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

The N,N -bidentate ligand 2-{( N -2,6-diisopropylphenyl)iminomethyl)}pyrrole ( L 1 ) and the N,N,P -tridentate ligand 2-{( N -2-diphenylphosphinophenyl)iminomethyl)}pyrrole ( L 2 ) have been prepared. Their reactions with homoleptic yttrium tris(alkyl) compound Y(CH 2 SiMe 3 ) 3 (THF) 2 have been investigated. Treatment of Y(CH 2 SiMe 3 ) 3 (THF) 2 with 1 equiv of L 1 generated a THF-solvated bimetallic (pyrrolylaldiminato)yttrium mono(alkyl) complex ( 1 ) of central symmetry. In this process, L 1 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 η 5 /η 1:κ 1 mode. The pyrrolyl ring behaves as a heterocyclopentadienyl ligand. Reaction of Y(CH 2 SiMe 3 ) 3 (THF) 2 with 2 equiv of L 1 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 η 1:κ 1 mode. The homoleptic tris(η 1:κ 1 -pyrrolylaldiminato)yttrium complex ( 4 ) was isolated when the molar ratio of L 1 to Y(CH 2 SiMe 3 ) 3 (THF) 2 increases to 3:1. Reaction of L 2 with equimolar Y(CH 2 SiMe 3 ) 3 (THF) 2 afforded an asymmetric binuclear complex ( 5 ). The dianionic N,N,P tridentate moieties derived from L 2 coordinate to yttrium atoms in η 5 /η 1:κ 2 modes, generating tetrahedron and trigonal/pyramidal geometry around the two metal centers, respectively. Both alkylation of the imino C N group of ligand L 2 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.