CRYSTAL GROWTH & DESIGN
A Hybrid Organic–Inorganic Conductor K′-BETS2FeCl4, BETS ) Bis(ethylenedithio)tetraselenafulvalene
2007 VOL. 7, NO. 12 2548–2552
Bin Zhang,*,† Francis Laurence Pratt,‡ Mohamedally Kurmoo,§ Yoshinori Okano, Hayao Kobayashi, and Daoben Zhu† Organic Solid Laboratory, CMS and Institute of Chemistry, The Chinese Academy of Science, Beijing, 100080, People’s Republic of China, ISIS, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom, Laboratoire de Chimie de Coordination Organique, UMR7140, Tectonique Moleculaire du Solide, UniVersite Louis Pasteur, Institut Le Bel, 4 rue Blaise Pascal, 67000 Strasbourg Cedex, France, and Institute for Molecular Science, Myodaiji, Okazaki 444, Japan ReceiVed May 17, 2007; ReVised Manuscript ReceiVed September 22, 2007
ABSTRACT: A hybrid organic–inorganic conductor κ′-BETS2FeCl4 is obtained by an electrocrystallization method from BETS and (Me4N)3Fe(C2O4)3 dissolved in a mixture of C6H5Cl and C2H5OH under 2.0 V for 4 weeks. It belongs to the monoclinic space group C2/c, with cell parameters a ) 37.783(9), b ) 11.207(3), c ) 8.530(2) Å, β ) 94.835(1)°, V ) 3598.8(1.4) Å3, and Z ) 4. The donor arrangement resembles the κ-phase but is different to that of the previously reported antiferromagnetic superconductor κ-BETS2FeCl4, which is orthorhombic and has been studied for 10 years. The inorganic anion exists in two opposite configurations in the anion sheet of κ′-BETS2FeCl4, and its crystal structure and electronic structure are different from those of the polymorphs λ-BETS2FeCl4 and κ-BETS2FeCl4. Crystal engineering on organic–inorganic hybrids has produced success in the construction of molecular conductors and magnetic molecular conductors.1 It has supplied a wide range of charge-transfer complexes with different arrangements of TTF and its derivatives in crystals exhibiting polymorphism and having a variety of electronic and magnetic ground states:2 The electronic ground states that are controlled by the TTF moiety vary from insulating to narrow gap semiconducting to metallic and in some cases superconducting, and the magnetic properties are described by models ranging from low dimensional antiferromagnetic behavior to Pauli paramagnetism and in a few cases to canted antiferromagnetism.3 The presence of polymorphism due to different donor arrangements has deepened our understanding of the relationship between structure (including crystal structure and electron structure) and physical properties.4 For example, BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene] crystallizes in more than 10 known crystal structures with the tri-iodide anion. While the conductivity is controlled by the arrangement of the donor molecules in the crystal, small variation of the donor arrangement can cause a difference in the physical properties of a crystal, for example, as l-mode and d-mode in the θ-phase, R and R′′ in the R -phase, and β and β′′ in the β-phase charge-transfer complexes with conductivity ranges from semiconductor to conductor to superconductor. For BEDT-TSF [bis(ethylenedithio)tetraselenafulvalene], this polymorphism is less extensive.5 Our interests have been focused on charge-transfer complexes with magnetic anions, for example, FeX4- (X ) Br, Cl), due to the possible interaction between the paramagnetic moment of the inorganic anion and the conduction electron on the organic moiety, also known as π-d interaction.5,6 Two charge-transfer complexes of BETS with FeX4- in the form of λ-BETS2FeX4 and κ-BETS2FeX4 have been reported.7 Their different donor and anion arrangements cause quite different conductivity and * To whom correspondence should be addressed. Tel: 86-10-62558982. Fax: 86-10-62559373. E-mail:
[email protected]. † The Chinese Academy of Science. ‡ Rutherford Appleton Laboratory. § Universite Louis Pasteur. Institute for Molecular Science.
