Intermolecular interactions interstack

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... 

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. 

The crystal structures of 3,6-bis(pyridin-3-yl)-l,2,4,5-tetrazine, 3,6-bis(pyridin-4-yl)-l,2,4,5-tetrazine, and 3,6-bis(pyrazin-2-yl)-l,2,4,5-tetrazine have been reported. The packing arrangements of these three compounds are affected by a subtle interplay of competing weak forces, most notably Jt-Jt-interactions and C-H- - -N hydrogen bonding. In most instances, the tetrazine- - -phenyl/pyridyl interaction is the dominant intermolecular interaction. This is affected by intramolecular torsions which depend upon the precise nature of the tetrazine substituents. Interstack arrangements are controlled by C-H- - -N interactions <2003CE082>. 

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).

---