Thermally Stable Terbium(II) and Dysprosium(II) Bis-amidinate Complexes

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.