Preparation, X-ray crystal structures and properties of α-(BEDT-TTF)₂[Fe^III(phen)(NCS)₄]·2CH₂Cl₂ and (BEDT-TTF)[Cr^III(isoq)₂(NCS)₄] (phen = 1,10-phenanthroline; isoq = isoquinoline)

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Abstract

This paper reports the preparation, X-ray crystal structure, conducting and magnetic properties of α-(BEDT-TTF)₂[Fe^III(phen)(NCS)₄] (1) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, phen = 1,10-phenanthroline] together with the crystal structure of (BEDT-TTF)[Cr^III(isoq)₂(NCS)₄] (2) (isoq = isoquinoline) for which the physical properties have been reported previously. Compound 1 crystallizes in the triclinic space group P1̄ (no. 2), a = 12.1528(1), b = 16.8269(3), c = 27.0703(4) Å, α = 95.726(1), β = 95.834(1), γ = 108.080(1)°, Z = 4, R = 0.0610 for 9764 reflections with I > 2σ(I); 2 crystallizes in the monoclinic space group P2₁/c (no. 14), a =10.623(5), b = 14.656(8), c = 12.701(7) Å, β = 100.19(2)°, Z = 2, R = 0.0737 for 2747 reflections with I > 2σ(I). The magnetic and transport properties have shown that compound 1 is a paramagnetic semiconductor with σ_{300 K} = 2.2 × 10⁻² Ω⁻¹ cm⁻¹.

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Introduction

Radical cation salts and charge transfer complexes based on tetrathiafulvalene (TTF) and its derivatives constitute a wide class of organic materials with transport properties ranging from insulating to superconducting.¹ The relative arrangement of the molecules in the solid state was found to be a crucial parameter in determining the properties of the target compound.² Investigations of organic/inorganic hybrid molecular materials combining conducting electrons and localized spins are of great interest.^3–10 When combining the two properties one of the aims is to obtain long-range magnetic coupling between isolated localized spins of the inorganic networks that contain transition metals (d-electrons) via the mobile electrons within the organic networks (π-electrons). To satisfy this model, which is based on the synergy between electrical conductivity and magnetic interactions, two conditions are needed, namely good electrical conductivity and strong interactions between the organic and inorganic networks. In order to establish magnetic and/or structural interactions between the organic and inorganic sublattices, several methods are under investigation. Recently, TTF-derived salts containing [Cr(NCS)₄(L)_n]⁻, L = 1,10-phenanthroline (phen) or isoquinoline (isoq), have been reported.¹⁰ These salts were designed with the idea that magnetic interactions would be induced by both S⋯S contacts and π-stacking overlap between the paramagnetic anions and organic radicals. Indeed, a large number of the salts showed long-range ferrimagnetic order with T_c values ranging from 4.2 to 8.9 K. Other work in this area is based on interactions through –N⋯I– contacts¹¹ or covalent bonding between the conducting and the paramagnetic networks.¹² Continuing our efforts in this line, we report here the preparation, X-ray crystal structure, transport and magnetic properties of α-(BEDT-TTF)₂[Fe^III(phen)(NCS)₄] (1) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, phen = 1,10-phenanthroline] as well as the crystal structure of (BEDT-TTF)[Cr^III(isoq)₂(NCS)₄] (2) (isoq = isoquinoline) for which the physical properties have been reported previously for a microcrystalline sample.¹⁰

Experimental

Synthesis

All experiments were conducted under nitrogen or argon. The solvents were distilled before use and the starting reagents were used as received. [Bu₄N][Fe(phen)(NCS)₄] (phen = C₁₂H₈N₂) and [isoqH][Cr(isoq)₂(NCS)₄]·3H₂O (isoq = C₉H₇N) were prepared following the published methods.¹⁰ The stoichiometries of the reported materials were identified by X-ray crystal structure analysis, since the electrochemical technique produces small quantities, unsuitable for elemental analysis.

