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C—I...NC halogen bonding in two polymorphs of the mixed‐valence 2:1 charge‐transfer salt (EDT‐TTF‐I 2 ) 2 (TCNQF 4 ), with segregated versus alternated stacks
Author(s) -
Lieffrig Julien,
Jeannin Olivier,
Vacher Antoine,
Lorcy Dominique,
AubanSenzier Pascale,
Fourmigué Marc
Publication year - 2014
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520613032629
Subject(s) - crystallography , chemistry , intramolecular force , acceptor , molecule , halogen , halogen bond , valence (chemistry) , charge (physics) , stereochemistry , hydrogen bond , alkyl , physics , organic chemistry , condensed matter physics , quantum mechanics
Oxidation of diiodoethylenedithiotetrathiafulvalene (EDT‐TTF‐I 2 ), C 8 H 4 I 2 S 6 , with the strong oxidizer tetrafluorotetracyanoquinodimethane (TCNQF 4 ), C 12 F 4 N 4 , affords, depending on the crystallization solvent, two polymorphs of the 2:1 charge‐transfer salt (EDT‐TTF‐I 2 ) 2 (TCNQF 4 ), represented as D 2 A . In both salts, the TCNQF 4 is reduced to the radical anion state, and is associated through short C—I...NC halogen bonds to two EDT‐TTF‐I 2 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‐I 2 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‐I 2 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 A 2 2− and D 0 – D 2 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 A 2 2− and D 2 2+ dyads.