Multi-component molecular conductors with supramolecular assembly based on ET radical cation salts with (NO3)− anion

Synthetic Metals 151 (2005) 156–164

Multi-component molecular conductors with supramolecular assembly based on ET radical cation salts with (NO3)− anion N.D. Kushch a,b,∗ , A.V. Kazakova a , A.N. Chekhlov a , L.I. Buravov a a

Institute of Problems of Chemical Physics RAS, Chernogolovka, MD 142432, Russia Walther-Meissner-Institute, Bayer Academy Science, D-85748 Garching, Germany

b

Received 28 February 2005; received in revised form 1 April 2005; accepted 4 April 2005 Available online 23 May 2005

Abstract Synthesis, structure and conducting properties of two new multi-component radical cation ET salts with the (NO3 )− counterion: (ET)2 (NO3 )·C2 H4 (OH)2 (I) and (ET)2 (NO3 )·0.5[C2 H4 (OH)2 ]·H2 O (II) are described. Both salts have layered structures with the distinguishing feature of hydrogen bonds being present between the radical cation layers and anion sheets. It was found that introduction of glycol molecules to compositions of I and II significantly affects the structure of their cation and anion layers and, as a result, their conducting properties. I is a semiconductor, while II demonstrates metallic conductivity down to 4.2 K. © 2005 Elsevier B.V. All rights reserved. Keywords: ET radical cation salts; Electrocrystallization; Structure; Conductivity

1. Introduction Radical cation ET salts with a simple inorganic (NO3 )− counterion were first prepared almost 20 years ago [1]. However, the composition and structure of only one of the three salts synthesized has been established by the authors as ␣-(ET)3 (NO3 )2 [1]. Recently good quality crystals of this salt have been prepared allowing its structure refinement [2]. In particular, it was found [2] that the ET stacks in the salt were built up of cation “triads”, in which two ET molecules have the formal charge (+0.5), and one ET molecule is (+1) charged. Thus, every “triad” of the cations has a charge of (+2). This charge distribution on the ET molecules is in good agreement with the conductive properties of the ␣-(ET)3 (NO3 )2 crystals in contrast to those in [1]. The analysis of the structure of radical cation layers in the ␣-(ET)3 (NO3 )2 crystals shows the ␤ -type ET arrangement and, hence, the above mentioned salts should be classified as ␤ -(ET)3 (NO3 )2 . It should also be noted that two other salts called by the authors as ␤-(ET)3 (NO3 )2 ∗

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and ␥-(ET)3 (NO3 )2 [1] were prepared simultaneously with the ␤ -(ET)3 (NO3 )2 crystals. As in case of the ␣-phase, the “␤”- and “␥”-symbols were used only to identify the salts with different crystallographic parameters. Low quality of the crystals of these salts did not allow their structures to be solved. Comparison of the crystallographic parameters shows that the ␥-(ET)3 (NO3 )2 crystals described in [1] are identical with those of the salt (ET)2 (NO3 )0.9 (NO2 )0.1 . This implies that in reality the ␥-salt has the composition (ET)2 (NO3 ) and ␦-type structure of the radical cation layers found for (ET)2 (NO3 )0.9 (NO2 )0.1 [3]. Recently, two additional types of crystals containing only (NO3 )− anions and some glycol in their structure instead of the expected rare-earth metal complex anions have been obtained when we investigated electrooxidation of ET in the presence of rare–earth metal complexes K3 M[(SCM)4 (NO3 )2 ] (M = Dy, Y, Ho, Gd) in chlorobenzene with small admixtures of glycol [4]. One kind of crystal showed a semiconductor type temperature dependence of conductivity and the other one exhibited a metallic behavior of resistance down to 4.2 K. However, the crystals of both types were of low quality, thus it was not possible to determine their structure and the exact composition. In this work we studied electrooxidation of

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ET in chlorobenzene–glycol medium using (NO3 )− anion, as counterion, and glycol, as modifying component. The inclusion of a neutral glycol molecule to the ET radical cation salts with the (NO3 )− anion is of interest in respect to the modification of anion layers through the formation of hydrogen bonds between the oxygen atoms of the (NO3 )− group and the hydrogen atoms of glycol. One would expect that such a modification of anion layer should lead to a novel donor arrangement. In this work we report novel multi-component molecular conductors based on ET salts, their crystal structures and conducting properties.