Table 1. Crystallographic Data of Three BETS2FeCl4 color morphology cell parameters
crystal system space group Z
κ′-BETS2FeCl4
κ-BETS2FeCl4
λ-BETS2FeCl4
black plate a ) 37.783(9)Å b ) 11.207(2)Å c ) 8.530(2)Å R ) 90° β ) 94.835(1)° γ ) 90° V ) 3598(1)Å3 monoclinic C 2/c 4
black plate a ) 11.693(5)Å b ) 35.945(9)Å c ) 8.4914(3)Å R ) 90° β ) 90° γ ) 90° V ) 3569(2)Å3 orthohombic P nma 4
black needle a ) 16.164(3)Å b ) 18.538(3)Å c ) 6.5928(8)Å R ) 98.40(1)° β ) 96.67(1)° γ ) 112.52(1)° V ) 1773.0(5)Å3 triclinic P 1j 2
magnetic behaviors, and they have been extensively studied in characterizing magnetism-conduction electron interactions. The conductivity of λ-BETS2FeCl4 decreases from room temperature to 80 K, and then, it shows metallic behavior below 80 K and turns into an antiferromagnetic insulator at 7 K. The κ-BETS2FeX4 series consists of antiferromagnetic metals with TN ) 2.5 K (X ) Br) and TN ) 0.45 K (X ) Cl), becoming magnetic superconductors at Tc ) 1 K (X ) Br) and Tc∼0.15 K (X ) Cl).8 A new modified κ-phase BETS2FeCl4 (we call it κ′-BETS2FeCl4 here) was found. Its crystal and electronic structure were studied and compared with κ-BETS2FeCl4. The neutral BETS was synthesized by the literature method.9 Shiny black platelike crystals of κ′-BETS2FeCl4 were obtained by electrocrystallization of 5 mg of BETS in a C6H5Cl solution
Figure 1. ORTEP drawing of the asymmetric unit of κ′-BETS2FeCl4. Thermal ellipsoids are drawn at the 30% probability level.
10.1021/cg0704605 CCC: $37.00 2007 American Chemical Society Published on Web 11/03/2007
A Hybrid Organic–Inorganic Conductor κ′-BETS2FeCl4
Crystal Growth & Design, Vol. 7, No. 12, 2007 2549
Table 2. Bond Lengths of the TSeF Core on the BETS Molecule (a-d Were Defined As Follows)
a κ′-BETS2FeCl4 1.349(8) κ-BETS2FeCl4 1.31(3) λ-BETS2FeCl4 1.360(7) 1.370(7)
b
c
1.881(6) 1.898(5) 1.875(5) 1.889(6) 1.94(2) 1.78(2) 1.90(2) 1.88(2) 1.897(5) 1.878(5) 1.871(5) 1.890(4) 1.891(5) 1.876(5) 1.872(5) 1.889(4)
1.892(5) 1.904(5) 1.906(6) 1.898(5) 1.84(2) 1.90(2) 1.91(2) 1.89(2) 1.898(5) 1.896(5) 1.894(4) 1.892(4) 1.888(5) 1.898(5) 1.892(5) 1.905(5)
d
Fe-Cl
1.333(8) 1.335(8)
2.165(2) 2.197(3)
1.35(3) 1.23(3)
2.178(9) 2.168(13) 2.166(10)
1.364(7) 1.358(7) 1.343(7) 1.361(7)
2.186(3) 2.179(3) 2.193(3)
of 20 mg [(CH3)4N]3Fe(C2O4)3 under a constant voltage of 2.0 V for 4 weeks. Elemental analysis using a JXA-8100 Electron Probe Microanalyzer gave a composition ratio of Se:S:Fe:Cl ) 8:8:1:4, which is in good agreement with the formula BETS2FeCl4. The chlorine comes from the solvent as in (BEDTTTF)3Cl2(H2O)2.10 X-ray diffraction data were collected on a Rigaku AFC8R diffractometer with Mo KR, λ ) 0.71073 Å at room temperature on a crystal with dimensions of 0.07 × 0.07 × 0.01 mm3. The data were numerically corrected for absorption. The structure
Figure 2. Packing diagram of κ′-BETS2FeCl4 viewed along the c-axis.
