4,2-Ribbon like ferromagnetic cyano-bridged FeIII2NiII chains: a magneto-structural study

Abstract

The low-spin iron(III) complex AsPh₄[Fe^III(bpy)(CN)₄]·CH₃CN (1) [AsPh₄ = tetraphenylarsonium cation] and the heterobimetallic chains [{Fe^III(L)(CN)₄}₂Ni^II(H₂O)₂]·4H₂O with L = bpy (2) and phen (3) [bpy = 2,2′-bipyridine and phen = 1,10-phenanthroline] have been prepared and their structures determined by X-ray diffraction methods. The structure of 1 consists of mononuclear [Fe(bpy)(CN)₄]⁻ anions, tetraphenylarsonium cations and acetonitrile molecules of crystallization. The iron(III) is hexacoordinated with two nitrogen atoms of the bidentate bpy and four carbon atoms of four terminal cyanide groups building a distorted octahedral surrounding around the metal atom. 2 and 3 are isomorphous compounds whose structure is made up of neutral 4,2-ribbon like bimetallic chains of formula [{Fe^III(L)(CN)₄}₂Ni^II(H₂O)₂] where the [Fe^III(L)(CN)₄]⁻ unit acts as a bis-monodentate bridging ligand toward the trans-diaquanickel(II) units through two of its four cyanide groups in cis positions. The chains exhibit two orientations in the unit cell and they interact with each other through hydrogen bonds involving the coordination and crystallization water molecules together with the uncoordinated cyanide nitrogen atoms of the [Fe(L)(CN)₄]⁻ units. Compounds 2 and 3 behave as ferromagnetic Fe^III₂Ni^II chains which interact ferromagnetically at very low temperatures in the case of 2, whereas metamagnetic-like behaviour is observed for 3 with a critical field (H_c) around 200 G. For H > H_c the ferromagnetic Fe^III₂Ni^II chains of 3 exhibit a frequency dependence of the out-of-phase ac susceptibility signal at T < 3.5 K.