Selective formation of homoleptic and heteroleptic 2,5-bis(N-aryliminomethyl)pyrrolyl yttrium complexes and their performance as initiators of epsilon-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-dimet hylphenyl; le, aryl = 2,6-diisopropylphenyl) have been prepared, and their reactions with a homoleptic Y{N(SiMe3)(2)}(3) (2) have been investigated. T he number of pyrrolyl ligands introduced to an yttrium atom can be controll ed by varying the bulkiness of the ligand to give mono(pyrrolyl) (3), bis(p yrrolyl) (4), or tris(pyrrolyl) complexes (5) upon aminolysis. When bulky l igands 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, DIP 2-pyr = 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl) were predomi nantly obtained, respectively, with the release of hexamethyldisilazane. Bo th complexes adopt five-coordination geometries, a distorted trigonal bipyr amidal mode for 3d and a distorted square-pyramidal one for 4e, in which th e 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 subst ituents. In the case of p-substituted ligands, la and 1b, we obtained homol eptic tris(pyrrolyl) yttrium complexes 5a and 5b, respectively. The complex 5a has three N,N ' ,N " -tridentate pyrrolyl ligands, and the yttrium cent er adopts a three-face-centered trigonal prismatic mode of nine-coordinatio n, 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-, eig ht-, and nine-coordination number, depending on the congestion by the bis(a ryliminomethyl)pyrrolyl ligands. We also found that these newly prepared mo no(pyrrolyl) (3d) and bis(pyrrolyl) (4e) complexes catalyzed polymerization of epsilon -caprolactone and that complex 4e acted as a single-site cataly st to give a polyester with a narrow molecular weight distribution (M-w/M-n = 1.2).