Synthesis, chirality, and magnetic properties of bimetallic cyanide-bridged two-dimensional ferromagnets

The assembly of hexacyanoferrate(III) anions and nickel(II) bis-diamino complexes of the chiral ligand trans-cyclohexane-1,2-diamine (trans-chxn) yields cyanide-bridged two-dimensional ferromagnets of the general formula [Ni(trans-chxn)(2)](3)[Fe(CN)(6)](2)center dot 2H2O. Their crystal structure is built from cyanide-bridged bimetallic planes separated by the bulky chxn ligands, giving rise to a large interlayer distance (d = 11.7 angstrom). These materials order ferromagnetically at the Curie temperature T-C = 14 K. AC susceptibility measurements evidence an unusual magnetic behavior below TC, with a marked frequency dependence. A thorough magnetic analysis demonstrates that this complex behavior is due to the pinning of the domain walls which results from the coexistence of the layered structure and the strong magnetic anisotropy. An activation energy for domain wall movement E-a = 116 K has been determined. Ferromagnetic materials of this kind that incorporate structural chirality have been obtained by using the enantiopure ligands trans-(1S,2S)-chxn and trans-(1R,2R)-chxn, as evidenced by circular dichroism spectroscopy. The ligand cis-cyclohexane-1,2-diamine affords also a two-dimensional compound of formula [Ni(cis-chxn)(2)](3)[Fe(CN)(6)](2)center dot 2H2O. This material orders ferromagnetically at T-C = 11 K and also exhibits slow relaxation behavior.