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 Cu^II_2+x{Cu^II₄[W^V(CN)₈]_4−2x[W^IV(CN)₈]_2x}·yH₂O were obtained according to two different synthetic strategies. The heterogeneous reaction between solid 2D cyano-bridged network (dienH₃){Cu^II[W^V(CN)₈]}₃·4H₂O (1) (dienH₃³⁺ = protonated diethylenetriamine) and an aqueous solution of Dy^III(NO₃)₃ results in the removal of dienH₃³⁺ cations and the formation of a 3D cyano-bridged Cu^II_2.44{Cu^II₄[W^V(CN)₈]_3.12[W^IV(CN)₈]_0.88}·5H₂O (2) network. The direct combination of [Cu^II(H₂O)₆]²⁺ and [W^V(CN)₈]³⁻ in aqueous media leads to the structurally related Cu^II_2.97{Cu^II₄[W^V(CN)₈]_2.06[W^IV(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) Å; c = 28.268(5) Å; Z = 2 (2) and a = b = 7.2858(9) Å, c = 28.282(5) Å, Z = 2 (3). Both networks are characterised by the increase of T_C 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.