Structural, Electrical, and Magnetic Properties of a Series of Molecular Conductors Based on BDT-TTP and Lanthanoid Nitrate Complex Anions (BDT-TTP = 2,5-Bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene)

The platelike crystals of a series of novel molecular conductors, which are based on the π-donor molecules BDT-TTP (2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene) with a tetrathiapentalene skeleton and lanthanide nitrate complex anions [Ln(NO₃)_x]^3-x (Ln = La, Ce, (Pr), Tb, Dy, Ho, Er, Tm, Yb, Lu) with localized 4f magnetic moments, were synthesized. Except for the Ce complex, the salts were composed of (BDT-TTP)₅[Ln(NO₃)₅] and were isostructural. Even though the Ce crystal had a different composition, (BDT-TTP)₆[Ce(NO₃)₆](C₂H₅OH)_x (x ≈ 3), the crystals all had the space group P1̄. Although the X-ray examination of the Pr salts was insufficient, the existence of two modifications was suggested in these systems by preliminary X-ray examination. Previously, we reported the crystal structures and unique magnetic properties of (BDT-TTP)₅[Ln(NO₃)₅] (Ln = Sm, Eu, Nd, Gd). Thus, by combining the results of this work with previous one, we for the first time succeeded in obtaining a complete set of organic conductors composed of the identical π-donors (BDT-TTP in this case) and all the lanthanide nitrate complex anions (except the complex with Pm³⁺). The crystals were all metallic down to 2 K. Electronic band structure calculations resulted in two-dimensional Fermi surfaces, which was consistent with their stable metallic states. Except for the Lu complex, which lacked paramagnetic moments, the magnetic susceptibilities were measured on the six heavy lanthanide ion complex salts by a SQUID magnetometer (Ln = Tb, Dy, Ho, Er, Tm, Yb). The large paramagnetic susceptibilities, which were caused by the paramagnetic moments of the rare-earth ions, were obtained. The Curie−Weiss law fairly accurately reproduced the temperature dependence of the magnetic susceptibilities of (BDT-TTP)₅[Ho(NO₃)₅] in the experimental temperature range (2−300 K) and a comparatively large Weiss temperature (|ϑ|) was obtained (ϑ(Ho) = −15 K). A Weiss temperature (ϑ(Tm) = −8 K) was also obtained for Tm. The |ϑ| values of other (BDT-TTP)₅[Ln(NO₃)₅] salts and (BDT-TTP)₆[Ce(NO₃)₆](C₂H₅OH)_x(x≈3) were as follows: |ϑ|/K = 4 (Er), ≤2 (Ce, Tb, Dy, Yb). The comparatively strong intermolecular magnetic interaction between Ho³⁺ ions, which was suggested by the |ϑ| value, is inconsistent with the traditional image of strongly localized 4f orbitals shielded by the electrons in the outer 5s and 5p orbitals. The dipole interactions between Ln³⁺ ions causing the Curie−Weiss behavior and the comparatively large ϑ value of (BDT-TTP)₅[Ho(NO₃)₅] is inconsistent with the data, since the complexes exhibit isostructural properties and there is not a clear relationship between the magnitudes of ϑ values and those of magnetic moments. Therefore, it is possible that the 4f orbitals of Ho atom are sensitive to the ligand field, which will have an effect on the orbital moment of the Ho³⁺ ion and/or produce a small amount of mixing between 4f and ligand orbitals to give rise to “real” intermolecular antiferromagnetic interaction through intermolecular overlapping between π (BDT-TTP) and ligand orbitals of lanthanide nitrate complex anions.