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An organic charge transfer salt (TCN-DBTTF)[Fe(H2O)6][FeBr4]3: Synthesis, crystal structure and physical properties
Polyhedron 25 (2006) 1613–1617 www.elsevier.com/locate/poly
An organic charge transfer salt (TCN-DBTTF)[Fe(H2O)6][FeBr4]3: Synthesis, crystal structure and physical properties Chunyang Jia a, Shi-Xia Liu a,*, Christina Ambrus a, Ga¨el Labat b, Antonia Neels b, Silvio Decurtins a a
Departement fu¨r Chemie und Biochemie, Universita¨t Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Institut de Chimie, Universite´ de Neuchaˆtel, Avenue Bellevaux 51, CH-2007 Neuchaˆtel, Switzerland
b
Received 18 August 2005; accepted 27 October 2005 Available online 28 December 2005
Abstract A new charge transfer salt (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1) (TCN-DBTTF:tetracyanodibenzotetrathiafulvalene) has been synthesized by electrocrystallization and structurally characterized. The compound crystallizes in the monoclinic space group C2/c, ˚ , b = 9.9966(6) A ˚ , c = 12.1129(10) A ˚ , b = 98.350(7), V = 4284.1(6) A ˚ 3 and Z = 4. The compound is M = 1694.91, a = 35.759(3) A composed of three types of paramagnetic spin carriers: oxidized TCN-DBTTF radicals (S = 1/2), [Fe(H2O)6]2+ cations (S = 2) and [FeBr4] anions (S = 5/2). Due to the specific crystal packing, an electrical conductivity at room temperature in the order of only 107 S cm1 is observed and magnetic measurements reveal overall antiferromagnetic interactions among the three different spin centers. NC
S
S
CN
NC
S
S
CN
Tetracyanodibenzotetrathiafulvalene (TCN-DBTTF)
2005 Elsevier Ltd. All rights reserved. Keywords: Tetrathiafulvalene; Charge-transfer salt; Antiferromagnetic interactions
1. Introduction Since the discovery of the first one-dimensional organic metal TTF-TCNQ, the development of organic conductors and superconductors has attracted a great deal of attention in the field of solid-state chemistry, mainly due to a large variety of crystal structures and novel physical properties [1–3]. Among these investigations, a large synthetic effort *
Corresponding author. Tel.: +41 31 6314296; fax: +41 31 6313993. E-mail address: [email protected] (S.-X. Liu).
0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.10.037
has been devoted to the preparation of magnetic organic conductors by the combination of organic p-donors with magnetic anions, which leads to so-called p–d cooperative systems [4]. For example, the first molecule-based paramagnetic organic superconductor b00 -(BEDT-TTF)4(H2O)[Fe(C2O4)3] Æ C6H5CN was prepared in 1995 [5]. Recently, Fujiwara et al. have reported a novel antiferromagnetic organic superconductor j-(BETS)2FeBr4, which is strongly anisotropic in the conduction plane because of the existence of an induced internal field (metamagnet) based on the antiferromagnetically ordering of the Fe3+ 3d spins
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[6]. In such hybrid systems, it has been found that the crystal packing pattern and charge state of the organic cations greatly depend on the combination of the organic cation and complex anions, resulting in a remarkable influence on the magnetic interaction between the anionic transition metal complexes via the organic cations [7,8]. Consequently, it is still a challenge for chemists to construct dual-property materials, seeking to establish for instance a coupling between the conduction electrons and the magnetic spin moments. Therefore, intense investigations are devoted to the chemical modification of the donor atoms (S) of tetrathiafulvalene (TTF) series [9,10] or the attachment of functional groups to the core of the donor framework [11–18] for an enhancement of the intermolecular interactions among electron donors and acceptors (or counter anions) which is essential to stabilize the conducting and superconducting states (suppressing the Peierls distortions) at low temperature. Moreover, it is well known that cyano groups are able to engage in weak intermolecular interactions such as hydrogen bonding and halogen bonding with halogenated anions. In this context, a new organic donor tetracyanodibenzotetrathiafulvalene (TCNDBTTF) has now been synthesized via triethyl phosphite mediated coupling reaction of 5,6-dicyanobenzene-1,3dithiole-2-one; the synthesis of the latter has been published in Ref. [19]. In this paper, we describe the synthesis of the donor molecule TCN-DBTTF and also the crystal structure, electrical and magnetic properties of its charge transfer salt (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1). 2. Experimental
