A ferrimagnetic cyclic tetranuclear copper(ii) complex: cyclo-[tetrakis(μ-η3-hydroxyethanoato-1κO:2κ2O′,O″)tetrakis(1,10-phenanthroline)tetracopper] tetranitrate dihydrate. Structural and magnetic properties

Abstract

The crystal structure and magnetic analysis of a cyclic tetranuclear copper(II) complex with formula [Cu₄(phen)₄(glyOH)₄](NO₃)₄·2H₂O, where phen = 1,10-phenanthroline and glyOH = glycolate are described. Copper atoms are connected through bridging carboxylate groups from chelated glycolate units, forming a butterfly-type framework. Magnetic measurements as a function of temperature (2–300 K) and at 10, 100 and 1000 G were performed. The behaviour of the T–T curves is characteristic of ferrimagnetic-type coupling. Several theoretical magnetic models were tested to study the experimental data. A tetranuclear model of S = 1/2 coupled spins in a C_2v symmetry, is the one that better describes the magnetic response of the solid product over the whole range of experimental conditions explored. This model allows us to suggest the existence of a very weak antiferromagnetic coupling for two different pairs of Cu(II) centers, and a weak ferromagnetic coupling between those pairs. Besides this, negligible magnetic coupling at the mean field level is proposed. It is important to point out that although the system is homonuclear, with coordination spheres which are chemically equivalent for all the metallic centers, and which shows (at room temperature) equivalent internuclear distances between first neighbouring copper atoms, different types of intramolecular magnetic coupling are inferred. To our knowledge, the complex here studied appears to be the first case of a cyclic tetranuclear Cu(II) system showing global ferrimagnetic coupling.