α-(BEDT-TTF)₂[Fe^III(phen)(NCS)₄] (1). Black, slightly elongated, plate crystals were obtained by galvanostatic oxidation of BEDT-TTF (8 mg, 2.1 × 10⁻² mmol) over one week, using platinum wire electrodes (∅, 1 mm) and a constant current of ca. 1.0 μA. A solution of [Bu₄N][Fe(phen)(NCS)₄] (100 mg, 7 × 10⁻² mmol) in CH₂Cl₂ (20 ml) was used as electrolyte.

(BEDT-TTF)[Cr^III(isoq)₂(NCS)₄] (2). Black parallelopipedic crystals were obtained by an identical method as used above: BEDT-TTF (8 mg, 2.1 × 10⁻² mmol), platinum wire electrode (∅, 1 mm), I = 1.0 μA, [isoqH][Cr(isoq)₂(NCS)₄]·3H₂O (100 mg, 6.8 × 10⁻² mmol) in CH₂Cl₂ (20 ml).

Crystallographic data collection and structure determination

Single crystals of compounds 1 and 2 were mounted on an Enraf-Nonius four circle diffractometer equipped with a CCD camera and using a graphite monochromated MoKα radiation source (λ = 0.71073 Å). Data collection was performed at room temperature. No absorption correction was performed and the structures were solved with SHELXS-97¹³ and refined with the SHELXL-97¹³ program, by the full-matrix, least squares method on F².

Collection parameters and crystallographic data are summarized in Table 1.

Table 1 Crystal data and structure refinement for 1 and 2^a

Parameter	1	2
a Click b110466g.txt for full crystallographic data (CCDC 165901 and 165902). b R 1 = Σ||Fo| − |Fc||/Σ|Fo|. c wR 2 = {Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]}^1/2.
Empirical formula	C37H26Cl2FeN6S20	C32H22CrN6S12
M	1322.59	927.28
Crystal system	Triclinic	Monoclinic
Space group	P1̄ (no. 2)	P21/c (no. 14)
a/Å	12.1528(1)	10.623(5)
b/Å	16.8269(3)	14.656(8)
c/Å	27.0703(4)	12.701(7)
α/°	95.726(1)	90
β/°	95.834(1)	100.19(2)
γ/°	108.080(1)	90
V/Å^3	5184.4(13)	1946.3(18)
Z	4	2
T/K	293(2)	120(2)
μ/mm^−1	1.238	0.972
Radiation, λ/Å	0.71073	0.71073
Reflections collected	36 905	7538
Independent reflections	23 691	4466
R 1 ^b [I > 2σ(I)]	0.0610 [9764]	0.0737 [2747]
wR 2 ^c	0.1297	0.1899

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Magnetic and transport measurements

Magnetic studies were carried out on powdered microcrystalline samples enclosed in a gelatin cap. Magnetic susceptibility measurements were performed with a Quantum Design MPMS-7 SQUID magnetometer, working in dc mode and down to a temperature of 2 K and up to an applied field of 7 T. Diamagnetic corrections were estimated using Pascal's constants. Four- or two-probe dc transport measurments were made using an Oxford Instruments Maglab 2000 equipped with an EP probe, operating from 2 to 350 K. Gold wire electrodes (∅, 0.025 mm) were attached directly to the relevant crystal face using Au paste.

Results and discussion

Crystal structures

Standard ORTEP drawings of the molecular structures, with the atom numbering scheme and 50% thermal ellipsoids, are shown in Figs. 1 and 3a for compounds 1 and 2, respectively.

Fig. 1 ORTEP diagram of compound 1 with 50% probability thermal ellipsoids and the atom numbering scheme. Click image or 1.htm to access a 3D representation [molecules C and D show a degree of disorder (see text)].