and washed with methanol (0.74 g, 80%): m.p. >250 C; EI-MS: 404(M+); 1H NMR (300 MHz, DMSO-d6): d = 8.33 (s, 2H); IR (KBr, cm1): m = 2232 (s), 1566 (w), 1354 (s), 1218 (m), 891 (w). Anal. Calc. for C18H4N4S4: C, 53.40; H, 1.00; N, 13.80. Found: C, 53.10; H, 0.80; N, 13.60%. 2.3. Synthesis of (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1) In a two-compartment H-shaped cell, single crystals of 1 were grown by electrochemical oxidation in CH2Cl2 in the presence of TCN-DBTTF and TEA Æ FeBr4 under a constant current of 1.5 lA with Pt electrodes. The anodic and cathodic chambers are separated by a porous frit. Brown block-shaped crystals were observed on the bottom of the anodic chamber after 20 days. The crystals were collected and washed with CH2Cl2. IR (KBr cm1): 2244 (m), 1616 (m), 1359 (w), 1222 (w), 892 (m). Anal. Calc. for C18H16Br12Fe4N4O6S4: C, 12.80; H, 0.95; N, 3.30. Found: C, 13.07; H, 0.97; N, 3.16%. 2.4. Crystallography A brown crystal of 1 was mounted on a Stoe Mark IIImaging Plate Diffractometer System (Stoe & Cie, 2002) equipped with a graphite-monochromator. Data collection was performed at 120 C using Mo Ka radiation ˚ ). One hundred and ninety-seven exposures (k = 0.71073 A (3 min per exposure) were obtained at an image plate distance of 100 mm, 180 frames with u = 0 and 0 < x < 180, and 17 frames with u = 90 and 0 < x < 17, with the crystal oscillating through 1.5 in x. The resolution ˚ . The structure was solved was Dmin–Dmax: 17.78–0.72 A
2.1. Materials and equipments Unless stated otherwise, all chemicals were purchased from commercial sources and used as received. Infrared spectra were obtained on a Perkin–Elmer SYSTEM 2000 FT-IR spectrometer. 1H NMR (300 MHz) spectra were recorded on a Bruker AC-300 NMR spectrometer using TMS as an internal standard. Mass spectra were measured on an AutoSpec Q MS spectrometer. Elemental analyses were performed on a Carlo-Erba-1106 instrument. Tetraethylammonium iron(III) tetrabromide (TEA Æ FeBr4) was prepared as described in the literature [6]. 2.2. Synthesis of TCN-DBTTF The compound 5,6-dicyanobenzene-1,3-dithiole-2-one (1 g, 4.6 mmol) was suspended in the mixture of triethyl phosphite (10 ml) and toluene (10 ml). The mixture was heated up to 120 C under argon atmosphere and stirred for 3 h. During the reaction, a red precipitate was formed. After cooling to room temperature, 20 ml of methanol was added to the reaction mixture to complete precipitation. The product was isolated by filtration as a red powder
Table 1 Crystallographic data for 1 1 Molecular formula Molecular weight Crystal dimensions (mm3) Temperature (K) Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A b () ˚ 3) V (A Z Density calculated (g cm3) Absorption coefficient (mm1) Rint Measured reflections Independent reflections Reflections with I > 2r(I) R1 (observed data) wR2 (all data) GOF
C18H16Br12Fe4N4O6S4 1694.91 0.20 · 0.167 · 0.10 153(2) monoclinic C2/c 35.759(3) 9.9966(6) 12.1129(10) 98.350(7) 4284.1(6) 4 2.628 12.752 0.0854 22 492 5952 3968 0.0620 0.1492 1.008
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Table 2 ˚ ) and bond angles () for 1 Selected bond lengths (A Bond lengths (A) S(1)–C(1) S(2)–C(3) C(9)–N(2)
1.721(8) 1.743(8) 1.137(11)
S(1)–C(2) C(2)–C(3) C(1)–C(1)i
1.744(7) 1.391(9) 1.381(15)
S(2)–C(1) C(8)–N(1) C(5)–C(8)
1.718(7) 1.140(10) 1.435(10)
Fe(1)–Br(1) Fe(1)–Br(2)ii Fe(2)–Br(5) Fe(3)–O(3)iii Fe(3)–O(1B)
2.3283(11) 2.3340(12) 2.3316(14) 2.089(5) 2.14(13)
Fe(1)–Br(2) Fe(2)–Br(3) Fe(2)–Br(6) Fe(3)–O(2B) Fe(3)–O(1A)
2.3340(12) 2.3648(14) 2.3082(14) 2.09(2) 2.139(9)
Fe(1)–Br(1)ii Fe(2)–Br(4) Fe(3)–O(3) Fe(3)–O(2B)iii Fe(3)–O(2A)
2.3283(11) 2.3281(13) 2.089(5) 2.09(2) 2.106(9)
Bond angles () S(1)–C(1)–S(2) C(4)–C(5)–C(8) C(6)–C(9)–N(2)
116.6(4) 121.4(6) 178.1(8)
C(1)–S(1)–C(2) C(7)–C(6)–C(9)
95.9(3) 120.5(6)
C(1)–S(2)–C(3) C(5)–C(8)–N(1)
95.2(3) 177.7(10)
Symmetry transformations used to generate equivalent atoms: i, x + 2, y + 1, z; ii, x + 2, y, z + 1/2; iii, x + 5/2, y 1/2, z.
