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; le, aryl = 2,6-diisopropylphenyl) have been prepared, and their reactions with a homoleptic Y{N(SiMe3)(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 Id and le were used, a mono(pyrrolyl) complex Y(Xyl(2)-pyr){N(SiMe3)(2)}(2) (3d, Xyl(2)-pyr = 2,5-bis{N-(2,6-dimethylphenyl)iminomethyl}pyrrolyl) and a bis(pyrrolyl) complex Y(DIP2-pyr)(2){N(SiMe3)(2)} (4e, DIP2-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 NN-bidentate coordination of le 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, la 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 epsilon -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).