α-(BEDT-TTF)₂[Fe^III(phen)(NCS)₄] (1). The asymmetric unit contains two complex anions, four BEDT-TTF molecules, labelled A, B, C and D, and two CH₂Cl₂ solvent molecules, both in general positions. The ORTEP diagram (Fig. 1) shows that the terminal ethylenic carbon atoms of both C and D molecules are disordered onto two sites each, C51a/C51b, C52a/C52b, C71a/C71b and C72a/C72b. The Fe–N (of NCS) distances [mean value 2.035(4) Å] are slightly shorter than those to the phenanthroline ligand [mean value 2.188(4) Å]. The Fe–N–CS angles range from 136.7(5) to 166.7(4)° and deviate slightly from linearity. In common with many BEDT-TTF charge transfer salts, the crystal packing [Fig. 2(a)] consists of alternate layers of organic and inorganic ions. The structure of the organic layer [Fig. 2(b)] is reminiscent of the well known α-type in the BEDT-TTF series.¹ The shortest S⋯S contacts between donors (≤3.60 Å, the sum of S⋯S van der Waals radii) are established between adjacent chains as usually observed in this kind of 2D compound [S9–S39 = 3.472(2) Å; S23–S25 = 3.489(2) Å; S10–S38 = 3.478(2) Å]. The packing in the inorganic layer is similar to that found in (TMTSF)₅[Cr^III(phen)(NCS)₄]₂ (TMTSF = tetramethyltetraselenafulvalene).¹⁰ Anion–anion interactions are present in the form of π-stacking between nearest neighbour phen ligands with interplanar separations of 3.51 Å and an interplanar angle of 2.36°. With this arrangement all the NCS ligands are oriented towards the organic layers [Fig. 2(a)] yielding several S⋯S contacts (≤3.80 Å) between the anionic and organic parts: S2–S40 = 3.660(2) Å; S3–S16 = 3.778(2) Å; S7–S24 = 3.790(2) Å; S5–S40 = 3.770(3) Å.

Fig. 2 (a) Crystal structure of compound 1 viewed along the [010] direction, showing the donor–acceptor layered structure and the π-overlap between the phenanthroline ligands of the anionic sheet. Shortest S⋯S contact values between the anionic and organic parts are shown: S2–S40 3.660(2) Å; S3–S16 3.778(2) Å; S7–S24 3.790(2) Å; S5–S40 3.770(3) Å. (b) α-Type of the organic layer. Shortest S⋯S contact measurements between organic parts are shown: S9–S39 3.472(2) Å; S23–S25 3.489(2) Å; S10–S38 3.478(2) Å; S31–S24 3.603(2) Å; S18–S30 3.612(2) Å; S26–S16 3.527(2) Å; S10–S38 3.478(2) Å; S34–S11 3.508(2) Å; S33–S21 3.507(2) Å.

(BEDT-TTF)[Cr^III(isoq)₂(NCS)₄] (2). The two independent components are lying on an inversion center, (1/2 0 1/2) for the anion and (0 0 0) for the BEDT-TTF moiety. The ORTEP diagram shows that the terminal ethylenic carbon atoms are disordered onto two sites, each C15a/C15b and C16a/C16b. The Cr–N (of NCS) distances [mean value 1.983(5) Å] are slightly shorter than those of the isoquinoline ligand [Cr–N3 = 2.079(5) Å]. From the 1∶1 stoichiometry, the charge on the BEDT-TTF molecule is assigned as +1. The crystal structure [Fig. 3(b)] can be viewed as alternate organic and inorganic layers in the a direction. The organic layer [Fig. 4(a)] consists of mixed columns parallel to the c direction of BEDT-TTF radical cations and isoquinoline ligands, the angle between BEDT-TTF and isoquinoline planes being 3.67°. The shortest interplanar separations between adjacent BEDT-TTF–isoquinoline and isoquinoline–isoquinoline units are 3.411(8) and 3.527(8) Å, respectively. In the projection of the structure in the ac plane [Fig. 4(b)], each BEDT-TTF molecule overlaps with another BEDT-TTF molecule on one side and it overlaps with the isoquinoline ligand on the other side, the shortest intermolecular S⋯S contact between adjacent BEDT-TTF units being S6–S6 = 3.755(4) Å. In a similar manner, each isoquinoline ligand overlaps on one face with an isoquinoline ligand of a neighbouring anion and with a BEDT-TTF unit on its other face. Short contacts are also present between BEDT-TTF molecules and the anions: S2⋯S3 = 3.496(3) Å, and S2⋯S6 = 3.482(3) Å.

