Allyl complexes of the heavy alkaline-earth metals: molecular structure and catalytic behavior

Abstract

Reaction of two equivalents of K[1,3-(SiMe₃)₂C₃H₃] (= KA′) with SrI₂ in thf produces the allyl complex SrA′₂(thf)₂. In the solid state, the monomeric complex is isostructural with its calcium counterpart; the allyl ligands are bound in a symmetrical trihapto manner to the metals, with an average Sr–C distance of 2.801(5) Å. The reaction of KA′ and BaI₂ does not result in the expected bis(allyl)barium complex, but rather in a heterometallic barium/potassium species, K(thf)Ba₂A′₅. The allyls are in fast exchange in solution, and in the solid state, the complex forms a polymeric chiral chain. Each barium center is coordinated by one terminal and two bridging allyl ligands; the potassium center is coordinated by two bridging allyl ligands and one thf molecule. On reaction with iodine, C–C coupling occurs with CaA′₂(thf)₂, SrA′₂(thf)₂ and K(thf)Ba₂A′₅ to give the 1,5-hexadiene, [(SiMe₃)₂C₃H₃]₂. CaA′₂(thf)₂, SrA′₂(thf)₂ and K(thf)Ba₂A′₅ are initiators for methyl methacrylate polymerization; the analogous magnesium complex is not active.