Sterically Hindered Lanthanide Allyl Complexes and Their Use as Single-Component Catalysts for the Polymerization of Methyl Methacrylate and ε-Caprolactone

Abstract

The reaction of KL3 (L3 = 1,3-C3H3(SiMe3)2) with NdI3(THF)3.5 affords two products, L3NdI2(THF)1.25 and (L3)2NdI(THF)2 (1). The latter has been crystallographically characterized; it has a distorted trigonal-bipyramidal structure with the THF ligands in trans position. The reaction of YCl3 with 2 equiv of the ansa-bis(allyl) ligand K2[3-(C3H3SiMe3-1)2SiPh2] in tetrahydrofuran followed by recrystallization from diethyl ether gives [Y{3-(η3-C3H3SiMe3-1)2SiPh2}2{μ-K}(THF)0.5(Et2O)1.5]∞ (2) as a bright orange solid. Reaction of Cp‘ ‘2LnCl2Li(THF)2 (Cp‘ ‘ = 1,3-C5H3(SiMe3)2); Ln = Y or Sm) with [K2{3-(η3-C3H3SiMe3-1)2SiMe2] in THF leads to ligand redistribution to give [Li(OEt2)(THF)3][Ln{3-(η3-C3H3SiMe3-1)2SiMe2}2] (3, Ln = Y; 4, Ln = Sm). The identity of 3 was confirmed by single-crystal X-ray diffraction. Reaction of Cp‘ ‘2YCl2Li(THF)2 with [K2{3-(η3-C3H3SiMe3-1)2SiPh2] affords 2, while Cp‘ ‘2SmCl2Li(THF)2 and [K2{3-(η3-C3H3SiMe3-1)2SiPh2] give a red crystalline solid, identified by single-crystal X-ray analysis as the mono-Cp complex [Cp‘ ‘Sm{3-(η3-C3H3SiMe3-1)2SiPh2}(μ-Cl)Li(THF)3}] (5). Above 50 °C complexes 1 as well as (L3)2LnCl(THF) (7) and (L3)2YCl (8) initiate the rapid polymerization of ε-caprolactone. The anionic complexes 2, 3, 4, and [Y{3-(η3-C3H3SiMe3-1)SiMe2}2{μ-K(THF)}·(THF)n]∞ (6) are very active for both ε-caprolactone and methyl methacrylate polymerization at room temperature. The stereoselectivity is dependent on the solvent and the countercation, with lithium salts of 3 and 4 in THF producing syndio-rich poly(methyl methacrylate).