Role of the Diamagnetic Zinc(II) Ion in Determining the Electronic Structure of Lanthanide Single‐Ion Magnets

Abstract

Four complexes containing Dy^III and Pr^III ions and their Ln^III -Zn^II analogs have been synthesized in order to study the influence that a diamagnetic Zn^II ion has on the electronic structure and hence, the magnetic properties of the Dy^III and Pr^III single ions. Single-crystal X-ray diffraction revealed the molecular structures as [Dy^III (HL)₂ (NO₃ )₃ ] (1), [Pr^III (HL)₂ (NO₃ )₃ ] (2), [Zn^II Dy^III (L)₂ (CH₃ CO₂ )(NO₃ )₂ ] (3) and [Zn^II₂ Pr^III (L)₂ (CH₃ CO₂ )₄ (NO₃ )] (4) (where HL=2-methoxy-6-[(E)-phenyliminomethyl]phenol). The dc and ac magnetic data were collected for all four complexes. Compounds 1 and 3 display frequency dependent out-of-phase susceptibility signals (χ_M "), which is a characteristic signature for a single-molecule magnet (SMM). Although 1 and 3 are chemically similar, a fivefold increase in the anisotropic barrier (U_eff ) is observed experimentally for 3 (83 cm^-1 ), compared to 1 (16 cm^-1 ). To rationalize the larger anisotropic barrier (1 vs. 3), detailed ab initio calculations were performed. Although the ground state Kramer's doublet in both 1 and 3 are axial in nature (g_zz =19.443 for 1 and 18.82 for 3), a significant difference in the energy gap (U_eff ) between the ground and first excited Kramer's doublet is calculated. This energy gap is governed by the electrostatic repulsion between the Dy^III ion and the additional charge density found for the phenoxo bridging ligand in 3. This extra charge density was found to be a consequence of the presence of the diamagnetic Zn^II ion present in the complex. To explore the influence of diamagnetic ions on the magnetic properties further, previously reported and structurally related Zn-Dy^III complexes were analyzed. These structurally analogous complexes unambiguously suggest that the electrostatic repulsion is found to be maximal when the Zn-O-Dy-O dihedral angle is small, which is an ideal condition to maximize the anisotropic barrier in Dy^III complexes.