# A Construction Principle of the K-Phase Based on the Efficient (O-H)dono\*r - Oanion Structural Functionality: The Examdes of K-(EDT-TTF(CH2 OH)),X (x =CIO4 and ReO4

Philippe Blanchard, K. Boubekeur, M. Sallé, G. Duguay, M. Jubault, Alain

Gorgues, James D. Martin, Enric Canadell, Pascale Auban-Senzier, Denis

Jerome, et al.

► To cite this version:

Philippe Blanchard, K. Boubekeur, M. Sallé, G. Duguay, M. Jubault, et al. A Construction Principle of the K-Phase Based on the Efficient (O-H)dono\*r - Oanion Structural Functionality: The Examdes of K-(EDT-TTF(CH2 OH)),X (x =CIO4 and ReO4. Advanced Materials, 1992. hal-04983412

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temperature factors were used. Owing to the nearly isotropic shape of the crystal (ca. 0.3 × 0.3 × 0.2 mm) and an absorption coefficient $\mu$ = 24.6 cm⁻¹, no absorption correction was applied. Using weights $w$ = 1/σ²(*F*) the final residuals were $R$ = 0.0380 and $R_s$ = 0.0276 and the goodness of fit $S$ = 2.14. Additional material on the crystal structure investigation is available from the Cambridge Crystallographic Data Centre, University, Chemical Laboratory, Lensfield Rd, Cambridge CB2 IEW, UK, on citation of the names of the authors and the journal reference.

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A Construction Principle of the κ-Phase Based on the Efficient (O-H)donor···Oanion Structural Functionality: The Examples of κ-(EDT-TTF(CH₂OH))₂X (X = ClO₄⁻ and ReO₄⁻)**

By Philippe Blanchard, Kamal Boubekeur, Marc Sallé, Guy Duguay, Michel Jubault,* Alain Gorgues, James D. Martin, Enric Canadell, Pascale Auban-Senzier, Denis Jérôme, and Patrick Batail*

Among the variety of structural types of conducting and superconducting organic cation radical salts[1] revealed over the past ten years or so, the κ-phase is perhaps the most

remarkable and the least understood. Its central structural feature is that of a chess-board arrangement of weakly coupled orthogonal dimers which in principle seems not to be well suited for extensive electronic delocalization. Yet, the few examples known are all metallic at ambient pressure[2] or at the small hydrostatic pressure of 340 bar in the case of κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl.[3] In addition, the production of a κ-phase quickly became a passport to the organic superconductivity kingdom since all but one such phases prepared so far indeed exhibit a transition, one of which being the highest $T_c$ for an organic based superconductor yet recorded.[2]

The search for novel κ-phases suffers, however, from the lack of knowledge as to which are the characteristics required for the donor molecule if this structure is to be generated. The current observations indicate that this structure is found (although it must still be considered as rare), for the centrosymmetrical molecule BEDT-TTF and also for the non-centrosymmetrical donors (methylenedithio)tetrathiafulvalene[4] (MDT-TFF or simply MD) and (S,S)-dimethyl-bis(ethylenedithio)tetrathiafulvalene[5] (S,S-DMBEDT-TTF).

In this communication we report the synthesis and the structural and electronic characterization of the title compounds **1** and **2**, two new examples of a κ-phase, purposely designed to associate the hydroxymethyl-functionalized non-centrosymmetrical donor, ethylenedithiohydroxymethyl-tetrathiafulvalene [EDT-TTF(CH₂OH)] with the discrete tetrahedral perchlorate and perrhenate anions.

<figure><img src="image_2.png" /><figcaption>Scheme 1. EDT-TTF(CH₂OH).</figcaption></figure>

Central to the design of organized materials is the possibility of monitoring the orientation of molecular units through directional, non-covalent intermolecular interactions such as hydrogen bonds.[6] In particular, C-H···O interactions have been shown recently to be a dominating intermolecular interaction in the structure of the salts derived from the bis(ethylenedioxa)tetrathiafulvalene,[7] serving as a relay of the orientation effect inherent to the tetrahedral geometry of the anion and resulting in the imposition of the acentric structure of (BEDT-TTF)₈SiW₁₂O₄₀.[8] On the basis of these concepts and observations, the synthesis of the title compounds was designed to explore the potential of strong O-H···O hydrogen bonding between the planar cation radical and a tetrahedral oxide anion. It was expected that the orientation effect of the tetrahedral anions will ultimately result in a non-coplanar arrangement of the organic donor molecular planes via the strong directionality of the hydrogen bond.