2. Experimental The crystals of I and II were simultaneously prepared by electrocrystallization of ET (C3 = 10−3 mol/l) in chlorobenzene containing a small amount of glycol (5 vol.%) on a Pt anode under galvanostatic conditions. The complex of KNO3 and 18-crown-6 ether was used as an electrolyte. Constant current of 0.3–0.5 ␮A and temperature of 25 ◦ C were optimal for the crystal growth. The crystals grew as rectangular black strips and irregular hexagon plates for 2–5 weeks depending on the current applied. The compositions of the crystals I–II were determined by the X-ray analysis. The X-ray analysis of rectangularshape crystals showed that they differ in their crystallographic ˚ data. Crystals with crystallographic data: a = 7.89(1) A, ˚ c = 30.60(3) A, ˚ β = 97.3(1)◦ , V = 1593(2) A ˚3 b = 6.654(4) A, and Pn space group (the salt III) had poor quality for complete X-ray analysis. Other rectangular crystals were characterized as salt I. The irregular hexagon-like crystals were determined as salt II. The X-ray analysis was carried out on an Enraf–Nonius CAD-4 diffractometer with graphite-monochromated radiation using the ω/2θ scanning technique. The crystallographic data and structure determination for the salts are summarized in Table 1. The crystal structures were solved by direct methods and subsequent Fourier synthesis using SHELXS86 [5] and SHELXL-93 [6] software package. The structures were refined by full-matrix least-squares procedures by the SHELXL-93 program [6]. The hydrogen atoms (excluding those belonging to hydroxyl groups of glycol molecule in salt I and water molecule in salt II) were placed at geometrically calculated positions and a riding model was used for their refinement. Co-ordinates of hydrogen atoms of hydroxyl groups of glycol molecule in salt I were refined by least-squares procedures by the SHELXL-93 program [6]. The hydrogen atoms of the H2 O molecule were not localized. Full crystallographic details were deposited at the Cambridge Crystallographic Data Centre (CCDC 270633 and 270634, respectively for I and II). The electrical resistance of the crystals in the conducting radical cation planes (parallel to the layers) was measured by the standard dc-four-probe method from a room temperature

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Table 1 Crystallographic data of the salts (ET)2 (NO3 )·C2 H4 (OH)2 (I) and (ET)2 (NO3 )·0.5C2 H4 (OH)2 ·H2 O (II) Compounds Chemical formula Chemical formula weight Crystal system Space group Temperature (K) ˚ a (A) ˚ b (A) ˚ c (A) α (◦ ) β (◦ ) γ (◦ ) ˚ 3) V (A Z Dcalc. (g cm−3 ) Radiation type µ (mm−1 ) Diffractometer Data collection method Total no. of reflections No. of unique reflections No. of observed reflections No. of refined parameters θ max (◦ ) Range of h, k, l

R, wR

I C22 H22 NO5 S16 893.37 Monoclinic C2/c 295 30.392(6) 13.418(3) 8.271(3) 90.00 91.83 90.00 3371.2(16) 4 1.760 Mo K␣ 1.063 Enraf–Nonius, CAD-4 ω/2θ 3185 2956 2061 221 24.96 −36 → h → 36, 0 → k → 15, 0→l→9 0.0363, 0.0895

II C21 H21 NO5 S16 880.35 Triclinic P-1 295 7.724(3) 12.997(5) 17.924(8) 69.53(4) 79.87(3) 72.98(3) 1606.5(11) 4 1.820 Mo K␣ 1.114 Enraf–Nonius, CAD-4 ω/2θ 4403 4182 2228 421 22.48 −8 → h → 7, −13 → k → 0, −19 → l → 18 0.0576, 0.1262

down to 4.2 K. Resistance in the direction perpendicular to the conducting layers was studied as described in [7].

