Key role of size and electronic configuration on the sign and strength of the magnetic coupling in a series of Cu₂Ln Trimers (Ln = Ce, Gd, Tb, Dy and Er)

Abstract

Five new trinuclear complexes with formula [(CuL^α−Me) ₂Ce(NO₃)₃] (1) and [(CuL^α−Me) ₂Ln(H₂O)(NO₃) ₂](NO₃)•2(CH₃OH) (Ln = Gd(2), Tb(3), Dy(4) and Er(5)) have been synthesized using the bidentate N₂O₂ donor metalloligand [CuL^α−Me] (H₂L^α−Me = N,N′-bis(α-methylsalicylidene)-1,3-propanediamine) and structurally characterized. In the case of compound 1, the larger ionic radius of Ce(III) leads to a neutral trinuclear complex with an asymmetric CeO₁₀ tetradecahedron coordination geometry formed by four oxygen atoms from two (CuL^α−Me) units and three bidentate NO₃⁻ ligands. In contrast, the isomorphic complexes 2–5, with smaller Ln(III) ions, give rise to monocationic trinuclear complexes with a non-coordinated nitrate as a counter ion. In these complexes, the Ln(III) ions show a LnO₉ tricapped trigonal prismatic coordination geometry with C₂ symmetry formed by four oxygen atoms from two (CuL^α−Me) units, two bidentate NO₃⁻ ligands and a water molecule. The magnetic properties show the presence of weak antiferromagnetic interactions in 1 and weak ferromagnetic interactions in 2–5. The fit of the magnetic properties of compounds 2–5 to a simple isotropic-exchange symmetric trimer model, including the anisotropy of the Ln(III) ions, shows that in all cases the Cu-Ln magnetic coupling is weak (J_Cu-Ln = 1.81, 1.27, 0.88 and 0.31 cm⁻¹ for 2–5, respectively) and linearly decreases as the number of unpaired f electrons of the Ln(III) decreases. The value found in compound 2 nicely fits with the previously established correlation between the dihedral Cu–O–O–Gd angle and the J value.