High-Temperature Molecular Magnets Based on Cyanovanadate Building Blocks: Spontaneous Magnetization at 230 K

TLDR

The authors synthesized two cyanovanadate-based magnetic salts, Cs₂MnV(CN)₆ (1) and (Et₄N)₀.₅Mn₁.₂₅[V(CN)₅]·2H₂O (2), by adding Mn(II) triflate to cold aqueous hexacyanovanadate(II) solutions. Both compounds crystallize in distinct lattices and behave as three‑dimensional ferrimagnets, with Néel temperatures of 125 K for the cesium salt and 230 K for the tetraethylammonium salt—making the latter the second‑highest‑temperature molecular magnet—while antiferromagnetic coupling between V II and high‑spin Mn II centers yields soft‑magnet hysteresis loops and demonstrates that early transition‑metal incorporation enhances magnetic coupling via cyanide π* backbonding.

Abstract

The molecular-based magnetic materials Cs 2 Mn II [V II (CN) 6 ] (1) and (Et 4 N) 0.5 Mn 1.25 - [V(CN) 5 ]⋅ ⋅2H 2 O (2) (where Et is ethyl) were prepared by the addition of manganese(II) triflate to aqueous solutions of the hexacyanovanadate(II) ion at 0°C. Whereas 1 crystallizes in a face-centered cubic lattice, 2 crystallizes in a noncubic space group. The cesium salt (1) has features characteristic of a three-dimensional ferrimagnet with a Néel transition at 125 kelvin. The tetraethylammonium salt (2) also behaves as a three-dimensional ferrimagnet with a Néel temperature of 230 kelvin; only two other molecular magnets have higher magnetic ordering temperatures. Saturation magnetization measurements indicate that in both compounds the V II and high-spin Mn II centers are antiferromagnetically coupled. Both 1 and 2 exhibit hysteresis loops characteristic of soft magnets below their magnetic phase-transition temperatures. The high magnetic ordering temperatures of these cyano-bridged solids confirm that the incorporation of early transition elements into the lattice promotes stronger magnetic coupling by enhancing the backbonding into the cyanide π* orbitals.