by direct methods using the program SHELXS-97 [20] and refined by full matrix least squares on F2 with SHELXL97 [21]. The hydrogen atoms were included in calculated positions and treated as riding atoms using SHELXL-97 default parameters. All non-hydrogen atoms were refined anisotropically. Four of the six water molecules in the [Fe(H2O)6]2+ cation were disordered over two positions and refined with half occupations; no H-atoms were found for the water molecules in reasonable positions. A semiempirical absorption correction was applied using MULABS (PLATON03 [22], Tmin = 0.146, Tmax = 0.266). Crystallographic data of 1 are collected in Table 1. Selected bond lengths and angles are given in Table 2. 2.5. Physical measurements Magnetic susceptibility data of a crystalline sample of 1 were collected on a MPMS Quantum Design SQUID magnetometer (XL-5) in the temperature range of 300–1.8 K at a field of 1000 G. The sample was wrapped in a saran bag and put into a straw, which served as sample holder. The molar magnetic susceptibility has been corrected for the diamagnetic contributions of the sample holder and of the sample, the latter is calculated from Pascals constants. 3. Results and discussion 3.1. Structure of (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1) The compound crystallizes in a monoclinic space group (C2/c) and an ORTEP plot of the molecular structure with the atomic numbering scheme is shown in Fig. 1. The unit cell contains four oxidized TCN-DBTTF molecules, four [Fe(H2O)6]2+ cations and 12 [FeBr4] anions. In the crystal structure, the donor molecule shows approximate D2h symmetry, whereby only a center of inversion at the midpoint of the C(1)@C(1b) bond is imposed by the exact space group symmetry. The TCNDBTTF molecule is nearly planar (the dihedral angle
between the benzene ring and the central TTF plane is 1.5), and the four peripheral cyano groups lie also in the plane of the dibenzotetrathiafulvalene skeleton, which is quite similar to other dibenzotetrathiafulvalene derivatives [23]. One [FeBr4] anion (Fe(1)) is located on a site with a twofold rotation axis and one [FeBr4] anion (Fe(2)) lies on a general position. The Fe–Br bond lengths of the ˚ for Fe(1) and [FeBr4] anions, 2.3283(11)–2.3340(12) A ˚ for Fe(2), are similar to other 2.3082(14)–2.3648(14) A [FeBr4] salts [24]. The Fe(3) center of the [Fe(H2O)6]2+ cation lies on an inversion center. Four of the six water molecules of the [Fe(H2O)6]2+ cations are disordered over two positions and refined with half occupations and the Fe–O bond lengths of [Fe(H2O)6]2+ are in the range of ˚. range of 2.089–2.14 A Importantly, the bond lengths of the central C@C bond, the C–S bonds near the molecular center and the outer C@C bonds within the TTF molecule are related to the degree of ionicity of the donor molecule. The central C@C bond length of TCN-DBTTF moiety of 1 is ˚ . Thus, it can be inferred that the TCN-DBTTF 1.381(15) A moiety in 1 is essentially fully oxidized according to the correlation between the oxidation states of TTF derivatives and bond lengths of central C@C bonds [25], which is fully in accordance with the stoichiometry of 1. In the crystal lattice of 1 (Fig. 2), the donor molecules are arranged in layers parallel to the bc plane. Along the c-axis, the direction of the molecular planes of two adjacent TCN-DBTTF molecules alternate with a dihedral angle of 77.4. They are separated from each other by tetrahedral [FeBr4] anions; no specific CN Br short contacts can be identified. However, weak intermolecular interactions ˚ ], Br C [3.487–3.533 A ˚ ] and of S Br [3.566–3.741 A ˚ Br Br [3.651–3.871 A] as well as hydrogen bonds C– ˚ ; C–H Br 178] are observed. H Br [C Br 3.742 A Along the b-direction, the donor molecules are parallel and coplanar to each other forming a columnar arrange˚ . In addition, ment with S S distances of 10 A 2+ [Fe(H2O)6] cations are located in-between the donor
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Fig. 1. ORTEP plot (50% probability ellipsoids) of 1.