3. Results and discussion The crystals of salts I–II have layered structures. They are characterized by the alternation of conducting radical cation layers and insulating anion sheets. The arrangement of the radical cation layers and the anion sheets in both complexes are different. Fig. 1 shows the crystal structure of salt I. The conducting layers of the ET radical cations alternate along the a axis with the anion sheets consisting of the (NO3 )− anions and glycol molecules. One crystallographically independent ET molecule (atom labels are shown in Fig. 2a) is in a general position and the (NO)3 − anion and glycol molecule, C2 H4 (OH)2 , are located on a two-fold axis. Conducting layers are formed by equidistant stacks in which the ET molecules are parallel (according to symmetry condi˚ from each other. tions) and arranged at a distance of 4.07 A The neighbouring radical cations in the stacks overlap with a longitudinal shift of approximately 3/2 of the central C C bond length and they are tilted with respect to each other with rotation angle of 27.2◦ (Fig. 3). The ET radical cation is not fully planar. The maximal deviation of carbon atoms from the ˚ One terminal ethylene average plane of a molecule is 0.47 A. ˚ group is disordered. The central C C bond length is 1.360 A.

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Fig. 1. Crystal structure of the radical cation salt (ET)2 (NO3 )·C2 H4 (OH)2 .

This value corresponds to (+0.5) cation charge [8]. There are no short contacts between heteroatoms from adjacent radical cations within the stacks. However, each radical cation is bonded by eight short side-by-side S· · ·S contacts with the ET cations from the neighbouring stacks (Fig. 4). The short ˚ (Table 2). It was noticed, contacts are equal to 3.411–3.591 A that only six of the eight sulfur atoms of the ET molecule are involved in the formation of the short contacts. The same construction of the radical cation layer was found in the ET(SCN)0.77 crystals [9]. Like salt I, in ET(SCN)0.77 the radical cations in the stacks are parallel and overlap each other according to the same mode. The rotation angle between the long axis of two neighbouring radical cations within the stack is equal to 26.4◦ . The interaction of the ET radical cations from the adjacent stacks is realized through short side-by-side S· · ·S contacts. The cation and anion layers in I are bonded by various weak hydrogen bonds formed between terminal ethylene groups of the ET molecules and oxygen atoms of the (NO3 )− anion or of the glycol molecule with C H· · ·O distances of ˚ respectively. 2.9–3.5 and 3.5 A, In I the anion sheets are built up of polymer chains consisting of periodic [ O (CH2 ) OH O(NO2 ) H ]n units (Fig. 5). As it is seen from Fig. 5, the polymeric chains are constructed of an oxygen atom, O(1), of the NO3 group and hydrogen atoms belonging to hydroxyl groups of two different glycol molecules owing to hydrogen bonds formation. ˚ It is a short The length of hydrogen bonds OH· · ·O is 1.93 A. length and it is evidence of the strong interaction between the anion and glycol molecules. This value is notably less

than the distance between the hydrogen and oxygen atoms involved in the formation of hydrogen bonds between the ET radical cations and the anion sheets. The (NO3 )− anion is practically flat. The N O bond ˚ and O N O valence angles (118.7, lengths (1.221, 1.243 A) 122.7◦ ) are close to the ordinary values. The presence of glycol molecules in the anion sheets of the complexes ET2 Br·C2 H4 (OH)2 [10], ET2 Cl·[C2 H4 (OH)2 ]0.5 [11], ET2 Br·C3 H6 (OH)2 [11], and BETS2 Br·C2 H4 (OH)2 [12] promotes the hydrogen bonds formation also. The hydrogen bonds in these salts are formed by the halogen atoms and hydroxyl groups from the glycol molecules. Thus, the structure of I is specified by intermolecular cation–cation, cation–anion, and anion–anion contacts, which result from the formation of a three-dimensional (3D) network of intermolecular interactions. Recently a 3D network of intermolecular interactions has been found in ET(SCN)0.77 [9]. In contrast to I, in ET(SCN)0.77 a network is formed by short contacts between the SCN groups in anion sheets, the S· · ·S short contacts inside of conducting layers and the S· · ·S contacts between the ET radical cations and the SCN anions. It is known that the formation of a 3D network of intermolecular interactions in organic conductors is observed rather rarely. The projection of the structure of salt II is shown in Fig. 6. It is characterized by the alternation of conducting ET layers and insulating anion sheets including the (NO3 )− anions as well as the C2 H4 (OH)2 and H2 O molecules along the c axis. The radical cation layers contain two crystallographically independent ET molecules (cations A and B, atom labels are

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159

Fig. 2. Atom labeling in the radical cations A and B and (NO3 )− anion.

