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

Abstract

The field of molecular spintronics has gained significant attention for the development of second-generation spintronic devices. Therefore, an electrically conducting molecular crystal, Ph₄P[Fe^III(1,2-Nc)(CN)₂]₂ (Ph₄P = tetraphenylphosphonium and 1,2-Nc = C_4h 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[Fe^III(1,2-Nc)(CN)₂]₂ was isostructural to its non-magnetic analogue Ph₄P[Co^III(1,2-Nc)(CN)₂]₂ but possessed higher electrical resistivity, indicating that the strong intramolecular π–d interaction is present in the [Fe^III(1,2-Nc)(CN)₂] unit. Although the magnetic interaction between π-conduction electrons and Fe^III-d spins (π–d interaction) is crucial for the emergence of a negative magnetoresistance effect, the negative magnetoresistance effect of Ph₄P[Fe^III(1,2-Nc)(CN)₂]₂ was significantly smaller (−6% at 30 K under a static 9 T magnetic field) than those of Ph₄P[Fe^III(Pc)(CN)₂]₂ (−32%) and Ph₄P[Fe^III(tbp)(CN)₂]₂ (−13%) analogues (Pc = phthalocyanine and tbp = tetrabenzoporphyrin). This small negative magnetoresistance effect of Ph₄P[Fe^III(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.