Fig. 2. Projection of the crystal structure of 1. Hydrogen atoms are omitted for clarity.
˚] molecule assemblies and there are CN O [2.881–3.025 A ˚ ] short contacts. and O Br [3.316 A 3.2. Electrical conductivity The compound 1 shows a low electrical conductivity with a room temperature value r = 4.3 · 107 S cm1. This observation corresponds to its structural feature of the crystal packing which does not show any specifically close stacking arrangements of the donor and acceptor units. 3.3. Magnetic properties The compound 1 is composed of three types of paramagnetic spin carriers: oxidized TCN-DBTTF radicals
(S = 1/2), [Fe(H2O)6]2+ cations (S = 2) and [FeBr4] anions (S = 5/2), the latter ones are located on two crystallographically different sites. The thermal dependence of the magnetic susceptibility of 1 is shown in form of a vMT versus T plot (Fig. 3). The calculated vMT value is 17.9 emu K/mol per stoichiometric formula unit, with g (rad) = 2; g (Fe2+) = 2.2 which is typical for HS Fe2+; g (Fe3+) = 2.06 [16]. This vMT value compares fairly well with the experimental vMT value at room temperature of 16.9 emu K/mol. With decreasing temperature, the vMT values decrease continuously, indicating overall antiferromagnetic interactions among these different paramagnetic centers. Clearly, the whole crystal structure is rather complex so that many different magnetic exchange pathways, extended in all three directions, are present. This situation
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the web www: http//www.ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2005.10.037. References
Fig. 3. Temperature dependence of vMT for compound 1.
renders a meaningful analysis with the assignment of specific exchange coupling parameters as impossible. Additionally, there is no signature of a magnetically ordered phase down to 1.8 K. 4. Conclusions A charge transfer salt (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1) based on the new donor TCN-DBTTF was synthesized by electrocrystallization. In the crystal lattice, intermolecular CN O interactions are observed rather than otherwise favorable CN Br interactions. As a result of the specific crystal packing, the compound shows a low electrical conductivity in the order of 107 S cm1 and the magnetic measurements exhibit overall antiferromagnetic exchange interactions among the three different spin carriers. Acknowledgments This work was supported by the Swiss National Science Foundation (Grant No. 200020-107589) as well as by the ESF Programme – SONS (NANOSYN). Appendix A. Supplementary data Crystallographic data for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 271519 for 1. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail: [email protected] or on
[1] J.M. Williams, J.R. Ferraro, R.J. Thorn, K.D. Carlson, U. Geiser, H.H. Wang, A.M. Kini, M.-H. Whangbo, Organic Superconductors (Including Fullerenes), Prentice Hall, Englewood Cliffs, 1992. [2] E. Coronado, P. Day, Chem. Rev. 104 (2004) 5419. [3] T. Enoki, A. Miyazaki, Chem. Rev. 104 (2004) 5449. [4] L. Ouahab, T. Enoki, Eur. J. Inorg. Chem. (2004) 933, and references therein. [5] A.W. Graham, M. Kurmoo, P. Day, J. Chem. Soc., Chem. Commun. (1995) 2061. [6] H. Fujiwara, E. Fujiwara, Y. Nakazawa, B.Zh. Narymbetov, K. Kato, H. Kobayashi, M. Tokumoto, P. Cassoux, A. Kobayashi, J. Am. Chem. Soc. 123 (2001) 306. [7] E. Fujiwara, H. Fujiwara, H. Kobayashi, T. Otsuka, A. Kobayashi, Adv. Mater. 14 (2002) 1376. [8] M. Katsuhara, M. Aragaki, T. Mori, Y. Misaki, K. Tanaka, Chem. Mater. 12 (2000) 3186. [9] D. Je´rome, A. Mazaud, M. Ribault, K. Bechgaard, J. Phys. Lett. (France) 41 (1980) 95. [10] D. Je´rome, Science 252 (1991) 1509. [11] S.