Anti-Bimetallic Complexes of Divalent Lanthanides with Silylated Pentalene and Cyclooctatetraenyl Bridging Ligands as Molecular Models for Lanthanide-Based Polymers

The new pentalene antibimetallic compounds [MCp*(THF)]₂(μ,η⁵:η⁵-C₈H₄^1,4-SiiPr₃) were prepared for M = Eu (1), Yb (2) from the one-pot reaction of MI₂(THF)_x and KCp* and the subsequent addition of ¹/₂ equiv of C₈H₄^1,4-SiiPr₃[K]₂ in THF. The related series of COT^1,4-SiiPr₃-bridged triple-deckers [MCp*(THF)_x]₂(μ,η⁸:η⁸-COT^1,4-SiiPr₃) (M = Eu, x = 0 (3); M = Yb, x = 0 (4); M = Sm, x = 1 (5)) were synthesized similarly; additionally, the base-free derivative with M = Sm and x = 0 (6) could be prepared by reaction of [SmCp*(μ-I)(THF)₂]₂ with COT^1,4-SiiPr₃[K]₂ in toluene with heating. The solid-state structures, as determined by X-ray diffraction, show antibimetallic arrangements in which the divalent lanthanide centers are held on opposing sides of a planar bridging ligand. The pentalene ligand coordinates in an approximate η⁵:η⁵ mode, with the metal centers slipped toward the wingtip carbons, whereas the COT ligand is bound in an η⁸:η⁸ fashion with the metal centers aligned with the centroid of the bridging ligand. Electronic spectroscopy suggests the Eu and Yb pentalene complexes have a smaller f−d gap than their COT analogues, indicating a greater extent of through-ligand metal−metal interaction in the pentalene species. [EuCp*(THF)_x]₂(μ-COT^1,4-SiiPr₃) displays a weak green-yellow emission in THF solution (λ_max 509 nm, Φ_em < 0.1%) upon excitation in the UV, consistent with a 4f⁶5d¹ → 4f⁷ emission process, with a short lifetime indicative of Eu−Eu coupling through the bridging COT ligand. Cyclic voltammetry reveals that 1, 3, and 4 decompose rapidly upon oxidation, although the monocation 2⁺ appears to be stable in THF solution. Through-ligand Yb−Yb coupling is suggested by the electrochemical data for 2, of magnitude similar to that observed for its transition-metal analogues. However, unfortunately, attempts to further quantify this conclusion by spectroscopic investigation of 2⁺ were unsuccessful.