz (Hewlett-Packard HP4194A). Electrical contacts were prepared using gold paste (Tokuriki 8560) to attach 10-μm-diameter gold wires to the single crystal, which were then placed into a cryogenic refrigeration system (Daikin PS24SS).

Results and Discussion The cation exchange reaction of (n-Bu4N)[Ni(dmit)2] with CDHA2þ in the presence of DCH[18]crown-6 formed the crystal polymorphs 1 (plate) and 2 (rhombic). 1 and 2 were assigned to the space groups P21/n and P1, respectively, where the cation-anion arrangements in the unit cells were completely different from each other. Crystal 1, with higher symmetry, was the major product (55% yield) compared to crystal 2 (14% yield). The density of crystal 1 (Dcalc = 1.60 g cm-3) is larger than that of 2 (Dcalc = 1.51 g cm-3), which indicates the dense cation-anion packing in 1. Deformed sandwich-type supramolecular cationic structures of (CHDA2þ)(DCH[18]crown-6)2 were observed in both crystals. However, the distinct difference in the [Ni(dmit)2]- anion arrangements of 1 and 2 resulted in completely different magnetic properties. Cation Conformation. Both CHDA2þ and DCH[18]crown-6 have conformational flexibility.16 Therefore, the conformational changes in the (CHDA2þ)(DCH[18]crown-6)2 structures of 1 and 2 (Figure 1) affect the [Ni(dmit)2]- anion arrangements in the crystals. Figure 2 shows the supramolecular cation structures of 1 and 2, which have crystallographically independent structural units within the unit cell. Two ammonium (-NH3þ) moieties of the CHDA2þ cations in the

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cis-configuration at axial- and equatorial-positions were included in the cavities of DCH[18]crown-6 molecules. Table 2 summarizes the selected structural parameters of the (CHDA2þ)(DCH[18]crown-6)2 cation. The average nitrogenoxygen distance of the N-Hþ 3 3 3 O hydrogen-bonding of DCH[18]crown-6 molecules to CHDA2þ at the axial position (dN-O = 2.962 A˚) in 1 was almost the same as that at the equatorial position (dN-O = 2.982 A˚), which indicates similar hydrogen-bonding strength.17 The dN-O values for the axial and equatorial -NH3þ groups in 2 were 2.975 and 2.937 A˚, respectively. The directions of two C-N bonds in cis-CHDA2þ formed angles of θ = 62.1 and 61.8° for 1 and 2, respectively; therefore, rather deformed sandwich-type (CHDA2þ)(DCH[18]crown-6)2 assemblies were observed, which is distinctly different from trans-CHDA2þ forming a symmetrical spoolshaped sandwich-type supramolecular structure of (transCHDA2þ)([18]crown-6)2 in [Ni(dmit)2]- salts.10b The deviation in the distance of the nitrogen atom from the mean six oxygen atoms in the plane of DCH[18]crown-6 (dN-X) was 1.04-1.11 A˚ (Table 2), which is comparable to that of (NH4þ)([18]crown-6) (1.02 A˚).18 In addition, disorder of the terminal cyclohexanering at 100 K was confirmed from structural analyses (see Figures S3 and S4). [Ni(dmit)2 ]- Arrangement and Crystal Packing. Each [Ni(dmit)2]- anion has one S= 1/2 spin, of which the arrangements, in terms of intermolecular interactions, directly determine the magnetic properties of the crystal. The magnitude of the intermolecular interaction was evaluated from the intermolecular transfer integrals (t) based on extended H€ uckel calculations of the [Ni(dmit)2]- anion LUMO orbital.15 Table 3 summarizes the t-values for 1 and 2. The magnetic

Figure 1. Supramolecular cationic structures of (CHDA2þ)(DCH[18]crown-6)2 in (a) 1 and (b) 2 (T = 100 K). The disorder in the cyclohexane rings of DCH[18]crown-6 molecules is omitted in the figures.

