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. (Figure Presented).