Synthesis, structure, magnetism and theoretical study of a series of complexes with a decanuclear core [Ln(iii)2Cu(ii)8] (Ln = Y, Gd, Tb, Dy)

Abstract

A family of four isomorph complexes with a decanuclear [Cu8Ln2] core of general formula [Ln2Cu8(μ-PyO)12(μ4-O)2(μ-Cl)2Cl4(H2O)2]·nH2O [Ln(III) = Y(III) (1), Gd(III) (2), Tb(III) (3), Dy(III) (4); 2-PyOH = 2-hydroxypyridine] was isolated and structurally characterized. All compounds are isomorphs and may be viewed as a hexanuclear central core sandwiched in between two lateral dinuclear copper units. The temperature dependence of the magnetic susceptibility and the field dependence of the magnetization were investigated on polycrystalline samples. The yttrium compound 1 showed an overall behavior dominated by an antiferromagnetic interaction between the copper ions, while for compounds 2–4 the magnetic behavior indicated the addition of a ferromagnetic interaction with the lanthanide ions. The magnetic properties were computationally studied by means of fragment ab initio calculations. The calculation on the yttrium complex allowed determining the strength and sign of the Cu⋯Cu magnetic interactions considering three antiferromagnetic coupling constants: two within the central (J3 = −44 cm−1) and the lateral (J4 = −40 cm−1) copper dinuclear unit, and one (J5 = −24 cm−1) between the lateral and the central copper. Simulation of the magnetic behavior of the Dy (4) compound gave J1 = +0.25 cm−1 for Dy–Dy and J2 = +2.0 cm−1 for Dy–Cu pairs. The calculated g tensors of the copper(II) ions were found to be quite anisotropic and contributed via anisotropic exchange interactions, together with zero-field (crystal field) splitting on Ln, to the weak single-molecule magnet (SMM) behavior of 2, 3 and 4. Among them, the highest coercivity was found in the gadolinium complex (2), despite the fact that it is much less anisotropic than the other two. We explain this surprising result by a higher multiplicity of the ground spin term in 2 compared to the ground manifolds of states in 3 and 4. Besides, due to relatively large Cu–Gd interaction, the ground exchange term in 2 has enough separation from excited exchange terms, which makes the barrier of reversal of magnetization efficient in this complex.