was solved by the direct method, and hydrogen atoms were added by calculation. All of the nonhydrogen atoms were refined anisotropically by the full-matrix method with the SHELX97-2 software on a PC with a final R ) 0.0518, wR2 ) 0.1245, and GOF ) 0.970 for 2766 observed reflections with I g 2σ(I0) in 4126 unique reflections.11 The data have been deposited at the Cambridge Crystallographic Data Center as CCDC265985. The crystallographic data of κ′-BETS2FeCl4 and its two polymorphs κ-BETS2FeCl4 and λ-BETS2FeCl4 are listed in Table 1. They belong to different crystal systems. The CdC and C-S bond lengths of the TTF core of the donor molecules are listed in Table 2. Accordingly, they all fall in the same range, and the charge per donor is +0.5 in each case. There is one unique BETS and one-half of the FeCl4- in the asymmetric unit, and the Fe sits on a special position as show in Figure 1. The packing is generated by the symmetry operation within the C2/c space group as shown in Figure 2. There are eight BETS and four FeCl4 units in one unit cell, as in κ-BETS2FeCl4. The unit cell parameters of κ′-BETS2FeCl4 are related to those of κ-BETS2FeCl4 by the transfer matrix (0,1,0; -1,0,0; 0,0,1) with one angle distorted from 90 to 94.83° as listed in Table 1 and the space group going from Pnma to C2/c. As compared to κ-BETS2FeCl4, the two axes parallel to the conducting layers in κ′-BETS2FeCl4 contract by 4.03 and 5.78%, respectively, while the axis perpendicular to the conducting layers expands by 5.17%. Consequently, the cell volume of κ′-BETS2FeCl4 is increased by 40 Å3, corresponding to an expansion of 1.12%. This indicates that κ-BETS2FeCl4 is more efficiently packed as compared to κ′-BETS2FeCl4 and may be interpreted as being a higher thermodynamic stability for κ-BETS2FeCl4. The key feature of the structure is the double BETS layer arrangement within one unit cell separated by the FeCl4- units along the a-axis. The donor arrangement within one layer is similar to what is commonly described as the κ-phase (Figure 3) with an intrapair distance of 3.519 Å. The dihedral angle between two nearest neighboring pairs is expanded from 77.1° in κ-BETS2FeCl4 to 79.6(1)° in κ′-BETS2FeCl4. The arrangement of adjacent donor layers in each crystal is different: In κ-BETS2FeCl4, molecules show different orientations of adjacent layers, making 30° to the longest axis of the crystal (Figure 4), whereas the longer directions of the donor molecules are all nearly parallel to the longest axis in κ′-BETS2FeCl4. This difference causes the elongation of the b-axis from 35.925 Å in κ-BETS2FeCl4 to 37.783Å in κ′-BETS2FeCl4. Except for the two peripheral ethylene groups, all of the atoms in a donor molecule are coplanar with maximum deviation from the mean plane of 0.12 Å. The shortest Se · · · Se, Se · · · S, and S · · · S
Figure 3. Donor arrangement in a layer of (a) κ′-BETS2FeCl4 and (b) κ-BETS2FeCl4. Dashed lines are intermolecular interactions.
2550 Crystal Growth & Design, Vol. 7, No. 12, 2007
Zhang et al.
Figure 4. Packing diagram of κ-BETS2FeCl4 viewed along the c-axis. Figure 6. Definition of overlap integrals in the donor layer.
Figure 5. Anion layer of (a) κ′-BETS2FeCl4 and (b) κ-BETS2FeCl4. Table 3. Transfer Integrals and Width of the Upper Band of K′-BETS2FeCl4 and K-BETS2FeCl4
κ-BETS2FeCl4
κ′-BETS2FeCl4
intermolecular overlap integrals of HOMO
width of upper band (eV)
a) -9.2386 c) 25.0496 p) 32.7874 q) 7.9239 a)-10.8639 c) 28.2594 p) 36.0079 q) 7.8840
0.90
Figure 7. Band structure and Fermi surface of (a) κ′-BETS2FeCl4 and (b) κ-BETS2FeCl4. In part a, the band structure matches closely to the measured plasma frequencies. In part b, the dashed line is the initial calculated band structure for κ-BETS2FeCl4, and the solid line is scaled to match the measurement.
