Influence of the anions on the structure and magnetic properties of a series of bis(μ-diphenoxo)-bridged linear trinuclear copper(II) complexes: an experimental and theoretical study

Abstract

The reaction of H₂L (N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)-ethylenediamine) with different copper salts, in methanol and using a H₂L/Cu = 2 : 3 molar ratio, led to four new bis(μ-diphenoxo)-bridged Cu₃ complexes of general formula [{Cu(S)(μ-L)}₂Cu(H₂O)_2n]X₂ (S = CH₃OH, n = 1 and X = BF₄⁻ for (1) or ClO₄⁻ for (2); S = Br₃⁻ anion and n = 1 without any X species for (3); S = H₂O, n = 0 and X = NO₃⁻ for (4)). The use in the same reaction conditions of 4,4′-bipyridine (4,4′-bipy) as connector led to the chain complex [{Cu(μ-4,4′-bipy)_0.5(μ-L)}₂Cu(H₂O)_2n](ClO₄)₂·17H₂O (5). The structure of the centrosymmetric trinuclear unit in (1)–(5) consists of two [Cu(L)] fragments connected through two phenoxo bridging groups to the central copper(II) ion giving rise to a linear arrangement of the copper(II) ions, where the ligand acts in a compartmental form wrapping the metal centre with a N₂O₂ tetradentate bridging mode. The coordination polyhedron of the symmetrically related external copper atoms exhibits a geometry very close to square-pyramidal, whereas the central copper(II) atom displays either a tetragonally elongated octahedral geometry or a square-planar geometry. Owing to the steric hindrance promoted by the methoxy groups at the phenyl rings, the whole Cu₃ structure is not planar but folded along the line connecting the phenoxo bridging oxygen atoms of the same ligand. Temperature dependence of the magnetic susceptibility of complexes (1)–(5) was measured, showing strong antiferromagnetic interactions between the central and external atoms through the bis(μ-phenoxo) groups. DFT calculations were also performed (a) to support the experimental values of the coupling constant (J₁) between the nearest-neighbouring copper atoms, (b) to determine the magnitude of the interactions between next-nearest copper(II) atoms (J₂) and (c) to study magneto-structural correlations for this kind of bis(μ-diphenoxo) trinuclear copper(II) complex.