Cation radical salts of cyano(ethylenedithio)tetrathiafulvalene with halogenated anions: annihilation of the CN···Hal interaction and stabilisation of conducting, antiferromagnetic square or chain-type salts

Abstract

The synthesis, X-ray crystal structure and electrochemical properties of 3-cyano-3′,4′-ethylenedithiotetrathiafulvalene (EDT-TTF-CN, 1) are described together with electrocrystallisation experiments in the presence of various halogenated anions, I₃⁻, FeBr₄⁻, InBr₄⁻, AuBr₄⁻, Mo₆Br₁₄²⁻, Mo₆Cl₈Br₆²⁻. X-Ray crystal structures are described for each salt, characterised by a systematic absence of any short and directional CN···Hal interactions, convincing evidence of the electrostatic character of this interaction, which is enhanced in the opposite situation where cationic halogenated tetrathiafulvalenes are associated with cyanide anions such as Ag(CN)₂⁻. This behaviour is further rationalised by atomic population calculations on TTF-CN and [TTF-CN]^•+, which confirm the strong decrease of the negatively charged character of the nitrogen atom upon TTF-CN oxidation to [TTF-CN]^•+. A conducting mixed-valence salt is obtained in [1]₂[I₃], which exhibits an unusual structure with the I₃⁻ anions embedded in the organic conducting layer. All the other salts contain fully oxidised [1]^•+ radical cations, associated into diamagnetic diads in [1][I₃], [1]₂[Mo₆Br₁₄][CH₃CN]₄ and [1]₂[Mo₆Cl₈Br₆][CH₃CN]₄, into spin chains in the two isostructural [1][FeBr₄] and [1][InBr₄] salts and into a quadratic two-dimensional spin system in [1][AuBr₄].