Synthesis and crystal engineering of new halogenated tetrathiafulvalene (TTF) derivatives and their charge transfer complexes and radical ion salts

Abstract

Efficient syntheses are reported for tetraiodotetrathiafulvalene 2, 4-iodo-5-methyl-4′,5′-bis(methylsulfanyl)TTF 3, and 4-iodo-4′,5′-bis(methylsulfanyl)TTF 4 by iodination, using perfluorohexyl iodide, of lithiated derivatives of the corresponding TTF system. Bromination and chlorination of lithiotrimethylTTF using 1,2-dibromotetrafluoroethane and hexachloroethane gave 4-bromo- and 4-chloro-4′,5,5′-trimethylTTF 6 and 7, respectively. Phosphite-induced self-coupling or cross-coupling reactions of 4-iodo-1,3-dithiole-2-thione or 4,5-diiodo-1,3-dithiole-2-thione(one) half-units resulted in TTF derivatives with partial loss of the iodine substituent(s). 4,5-Dibromo-4′,5′-bis(cyanoethylsulfanyl)TTF 15 was prepared by cross-coupling methodology, and converted into 4,5-dibromo-4′,5′-bis(methylsulfanyl)TTF 16 by reaction with caesium hydroxide and then methyl iodide. EPR data are reported for the electrochemically generated cation radicals of trimethylTTFX derivatives (X = I, Br and Cl) 5–7, respectively. For the neutral donors, the X-ray crystal structures are reported for 2, 5, 6, tetramethylTTF 8 and 15. Structure 2 is characterised by a particularly dense packing with continuous chains of intra-stack I⋯I contacts (4.17–4.19 A). The crystals of 6 and 8 are isomorphous, while the structure of 5 is different. The iodo-substituent in 5 affects the packing in a way the bromo-substituent in 6 does not, due to differences in specific interactions rather than steric demands of I and Br, which are similar. Structure 15 comprises face-to-face dimers with inter-dimer Br⋯Br (3.57 A) and Br⋯S (3.55 A) contacts: a remarkable difference in bond distances between the Br and S-substituted dithiole rings is observed. The 1 ∶ 1 charge-transfer (CT) complexes 3·TCNQ and 4·TCNQ (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, 17) display mixed stair-like stacks of alternating D and A moieties: the overall degree of CT is estimated from bond length analysis to be 0.2 e and 0.3 e, respectively. In 3·TCNQ either position of the disordered iodine atom has one short (inter-stack, but intra-layer) contact with a cyano group (I⋯N distances of 3.14 and 3.18 A). In 4·TCNQ a similar I⋯N contact is much longer (3.35 A). In the structure of 5+·I3−·½I2 the cation radical is disordered; dimeric cation radicals display short intra-dimer contacts (S⋯S 3.38–3.39 A, C⋯C 3.35 A) consistent with electron coupling. Each dimer is surrounded by four I3− anions. The crystal structure of 16+·I3− is comprised of layers with interplanar separations of 3.55 A. Cations of one layer overlap with anions of the next, and the packing can be described as mixed stacks parallel to the a axis. The remarkably high conductivity of this salt for a system of 1 ∶ 1 stoichiometry (σrt = 8 × 10−2 S cm−1) is ascribed to partial charge transfer (the charge on the TTF moiety is estimated as +⅔ from bond length analysis) and a continuous system of short non-bonding contacts.