Fig. 3. Overlap modes of the ET radical cations in stacks of the radical cation salt (ET)2 (NO3 )·C2 H4 (OH)2 .

noted in Fig. 2b and c, respectively). Both ET molecules, the NO3 group and the water molecule are located in general positions. The glycol molecule is in special position on the inversion center. In the conducting layers the A and B radical cations form stacks approximately along the [2 1 0] direction and ribbons along the b axis (Fig. 7). The stacks consist of radical cations A and B alternating in the [–A–A–B–B–] sequence. In the ribbons there is another sequence of the A and B radical cations: [–A–B–A–B–]. The distances between the adjacent radical cations in the stacks are 3.713, 4.176, and ˚ for the A–A, A–B, and B–B pairs, respectively. Thus, 3.781 A one could speak about dimerization of the radical cations of the same type in the stacks. The A and B radical cations are almost parallel since the angle between their average planes

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Fig. 4. Side-by-side short S· · ·S contacts between the ET radical cations in the conducting layers of the radical cation salt (ET)2 (NO3 )·C2 H4 (OH)2 .

is 2.0◦ . The central bond lengths in the radical cations of both ˚ (A) and 1.371 A ˚ (B). In contypes are close and equal 1.376 A trast to the A radical cation, whose terminal ethylene groups are disordered, in B they are ordered and have “eclipsed” conformation. The radical cations overlapping modes in the stacks are shown in Fig. 8. The radical cations in the A–A and B–B pairs are located one over another with a transverse shift of the “ring over atom” (RA) manner [8,13]. In the A–A pair there is a longitudinal shift of radical cations with respect to each other additionally to the transverse one (Fig. 8a). The Table 2 ˚ [14]) between the radical cations in the salts Short contacts: S· · ·S (r ≤ 3.68 A (ET)2 (NO)3 ·C2 H4 (OH)2 (I) and (ET)2 (NO3 )·0.5C2 H4 (OH)2 ·H2 O (II) and ˚ [14]) between the radical cations and anion (or glycol) S· · ·O (r ≤ 3.25 A II Contact S1· · ·S71

S4· · ·S62 S5· · ·S71 S6· · ·S82

III ˚ Distance (A)

Contact

˚ Distance (A)

3.591 3.411 3.440 3.465

S1A· · ·S6B3,5

3.549 3.563 3.470 3.438 3.515 3.432 3.654 3.591 3.632 3.461 3.205 3.138 3.138

S2A· · ·S5B S5A· · ·S6B3,5 S6A· · ·S5B S7A· · ·S4B3,5 S7A· · ·S8B3,5 S8A· · ·S2B4 S8A· · ·S3B S8A· · ·S6B4 S8A· · ·S7B S6A· · ·O16 S7B· · ·O7 S8B· · ·O38

Symmetry codes for the salt I: 1 (0.5 − x; 0.5 − y; 1 − z); 2 (0.5 − x; 1.5 − y; 1 − z). Symmetry codes for the salt II: 3 (x; y − 1; z); 4 (1 − x; 1 − y; 1 − z); 5 (x; 1 + y; z); 6 (−x; 1 − y; −z); 7 (x; y; z − 1); 8 (−x; 1 − y; 1 − z).

