Planar Trimethylenemethane Dianion Chemistry of Lanthanide Metallocenes:  Synthesis, Structure, Density Functional Theory Analysis, and Reactivity of [(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>Ln]<sub>2</sub>[μ-η<sup>3</sup>:η<sup>3</sup>-C(CH<sub>2</sub>)<sub>3</sub>] Complexes

The unusual formation of planar trimethylenemethane (TMM) dianion complexes of lanthanide metallocenes, [(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>Ln]<sub>2</sub>[μ-η<sup>3</sup>:η<sup>3</sup>-C(CH<sub>2</sub>)<sub>3</sub>] (Ln = Sm, <b>1</b>; La, <b>2</b>; Pr, <b>3</b>; Nd, <b>4</b>; Y, <b>5</b>) 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 <b>1</b><b>−</b><b>4</b> have been expanded from the [(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>Ln][(μ-Ph)<sub>2</sub>BPh<sub>2</sub>]/neopentyl lithium reaction involving β-methyl elimination to a [(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>Ln][(μ-Ph)<sub>2</sub>BPh<sub>2</sub>]/isobutyl lithium route. The synthetic pathways are sensitive to metal radius. To obtain <b>5</b>, the methylallyl complex, (C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>Y[CH<sub>2</sub>C(Me)CH<sub>2</sub>], <b>6</b>, was synthesized and treated with [(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>YH]<i><sub>x</sub></i>. In the Lu case, the neopentyl complex [(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>LuCH<sub>2</sub>C(CH<sub>3</sub>)<sub>3</sub>]<i><sub>x</sub></i>, <b>7</b>, 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 <b>1</b> to expand the original four-carbon TMM skeleton to a dianionic bis(alkoxide) ligand containing a symmetrically substituted CCH<sub>2</sub> moiety in [(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>Sm]<sub>2</sub>[μ-(OCPh<sub>2</sub>CH<sub>2</sub>)<sub>2</sub>CCH<sub>2</sub>], <b>8</b>. In this reaction, the TMM complex reacts as a bifunctional bisallylic reagent that generates an organic framework containing a central vinyl group.