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

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 ε_a= 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)].