Synthesis, electrochemistry, structure, and magnetic susceptibility of 5-tert-butyl-1,3-bis-(1,2,3,5-dithiadiazolyl)benzene.: Structural effect of the bulky substituent

The crystal structure of the title compound was determined at 250 K in space group I (4) over bar 2m, a = 20.661(5) Angstrom, c = = 6.764(7) Angstrom, Z = 8. The individual dithiadiazole rings form two sets of contrarotatory 4-member pinwheels clustered around a 4-fold rotation-inversion axis located halfway along the unit cell edges, describing an infinite channel lined with sulfur atoms but in which there are short intra-stack contacts through only one S atom of each CN(2)S(2) group. The double-layer stacking occurs in order to accommodate the bulk of the (t)Bu group, and the spacing between layers is very regular, with short and long S ... S contacts of 3.48(2) and 3.61(2) Angstrom and considerable thermal motion in the c direction. The title compound and its SbF(6)(-) salt are oxidized at +0.81 V (in CH(2)Cl(2)) and at +0.61 V (in CH(3)CN), while a reduction process is observed only in CH(2)Cl(2) at -0.73 V vs. SCE. Magnetic susceptibility data between 5 and 400 K demonstrate at very low temperature that the sample follows the Curie-Weiss law, theta = 0 K, and chi(0) =-156 ppm emu mol(-1). The free-spin concentration at T = 0 K is approximate to 1.3%, due to paramagnetic defects in an essentially diamagnetic structure. The diamagnetism starts to lift above 210 K; above 260 K, a strong antiferromagnetic exchange is operative. These results are consistent with the lifting of the Peierls distortion in this structure, starting above approximate to 200 K. The crystal structure of the parent diamidine 5-tert-butyl-1,3-[(Me(3)Si)(2)NCNSiMe(3)](2)C(6)H(3) was determined in C2/c with a = 10.0788(3), b = 21.328(5), c = 20.876(5) Angstrom, beta = 99.41(2)degrees, Z = 4. The two amidine functional groups are equivalent by crystal symmetry.