-X. Liu, S. Dolder, M. Pilkington, S. Decurtins, J. Org. Chem. 67 (2002) 3160. [12] C.Y. Jia, D.Q. Zhang, Y. Xu, W. Xu, H.M. Hu, D.B. Zhu, Synth. Met. 132 (2003) 249. [13] A. Dolbecq, A. Guirauden, M. Fourmigue´, K. Boubekeur, P. Batail, M.-M. Rohmer, M. Be´nard, C. Coulon, M. Salle´, P. Blanchard, J. Chem. Soc., Dalton Trans. (1999) 1241. [14] A.S. Batsanov, A.J. Moore, N. Roberston, A. Green, M.R. Bryce, J.A.K. Howard, A.E. Underhill, J. Mater. Chem. 7 (1997) 387. [15] B. Domercq, T. Devic, M. Fourmigue´, P. Auban-Senzier, E. Canadell, J. Mater. Chem. 11 (2001) 1570. [16] T. Devic, J.N. Bertran, B. Domercq, E. Canadell, N. Avarvari, P. Auban-Senzier, M. Fourmigue, New J. Chem. 25 (2001) 1418. [17] G. Cooke, A.K. Powell, S.L. Heath, Synthesis 11 (1995) 1411. [18] N. Terkia-Derdra, R. Andreu, M. Salle´, E. Levillain, J. Orduna, J. Garı´n, E. Ortı´, R. Viruela, R. Pou-Ame´rigo, B. Sahraoui, A. Gorgues, J.-F. Favard, A. Riou, Chem. Eur. J. 6 (2000) 1199. [19] C. Loosli, C.Y. Jia, S.-X. Liu, M. Haas, M. Dias, E. Levillain, A. Neels, H.S. Evans, A. Hauser, S. Decurtins, J. Org. Chem. 70 (2005) 4988. [20] G.M. Sheldrick, Acta Crystallogr., Sect. A 46 (1990) 467. [21] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement, University of Go¨ttingen, Germany, 1997. [22] A.L. Spek, J. Appl. Cryst. 36 (2003) 7. [23] T. Inayoshi, I. Ono, Synth. Met. 110 (2000) 153. [24] T. Mallah, C. Hollis, S. Bott, M. Kurmoo, P. Day, M. Allan, R.H. Friend, J. Chem. Soc., Dalton Trans. (1990) 859. [25] M.B. Inoue, M. Inoue, M.A. Bruck, Q. Fernando, J. Chem. Soc., Chem. Commun. (1992) 515.
An organic charge transfer salt (TCN-DBTTF)[Fe(H2O)6][FeBr4]3: Synthesis, crystal structure and physical properties Chunyang Jia a, Shi-Xia Liu a,*, Christina Ambrus a, Ga¨el Labat b, Antonia Neels b, Silvio Decurtins a a
Departement fu¨r Chemie und Biochemie, Universita¨t Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Institut de Chimie, Universite´ de Neuchaˆtel, Avenue Bellevaux 51, CH-2007 Neuchaˆtel, Switzerland
b
Received 18 August 2005; accepted 27 October 2005 Available online 28 December 2005
Abstract A new charge transfer salt (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1) (TCN-DBTTF:tetracyanodibenzotetrathiafulvalene) has been synthesized by electrocrystallization and structurally characterized. The compound crystallizes in the monoclinic space group C2/c, ˚ , b = 9.9966(6) A ˚ , c = 12.1129(10) A ˚ , b = 98.350(7), V = 4284.1(6) A ˚ 3 and Z = 4. The compound is M = 1694.91, a = 35.759(3) A composed of three types of paramagnetic spin carriers: oxidized TCN-DBTTF radicals (S = 1/2), [Fe(H2O)6]2+ cations (S = 2) and [FeBr4] anions (S = 5/2). Due to the specific crystal packing, an electrical conductivity at room temperature in the order of only 107 S cm1 is observed and magnetic measurements reveal overall antiferromagnetic interactions among the three different spin centers. NC
S
S
CN
NC
S
S
CN
Tetracyanodibenzotetrathiafulvalene (TCN-DBTTF)
2005 Elsevier Ltd. All rights reserved. Keywords: Tetrathiafulvalene; Charge-transfer salt; Antiferromagnetic interactions
1. Introduction Since the discovery of the first one-dimensional organic metal TTF-TCNQ, the development of organic conductors and superconductors has attracted a great deal of attention in the field of solid-state chemistry, mainly due to a large variety of crystal structures and novel physical properties [1–3]. Among these investigations, a large synthetic effort *
Corresponding author. Tel.: +41 31 6314296; fax: +41 31 6313993. E-mail address: [email protected] (S.-X. Liu).