<footnote>[*] Prof. M. Jubault, Dr. Marc Sallé, Prof. Alain Gorgues
Laboratoire de Synthèse Organique et d'Electrochimie
Université d'Angers, F-49045 Angers (France)

Dr. P. Batail, Dr. K. Boubekeur, Dr. P. Auban, Dr. D. Jérôme
Laboratoire de Physique des Solides Associé au CNRS
Université de Paris-Sud, F-91405 Orsay (France)

P. Blanchard, Prof. Dr. G. Duguay
Laboratoire de Synthèse Organique Associé au CNRS
Université de Nantes, F-44072 Nantes (France)

Dr. J. D. Martin, Dr. E. Canadell
Laboratoire de Chimie Théorique Associé au CNRS
Université de Paris-Sud, F-91405 Orsay (France)</footnote>

<footnote>[**] This work was supported by the CNRS and the ESPRIT Basic Research Action MOLCOM (n° 3121). P. Bl. and M. S. thank the Région Pays de Loire and the Ministère de la Recherche Scientifique (France), respectively, for Thesis Fellowships. J. D. M. thanks the National Science Foundation Office of International Programs (USA) for a postdoctoral fellowship.</footnote>
The donor molecule was synthesized as recently reported by us[9] and plate-like single crystals of the $ClO_4^-$ and $ReO_4^{2-}$ salts **1** and **2**, respectively, were grown electrochemically. The two salts are isostructural[10] and their structure (Fig. 1a) is that of the superconducting $\kappa$-phase salts.[11] In particular, the centrosymmetrically related donor molecules of a dimer have a bond-over-ring arrangement and an intradimer spacing of 3.35 Å. Note that **1** and **2** are, with $\kappa$-((S,S)-DMBEDT-TTF)$_2$ClO$_4$ reported very recently by Zambounis, Hilti et al.,[5] the only examples of a $\kappa$-phase with discrete, nonlinear anions. The anions experience high thermal motion although only one equilibrium position is detected. Likewise, the oxygen atom of the hydroxy group is affected by a significant thermal motion and its average position is pointing towards one oxygen atom of the anion (Fig. 1b) with an O···O separation of 3.33 Å. Note that the hydrogen

<figure><img src="image_3.png" /><figcaption>Fig. 1. a) Typical $\kappa$-phase network for a single slab of EDT-TTF(CH$_2$OH) molecules in **1** and **2**. The unit-cell boundaries are indicated. b) Fragment of two donor slabs, separated by the tetrahedral anions and exemplifying the relative orientations of adjacent EDT-TTF(CH$_2$OH) molecules and the anions. Dotted lines indicate the O(H)···O hydrogen bonds.</figcaption></figure>

atom could not be located which precludes any further meaningful interpretation of the strength of this hydrogen bond, solely[12] on the basis of the O···O distance. The relative orientation (Fig. 1b) of the hydroxy group and one oxygen atom of the anion leaves no doubt however of the occurrence of hydrogen bonding in the materials. As exemplified in Figure 1b, adjacent orthogonal donor molecules in one organic layer are associated via this hydrogen bond with anions belonging to different layers. Note the relative quasi-orthogonal orientation of the latter as imposed by glide plane symmetry and hence, the subsequent orthogonal arrangement of the donor planes in one layer as anticipated.

Clearly, the production of the $\kappa$-phase geometry is not uniquely determined in the present salts by the orientation effect of a single tetrahedral oxide anion. Nevertheless, it appears that by forcing adjacent donor molecules to adopt this mutual, essentially orthogonal arrangement, the construction of the typical $\kappa$-phase layer follows in a rather logical and rewarding way. More examples are however needed to test this principle.

It should be noted that although the crystal structure determines the electronic band structure, the latter is very sensitive to small differences in orientation of the donor molecules because of the strong directionality of the $\pi$-type orbitals. Therefore, besides the comparative analysis of the crystal structures of a series of materials, the detailed analysis of their band structures provides a stronger test for their similarity. The dispersion relations[13] for the four bands built from the HOMO of EDT-TTF(CH$_2$OH) are shown in Figure 2. For each dimer of the $\kappa$-phase there is one bonding

<figure><img src="image_4.png" /><figcaption>Fig. 2. Dispersion relations for the highest four occupied bands of **1** where $\Gamma$, $X$, $Z$ and $M$ refer to (0, 0), ($a^*/2$, 0), (0, $c^*/2$) and ($a^*/2$, $c^*/2$), respectively. The dashed line indicates the Fermi level for a metallic filling of these bands.</figcaption></figure>

($\Psi_+$) and one antibonding ($\Psi_-$) combination of the HOMOs. The pair of the upper/lower bands of Figure 2 are built from the $\Psi_-$/$\Psi_+$ orbitals of the two dimers, respectively. This band structure is practically identical to that of the metallic $\kappa$-(MD)$_2$AuI$_2$ salt[11] which becomes superconducting at 4.5 K.[2, 4] The calculated Fermi surfaces for $\kappa$-(EDT-TTF(CH$_2$OH))$_2$ClO$_4$ and ReO$_4$, assuming a metallic filling of the bands of Figure 2, and $\kappa$-(MD)$_2$AuI$_2$ are also virtually indistinguishable. In consequence, we conclude that the new salts **1** and **2** fulfill the crystal and electronic requirements for metallic and possibly superconducting properties of the $\kappa$-phase.

In fact, and as seen in Figure 3, both salts exhibit activated conductivity at ambient pressure with room-temperature conductivities of 3.0 and 0.27 S/cm for **1** and **2**, respectively. Furthermore, the activated behavior is not suppressed in **1** by applying moderate gas pressures, a remarkable unprecedented observation in this class of materials.

