Rare earth benzene tetraanion-bridged amidinate complexes.

The benzene tetraanion-bridged rare earth inverse arene amidinate complexes [{Ln(κ:η-Piso)}(μ-η:η-CH)] (2-Ln, Ln = Gd, Tb, Dy, Y; Piso = {(NDipp)C Bu}, Dipp = CH Pr-2,6) were prepared by the reduction of parent Ln(iii) bis-amidinate halide precursors [Ln(Piso)X] (Ln = Tb, Dy; X = Cl, I) or [Ln(Piso)I] (Ln = Gd, Y) with 3 eq. KC in benzene, or by the reaction of the homoleptic Ln(ii) complexes [Ln(Piso)] (Ln = Tb, Dy) with 2 eq. KC in benzene. The arene exchange reaction of 2-Tb with toluene gave crystals of [{Tb(κ:η-Piso)}(μ-η:η-CH)] (3-Tb), while no reactions were observed when CD 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 calculations. These data consistently show that 2-Ln formally contain Ln(iii) centres with arene-capped inverse-sandwich Dipp-Ln(iii)-(CH)-Ln(iii)-Dipp configurations, and DFT calculations on a model of 2-Y revealed strong Y-(CH) 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 ( = -6.84 cm) was evidenced by a step in a magnetization field plot of 2-Tb at . 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 = -0.25(1) cm for 2-Gd.