Ye et al.

exchange energy (|J|) was estimated from the equation |J| ∼ 4t2/ Ueff,19 where Ueff is the effective on-site repulsive Coulombic energy of a [Ni(dmit)2]- anion. Two crystallographically independent [Ni(dmit)2]- anions (A and B) were observed in 1. The B anion has an almost planar conformation with a dihedral angle of φNi = 4.5° between the two C3S5-planes, whereas a distorted nonplanar conformation with φNi = 21.1° was observed for the A anion (Figure S5). Figure 2a shows the unit cell of 1 viewed along the a-axis. Weak interactions between CHD[18]crown-6 and the lateral sulfur atoms of the two nearest-neighboring [Ni(dmit)2]- A and B anions were observed along the c-axis. The dihedral angle between the π-planes of A and B was 42°. Intermolecular interaction between the π-plane of B and the lateral sulfur atoms of A was observed along the a-c axis. Figure 2b shows the [Ni(dmit)2]- anion arrangement within the (11-1) plane. The lateral [Ni(dmit)2]- anion arrangements along the short axis of A and B are elongated along the a-c axis with t1 = -24.8 meV and t2 = 1.22 meV, respectively. Three weak interactions of t3 = -2.72 meV, t4 = -1.03 meV, and t5 = 1.70 meV were observed along the b-axis. Among these interactions, the lateral t1-interation of the A-B pair had the effective magnitude, which should predominantly determine the magnetic properties of the crystal. Although the stoichiometry of 2 was the same as that of 1, completely different cation-anion arrangements were achieved in the crystal. Figure 3a shows the unit cell of 2 viewed along the a-axis. The two C and D [Ni(dmit)2]anions are the crystallographically independent structural units of 2. The [Ni(dmit)2]- anion layers are separated by (CHDA2þ)(DCH[18]crown-6)2 supramolecules along the b-c axis. Two types of [Ni(dmit)2]- π-dimers of C-C and D-D units are observed within the crystal (Figure 3b). Relatively strong π-dimerization occurs in the D-D dimer with t2 = -152 meV, which is three times larger than that in the C-C dimer (t1 = -45.9 meV). The C-C and D-D dimers interact with each other through weak intermolecular interactions of t3 = -1.01 and t4 = -7.25 meV along the -a þ b þ c axis. The effective intermolecular interactions of the C-C and D-D units dominate the magnetic properties of 2. Magnetic Properties. Figure 4 shows χmolT-T plots of 1 and 2 (χmol is molar magnetic susceptibility per two [Ni(dmit)2]- anions). The χmolT value (0.700 emu K mol-1) of 1 at 300 K was almost consistent with the two S=1/2 spins. Upon cooling to 25 K, the χmolT value remained constant, but it gradually decreased below 25 K. The χmolT-T behavior of 1 was well fitted by the Curie-Weiss model using a Curie constant of 0.390 emu K mol-1 for each [Ni(dmit)2]-

Figure 2. Crystal structure of 1. (a) Unit cell viewed along the a-axis. Hydrogen atoms are omitted for clarity. Two independent [Ni(dmit)2]anions (A and B) are present in the unit cell. (b) A and B [Ni(dmit)2]- anion arrangements within the (11-1) plane.

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Table 2. Average Hydrogen-Bonding Nitrogen-Oxygen Distance (dN-O, A˚), Distance between the Nitrogen Atom and the Mean Plane of Six Oxygen Atoms of DCH[18]crown-6 (dN-X, A˚), and Dihedral Angle between the Upper and Lower Mean Planes of DCH[18]crown-6 (θ, deg) in 1 and 2 at 100 K crystal

positiona

dN-O, A˚

dN-X, A˚

θ, deg b

1

axial equatorial axial equatorial

2.962 2.982 2.975 2.937

1.11 1.04 1.01 1.06

62.1

2

61.8

a The axial and equatorial NH3þ groups at the 1- and 4-positions of the cyclohexane ring. b The average planes of upper and lower CHD[18]crown-6 were defined by the six oxygen atoms.

Table 3. Selected Transfer Integrals (t, meV) of 1 and 2 at 100 Ka t1 t2 t3 t4 t5

1

2

-24.8 1.22 -2.72 -1.03 1.70

-45.9 -152 -1.01 -7.25

a The transfer integrals (t) were obtained by the LUMO of uckel calculation (t=-10S eV, [Ni(dmit)2]- based on the extended H€ where S is the overlap integral).

Figure 3. Crystal structure of 2. (a) Unit cell viewed along the a-axis. Hydrogen atoms are omitted in the figure. Two independent [Ni(dmit)2]- anions (C and D) are present in the unit cell. (b) [Ni(dmit)2]- anion arrangements of C and D along the -a þ b - c direction.

anion (A and B) and a Weiss temperature (θ) of -4.52 K. The t1-interaction of -24.8 meV in the A-B pair yielded weak antiferromagnetic coupling. The χmolT-T plots for 2 show stepwise decreases of χmolT at 150 and 20 K, which suggest that two distinct magnetic interactions occur in the crystal. From the crystal structure, the weak C-C and strong D-D π-dimer should mainly contribute to the χmolT-T behavior of 2. The ratio of magnetic exchange energy (J) for the C-C and D-D dimers was approximately 1:10, assuming the relation J ∼ t2. We used the sum of singlet-triplet thermal excitation models with different J-values in order to reproduce the stepwise

Figure 4. Temperature dependent magnetic susceptibility of 1 and 2; χmolT-T plots of 1 (black) and 2 (red). The black and red lines represent a Curie-Weiss model for 1 and a double singlet-triplet thermal excitation model for 2, respectively (see text).

