Elsevier

Solid State Communications

Volume 140, Issue 5, November 2006, Pages 256-260
Solid State Communications

A novel BEDT-TTF-based purely organic magnetic conductor, α - (BEDT - TTF)2(TEMPO - N(CH3)COCH2SO3) ⋅ 3H2O

https://doi.org/10.1016/j.ssc.2006.07.034Get rights and content

Abstract

A new BEDT-TTF-based salt with an organic free radical TEMPO has been prepared. The salt consists of alternate layers of conducting and magnetic sheets, between which are short S(BEDT-TTF) …O(TEMPO) contacts. The magnetic susceptibility is well modelled by the combination of a 2D Heisenberg and a Curie–Weiss model with J=89K and θ=+0.05K.

Introduction

Over the past decade, molecule-based magnetic conductors have attracted great interest due to the discovery of their unique physical properties. For example, paramagnetic superconductivity [1], [2], [3], a paramagnetic metallic state [3], metallic antiferromagnetism [3], [4], metallic ferromagnetism [3], [5], a field induced metallic/superconducting state [6], etc. have been reported. Their interesting properties emerge from interactions between magnetic centres incorporated into conducting lattices. In many cases, the magnetic source is a transition metal compound. However, magnetic interactions in these salts are usually very small. This is because the transition metals are usually surrounded by ligands such as Cl, Br, C2O42−, CN, etc., through which the metals must interact indirectly with one other.

An alternative source of localized spins are stable organic free radicals. The spin is localized on one or a few atoms, which are not completely surrounded by other atoms. In other words, the spin is located on a ‘bare’ orbital, which can potentially overlap directly with another orbital which contains a localized spin or conduction electron. Therefore many researchers have introduced organic radicals in organic conducting salts. For example, charge-transfer (CT) salts of free radical substituted organochalcogen donors [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], CT complexes of free radicals and acceptors [17], [18], CT salts of free radical substituted cations and acceptors [19], [20], [21], [22], [23], have been reported.

For more than five years, we have focused on preparing new organic magnetic anions that include the organic free radical tetramethylpiperidine-1-oxyl (TEMPO) with sulfonate (−SO3) for use as a counter-ion in (donor)n(anion)-type organic conductors [24], [25], [26], [27], [28], [29], [30], [31]. Recently, we prepared a sulfo derivative of TEMPO, TEMPO-NHCOCH2SO3 (1). 1 forms a CT salt of the donor, bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with stoichiometry α-(BEDT-TTF)3(TEMPO-NHCOCH2SO3)2⋅6H2O [31]. This salt is a semiconductor and has a TEMPO …TEMPO interaction to form a 1D ferromagnetic chain. Its temperature-dependent magnetic susceptibility is well described by a 1D ferromagnetic Heisenberg model with J=+0.42K.

In this paper, we report the structures and physical properties of the N-methyl derivative of 1, TEMPO-N(CH3)COCH2SO3 (2) prepared as a tetraphenylphosphonium (PPh4) salt (3) and its electrochemically synthesized salt with BEDT-TTF, α-(BEDT-TTF)2(2)⋅3H2O(4). We also report the temperature dependence of electrical resistivity of 4 under high pressure.

Section snippets

Experimental

4-methylamino-TEMPO was prepared according to the literature method [32] (yield 66%). The acidic TEMPO-N(CH3)COCH2SO3H was prepared by reacting 4-methylamino-TEMPO (1.5 g, 7.8 mmol) with sulfoacetic acid (1.3 g, 9.4 mmol) in the presence of N,N-dicyclohexylcarbodiimide (DCC, 3.9 g, 19 mmol) and 4-dimethylaminopyridine (DMAP, 2.3 g, 19 mmol) in 30 mL of CH2Cl2 at room temperature with stirring overnight. Metathesis of the acid 1 with PPh4⋅Br gave 3 as orange crystals, which were then

Results and discussion

The crystal data of the PPh4 salt (3) are as follows: 299 K, triclinic P1̄, a=12.074(2), b=13.910(3), c=10.616(5)Å, α=98.05(3), β=98.49(3), γ=100.05(2), V=1711(1)Å3, R=0.065, Rw=0.068. One PPh4 cation and one anion of 2 are crystallographically independent. In the crystal, there is a TEMPO spin dimer about the center of symmetry with a short O…O contact of 4.775(5) Å. Actually, the temperature-dependent magnetic susceptibility is well explained by a Singlet–Triplet model [35] with C=0.378emu K/

Acknowledgement

This work was partially supported by a Grant-in-Aid for Scientific Research (No. 14740330) from Japan Society for the Promotion of Science.

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