Role of Magnetic Exchange Interactions in the Magnetization Relaxation of {3d–4f} Single‐Molecule Magnets: A Theoretical Perspective

Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni 3 III Ln III } star‐type complexes [Ln=Gd ( 1 ), Dy ( 2 )] to shed light on the mechanism of magnetic exchange in 1 and the origin of the slow magnetization relaxation in complex  2 . DFT calculations correctly reproduce the sign and magnitude of the J values compared to the experiments for complex  1 . Acute ∢NiOGd bond angles present in 1 instigate a significant interaction between the 4f xyz orbital of the Gd III ion and 3d ${{_{x{^{2}}- y{^{2$ orbital of the Ni II ions, leading to rare and strong antiferromagnetic Ni⋅⋅⋅Gd interactions. Calculations reveal the presence of a strong next‐nearest‐neighbour Ni⋅⋅⋅Ni antiferromagnetic interaction in complex  1 leading to spin frustration behavior. CASSCF+RASSI‐SO calculations performed on complex  2 suggest that the octahedral environment around the Dy III ion is neither strong enough to stabilize the m J |±15/2〉 as the ground state nor able to achieve a large ground‐state–first‐excited‐state gap. The ground‐state Kramers doublet for the Dy III ion is found to be the m J |±13/2〉 state with a significant transverse anisotropy, leading to very strong quantum tunneling of magnetization (QTM). Using the POLY_ANISO program, we have extracted the J NiDy interaction as −1.45 cm −1 . The strong Ni⋅⋅⋅Dy and next‐nearest‐neighbour Ni⋅⋅⋅Ni interactions are found to quench the QTM to a certain extent, resulting in zero‐field SMM behavior for complex  2 . The absence of any ac signals at zero field for the structurally similar [Dy(AlMe 4 ) 3 ] highlights the importance of both the Ni⋅⋅⋅Dy and the Ni⋅⋅⋅Ni interactions in the magnetization relaxation of complex  2 . To the best of our knowledge, this is the first time that the roles of both the Ni⋅⋅⋅Dy and Ni⋅⋅⋅Ni interactions in magnetization relaxation of a {3d–4f} molecular magnet have been established.