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

The thermostable four-coordinate divalent lanthanide (Ln) bis-amidinate complexes [Ln(Piso)] (Ln = Tb, Dy; Piso = {(NDipp)CBu}, Dipp = CHPr-2,6) were prepared by the reduction of parent five-coordinate Ln(III) precursors [Ln(Piso)I] (Ln = Tb, Dy) with KC; halide abstraction of [Ln(Piso)I] with [H(SiEt)][B(CF)] gave the respective Ln(III) complexes [Ln(Piso)][B(CF)]. All complexes were characterized by X-ray diffraction, ICP-MS, elemental analysis, SQUID magnetometry, UV-vis-NIR, ATR-IR, NMR, and EPR spectroscopy and CASSCF-SO calculations. These data consistently show that [Ln(Piso)] formally exhibit Ln(II) centers with 4f5d (Ln = Tb, = 8; Dy, = 9) valence electron configurations. We show that simple assignments of the f-d coupling to either - or - schemes are an oversimplification, especially in the presence of significant crystal field splitting. The coordination geometry of [Ln(Piso)] is intermediate between square planar and tetrahedral. Projecting from the quaternary carbon atoms of the CN 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(CPr)]. We tentatively attribute the fast zero-field magnetic relaxation for these complexes at low temperatures to transverse fields, resulting in considerable mixing of states.