Magnetism and electrical conductivity of molecular semiconductor, [FeII(DMF)4(TCNQ)2](TCNQ)2, with fractionally charged TCNQ units

We report the synthesis of a semiconducting material, [FeII(DMF)4(TCNQ)2](TCNQ)2 (1), which contains partially charged TCNQδ− anions (TCNQ = 7,7,8,8-tetracyanoquinodimethane, 0 < δ < 1). Compound 1 was prepared by different routes, using (Et3NH)(TCNQ)2, a mixture of TCNQ and LiTCNQ, or a mixture of TCNQ and NaTCNQ, to obtain the structure with fractionally charged TCNQδ− anions. The crystal structure of 1 is formed by mononuclear cations [Fe(DMF)4(TCNQ)2]2δ+, the TCNQ units of which form infinite π-π stacks with non-coordinated TCNQδ− counter ions. A detailed examination of X-ray diffraction data, supplemented by inductively coupled plasma mass spectrometry, indicates that some of the non-coordinated TCNQδ− anions, likely, form bonds to a partially occupied Na+ site, thus causing slightly different crystal packing in the vicinity of some Fe(II) centers. This finding is indirectly corroborated by the results of Mössbauer spectroscopy, which shows that the transition metal site corresponds to the high-spin Fe(II) ion, but two such sites are present, with slightly different quadrupole splitting parameters. Temperature-dependent conductivity measurements reveal semiconducting behavior, with the room-temperature conductivity of 0.125 S/cm and the charge-hopping activation energy of 180 meV.