Role of Single-Ion Anisotropy and Magnetic Exchange Interactions in Suppressing Zero-Field Tunnelling in {3d-4f} Single Molecule Magnets.

Extensive ab initio CASSCF/RASSI-SO/SINGLE_ANISO/POLY_ANISO calculations have been undertaken on eight structurally similar previously synthesized [CuII(L)(C3H6O)LnIII(NO3)3] (Ln = Dy (1), Tb (3), Ho (5), and Er (7)) and [VIVO(L)(C3H6O)LnIII(NO3)3] (Ln = Dy (2), Tb (4), Ho (6), and Er (8)) (here H2L = N,N'-bis(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane) complexes (crystal structures reported earlier). Our estimated exchange interactions (J) using the Lines model for complexes 1-8 (1.55 cm-1, 0.15 cm-1, 5.30 cm-1, 0.06 cm-1, 1.05 cm-1, -0.18 cm-1, 0.24 cm-1, and -0.02 cm-1 for complexes 1-8 respectively) match well with the experimental values (HE-EPR and pulse magnetization technique) reported earlier and offer confidence in the methodology employed. We have established the mechanism of magnetic coupling for this series to rationalize the observation that LnCu complexes are strongly coupled compared to LnV complexes. Besides, the results procured based on the BS-DFT method imply a crucial role of overlap between the 3d and 4f orbitals, the formally empty 5d/6s/6p orbitals of LnIII ion in the exchange coupling mechanism. To probe the origin/absence of magnetization relaxation observed in these complexes 1-8, both the single-ion and the exchange anisotropy are analyzed. Our calculations reveal that stronger exchange interaction quenches the quantum tunnelling of magnetization behavior in these complexes; however, for LnV complexes the exchange interaction was too small to offer a large blockade barrier. In the quest to obtain a stronger exchange interaction, we have assessed several models and have developed a magneto-structural correlation. An antagonizing behavior between the JCuDy and Ucal values are noted for the Dy-O-O-Cu dihedral angle correlation developed on complex 1. This highlights the subtle nature of the magnetic anisotropy in this class of complexes and postulates that both the single-ion anisotropy and the exchange interaction are needed to be targeted simultaneously to achieve a new generation {3d-4f} single molecule magnets (SMM).