The thermostable four-coordinate divalent lanthanide (Ln) bis-amidinate complexes [Ln(Piso)₂] (Ln = Tb, Dy; Piso = {(NDipp)₂C^tBu}, Dipp = C₆H₃ⁱPr₂-2,6) were prepared by the reduction of parent five-coordinate Ln(III) precursors [Ln(Piso)₂I] (Ln = Tb, Dy) with KC₈; halide abstraction of [Ln(Piso)₂I] with [H(SiEt₃)₂][B(C₆F₅)] gave the respective Ln(III) complexes [Ln(Piso)₂][B(C₆F₅)]. All complexes were characterized by X-ray diffraction, ICP-MS, elemental analysis, SQUID magnetometry, UV-vis-NIR, ATR-IR, NMR, and EPR spectroscopy and ab initio CASSCF-SO calculations. These data consistently show that [Ln(Piso)₂] formally exhibit Ln(II) centers with 4fⁿ5d_z2¹ (Ln = Tb, n = 8; Dy, n = 9) valence electron configurations. We show that simple assignments of the f-d coupling to either L-S or J-s schemes are an oversimplification, especially in the presence of significant crystal field splitting. The coordination geometry of [Ln(Piso)₂] is intermediate between square planar and tetrahedral. Projecting from the quaternary carbon atoms of the CN₂ ligand backbones shows near-linear C···Ln···C arrangements. This results in strong axial ligand fields to give effective energy barriers to magnetic reversal of 1920(91) K for the Tb(II) analogue and 1964(48) K for Dy(II), the highest values observed for mononuclear Ln(II) single-molecule magnets, eclipsing 1738 K for [Tb(C₅ⁱPr₅)₂]. We tentatively attribute the fast zero-field magnetic relaxation for these complexes at low temperatures to transverse fields, resulting in considerable mixing of m_J states.

Theoretical calculation plays an important role in the emerging field of single-molecule magnets (SMMs). It can not only explain experimental phenomena but also provide synthetic guidance. This review focuses on discussing the computational methods that have been used in this field in recent years. The most common and effective method is the complete active space self-consistent field (CASSCF) approach, which predicts mononuclear SMM property very well. For bi- and multi-nuclear SMMs, magnetic exchange needs to be considered, and the exchange coupling constants can be obtained by Monte Carlo (MC) simulation, ab initio calculation via the POLY_ANISO program and density functional theory combined with a broken-symmetry (DFT-BS) approach. Further application for these calculation methods to design high performance SMMs is also discussed.

Arene capped inverse-sandwich rare earth complexes stabilized by Ln–[C₆H₆]⁴⁻ covalent δ-bonding.