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

The reaction of K L 3 ( L 3 = 1,3-C 3 H 3 (SiMe 3 ) 2 ) with NdI 3 (THF) 3.5 affords two products, L 3 NdI 2 (THF) 1.25 and ( L 3 ) 2 NdI(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 YCl 3 with 2 equiv of the ansa -bis(allyl) ligand K 2 [3-(C 3 H 3 SiMe 3 -1) 2 SiPh 2 ] in tetrahydrofuran followed by recrystallization from diethyl ether gives [Y{3-(η 3 -C 3 H 3 SiMe 3 -1) 2 SiPh 2 } 2 {μ-K}(THF) 0.5 (Et 2 O) 1.5 ] ∞ ( 2 ) as a bright orange solid. Reaction of Cp‘ ‘ 2 LnCl 2 Li(THF) 2 (Cp‘ ‘ = 1,3-C 5 H 3 (SiMe 3 ) 2 ); Ln = Y or Sm) with [K 2 {3-(η 3 -C 3 H 3 SiMe 3 -1) 2 SiMe 2 ] in THF leads to ligand redistribution to give [Li(OEt 2 )(THF) 3 ][Ln{3-(η 3 -C 3 H 3 SiMe 3 -1) 2 SiMe 2 } 2 ] ( 3, Ln = Y; 4, Ln = Sm). The identity of 3 was confirmed by single-crystal X-ray diffraction. Reaction of Cp‘ ‘ 2 YCl 2 Li(THF) 2 with [K 2 {3-(η 3 -C 3 H 3 SiMe 3 -1) 2 SiPh 2 ] affords 2, while Cp‘ ‘ 2 SmCl 2 Li(THF) 2 and [K 2 {3-(η 3 -C 3 H 3 SiMe 3 -1) 2 SiPh 2 ] give a red crystalline solid, identified by single-crystal X-ray analysis as the mono-Cp complex [Cp‘ ‘Sm{3-(η 3 -C 3 H 3 SiMe 3 -1) 2 SiPh 2 }(μ-Cl)Li(THF) 3 }] ( 5 ). Above 50 °C complexes 1 as well as ( L 3 ) 2 LnCl(THF) ( 7 ) and ( L 3 ) 2 YCl ( 8 ) initiate the rapid polymerization of ε-caprolactone. The anionic complexes 2, 3, 4, and [Y{3-(η 3 -C 3 H 3 SiMe 3 -1)SiMe 2 } 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).