Cyano-bridged pentanuclear and honeycomblike M^IIICu^II (M = Fe, Cr) bimetallic assemblies: Structural variations modulated by side groups of macrocyclic ligands and magnetic properties

Three two-dimensional (2D) layered M(III)-Cu(II) (M = Fe, Cr) bimetallic complexes and a discrete cluster were prepared using [M(CN)₆] ^3- and respective Cu macrocycles with methyl (1, 3) or ethyl (2, 4) side groups. The crystal structures can be described as aesthetic honeycomb-like M₂Cu₃ neutral sheets (1, 3, 4) and a pentanuclear Fe ₂Cu₃ entity (2) with an isolated Cu(L2)²⁺ moiety. The steric hindrance induced by the pendant groups renders the internal parameters of the Cu-N_ax (where the "ax" subscript denotes axial) length and Cu-N_ax-C_ax angle to become elongated and acute. The magnetic data disclose that ferromagnetic couplings are transmitted between M(III) and Cu(II) ions through the CN links. For 4, particularly, there is a magnetically defined chain (J_c) composed of dimers (J _d) and trimers (J_t), based on the structural considerations. The intramolecular magnetic coupling constants are J_d = 3.73 cm^-1 and J_t = 4.08 cm^-1, while the J_c value corresponds to 0.17 cm^-1. From a magnetostructural point of view, it is reasoned that the Cu-N_ax bond length is probably a determining factor of the strength of magnetic couplings in 4.