Crystal structures and physical properties of bis(ethylenedithio)-tetrathiafulvalene charge-transfer salts with FeX4–(X = Cl or Br) anions

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Abstract
Two new charge-transfer salts [bettf]2[FeCl4](1) and [bettf][FeBr4](2) have been prepared and crystallised electrochemically [bettf = bis(ethylenedithio)tetrathiafulvalene]. The crystal structures of both salts have been refined in the P space group, the unit-cell parameters being: (1), a= 6.626(2), b= 15.025(2), c= 17.805(2), α= 82.80(2), β= 89.53(2), γ= 88.15(2), Z= 2, R= 0.044; (2), a= 8.634(2), b= 10.980(2), c= 11.773(2), Å, α= 91.91(2), β= 102.84(2), γ= 93.73(2), Z= 2, R= 0.054. The structure of (1) consists of stacks of dimerised bettf molecules with short ⋯ contacts, forming layers separated by sheets of FeCl4 –. In (2) there are no stacks of bettf molecules: the structure consists of discrete dimers separated by FeBr4 –. Compound (1) shows semi-conducting behaviour from 160 to 300 K with εa= 0.21 eV and σ≈ 10–2 S cm–1 at 300 K while (2) is a quasi-insulator at room temperature with σ≈ 10–6 S cm–1. The magnetic susceptibility of both salts from 5 to 300 K is dominated by the S=5//2 Fe3+ with small Weiss constants [–6(1) for (1) and –5(1 )K for (2)].