Ligand Effects in the Synthesis of Ln²⁺ Complexes by Reduction of Tris(cyclopentadienyl) Precursors Including C–H Bond Activation of an Indenyl Anion

The tris­(cyclopentadienyl) yttrium complexes Cp₃Y­(THF), Cp^Me₃Y­(THF), Cp″₃Y, Cp″₂YCp, and Cp″₂YCp^Me [Cp = C₅H₅, Cp^Me = C₅H₄Me, Cp″ = C₅H₃(SiMe₃)₂] have been treated with potassium graphite in the presence of 2.2.2-cryptand to search for more stable examples of complexes featuring the recently discovered Y²⁺ ion first isolated in [K­(18-crown-6)]­[Cp′₃Y] and [K­(2.2.2-cryptand)]­[Cp′₃Y], 1-Y (Cp′ = C₅H₄SiMe₃). Reduction of the tris­(cyclopentadienyl) complexes generates dark solutions like that of 1-Y, and the EPR spectra contain doublets with g values between 1.990 and 1.991 and hyperfine coupling constants of 34–47 gauss that are consistent with the presence of Y²⁺. [K­(2.2.2-cryptand)]­[Cp″₂YCp], 2-Y, was characterizable by X-ray crystallography. Reduction of the Cp″₃Gd, Cp″₂GdCp, and Cp″₂GdCp^Me complexes containing the larger metal gadolinium were also examined. In each case, dark solutions and EPR spectra like that of [K­(2.2.2-cryptand)]­[Cp′₃Gd], 1-Gd, were obtained, and [K­(2.2.2-cryptand)]­[Cp″₂GdCp], 2-Gd, was crystallographically characterizable. None of the new yttrium and gadolinium complexes displayed greater stability than 1-Y and 1-Gd. Exploration of this reduction chemistry with indenyl ligands did not give evidence for +2 complexes. The only definitive information obtained from reductions of the Cp^In₃Ln (Cp^In = C₉H₇, Ln = Y, Ho, Dy) complexes was the X-ray crystal structure of {K­(2.2.2-cryptand)}₂{[(C₉H₇)₂Dy­(μ–η⁵:η¹-C₉H₆)]₂}, a complex containing the first example of the indenyl dianion, (C₉H₆)^2–, derived from C–H bond activation of the (C₉H₇)^1– monoanion. Density functional theory analysis of these results provides an explanation for the observed hyperfine coupling constants in the yttrium complexes and for the C–H bond activation observed for the indenyl complex.