The semiconducting behavior of **1** and **2**, unaffected by the application of hydrostatic pressure, contradicts the predic-
<figure><img src="image_5.png" /><figcaption>Fig. 3. Four-probe single-crystal temperature dependence of the conductivity for 1 and 2 at several different pressures.</figcaption></figure>

tions based on the analysis of their crystal and electronic band structures and suggests that the κ-phases lie at the borderline of the validity range of the localized and delocalized descriptions of their electronic structure. In consequence, once such κ-phases are being successfully synthesized, their metallic vs. insulating properties should be very sensitive to pressure,[14] temperature, disorder or the occurrence of weak random potentials. A likely cause for the pressure-resistant electronic localization in the present salts could be the presence of disorder associated with the oxygen and hydrogen atoms of the O-H···O interactions.

## Experimental

8 mg of EDT-TTF(CH₂OH) were oxidized at a constant temperature of 14°C (or 2°C) and under constant low current (1 µA) at a platinum wire anode in 30 ml of a 1,1,2-trichloroethane solution containing 10% of ethanol under an inert atmosphere in the presence of 0.3·10⁻² mole of Bu₄NCIO₄ (ReO₄) as supporting electrolyte. Black, shiny distorted hexagonal plate-like single-crystals were harvested after 9 (or 6) days.

Received: May 20, 1992
Final version: June 22, 1992

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[3] Y. V. Sushko, V. A. Bondarenko, R. A. Petrosov, N. D. Kushch, E. B. Yagubskii, *Physica C* **1992**, *185-189*, 2683. J. M. Williams, A. M. Kini, H. H. Wang, K. D. Carlson, U. Geiser, L. K. Montgomery, G. J. Pyrka, D. M. Watkins, J. M. Kommers, S. J. Boryschuk, A. V. Strieby Crouch, W. K. Kwok, J. E. Schirber, D. L. Overmyer, D. Jung, M.-H. Whangbo, *Inorg. Chem.* **1990**, *29*, 3272.

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[10] X-ray structure analysis of 1 and 2 (data and results for the latter compound are given in parenthesis): C₁₈H₁₆O₆S₁₂Cl, 1 (C₁₈H₁₆O₆S₁₂Re, 2), M* = 748.55 (899.29), orthorhombic space group *Pnma*, *a* = 11.325(5) (11.319(4)), *b* = 30.758(7) (31.220(5)), *c* = 8.211(4) (8.247(4)) Å, *V* = 2860.2 (2914.5) Å³, *Z* = 4, *d*<sub>calc.</sub> = 1.74 (2.05) gcm⁻³, µ(Mo₆) = 10.09 (50.87) cm⁻¹; a black shiny plate-shaped crystal of approximate dimensions 0.36 × 0.24 × 0.03 mm³ (ca. 0.27 × 0.21 × 0.12 mm³) mounted on a glass fiber, was used for data collection on an Enraf-Nonius CAD4-F diffractometer at room temperature with monochromatized Mo₆ radia- tion (3226 (3631) reflections up to *θ*<sub>max</sub> = 26 (27)°, collected with ω/2θ scan technique). The structures were solved by direct methods (MULTAN 11/82). H-atoms were visible on Fourier-difference maps (except that of the terminal hydroxy group in both cases probably because of the high thermal motion of the oxygen atoms of the hydroxy group itself and of the anions) but simply introduced at calculated positions. Full matrix least-squares refinement converged at final *R* and *R*<sub>w</sub> values: 0.069 (0.074) and 0.107 (0.087) for 1352 (1143) reflections with *I* ≥ 3σ(*I*) (G.O.F = 2.052 (2.654)); highest peak on DF: −0.78, +1.31 (−1.78, +2.21) e.Å⁻³. Further details of the crystal structure are available on request from the Cambridge Crystallographic Data Centre, University Chemical Laboratory, Lensfield Rd, Cambridge CB2 IEW, UK, on quoting the names of authors and the journal citation.

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[12] T. Steiner, S. A. Mason, W. Saenger, *J. Am. Chem. Soc.* **1991**, *113*, 5676.

[13] Our calculations use an Extended Hückel-type hamiltonian with double-ζ type orbitals: M.-H. Whangbo, J. M. Williams, P. C. W. Leung, M. A. Beno, T. J. Emge, H. H. Wang, K. D. Carlson, G. W. Crabtree, *J. Am. Chem. Soc.* **1985**, *107*, 5815.

[14] Y. V. Sushko, V. A. Bondarenko, R. A. Petrosov, N. D. Kushch, E. B. Yagubskii, *J. Phys. I France* **1991**, 1375. J. E. Schirber, D. L. Overmyer, K. D. Carlson, J. M. Williams, A. M. Kini, H. H. Wang, H. A. Charlier, B. J. Love, D. M. Watkins, G. A. Yaconi, *Phys. Rev. B* **1991**, *44*, 4667.