Acta Crystallographica Section B

Structural Science, Crystal Engineering and Materials

Volume 70, Part 1 (February 2014)

crystal engineering

Acta Cryst. (2014). B70, 141-148    [ doi:10.1107/S2052520613032629 ]

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

J. Lieffrig, O. Jeannin, A. Vacher, D. Lorcy, P. Auban-Senzier and M. Fourmigué

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 D0-A^{{-}{\bullet}}-D^{{+}{\bullet}} motifs give rise, in a perpendicular direction, to diamagnetic A22- and D0-D22+-D0 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 [pi]-type radicals, into either delocalized, uniform chains with partial charge transfer and conducting behaviour, or localized association of radicals into face-to-face A22- and D22+ dyads.

CCDC references: 974605, 974606, 974607 and 974608

Keywords: charge-transfer salt; solid state; radical species.

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Structure factor file (CIF format) (206.5 kbytes)
[ doi:10.1107/S2052520613032629/bi5028NCCH22S2TTFI2sup2.hkl ]
Contains datablock {[NC(CH2)2S]2TTFI2}

hkldisplay filedownload file

Structure factor file (CIF format) (81.5 kbytes)
[ doi:10.1107/S2052520613032629/bi5028EDTTTFI2sup3.hkl ]
Contains datablock edtttfi2

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Structure factor file (CIF format) (217.2 kbytes)
[ doi:10.1107/S2052520613032629/bi5028Isup4.hkl ]
Contains datablock I

hkldisplay filedownload file

Structure factor file (CIF format) (428.0 kbytes)
[ doi:10.1107/S2052520613032629/bi5028IIsup5.hkl ]
Contains datablock II


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