An electrically conducting molecular crystal composed of a magnetic iron(III) complex (S = 1/2) with a large aromatic ligand, 1,2-naphthlalocyanine (C4h isomer): towards the development of molecular spintronics.

The field of molecular spintronics has gained significant attention for the development of second-generation spintronic devices. Therefore, an electrically conducting molecular crystal, Ph4P[FeIII(1,2-Nc)(CN)2]2 (Ph4P = tetraphenylphosphonium and 1,2-Nc = C4h isomer of 1,2-naphthalocyanine), was fabricated as a new coordination compound with a strong π-d interaction. Furthermore, it is a mixed-valence compound with a local spin of S = 1/2 at the center of the conduction path. Crystal structure analysis revealed that Ph4P[FeIII(1,2-Nc)(CN)2]2 was isostructural to its non-magnetic analogue Ph4P[CoIII(1,2-Nc)(CN)2]2 but possessed higher electrical resistivity, indicating that the strong intramolecular π-d interaction is present in the [FeIII(1,2-Nc)(CN)2] unit. Although the magnetic interaction between π-conduction electrons and FeIII-d spins (π-d interaction) is crucial for the emergence of a negative magnetoresistance effect, the negative magnetoresistance effect of Ph4P[FeIII(1,2-Nc)(CN)2]2 was significantly smaller (-6% at 30 K under a static 9 T magnetic field) than those of Ph4P[FeIII(Pc)(CN)2]2 (-32%) and Ph4P[FeIII(tbp)(CN)2]2 (-13%) analogues (Pc = phthalocyanine and tbp = tetrabenzoporphyrin). This small negative magnetoresistance effect of Ph4P[FeIII(Pc)(CN)2]2 could be ascribed to the weak intermolecular antiferromagnetic interaction between its d spins. Hence, this study showed that constructing a molecular design for strengthening the intermolecular antiferromagnetic interaction is key to enhancing the negative magnetoresistance effect.