Crystal structure and magnetic properties of [{Cu(cyclam)}3{Fe(CN)6}2] · 6H2O,a cyano-bridged assembly with a rope-ladder chain structure

The synthesis, crystal structure and magnetic properties of the cyano-bridged complex [{Cu(cyclam)}₃{Fe(CN)₆}₂] · 6H₂O are reported. Its structure is made up of centrosymmetric S-shaped pentanuclear [{Cu(cyclam}₃{Fe(CN)₆)}₂] units, in which three [Cu(cyclam)]²⁺ units are alternatively bridged by two trans-CN groups of [Fe(CN)₆]³⁻ anions and water molecules. The pentanuclear Fe₂Cu₃ units are held together by two complementary and very weak Fe–CN?Cu1 bonds, forming a rope-ladder chain along the c axis. The compound exhibits a ferromagnetic interaction between the Cu(II) and Fe(III) ions as a consequence of the orthogonality of their magnetic orbitals of σ and π nature, respectively. The magnetic data were fitted to the calculated magnetic susceptibility equation for a pentanuclear model, leading to the following magnetic parameters: J₁ = 9.0(3) cm⁻¹, J₂ = 3.8(4) cm⁻¹, g = 2.2, θ = −1.2 K. These results show that the interactions through the long Cu–N axial bonds are not so weak as is usually assumed.