0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.10.037
has been devoted to the preparation of magnetic organic conductors by the combination of organic p-donors with magnetic anions, which leads to so-called p–d cooperative systems [4]. For example, the first molecule-based paramagnetic organic superconductor b00 -(BEDT-TTF)4(H2O)[Fe(C2O4)3] Æ C6H5CN was prepared in 1995 [5]. Recently, Fujiwara et al. have reported a novel antiferromagnetic organic superconductor j-(BETS)2FeBr4, which is strongly anisotropic in the conduction plane because of the existence of an induced internal field (metamagnet) based on the antiferromagnetically ordering of the Fe3+ 3d spins
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[6]. In such hybrid systems, it has been found that the crystal packing pattern and charge state of the organic cations greatly depend on the combination of the organic cation and complex anions, resulting in a remarkable influence on the magnetic interaction between the anionic transition metal complexes via the organic cations [7,8]. Consequently, it is still a challenge for chemists to construct dual-property materials, seeking to establish for instance a coupling between the conduction electrons and the magnetic spin moments. Therefore, intense investigations are devoted to the chemical modification of the donor atoms (S) of tetrathiafulvalene (TTF) series [9,10] or the attachment of functional groups to the core of the donor framework [11–18] for an enhancement of the intermolecular interactions among electron donors and acceptors (or counter anions) which is essential to stabilize the conducting and superconducting states (suppressing the Peierls distortions) at low temperature. Moreover, it is well known that cyano groups are able to engage in weak intermolecular interactions such as hydrogen bonding and halogen bonding with halogenated anions. In this context, a new organic donor tetracyanodibenzotetrathiafulvalene (TCNDBTTF) has now been synthesized via triethyl phosphite mediated coupling reaction of 5,6-dicyanobenzene-1,3dithiole-2-one; the synthesis of the latter has been published in Ref. [19]. In this paper, we describe the synthesis of the donor molecule TCN-DBTTF and also the crystal structure, electrical and magnetic properties of its charge transfer salt (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1). 2. Experimental
and washed with methanol (0.74 g, 80%): m.p. >250 C; EI-MS: 404(M+); 1H NMR (300 MHz, DMSO-d6): d = 8.33 (s, 2H); IR (KBr, cm1): m = 2232 (s), 1566 (w), 1354 (s), 1218 (m), 891 (w). Anal. Calc. for C18H4N4S4: C, 53.40; H, 1.00; N, 13.80. Found: C, 53.10; H, 0.80; N, 13.60%. 2.3. Synthesis of (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1) In a two-compartment H-shaped cell, single crystals of 1 were grown by electrochemical oxidation in CH2Cl2 in the presence of TCN-DBTTF and TEA Æ FeBr4 under a constant current of 1.5 lA with Pt electrodes. The anodic and cathodic chambers are separated by a porous frit. Brown block-shaped crystals were observed on the bottom of the anodic chamber after 20 days. The crystals were collected and washed with CH2Cl2. IR (KBr cm1): 2244 (m), 1616 (m), 1359 (w), 1222 (w), 892 (m). Anal. Calc. for C18H16Br12Fe4N4O6S4: C, 12.80; H, 0.95; N, 3.30. Found: C, 13.07; H, 0.97; N, 3.16%. 2.4. Crystallography A brown crystal of 1 was mounted on a Stoe Mark IIImaging Plate Diffractometer System (Stoe & Cie, 2002) equipped with a graphite-monochromator. Data collection was performed at 120 C using Mo Ka radiation ˚ ). One hundred and ninety-seven exposures (k = 0.71073 A (3 min per exposure) were obtained at an image plate distance of 100 mm, 180 frames with u = 0 and 0 < x < 180, and 17 frames with u = 90 and 0 < x < 17, with the crystal oscillating through 1.5 in x. The resolution ˚ . The structure was solved was Dmin–Dmax: 17.78–0.72 A
2.1. Materials and equipments Unless stated otherwise, all chemicals were purchased from commercial sources and used as received. Infrared spectra were obtained on a Perkin–Elmer SYSTEM 2000 FT-IR spectrometer. 1H NMR (300 MHz) spectra were recorded on a Bruker AC-300 NMR spectrometer using TMS as an internal standard. Mass spectra were measured on an AutoSpec Q MS spectrometer. Elemental analyses were performed on a Carlo-Erba-1106 instrument. Tetraethylammonium iron(III) tetrabromide (TEA Æ FeBr4) was prepared as described in the literature [6]. 2.2. Synthesis of TCN-DBTTF The compound 5,6-dicyanobenzene-1,3-dithiole-2-one (1 g, 4.6 mmol) was suspended in the mixture of triethyl phosphite (10 ml) and toluene (10 ml). The mixture was heated up to 120 C under argon atmosphere and stirred for 3 h. During the reaction, a red precipitate was formed. After cooling to room temperature, 20 ml of methanol was added to the reaction mixture to complete precipitation. The product was isolated by filtration as a red powder