1.00
charge polarization induced by the anions. This effect is sometimes very pronounced for highly charged anions. For example, in (BEDT-TTF)4ReCl6 · C6H5CN,12 charge localization induced by the anion results in +1 charged donor molecules locating next to the ReCl62- and neutral donor molecules locating next to the neutral solvent. However, in the present compound, the situation is quite subtle. Here, we have all of the donor molecules carrying the same charge (+0.5), but they are aligned as a pair on the two sides of one FeCl4; thus, one can tentatively assign a positive end next to the anion and a neutral end next to another neutral donor end. This particular arrangement imposes a head-to-tail packing of the BETS molecules within the donor layers, where all of the heads form a zigzag chain, which alternates that of the tails. Consequently, a similar zigzag chain formation is seen in the anion layer. The relationship between the electronic structures of κ-BETS2FeCl4 and κ′-BETS2FeCl4 was studied by band calcula-
distances in κ′-BETS2FeCl4 are 3.983(1), 3.518(2), and 3.491(3) Å, respectively. They are on average longer than those for κ-BETS2FeCl4 (3.728, 3.615, and 3.347 Å). The shortest BETS-anion contact S · · · Cl shifts from 3.501(3) to 3.535 Å, in addition to several very weak hydrogen bonds, C7-H7A · · · Cl2 3.62 Å, 158.4°, and C8-H8A · · · Cl1 3.73 Å, 141.9°. The FeCl4- unit is close to being a perfect tetrahedron, as expected, with an average Fe-Cl distance of 2.181 Å, lying between those of κ-BETS2FeCl4 (2.171 Å) and λ-BETS2FeCl4 (2.186 Å); Cl-Fe-Cl angles are close to 109°. The units are arranged in zigzag chains along the c-axis and are different to κ-BETS2FeCl4 as shown in Figure 5. The Fe · · · Fe distances are 5.882 and 5.908 Å alternately within the chain and 8.481 Å between chains. The two anion layers within the unit cell are identical. They are related by the face-center symmetry operation. A commonly encountered feature of these charge transfer salts is the segregation of differently charged molecules due to the
Table 4. Electronic Properties Derived from Drude-Lorentz Fitting of Reflectivity Data for K′-BETS2FeCl4 and K-BETS2FeCl4 ωp 3
-1
γ calculated (eV)
3
conduction plane anisotropy -1
measured (10 cm )
measured (eV)
κ-BETS2FeCl4 E c E⊥c
10 cm
7.39(4) 5.35(3)
0.92 0.66
0.94 0.65
1.23(1) 1.02(1)
3.31(3) 3.31(4)
0.55
κ′-BETS2FeCl4 E c E⊥c
6.56(1) 4.58(1)
0.81 0.57
0.89(0.97a) 0.66(0.59a)
0.60(1) 0.96(1)
5.26(3) 4.19(3)
0.51
(ωp/ωp⊥)/(d/d⊥)
A Hybrid Organic–Inorganic Conductor κ′-BETS2FeCl4
Crystal Growth & Design, Vol. 7, No. 12, 2007 2551
Figure 8. Polarized IR reflectivity measured at room temperature for κ′-BETS2FeCl4 and κ-BETS2FeCl4. The solid lines show Drude-Lorentz fits to the data, which were made to extract the plasma frequencies in each case. The plasma frequencies and damping rates are listed in Table 3 and compared with the plasma frequencies derived from calculated band structures.
Figure 9. Relative temperature dependence of resistance of κ′BETS2FeCl4.
tions and polarized IR spectra analysis. The transfer integrals of donors in a layer were calculated using the extended Hückel method and used for determining the tight-binding band structure as shown in Figure 6 and Table 3.13 Values of the transfer integrals, band dispersions, and Fermi surface of κ′-BETS2FeCl4 are very similar to κ-BETS2FeCl4 as seen in Figure 7. The Fermi surface consists of a one-dimensional electron section closely connected to a 2D hole section. This is the typical Fermi surface for the κ-phases of both BEDT-TTF and BETS.14 The bandwidth of the κ′-phase is calculated to be ∼10% larger than that of the κ-phase. The calculated area of the closed pocket of κ′BETS2FeCl4 is close to 21% of the full area of the Brillouin zone, as in κ-BETS2FeCl4.
