Heterometallic Zn₃Ln₃ Ensembles Containing (μ₆-CO₃) Ligand and Triangular Disposition of Ln³⁺ ions: Analysis of Single-Molecule Toroic (SMT) and Single-Molecule Magnet (SMM) Behavior

Two new heterometallic Zn₃Ln₃ (Ln³⁺=Dy, Tb) complexes, with a double triangular topology of the metal ions, have been assembled from the polytopic Mannich base ligand 6,6′-{[2-(dimethylamino)ethylazanediyl]bis(methylene)}bis(2-methoxy-4-methylphenol) (H₂L) with the aid of an in situ generated carbonate ligand from atmospheric CO₂ fixation. Theoretical calculations indicate axial ground states for the Ln³⁺ ions in these complexes, with their local magnetic moments being almost coplanar and tangential to the Ln³⁺ atoms that define the equilateral triangle. Therefore, they can be considered as single-molecule toroics (SMTs) with almost zero total magnetic moment. Micro-SQUID measurements on the Dy³⁺ counterpart show hysteresis loops below 3 K that have an S-shape, with large coercive fields opening upon cooling. This behavior is typical of a single molecule magnet (SMM) with very slow zero-field relaxation. At around ±0.35 T, the loops have a broad step, which is due to a direct relaxation process and corresponds to an acceleration of the relaxation of the magnetization, also observed at this magnetic field from ac susceptibility measurements. Simulations suggest that the broad step corresponds to two level avoidance of crossing points where the spin chiral Kramers doublet meets excited states of the coupled manifold, whose position is defined by exchange and dipole interactions. The Tb³⁺ counterpart does not exhibit SMM behavior, which is due to the fact that the degeneracy of the ground state of the exchange coupled system is lifted at zero field, thus favoring quantum tunneling of magnetization (QTM).