Interactions interstacks

Dithiolene complexes with the maleonitriledithiolate (mnt) ligand form highly delocalized systems and are widespread in studies of conducting and magnetic materials. The electronic properties have been extensively studied with various computational methods including Hiickel and extended Hiickel approaches to identify the nature of the orbitals involved in intramolecular and intermolecular interactions. These structural properties allow the complexes to interact in the solid state via short stacking S, S and short interstack S---S contacts.10 4-1048... 

We saw in Section 12.2.3.1 that the presence of additional chalcogen atoms in BEDT-TTF/TCNQ promotes interstack interactions, suppressing the Peierls distortion and imparting upon the salt increased dimensionality compared to TTF/TCNQ. The result of including a different chalcogen into the TTF/TCNQ structure is shown in Table 2. Despite losing donor efficiency compared to TTF (Table 1) the TCNQ complexes of m/trans-diselenadithiafulvalene (DSDTF, 55/56) and TSF show an improvement in conductivity when two or four selenium atoms are incorporated. The reduced metal-insulator transition suggests that this effect is also caused by a suppression of the Peierls distortion. Increased Se-Se interstack contacts add dimensionality to the structure and limit the co-facial dimerisation typical of Peierls distortion. Wider conduction bands are afforded from the improved overlap of diffuse orbitals. 

DSDTeF, 521251400 Metallic at 4.2 K, quasi 1-D, strong interstack no transition interactions ... 

BTDM-TTF is one of the most powerfnl donors of the TTF class. Alkylthio-substitution, such as in BEDT-TTF (ET), leads to a slight redaction in its electron-donor ability. Nevertheless, salts of this cation-radical with tetracyanoqninodimethide (TCNQ) and its derivatives show significantly higher conductivity (by ca. 10 -10 ) than the analogons BTDM-TTF salts (Grossel and Weston 1994). This reflects some enhancement of intra and interstack interactions produced by the sulfur atoms at the edges of the donor skeleton. 

Figure 6 Room-temperature crystal packing of 0-(BEDT-TTF)2I3 (a) loose intrastack packing of BEDT-TTF molecules (ds s > 3.60 A) and I3 anions (b) corrugated sheet network of short (ds s < 3.60 A) interstack S—S interactions. Only S atoms of the BEDT-TTF molecules are given. (From Ref. 64.)...

TCNQ salts however, this was only a substitute for a more quantitative approach through the calculation of transfer integrals (see Section VII) or for the direct observation of the charge density between molecules (see Section XI). However, in series of isostructural materials such as the Bech-gaard salts, it is clearly possible to estimate intra- and interstack interactions... 

Possibly the most important structural feature that has been revealed from crystallographic studies performed at two temperatures (298 and 125 K) is the existence of an infinite sheet network (32) of Se-Se interactions as shown in Fig. 6. At room temperature the intermolecular intra- and inferstack Se-Se distances are all similar and have values of 3.9-4.9 A, compared to the van der Waals radius sum for the selenium atom (52) of 4.0 A. However, as the temperature is lowered (298 - 125 K) rather unusual changes occur, viz. the ratio of the decrease in the interstack mfrastack Se-Se distances is not unity but is approximately 2 1 (32, 40). Thus, the distances between the chains shown in Fig. 6 decrease, on the average, by twice as much as the distances between TMTSF molecules in each stack. This most certainly leads to increased interchain bonding and electronic delocalization through the selenium atom network as the temperature is decreased (42). 

Fig. 12. View of the intermolecular S S interactions in (ET)2Br04. The top figure indicates the interstack S S contact distances less than the van der Waals sum of 3.60 A (298/125 K) d, = 3.581(2)/3.505(2), d2 = 3.499(2)/3.448(2), d3 = 3.583(2)/3.483(2), d4 = 3.628(2)/3.550(2), d5 = 3.466(2)/3.402(2), d6 = 3.497(2)/3.450(2), d7 = 3.516(2)/3.434(2), and d8 = 3.475(2)/3.427(2) A. The S S contact distances, d9-d16 (bottom), are, by contrast, all longer than 3.60 A even at 125 K. In addition the loose zig-zag molecular packing of ET molecules is such that they are not equally spaced, D, = 4.01/3.95 A and D2 = 3.69/3.60 A. As a result of the (apparently) weak intrastack and strong interstack interactions, (ET)2X molecular metals are structurally different from the previously discovered (TMTSF)2X based organic superconductors. Almost identical S S distances and interplanar spacings are observed in (ET)2Re04 at both 298 and 125 K. Only theoretical calculations will reveal the extent, if any, of chemical bonding associated with the various S S distances observed in (ET) X systems.

Fig. 13. A stereoview of the short (< 3.60 A) intermolecular interstack S-S interactions in (ET)2Re04 and (ET)2Br04 which form a two-dimensional corrugated sheet network (56). This network, which is the principal pathway for electrical conduction, is much different from that observed in (TMTSF)2X salts, but similar to the network of interstack S-S interactions observed in ET2(C104)(TCE)0 5 (59).

Fig. 17. Stereoview of the novel sandwich or layered structure of / -(ET)2IBr2 composed of alternating two-dimensional sheets of linear (Br-I-Br) anions, between which a corrugated sheet network of short interstack S S interactions is inserted. Only the S atoms of the ET molecules are shown in the network and light lines indicate the interstack (ds s < 3.60 A) interactions. The —CH2 groups at ET protrude from both ends of the molecule (directly out of the plane of the page) and grasp the X3 " anions in a pincer hold. Therefore, by varying the length of the X3 " anion the interstack S S distances can be directly altered (108).

The main difference between the (3" structure compared to the / phase is the direction of the strong intermolecular interactions. Due to the smaller anion size the interaction directions are at 0°, 30°, and 60°, respectively, instead of face-to-face (90°) overlaps [335]. The more complicated interstack interaction results in a more anisotropic band structure with ID and 2D energy bands. There exists considerable disagreement between different band-structure calculations which might be caused by small differences in the transfer integral values [332, 335, 336]. One calculated FS based on the room temperature lattice parameters is shown in Fig. 4.27a [335]. Small 2D pockets occur around X and two ID open sheets run perpendicular to the a direction. In contrast, the calculation of [332] (not shown) revealed a rather large closed orbit around the F point. 

It should be mentioned that although Bulaevskii et al. find that (19) holds for the 5C of NMP-TCNQ at least down to about 0.15 K, Azevedo finds that (20) is not followed for below about IK. It seems clear from the structure of NMP-TCNQ that interstack interactions ultimately have to become important at low temperatures. Perhaps they are responsible for the upturn in C obtained by Azevedo. If so, NMP-TCNQ would be a very interesting three-dimensional spin glass of a novel type. 

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