Planar Trimethylenemethane Dianion Chemistry of Lanthanide Metallocenes:  Synthesis, Structure, Density Functional Theory Analysis, and Reactivity of [(C5Me5)2Ln]2[μ-η33-C(CH2)3] Complexes

The unusual formation of planar trimethylenemethane (TMM) dianion complexes of lanthanide metallocenes, [(C5Me5)2Ln]2[μ-η33-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 14 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.