Probing the Origin of Ferro-/Antiferromagnetic Exchange Interactions in Cu(II)-4f Complexes

The mechanistic investigations between Cu(II) and the anisotropic lanthanides (Ln(III)) are not much explored to date. This is due to the complicated energy spectrum which arises due to the orbital angular momentum of anisotropic lanthanides. Interestingly, the exchange coupling J in Ln(III)-Cu(II) systems was found to be antiferromagnetic for <4f⁷ metal ions and ferromagnetic for ≥4f⁷ metal ions, while the net magnitude of J_Total strength gradually decreases moving from f¹ to f¹³. While this is established in several examples, the reason for this intriguing trend is not rationalized. In this article, we have taken up these challenging tasks by synthesizing a family of complexes with the general molecular formula [Cu₂Ln(HL)₄(NO₃)](NO₃)₂, where Ln = La (1), Ce (2), Pr (3), Gd (4), Tb (5), Dy (6), and Ho (7) and HL = C₁₅H₁₅N₁O₃; (2-methoxy-6-[(E)-2'-hydroxymethyl-phenyliminomethyl]-phenolate) is a monodeprotonated tridentate Schiff base ligand. Detailed dc magnetic susceptibility measurements performed for all the complexes reveal that the Cu(II) ion is coupled ferromagnetically to the respective Ln(III) ion, which has more than seven electrons in the 4f shell, while an antiferromagnetic coupling is witnessed if Ln(III) has less than seven electrons. The strength of the exchange coupling constant was quantitatively determined for representative complexes from the high-field/high-frequency electron paramagnetic resonance spectroscopy which follows the order of 4 (1.50(10) cm^-1) > 5 (1.18(10) cm^-1) > 6 (0.56(10) cm^-1 based on the $$ spin Hamiltonian. The increased axiality in 5 and 6 due to the presence of 3d ions in the near vicinity of an oblate ion and the increased exchange coupling strength between Cu(II) and Tb(III) or Dy(III) is the ideal combination to stabilize magnetic bistability in these complexes in the absence of an external magnetic field with the effective energy barrier of 15.7 K (τ_o = 2.49 × 10^-6 s) and 12.6 K (τ_o = 1.70 × 10^-5 s), respectively. To rationalize this experimental trend, we have performed ab initio CASSCF and DFT calculations. To compute the J values, we have employed POLY_ANISO routines and utilized the computed data to establish the generic mechanism of magnetic coupling in {Cu-Ln-Cu} motifs. These mechanistic findings reveal the importance of 5d orbitals and their energy with respect to the d orbital of Cu(II) ions in controlling the magnetic coupling of {Cu-4f} complexes.