Synthesis, molecular structure and magnetic properties of tetranuclear copper(ii) complexes with pendant-arm macrocyclic ligands

Abstract

The condensation of sodium 2,6-diformyl-4-X-phenolate (X = Cl, CH₃) with the reduced 1 ∶ 1 Schiff base of 2,2′,2″-tris(aminoethyl)amine (tren) and 2-formylfuran (or benzaldehyde) followed by in situ transmetallation with copper(II) perchlorate results in the production of four novel tetranuclear copper(II) complexes of 2 ∶ 2 macrocyclic Schiff bases with functional pendant-arms (H₂L^1–4), which have been spectroscopically characterized. A typical complex [Cu₄L¹Cl₂(OH)₂](ClO₄)₂ (1) has been structurally studied by X-ray diffraction. In the cation of complex 1, [Cu₄L¹Cl₂(OH)₂]²⁺, a chair-shaped Cu₄O₄ core with two μ₃-hydroxide groups inside is composed of two kinds of crystallographically independent Cu(II) atoms, each of which possesses a distorted square pyramidal configuration. The four metal atoms are in an approximate parallelogram. Two pendant-arms, bonding in a monodentate fashion to their adjacent metal atoms, lie in the ‘trans’ position to the macrocycle. The variable temperature magnetic susceptibility of 1 has been measured over the temperature range 4–300 K. The best fitting with a quasi-butterfly magnetostructural model shows antiferromagnetic exchanges within this compound, with J_bb  =  −270.8 cm⁻¹, J_wb1  =  −40.3 cm⁻¹ and J_wb2  =  −37.5 cm⁻¹.