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

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posted on 26.10.2009, 00:00 by Owen T. Summerscales, Simon C. Jones, F. Geoffrey N. Cloke, Peter B. Hitchcock
The new pentalene antibimetallic compounds [MCp*(THF)]2(μ,η55-C8H41,4-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 C8H41,4-SiiPr3[K]2 in THF. The related series of COT1,4-SiiPr3-bridged triple-deckers [MCp*(THF)x]2(μ,η88-COT1,4-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 COT1,4-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 η55 mode, with the metal centers slipped toward the wingtip carbons, whereas the COT ligand is bound in an η88 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-COT1,4-SiiPr3) displays a weak green-yellow emission in THF solution (λmax 509 nm, Φem < 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.