RA overlapping type is one of the most efficient ones for realization of optimal S· · ·S contacts [13] and is characteristic of the ␤ -type structures. The conducting layers contain multiple side-by-side S· · ·S contacts between the radical cations ˚ Table 2). from the neighbouring stacks (3.438–3.654 A, The anion sheets consist of polymer chains in which the (NO3 )− anion bonded by hydrogen bonds with the water and glycol molecules (Fig. 9). It should be noted that the glycol molecules are disordered and there are two possible positions for the C C glycol fragment relative to a pseudotwo-fold symmetry axis drawing through the inversion center. In a polymer chain the oxygen atom from the anion, O(2), forms the O· · ·H bond with the hydrogen atom of the ( CH2 CH2 ) group of glycol (in salt I the hydroxyl groups form hydrogen bonds) and water (OW). Another hydrogen atom of the same ethylene group has hydrogen bond with a second anionic particle. Besides O(2), the water molecule is bonded by hydrogen bonds with the second oxygen atom, ˚ and O(1), of the anion (distance O(1)· · ·OW is equal 2.87 A) the hydroxyl group of one more glycol molecule. The bond lengths in the pair of glycol O(2), O(2) OW and OW glycol ˚ (C H· · ·O distance), 2.54 and 2.54 A ˚ are equal to 3.50 A (O· · ·O distance), respectively. As one can see, the difference in bond lengths is small. It indicates the uniform charge distribution along an anionic chain. Availability of hydrogen bonds was also observed between the cation and anion layers. They are formed: (1) between the terminal ethylene groups of the ET radical cations and oxygen atoms of the anions (C H· · ·O ˚ (2) between the terminal ethylene distances of 2.84–3.50 A); groups of the ET radical cations and glycol (C H· · ·O dis˚ (3) between terminal ethylene groups tances of 3.28–3.40 A);

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Fig. 5. Anion sheet in the radical cation salt (ET)2 (NO3 )·C2 H4 (OH)2 comprising chains formed by the (NO3 )− anions and glycol molecules.

of the ET radical cations and water (C H· · ·O distances of ˚ There are also short S· · ·O contacts (Table 2) 2.65–2.99 A). between the cation and anion layers owing to the interactions of the ET molecules with the anion and the ET molecule with glycol. Hence, in crystals II the cation–anion intermolecular interaction is realized both short S· · ·O interlayer contacts and the formation of various hydrogen bonds. Possibly, this is the first radical cation salt in which the formation of a 3D network of intermolecular interactions is provided by a great number of unequal molecules involved in different types of interactions. Thus, both salts (I and II) are multi-component molecular conductors with supramolecular assembly. Room temperature conductivities of single crystals of I, II, and III are 0.3–0.5, 20–40 and 2 × 10−2 −1 cm−1 , respectively. The temperature dependences of resistance in the plane of the ET layers are shown for the single crystals of I

and III in Fig. 10. The resistance of both samples increases exponentially with an activation energy of 0.13 eV (for III) and 10 meV (for I). In the range of 225–245 K salt I shows some peculiarity in the resistance behavior, which occurs to a certain degree in the measured samples. The reason of this peculiarity in the resistance behavior is unclear. To elucidate the reason of the observed phenomena, future investigations are needed. The resistivity anisotropy of I is about 1.5 × 103 at room temperature and almost does not change with cooling down 100 K. This value of the resistance anisotropy is typical for the organic conductors with quasi-two-dimensional structures [15]. Thus, the presence of a 3D network of intermolecular interactions in the structure of I does not affect the value of anisotropy of resistivity found, in contrast to that of ET(SCN)0.77 [4,9]. Obviously, the interaction between the radical cations layers and the anion sheets in crystals I is essentially weaker than the anion–anion interaction in the

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Fig. 6. Projection of the crystal structure of the radical cation salt (ET)2 (NO3 )·0.5[C2 H4 (OH)2 ]·H2 O viewed along the a axis.

