Hard magnets based on transition metal complexes with the dicyanamide anion, {N(CN)2}-,

Abstract

We present the crystal structures and magnetic properties of a series of magnetic compounds, M^II{N(CN)₂}₂, where M=Cu (1), Ni (2), Co (3) and Fe (4), and [Mn{N(CN)₂}₂(C₂H₅OH)₂]Z·(CH₃)₂CO (5). In the isostructural compounds 1–4, the dicyanamide anion is triply coordinating through its three nitrogen atoms. It bridges the metal ions to form infinite 3D metal-organic frameworks with a rutile-type structure. The framework contains doubly bridged M(–Nâ–·C–N–Câ–·N–)₂ ribbons that link approximately orthogonally through the amide nitrogen atoms. The Jahn–Teller distortion in 1 has a strong influence on the packing arrangement (M–N bond lengths: 1.98 and 2.47 Å for 1 and 2.10 and 2.15 Å for 3). On lowering the temperature the bond distances in 1 remain unchanged except for a decrease of the M–N_amide length to 2.45 Å. Magnetic data for 1 obey the Curie–Weiss law (Θ=-2.1 K). 2 and 3 are ferromagnets with Curie temperatures (T_C) of 9 and 21 K and are characterized by hysteresis loops of 710 and 7975 Oe at 2 K, remnant magnetization, magnetization approaching the expected saturation (gS) of 2 and 3 µ_B in high field, absorptive component (χ″) in the AC magnetization and λ peak in the heat capacity data. 4 is similarly characterized and shows behaviour that is characteristic of a canted antiferromagnet: the Weiss constant is temperature dependent (+3 K in the range 200–300 K), there is a sharper peak than for 1 or 2 in the AC magnetization and the isothermal magnetization at 3 K increases monotonically to ≈1.3 µ_B (expected to be 4 µ_B for ferromagnetic alignment of the spins) in a field of 8 T. Its coercive field (17800 Oe) is the largest observed for any metal-organic compound and exceeds those of alloys of SmCo₅ and Nd₂Fe₁₄B. The maximum energy product (B · H) is the highest for 3 and is comparable to alloys of Sm–Co. We attribute the large coercive field to a combination of single ion and particle shape anisotropies. 5 is paramagnetic at high temperature with Θ=-3 K. Below 16 K it behaves as a canted antiferromagnet with a very weak resultant spontaneous magnetization.