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

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 asD2A. In both salts, the TCNQF4is reduced to the radical anion state, and is associated through short C—I...NC halogen bonds to two EDT-TTF-I2molecules. The two polymorphs differ in the solid-state association of these trimericD–A–Dmotifs. In polymorph (I) the trimeric motif is located on an inversion centre, and hence both EDT-TTF-I2molecules 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-I2molecules 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-bondedD0–A^{{-}{\bullet}}–D^{{+}{\bullet}} motifs give rise, in a perpendicular direction, to diamagneticA22−andD0–D22+–D0entities, 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-bondedD–A–Dmotifs 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-faceA22−andD22+dyads.