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

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posted on 29.01.2007, 00:00 by Yi Yang, Shihui Li, Dongmei Cui, Xuesi Chen, Xiabin Jing
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 η511 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 η11 mode. The homoleptic tris(η11-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 η512 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 13 and 5 initiated polymerizations of d,l-lactide to give atactic polylactides with high molecular weights and narrow molecular weight distributions.