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)

Five new trinuclear complexes with formula [(CuL alpha-Me)(2)Ce(NO3)(3)] (1) and [(CuL alpha-Me)(2)Ln(H2O)(NO3)(2)](NO3)center dot 2(CH3OH) (Ln = Gd(2), Tb(3), Dy(4) and Er(5)) have been synthesized using the bidentate N202 donor metalloligand [CuL--me] (H2L(alpha-Me) = N,M-bis(a-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 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 LnO(9) tricapped trigonal prismatic coordination geometry with C-2 symmetry formed by four oxygen atoms from two (CuL. 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 (ku_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.