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) A. 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.