Fig. 3 Compound 2 (a) ORTEP diagram with 50% probability thermal ellipsoids and the atom numbering scheme. (b) Crystal structure viewed along the [001] direction, showing the donor–acceptor layered structure and the π-stacking overlap between the isoquinoline ligands and the BEDT-TTF molecules. Click image or 3b.htm to access a 3D representation.

Fig. 4 Compound 2 (a) projection of the organic layer in the bc plane showing the π-stacking between the isoquinoline ligands and the BEDT-TTF molecule. (b) Projection of the structure in the ac plane showing the shortest S⋯S contacts: S6–S6 = 3.755(4) Å; S2–S3 = 3.496(3); S2–S6 = 3.482(3) Å.

Magnetic and transport properties

The temperature dependences of the magnetic susceptibility for the title compounds were measured in the temperature range 2–300 K, with an applied field of 1000 G. The ac and dc magnetic susceptibility values of compound 2 were identical to that previously reported for a microcrystaline sample, i.e. it is a bulk ferrimagnet with a T_c of 4.2 K.¹⁰ Compound 1 is a simple paramagnet which obeys the Curie–Weiss law with a very small Weiss constant of −0.04 K and a Curie constant of 4.362 emu K mol⁻¹ (a spin only value of 4.377 is expected for high spin Fe³⁺). Thus the magnetization is dominated by the coordination metal centre.

The transport properties of 1 indicate a highly anisotropic semiconductor. The conductivity versus temperature data (Fig. 5) were measured in three directions with respect to the elongated plates. Perpendicular to the plate, a two probe measurement gave a conductivity at 302 K (σ_{302 K}) of 1.03 × 10⁻⁵ Ω⁻¹ cm⁻¹. However, in the plane of the plate, parallel to the longer dimension, the conductivity is three orders of magnitude higher; the high temperature value is σ_{280 K} = 3.2 × 10⁻² Ω⁻¹ cm⁻¹, although the activation energy changes slightly from 0.28 eV at high temperature to 0.17 eV at low temperatures. Parallel to the plane of the short dimension of the plate the conductivity is also relatively high, σ_{300 K} = 2.2 × 10⁻² Ω⁻¹ cm⁻¹. This indicates the two-dimensional nature of 1, as evident from the crystal structure.

Fig. 5 Curve of resistivity versus temperature for compound 1 measured in the most conductive direction parallel to the crystal plate.

Conclusion

A new charge transfer salt α-(BEDT-TTF)₂[Fe^III(phen)(NCS)₄] (1) has been obtained and structurally and magnetically characterized. Compound 1 is a paramagnetic semiconductor in which the magnetic characteristics are dominated by the high spin Fe^III centre. The absence of long-range magnetic order, as evidenced by compound 2, is most likely due to the absence of π-stacking interactions between the anion and cation components. The transport properties of 1 are highly anisotropic and show two-dimensional characteristics. Additionally, we grew single crystals of the salt (BEDT-TTF)[Cr^III(isoq)₂(NCS)₄] (2) and determined its crystal structure. The magnetic properties of 2 have already been discussed elsewhere¹⁰ and were identical to those of the sample reported here.

Acknowledgements

Financial support from the CNRS, the UK Science and Engineering Research Council and the TMR Program from the European Union (contract ERBFMRXCT98-0181). S. F. thanks the Algerian and French Ministries of Education for a PhD fellowship.

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