Selective Formation of Homoleptic and Heteroleptic 2,5-Bis(N-aryliminomethyl)pyrrolyl Yttrium Complexes and Their Performance as Initiators of ε-Caprolactone Polymerization

Tridentate 2,5-bis( N -aryliminomethyl)pyrroles ( 1a, aryl = 4-methoxylphenyl; 1b, aryl = 4-methylphenyl; 1c, aryl = 2-methylphenyl; 1d, aryl = 2,6-dimethylphenyl; 1e, aryl = 2,6-diisopropylphenyl) have been prepared, and their reactions with a homoleptic Y{N(SiMe 3 ) 2 } 3 ( 2 ) have been investigated. The number of pyrrolyl ligands introduced to an yttrium atom can be controlled by varying the bulkiness of the ligand to give mono(pyrrolyl) ( 3 ), bis(pyrrolyl) ( 4 ), or tris(pyrrolyl) complexes ( 5 ) upon aminolysis. When bulky ligands such as 1d and 1e were used, a mono(pyrrolyl) complex Y(Xyl 2 -pyr){N(SiMe 3 ) 2 } 2 ( 3d, Xyl 2 -pyr = 2,5-bis{ N -(2,6-dimethylphenyl)iminomethyl}pyrrolyl) and a bis(pyrrolyl) complex Y(DIP 2 -pyr) 2 {N(SiMe 3 ) 2 } ( 4e, DIP 2 -pyr = 2,5-bis{ N -(2,6-diisopropylphenyl)iminomethyl}pyrrolyl) were predominantly obtained, respectively, with the release of hexamethyldisilazane. Both complexes adopt five-coordination geometries, a distorted trigonal bipyramidal mode for 3d and a distorted square-pyramidal one for 4e, in which the N,N ‘ -bidentate coordination of 1e to the yttrium atom was found in solution and the solid state presumably due to the bulkiness of the isopropyl substituents. In the case of p -substituted ligands, 1a and 1b, we obtained homoleptic tris(pyrrolyl) yttrium complexes 5a and 5b, respectively. The complex 5a has three N, N ‘, N ‘ ‘-tridentate pyrrolyl ligands, and the yttrium center adopts a three-face-centered trigonal prismatic mode of nine-coordination, while we found the eight-coordinated, square-antiprismatic geometry for 5b with two N, N ‘, N ‘ ‘-tridentate and one N, N ‘-bidentate pyrrolyl ligand. Thus, we demonstrated that the yttrium atom was able to have a five-, eight-, and nine-coordination number, depending on the congestion by the bis(aryliminomethyl)pyrrolyl ligands. We also found that these newly prepared mono(pyrrolyl) ( 3d ) and bis(pyrrolyl) ( 4e ) complexes catalyzed polymerization of ε-caprolactone and that complex 4e acted as a single-site catalyst to give a polyester with a narrow molecular weight distribution ( M w / M n = 1.2).