The polarized IR reflectance spectra of a single crystal were recorded on the best developed surface. They are typical of the IR reflectivity spectra of a low-dimensional metal. The reflectance was fitted to the Drude-Lorentz model to allow for phonon and interband contributions well as the intraband Drude term as shown in Figure 8. The fitted Drude parameters are listed in Table 4. There are some differences between κ-BETS2FeCl4 and κ′-BETS2FeCl4. κ′-BETS2FeCl4 has a higher absolute reflectivity at low frequencies, and plasma frequencies are higher than those of κ-BETS2FeCl4, as expected from the larger calculated overlap integrals. Furthermore, the major vibration features around 1400 cm-1 are found to be weaker for κ′-BETS2FeCl4 than for κ-BETS2FeCl4. Another difference is in the optical anisotropy within the conducting plane, the κ′BETS2FeCl4 being slightly less anisotropic than the κ-BETS2FeCl4. The experimental plasma frequencies for κ′BETS2FeCl4 agree very well with those derived from the calculated band structure. The fitted dielectric constants and observed plasma edges and damping rates are broadly similar to those of other metallic κ-phase ET salts;14 however, for κ-BETS2FeCl4, the fitted plasma frequencies indicate a narrower bandwidth than suggested by the calculated transfer integrals. The difference between κ-BETS2FeCl4 and κ′-BETS2FeCl4 is similar to that between Z-Mode and L-Mode in the θ-phase.4c The four-probe dc-resistivity data on a single crystal show metallic behavior from 300 to 1.9 K (Figure 9) with σrt ) 10-100 S cm-1; this range of values is similar to that reported for κ-BETS2FeCl4 (100 S cm-1).8 The ratio F(300 K)/F(2 K) is 20 for κ′-BETS2FeCl4, which is considerably smaller than the value of 5000 for κ-BETS2FeCl4.
Figure 10. (a) Temperature-dependent magnetic susceptibility 2-300 K; empty square, experimental data; red line, fitting data. (b) Experimental isothermal magnetization at 2 K for κ′-BETS2FeCl4.
2552 Crystal Growth & Design, Vol. 7, No. 12, 2007
The temperature dependence of the magnetic susceptibility is that of a paramagnet from 2 to 300 K without any anomaly. The magnetic susceptibility data were fitted to the Curie–Weiss law with Curie constant C ) 4.23(3) emu K mol-1 and Weiss temperature θ ) –0.69(3) K (Figure 10a).15 The Curie constant is lower than the value 4.38 K emu mol-1 expected for an S ) 5/2 localized spin system with g ) 2.0 and the value of 4.7 K emu mol-1 reported for κ-BETS2FeCl4. The mean-field antiferromagnetic interaction is also smaller than that reported for κ-BETS2FeCl4 (θ ) -5.5 K).8 The exchange interactions were estimated using the extended Huckel method with Jπd ) -(2/ n/∆πd)Σiti2 (i ) 1, 2, 3... n), while ti is the overlap integral between the donor HOMO and the d-like orbital, ∆πd is the energy difference between the donor HOMO and the d-like orbital on anion, and Jdd ) -(2/n)Σiti2(i ) 1, 2, 3... n), while ti is the overlap integral between the d-like orbitals on two anions,16 obtaining Jπd ) 0.13 K and Jdd ) 0.08 K. Because the shortest Fe · · · Fe distance is 5.908(2) Å, which is longer than the sum of van der Waals distance for possible d-d interactions, the π-d interaction (Jπd) is 20 times lower than that of κ-BETS2FeCl4 (Jπd ) 2.54 K). So, a smaller θ value is expected in the title crystal.13,16 The field dependence of the magnetization at 2 K shows saturation above 20 kOe (Figure 10b). A new metallic, Curie–Weiss paramagnetic hybrid κ′BETS2FeCl4 has been prepared. Its crystal structure reveals pairwise interactions between BETS and FeCl4-, driving the formation of zigzag chains in the BETS and FeCl4- layers, which may be responsible for the lack of the long-range magnetic ordering, which is commonly found in related compounds. There are some subtle differences in both the crystal structure and the electronic structure between the two κ-phases of BETS2FeCl4. Acknowledgment. We thank Dr. Prof. Xiulian Jing and Li Lu (Institute of Physics, The Chinese Academy of Science) for help with the conductivity measurements. This research was supported by NSFC Nos. 20473095 and 20673120, CMSCX200626, and MOST Nos. 2006CB932102 and 2006CB601001. Supporting Information Available: X-ray crystallographic data supplied by CCDC265985. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG0704605
