Synthesis, Structure, and Olefin Polymerization Activity of Titanium Complexes Bearing Asymmetric Tetradentate [OSNO]-Type Bis(phenolato) Ligands

A series of chiral linear tetradentate bis(phenols) that contain both sulfur and nitrogen donors of the type [2,2′-(HOC6H2-6-tBu-4-R1)2SC6H10NR2] [R1 = Me (a), tBu (b); R2 = H (1), Me (2)], [2,2′-(HOC6H2-4,6-tBu2)2SC6H10NCH] (3), and [2,2′-(HOC6H2-4,6-tBu2)2SC6H10NHCH2] (4) were synthesized. The reaction of these bis(phenols) with TiX4 (X = Cl, OiPr) afforded the corresponding C1-symmetric titanium complexes [Ti{2,2′-(OC6H2-6-tBu-4-R1)2SC6H10NR2}X2] [R1 = Me (a), tBu (b); R2 = H, X = Cl (5a, 5b), OiPr (6a); R2 = Me, X = Cl (7a, 7b), OiPr (8a)], [Ti{2,2′-(OC6H2-4,6-tBu2)2SC6H10NCH}Cl2] (9), and [Ti{2,2′-(OC6H2-4,6-tBu2)2SC6H10NHCH2}Cl2] (10). The formation of titanium complexes 5−8 proceeded diastereoselectively, but a mixture of two isomers (a and b) was obtained for 9 and 10. The configuration of the ligand around the metal center was determined by a combination of NMR spectroscopy and single-crystal X-ray diffraction studies of 5b, 7b, 8a, 9b, 10a, and 10b. All titanium complexes were configurationally stable in solution up to 100 °C. For compounds 5−8, cis-α and cis-β2 coordination modes of the ligand were observed, depending on the nitrogen substituent and the auxiliary ligand. In compounds 9 and 10, both configurations coexist and do not interconvert at elevated temperatures, but HCl catalyzes the isomerization of 10a to 10b. Upon activation with methylaluminoxane, [Ti{OSNO}X2] complexes show moderate activity in the polymerization of styrene and trace activity in the polymerization of 1-hexene.