Planar Trimethylenemethane Dianion Chemistry of Lanthanide Metallocenes:  Synthesis, Structure, Density Functional Theory Analysis, and Reactivity of [(C₅Me₅)₂Ln]₂[μ-η³:η³-C(CH₂)₃] Complexes

The unusual formation of planar trimethylenemethane (TMM) dianion complexes of lanthanide metallocenes, [(C₅Me₅)₂Ln]₂[μ-η³:η³-C(CH₂)₃] (Ln = Sm, 1; La, 2; Pr, 3; Nd, 4; Y, 5) has been examined by synthesizing examples with metals from La to Y to examine the effects of radial size on structure and to provide closed shell examples for direct comparison with density functional theory (DFT) calculations. Synthetic routes to 1−4 have been expanded from the [(C₅Me₅)₂Ln][(μ-Ph)₂BPh₂]/neopentyl lithium reaction involving β-methyl elimination to a [(C₅Me₅)₂Ln][(μ-Ph)₂BPh₂]/isobutyl lithium route. The synthetic pathways are sensitive to metal radius. To obtain 5, the methylallyl complex, (C₅Me₅)₂Y[CH₂C(Me)CH₂], 6, was synthesized and treated with [(C₅Me₅)₂YH]_x. In the Lu case, the neopentyl complex [(C₅Me₅)₂LuCH₂C(CH₃)₃]_x, 7, was isolated instead of the TMM product. X-ray crystallography showed that the metrical parameters of the planar TMM dianions in each complex are similar. The structural data have been compared with DFT calculations on the closed-shell lanthanum and lutetium complexes that suggest only limited covalent interactions with the lanthanide ion. Benzophenone (2 equiv) reacts with 1 to expand the original four-carbon TMM skeleton to a dianionic bis(alkoxide) ligand containing a symmetrically substituted CCH₂ moiety in [(C₅Me₅)₂Sm]₂[μ-(OCPh₂CH₂)₂CCH₂], 8. In this reaction, the TMM complex reacts as a bifunctional bisallylic reagent that generates an organic framework containing a central vinyl group.