Rare earth benzene tetraanion-bridged amidinate complexes

The benzene tetraanion-bridged rare earth inverse arene amidinate complexes [{Ln(κ¹:η⁶-Piso)}₂(μ-η⁶:η⁶-C₆H₆)] (2-Ln, Ln = Gd, Tb, Dy, Y; Piso = {(NDipp)₂C ^t Bu}, Dipp = C₆H₃ ⁱPr₂-2,6) were prepared by the reduction of parent Ln(iii) bis-amidinate halide precursors [Ln(Piso)₂X] (Ln = Tb, Dy; X = Cl, I) or [Ln(Piso)₂I] (Ln = Gd, Y) with 3 eq. KC₈ in benzene, or by the reaction of the homoleptic Ln(ii) complexes [Ln(Piso)₂] (Ln = Tb, Dy) with 2 eq. KC₈ in benzene. The arene exchange reaction of 2-Tb with toluene gave crystals of [{Tb(κ¹:η⁶-Piso)}₂(μ-η⁶:η⁶-C₇H₈)] (3-Tb), while no reactions were observed when C₆D₆ solutions of 2-Y were separately treated with biphenyl, naphthalene or anthracene. The reactivity study shows that 2-Y can behave as a four-electron reductant to reduce 1,3,5,7-cyclooctatetraene (COT). Complexes 2-Ln were characterized by single crystal X-ray diffraction, elemental analysis, SQUID magnetometry, UV-vis-NIR, ATR-IR, NMR, density functional theory (DFT) and ab initio calculations. These data consistently show that 2-Ln formally contain Ln(iii) centres with arene-capped inverse-sandwich Dipp-Ln(iii)-(C₆H₆)^4--Ln(iii)-Dipp configurations, and DFT calculations on a model of 2-Y revealed strong Y-(C₆H₆)^4- δ-bonding interactions between the filled π-orbitals of the benzene tetraanion and vacant 4d orbitals of the Y(iii) ions. A strong intermolecular coupling interaction between the two Tb(iii) centres in 2-Tb (J _tot = -6.84 cm^-1) was evidenced by a step in a magnetization vs. field plot of 2-Tb at ca. 3.4 T at 2 K, which we attribute to an anti-ferromagnetic transition of the magnetic moment; we also determined an exchange coupling constant J _ex = -0.25(1) cm^-1 for 2-Gd.