Table 1 Crystallographic data for 1 1 Molecular formula Molecular weight Crystal dimensions (mm3) Temperature (K) Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A b () ˚ 3) V (A Z Density calculated (g cm3) Absorption coefficient (mm1) Rint Measured reflections Independent reflections Reflections with I > 2r(I) R1 (observed data) wR2 (all data) GOF
C18H16Br12Fe4N4O6S4 1694.91 0.20 · 0.167 · 0.10 153(2) monoclinic C2/c 35.759(3) 9.9966(6) 12.1129(10) 98.350(7) 4284.1(6) 4 2.628 12.752 0.0854 22 492 5952 3968 0.0620 0.1492 1.008
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Table 2 ˚ ) and bond angles () for 1 Selected bond lengths (A Bond lengths (A) S(1)–C(1) S(2)–C(3) C(9)–N(2)
1.721(8) 1.743(8) 1.137(11)
S(1)–C(2) C(2)–C(3) C(1)–C(1)i
1.744(7) 1.391(9) 1.381(15)
S(2)–C(1) C(8)–N(1) C(5)–C(8)
1.718(7) 1.140(10) 1.435(10)
Fe(1)–Br(1) Fe(1)–Br(2)ii Fe(2)–Br(5) Fe(3)–O(3)iii Fe(3)–O(1B)
2.3283(11) 2.3340(12) 2.3316(14) 2.089(5) 2.14(13)
Fe(1)–Br(2) Fe(2)–Br(3) Fe(2)–Br(6) Fe(3)–O(2B) Fe(3)–O(1A)
2.3340(12) 2.3648(14) 2.3082(14) 2.09(2) 2.139(9)
Fe(1)–Br(1)ii Fe(2)–Br(4) Fe(3)–O(3) Fe(3)–O(2B)iii Fe(3)–O(2A)
2.3283(11) 2.3281(13) 2.089(5) 2.09(2) 2.106(9)
Bond angles () S(1)–C(1)–S(2) C(4)–C(5)–C(8) C(6)–C(9)–N(2)
116.6(4) 121.4(6) 178.1(8)
C(1)–S(1)–C(2) C(7)–C(6)–C(9)
95.9(3) 120.5(6)
C(1)–S(2)–C(3) C(5)–C(8)–N(1)
95.2(3) 177.7(10)
Symmetry transformations used to generate equivalent atoms: i, x + 2, y + 1, z; ii, x + 2, y, z + 1/2; iii, x + 5/2, y 1/2, z.
by direct methods using the program SHELXS-97 [20] and refined by full matrix least squares on F2 with SHELXL97 [21]. The hydrogen atoms were included in calculated positions and treated as riding atoms using SHELXL-97 default parameters. All non-hydrogen atoms were refined anisotropically. Four of the six water molecules in the [Fe(H2O)6]2+ cation were disordered over two positions and refined with half occupations; no H-atoms were found for the water molecules in reasonable positions. A semiempirical absorption correction was applied using MULABS (PLATON03 [22], Tmin = 0.146, Tmax = 0.266). Crystallographic data of 1 are collected in Table 1. Selected bond lengths and angles are given in Table 2. 2.5. Physical measurements Magnetic susceptibility data of a crystalline sample of 1 were collected on a MPMS Quantum Design SQUID magnetometer (XL-5) in the temperature range of 300–1.8 K at a field of 1000 G. The sample was wrapped in a saran bag and put into a straw, which served as sample holder. The molar magnetic susceptibility has been corrected for the diamagnetic contributions of the sample holder and of the sample, the latter is calculated from Pascals constants. 3. Results and discussion 3.1. Structure of (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1) The compound crystallizes in a monoclinic space group (C2/c) and an ORTEP plot of the molecular structure with the atomic numbering scheme is shown in Fig. 1. The unit cell contains four oxidized TCN-DBTTF molecules, four [Fe(H2O)6]2+ cations and 12 [FeBr4] anions. In the crystal structure, the donor molecule shows approximate D2h symmetry, whereby only a center of inversion at the midpoint of the C(1)@C(1b) bond is imposed by the exact space group symmetry. The TCNDBTTF molecule is nearly planar (the dihedral angle
