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 isdue to the complicated energy spectrum which arises due to the orbitalangular momentum of anisotropic lanthanides. Interestingly, the exchangecouplingJin Ln(III)-Cu(II) systems was found to be antiferromagneticfor <4f7metal ions and ferromagnetic for >= 4f7metal ions, while the netmagnitude ofJTotalstrength gradually decreases moving fromf1tof13. Whilethis is established in several examples, the reason for this intriguing trend isnot rationalized. In this article, we have taken up these challenging tasks bysynthesizing a family of complexes with the general molecular formula[Cu2Ln(HL)4(NO3)](NO3)2, where Ln = La (1-La), Ce (2-Ce), Pr (3-Pr),Gd (4-Gd), Tb (5-Tb), Dy (6-Dy), and Ho (7-Ho) and HL = C15H15N1O3; (2-methoxy-6-[(E)-2 '-hydroxymethyl-phenyliminomethyl]-phenolate) is amonodeprotonated tridentate Schiffbase ligand. Detailed dc magneticsusceptibility measurements performed for all the complexes reveal that the Cu(II) ion is coupled ferromagnetically to the respectiveLn(III) ion, which has more than seven electrons in the 4f shell, while an antiferromagnetic coupling is witnessed if Ln(III) has lessthan seven electrons. The strength of the exchange coupling constant was quantitatively determined for representative complexesfrom the high-field/high-frequency electron paramagnetic resonance spectroscopy which follows the order of4-Gd(1.50(10) cm-1)>5-Tb(1.18(10) cm-1)>6-Dy(0.56(10) cm-1based on the-middot+middot-JSJSJ2()LnzzCuCuLnCuLn12spin Hamiltonian. The increasedaxiality in5-Tband6-Dydue to the presence of 3d ions in the near vicinity of an oblate ion and the increased exchange couplingstrength between Cu(II) and Tb(III) or Dy(III) is the ideal combination to stabilize magnetic bistability in these complexes in theabsence of an external magneticfield with the effective energy barrier of 15.7 K (Tau o= 2.49x10-6s) and 12.6 K (Tau o= 1.70x10-5s),respectively. To rationalize this experimental trend, we have performedab initioCASSCF and DFT calculations. To compute theJvalues, we have employed POLY_ANISO routines and utilized the computed data to establish the generic mechanism of magneticcoupling in {Cu-Ln-Cu} motifs. These mechanisticfindings reveal the importance of 5d orbitals and their energy with respect tothe dx2-y2orbital of Cu(II) ions in controlling the magnetic coupling of {Cu-4f} complexes