polymer chains as it derives from the above mentioned values of the hydrogen bonds. Salt II demonstrates a metallic behavior of resistivity from 300 down to 4.2 K. Resistance measured of the majority of the crystals in both the ET radical cation plane (1 in Fig. 11) and the perpendicular direction (2 in Fig. 11) decreases by ∼20–25 times at helium temperature. The resistance anisotropy is equal to (2.0–2.5) × 103 at room temperature and changes insignificantly with cooling down to 4.2 K. It rises by ∼25–30% with temperature decreasing down to 200 K and then begins to decrease, reaching the initial value at 60 K. Like salt I, the existence of the 3D network of intermolecular interactions in the structure of crystal II does not essentially affect the resistance anisotropy. However, the network stabilizes a metallic state in II down to helium tem-

perature. It should be noted, that the well known salt ␤ (NO3 )2 , having the same arrangement of conducting layers as in II, shows metallic type conductivity only down to 27 K [1,2]. Furthermore, conducting properties of ␤ -ET3 (NO3 )2 are observed to be unstable with time in contrast to those of II. In a year’s time the room temperature conductivity of the ␤ -ET3 (NO3 )2 crystals reduced from 200–800 down to 40–60 −1 cm−1 and the M–I transition observed shifted from 27 to 55 K (Fig. 12). Possibly, the conductivity change observed in ␤ -ET3 (NO3 )2 is a result of adsorption of water molecules by the crystals surface and the formation of hydrogen bonds between them and the discrete (NO3 )− anions. We analyzed the conducting properties (values of room temperature conductivities, temperature dependences of resistance and activation energies) of salts I–III, obtained in

Fig. 7. Packing motif for the conducting ET radical cation layers in the (ET)2 (NO3 )·0.5[C2 H4 (OH)2 ]·H2 O crystals.

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Fig. 10. Temperature dependence of resistance for a single crystal of (ET)2 (NO3 )·C2 H4 (OH)2 (1) and salt III (2) in the plane of conducting ET radical cation layers.

Fig. 8. Overlap modes of the ET radical cations in stacks of the radical cation salt (ET)2 (NO3 )·0.5[C2 H4 (OH)2 ]·H2 O: (a) inside of A–A pairs; (b) between the A and B radical cations; (c) inside of B–B pairs.

this work, and the ET salts with the (NO3 )− counterion, described us in [4] before. The compositions of the crystals in [4] were preliminary determined by the elemental analysis and their structures were not studied because of poor quality of the crystals. Now we can suppose that the crystals of II and the crystals with the metallic type of conductivity reported in [4] are the same salt with the composition (ET)2 (NO3 )·0.5[C2 H4 (OH)2 ]·H2 O. The conducting properties of salt III correspond to those of the rectangular crystals prepared with the use of metal complex anions of rare-earth elements in [4].

Fig. 9. Optimal view of the anion sheet in the radical cation salt (ET)2 (NO3 )·0.5[C2 H4 (OH)2 ]·H2 O comprising chains formed by the (NO3 )− anions and molecules of glycol and water.

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counterions. Polymer chains are constructed by anion and glycol molecules (in I) or anion, glycol and water molecules (in II) owing to hydrogen bonds formation. There is a great number of short S· · ·S contacts in the radical cation layers of these salts. In I conducting radical cation and anion layers are bonded by hydrogen bonds where as in II they are bonded by both hydrogen bonds and short S· · ·O contacts. Both salts are characterized by the presence of a 3D network of cation–cation, cation–anion, and anion–anion intermolecular interactions. I shows semiconductor properties with a small band gap (10 meV) and II demonstrates a metallic behavior of resistance down to 4.2 K.

Fig. 11. Temperature dependence of resistance for a single crystal of (ET)2 (NO3 )·0.5[C2 H4 (OH)2 ]·H2 O in the conducting ab plane (1) and perpendicular to the ab plane (2).

Acknowledgments The authors are grateful to E.B. Yagubskii for useful discussion and A.D. Dubrovskii for the discussion of crystal structures. The work was supported by RFBR (No. 03-0204023) and INTAS (No. 02-2212).

References

Fig. 12. Temperature dependence of resistance for a single crystal of ␤ ET3 (NO3 )2 along the a-axis as-prepared (1) and 1 year later (2).

4. Conclusion New multi-component radical cation salts: (ET)2 (NO3 )·C2 H4 (OH)2 and (ET)2 (NO3 )·0.5[C2 H4 (OH)2 ]·H2 O were synthesized and their structures and conducting properties were studied. Both salts have layered structures differing in packing motifs of radical cation layers. The anion sheets separating the conducting ET layers consist of polymer chains as a result of glycol insertion between the (NO3 )−

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