Pyrrolide-Ligated Organoyttrium Complexes. Synthesis, Characterization, and Lactide Polymerization Behavior

The N,N-bidentate ligand 2-{(N-2,6-diisopropylphenyl)iminomethyl)}pyrrole (L1) and the N,N,P-tridentate ligand 2-{(N-2-diphenylphosphinophenyl)iminomethyl)}pyrrole (L2) have been prepared. Their reactions with homoleptic yttrium tris(alkyl) compound Y(CH2SiMe3)3(THF)2 have been investigated. Treatment of Y(CH2SiMe3)3(THF)2 with 1 equiv of L1 generated a THF-solvated bimetallic (pyrrolylaldiminato)yttrium mono(alkyl) complex (1) of central symmetry. In this process, L1 is deprotonated by metal alkyl and its imino CN group is reduced to C−N by intramolecular alkylation, generating dianionic species that bridge two yttrium alkyl units in a unique η5/η1:κ1 mode. The pyrrolyl ring behaves as a heterocyclopentadienyl ligand. Reaction of Y(CH2SiMe3)3(THF)2 with 2 equiv of L1 afforded the monomeric bis(pyrrolylaldiminato)yttrium mono(alkyl) complex (2), selectively. Amination of 2 with 2,6-diisopropylaniline gave the corresponding yttrium amido complex (3). In 3 the pyrrolide ligand is monoanionic and bonds to the yttrium atom in a η1:κ1 mode. The homoleptic tris(η1:κ1-pyrrolylaldiminato)yttrium complex (4) was isolated when the molar ratio of L1 to Y(CH2SiMe3)3(THF)2 increases to 3:1. Reaction of L2 with equimolar Y(CH2SiMe3)3(THF)2 afforded an asymmetric binuclear complex (5). The dianionic N,N,P tridentate moieties derived from L2 coordinate to yttrium atoms in η5/η1:κ2 modes, generating tetrahedron and trigonal/pyramidal geometry around the two metal centers, respectively. Both alkylation of the imino CN group of ligand L2 and the pyrrole's ability to act similar to a heterocyclopentadienyl moiety were also found in complex 5. Complexes 1−3 and 5 initiated polymerizations of d,l-lactide to give atactic polylactides with high molecular weights and narrow molecular weight distributions.