Exploring the formation of 3D ferromagnetic cyano-bridged CuII2+x{CuII4[WV(CN)8]4–2x[WIV(CN)8]2x}·yH2O networks

Abstract

Two novel non-stoichiometric 3D cyano-bridged coordination networks of general formula CuII2+x{CuII4[WV(CN)8]4−2x[WIV(CN)8]2x}·yH2O were obtained according to two different synthetic strategies. The heterogeneous reaction between solid 2D cyano-bridged network (dienH3){CuII[WV(CN)8]}3·4H2O (1) (dienH33+ = protonated diethylenetriamine) and an aqueous solution of DyIII(NO3)3 results in the removal of dienH33+ cations and the formation of a 3D cyano-bridged CuII2.44{CuII4[WV(CN)8]3.12[WIV(CN)8]0.88}·5H2O (2) network. The direct combination of [CuII(H2O)6]2+ and [WV(CN)8]3− in aqueous media leads to the structurally related CuII2.97{CuII4[WV(CN)8]2.06[WIV(CN)8]1.94}·4H2O (3) assembly. The assemblies 2 and 3 were characterised by X-ray powder diffraction along with IR, X-ray absorption spectroscopy (XAS), proton induced X-ray emission (PIXE) and magnetic measurements. 2 and 3 crystallise in a tetragonal system, space group I4/mmm with cell parameters a = b = 7.2695(9) A; c = 28.268(5) A; Z = 2 (2) and a = b = 7.2858(9) A, c = 28.282(5) A, Z = 2 (3). Both networks are characterised by the increase of TC from 33 K to 40 K and coercivity from 0.2 to 2–2.5 kOe compared to 1. Despite their general structural and magnetic similarity, 2 and 3 reveal significant differences in magnetic dimensionality: compound 2 exhibits the features of a metamagnet with a threshold field of 1.8 kOe at 4.2 K, while compound 3 resembles a classical magnet with 3D ordering. This difference is discussed in terms of non-stoichiometry of the networks accompanied by the appearance of different numbers of non-magnetic “defects” due to the formation of diamagnetic W(IV) centres.