Cyano-Bridged Pentanuclear and Honeycomblike MIIICuII (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 Fe2Cu3 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-Ca-x 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 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.