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.