χmolT-T behavior of 2.20 The overall χmolT-T behavior was well fitted by using a Curie constant of 0.46 emu K mol-1 for the C-C and D-D dimers with JC-C = -153 and JD-D = -2.06 K, respectively. The much larger JC-C value than that for JD-D is consistent with the ratio of the t1 and t2 interactions. Dielectric Properties. Molecular motions in crystals are often responsible for the dielectric response.21 We have already reported large dielectric responses due to molecular rotation in (m-fluoroanilinium)(dibenzo[18]crown-6) and pendulum motion in (o-aminoanilinium)(dibenzo[18]crown-6) introduced in [Ni(dmit)2]- salts, which caused significant changes in the dipole moments of the crystals.11d Although disorder of the DCH[18]crown-6 cyclohexane ring was observed in 1 and 2, no evidence for molecular motion in the CHDA2þ cation was detected. The dielectric constants (ε1) of 1 and 2 did not show distinct temperature- or frequencydependent behaviors, which indicates the molecular motion of CHDA2þ is restricted within the supramolecular structures. However, dielectric anisotropy was observed, which corresponds to the [Ni(dmit)2]- anion arrangements. The temperature independent ε1 values below 150 K were mainly affected by the [Ni(dmit)2]- anions (Figure S7). The delocalized π-electron was extended along the long axis of the [Ni(dmit)2]- anion, which resulted in a decrease of the polarization magnitude of the [Ni(dmit)2]- anion in the order of the long-axis of the anion, the short-axis of the anion, to perpendicular to the π-plane of the anion. The temperatureindependent ε1-values along the a-, b-, and c-axes of 1 were 116, 135, and 52, respectively. The long axis of the [Ni(dmit)2]anion in 1 was almost arranged along the b-axis, so that the largest ε1-value was observed along the b-axis. In contrast, almost isotropic ε1-values were observed for 2 along the a-, b-, and c-axes (ca. 130; see Figure S7). The C-C and D-D dimers were elongated along the -a þ b þ c axes; therefore, the isotropic arrangement of the long axis of [Ni(dmit)2]- anions yielded almost the same ε1 along the a-, b-, and c-axes. Conclusions The organic diammonium cation of cis-CHDA2þ with DCH[18]crown-6 formed distorted sandwich-type (CHDA2þ)(DCH[18]crown-6)2 supramolecules through hydrogen-bonding. The flexible cation yielded two types of crystals 1 and 2 with the respective space groups P21/n and P1, but having the same formula, (CHDA2þ)(DCH[18]crown-6)2[Ni(dmit)2]2, in which the cation-anion arrangements were

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distinctively different from each other. The magnetic properties of 1 and 2 were dominated by the [Ni(dmit)2]- anion arrangement in the crystals. The weak lateral dimer along the short axis of [Ni(dmit)2]- resulted in the Curie-Weiss behavior of crystal 1, whereas two π-dimers with different intermolecular interactions caused the magnetism of 2 with a stepwise decrease of χmol with decreasing temperature. Flexible supramolecular cations have the potential to induce polymorphism, which largely affects the magnetic interactions in crystals. The supramolecular cations of organic ammoniumcrown ether assemblies have structural freedom in terms of overall cationic shape and conformation. The molecular crystals of supramolecular cations with [Ni(dmit)2]- anions were constructed from the electrostatic cation-anion interaction and weak intermolecular interactions. Slight differences in cationic structures modify the cation-anion packing structures, resulting in the crystal polymorph. Acknowledgment. This work was supported in part by a Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and the National Natural Science Foundation of China (20701007). Q.Y. (JSPS P08040) thanks the Japan Society for the Promotion of Science. Supporting Information Available: Atomic numbering scheme of 1 and 2, IR spectra, UV-vis-NIR spectra at room temperature, dielectric anisotropy, and cif file. This material is available free of charge via the Internet at http://pubs.acs.org.

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