C—I...NC halogen bonding in two polymorphs of the mixed-valence 2:1 charge-transfer salt (EDT-TTF-I₂)₂(TCNQF₄), with segregatedversusalternated stacks

Abstract

Oxidation of diiodoethylenedithiotetrathiafulvalene (EDT-TTF-I2), C8H4I2S6, with the strong oxidizer tetrafluorotetracyanoquinodimethane (TCNQF4), C12F4N4, affords, depending on the crystallization solvent, two polymorphs of the 2:1 charge-transfer salt (EDT-TTF-I2)2(TCNQF4), represented as D2A. In both salts, the TCNQF4 is reduced to the radical anion state, and is associated through short C-I...NC halogen bonds to two EDT-TTF-I2 molecules. The two polymorphs differ in the solid-state association of these trimeric D-A-D motifs. In polymorph (I) the trimeric motif is located on an inversion centre, and hence both EDT-TTF-I2 molecules have +0.5 charge. Together with segregation of the TTF and TCNQ derivatives into stacks, this leads to a charge-transfer salt with high conductivity. In polymorph (II) two crystallographically independent EDT-TTF-I2 molecules bear different charges, close to 0 and +1, as deduced from an established correlation between intramolecular bond lengths and charge. Overlap interactions between the halogen-bonded D(0)-A-∙D+∙ motifs give rise, in a perpendicular direction, to diamagnetic A2(2-) and D(0)-D2(2+)-D(0) entities, where the radical species are paired into the bonding combination of respectively the acceptor LUMOs and donor HOMOs. The strikingly different solid-state organization of the halogen-bonded D-A-D motifs provides an illustrative example of two modes of face-to-face interaction between π-type radicals, into either delocalized, uniform chains with partial charge transfer and conducting behaviour, or localized association of radicals into face-to-face A2(2-) and D2(2+) dyads.

Halogen Bonding in Supramolecular Chemistry

“…In a development of previous work in the exploitation of XB in tetrathiafulvalene-based molecular conductors, Fourmigué and co-workers recently reported a series of cocrystals incorporating diiodo-functionalized tetrathiafulvalene (Figure a) . This tecton was crystallized with various oxygen-based XB acceptors, including perchlorate (ClO 4 – ), racemic camphor sulfonate (Figure b), 1,5- and 2,6-naphthalenebis­(sulfonate) (Figure c,d), and 2,6-anthracenebis­(sulfonate) (Figure e).…”

Electrochemically driven interfacial halogen bonding on self-assembled monolayers for anion detection

“…1a was synthesized from the dissymmetrical TTF derivative 2a. 23 We also prepared a reference TTF derivative, 1b, in which the two iodine atoms were replaced by methylthio substituents, starting from the precursor 2b. 24 Compounds 2a-b are substituted on one side by two cyanoethylthio groups which can lead under basic conditions to the corresponding dithiolates.…”

Organizing Radical Species in the Solid State with Halogen Bonding

“…A similar series of compounds was obtained with the diiodo TTF derivative, EDT-TTFI 2 [87,88], where the competition with the C-HÁ Á ÁN hydrogen bond cannot take place. To establish a correlation between the halogen bond length and the degree of charge transfer, a complete series was crystallized from the less oxidizing TCNQ to the most oxidizing TCNQF 4 .…”