Three-Dimensional Network-Structured Cyanide-Based Magnets

Abstract

Magnets based on metal oxides have been important for hundreds of years. Magnetite, Fe3O4, Co-doped γ-Fe2O3, and CrO2 are important examples. The oxide (O2-) bridge between the magnetic metal ions has filled p orbitals (Figure 1a) that provide the pathway for strong spin coupling. Albeit with twice as many atoms, cyanide (C≡N−) can bridge between two metal ions via its pair of empty antibonding orbitals (Figure 1b) and filled nonbonding orbitals. Even prior to a detailed understanding of either their composition or structure, magnetic ordering of several cyanide complexes, although at low temperature, was noted. The differing atoms at each end of the cyanide ion have different binding affinities to metal ions, and simple coordination compounds, for example, [FeII(CN)6]2− (ferrocyanide), with alkali cations can easily be made. Replacement of the alkali cations with transition-metal cations affords insoluble materials, for example FeIII4[FeII(CN)6]3 (Prussian blue). Prussian blue has been used as a pigment and as an electrochromic and electrocatalyst material. The structure of Prussian blue was elucidated to be cubic (isotropic) with ⟶FeII⟵C≡N⟶FeIII⟵N≡C⟶FeII⟵ linkages along all three crystallographic directions (Figure 2). The FeII … FeIII separation is ∼5 Å. However, based on the composition, this is an idealized structure, as one FeII site per unit cell is missing. Water fills the vacant sites as well as the channels present in the structure. Due to the structural defects, it has been a challenge to grow single crystals.