between the benzene ring and the central TTF plane is 1.5), and the four peripheral cyano groups lie also in the plane of the dibenzotetrathiafulvalene skeleton, which is quite similar to other dibenzotetrathiafulvalene derivatives [23]. One [FeBr4] anion (Fe(1)) is located on a site with a twofold rotation axis and one [FeBr4] anion (Fe(2)) lies on a general position. The Fe–Br bond lengths of the ˚ for Fe(1) and [FeBr4] anions, 2.3283(11)–2.3340(12) A ˚ for Fe(2), are similar to other 2.3082(14)–2.3648(14) A [FeBr4] salts [24]. The Fe(3) center of the [Fe(H2O)6]2+ cation lies on an inversion center. Four of the six water molecules of the [Fe(H2O)6]2+ cations are disordered over two positions and refined with half occupations and the Fe–O bond lengths of [Fe(H2O)6]2+ are in the range of ˚. range of 2.089–2.14 A Importantly, the bond lengths of the central C@C bond, the C–S bonds near the molecular center and the outer C@C bonds within the TTF molecule are related to the degree of ionicity of the donor molecule. The central C@C bond length of TCN-DBTTF moiety of 1 is ˚ . Thus, it can be inferred that the TCN-DBTTF 1.381(15) A moiety in 1 is essentially fully oxidized according to the correlation between the oxidation states of TTF derivatives and bond lengths of central C@C bonds [25], which is fully in accordance with the stoichiometry of 1. In the crystal lattice of 1 (Fig. 2), the donor molecules are arranged in layers parallel to the bc plane. Along the c-axis, the direction of the molecular planes of two adjacent TCN-DBTTF molecules alternate with a dihedral angle of 77.4. They are separated from each other by tetrahedral [FeBr4] anions; no specific CN Br short contacts can be identified. However, weak intermolecular interactions ˚ ], Br C [3.487–3.533 A ˚ ] and of S Br [3.566–3.741 A ˚ Br Br [3.651–3.871 A] as well as hydrogen bonds C– ˚ ; C–H Br 178] are observed. H Br [C Br 3.742 A Along the b-direction, the donor molecules are parallel and coplanar to each other forming a columnar arrange˚ . In addition, ment with S S distances of 10 A 2+ [Fe(H2O)6] cations are located in-between the donor
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Fig. 1. ORTEP plot (50% probability ellipsoids) of 1.
Fig. 2. Projection of the crystal structure of 1. Hydrogen atoms are omitted for clarity.
˚] molecule assemblies and there are CN O [2.881–3.025 A ˚ ] short contacts. and O Br [3.316 A 3.2. Electrical conductivity The compound 1 shows a low electrical conductivity with a room temperature value r = 4.3 · 107 S cm1. This observation corresponds to its structural feature of the crystal packing which does not show any specifically close stacking arrangements of the donor and acceptor units. 3.3. Magnetic properties The compound 1 is composed of three types of paramagnetic spin carriers: oxidized TCN-DBTTF radicals
(S = 1/2), [Fe(H2O)6]2+ cations (S = 2) and [FeBr4] anions (S = 5/2), the latter ones are located on two crystallographically different sites. The thermal dependence of the magnetic susceptibility of 1 is shown in form of a vMT versus T plot (Fig. 3). The calculated vMT value is 17.9 emu K/mol per stoichiometric formula unit, with g (rad) = 2; g (Fe2+) = 2.2 which is typical for HS Fe2+; g (Fe3+) = 2.06 [16]. This vMT value compares fairly well with the experimental vMT value at room temperature of 16.9 emu K/mol. With decreasing temperature, the vMT values decrease continuously, indicating overall antiferromagnetic interactions among these different paramagnetic centers. Clearly, the whole crystal structure is rather complex so that many different magnetic exchange pathways, extended in all three directions, are present. This situation
C. Jia et al. / Polyhedron 25 (2006) 1613–1617
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the web www: http//www.ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2005.10.037. References
Fig. 3. Temperature dependence of vMT for compound 1.
renders a meaningful analysis with the assignment of specific exchange coupling parameters as impossible. Additionally, there is no signature of a magnetically ordered phase down to 1.8 K. 4. Conclusions A charge transfer salt (TCN-DBTTF)[Fe(H2O)6][FeBr4]3 (1) based on the new donor TCN-DBTTF was synthesized by electrocrystallization. In the crystal lattice, intermolecular CN O interactions are observed rather than otherwise favorable CN Br interactions. As a result of the specific crystal packing, the compound shows a low electrical conductivity in the order of 107 S cm1 and the magnetic measurements exhibit overall antiferromagnetic exchange interactions among the three different spin carriers. Acknowledgments This work was supported by the Swiss National Science Foundation (Grant No. 200020-107589) as well as by the ESF Programme – SONS (NANOSYN). Appendix A. Supplementary data Crystallographic data for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 271519 for 1. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail: [email protected] or on
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