Isospecific styrene polymerization by chiral titanium complexes that contain a tetradentate [OSSO]-type bis(phenolato) ligand

A series of titanium dichloro and di(isopropoxy) complexes with a 1,4-dithiabutanediyl-linked bis(4,6-disubstituted phenolato) ligand [Ti(OC6H2-6-R-1-4-R-2)(2){S(CH2)(2)S}X-2] (X = Cl, 2a-i, (OPr)-Pr-i, 3a-i; R-1 = H, Me, Pr-i, Bu-t, 2-phenyl-2-propyl; R-2 = H, Me, Bu-t, OMe, 2-phenyl-2-propyl) were synthesized by reacting the corresponding linked bis(phenol) (HOC6H2-6-R-1-4-R-2)(2){S(CH2)(2)S} (1a-i) with the titanium precursor TiX4. The NMR spectra of the dichloro complexes Ti(OC6H2-6-R-1-4-R-2)(2){S(CH2)(2)S}Cl-2] (2a-d) with small ortho substituents R-1 = H, Me, Pr-i are in agreement with a C-2-symmetrical helical structure, but the complexes become fluxional at higher temperatures. The corresponding di(isopropoxy) complexes [Ti(OC6H2-6-R-1-4-R-2)(2){S(CH2)(2)S}((OPr)-Pr-i)(2)] (3a-d) are fluxional in solution at room temperature due to rapid interconversion between the Delta and Delta isomers. In contrast, both dichloro and di(isopropoxy) complexes 2e-i and 3e-i with bulky ortho substituents R-1 = Bu-t and 2-phenyl-2-propyl exhibit a rigid C2-symmetrical helical structure in solution up to 100 C. The helical structure with trans-O,O, cis-S,S, cis-Cl,Cl ("alpha-cis") is confirmed by the single-crystal structure analysis of two dichloro complexes [Ti(OC6H2-6-R-1-4-R-2)(2){S(CH2)(2)S}Cl-2] (2e, R-1 = Bu-t, R-2 = Me; 2i, R-1, R-2 = 2-phenyl-2-propyl). Related dichloro and di(isopropoxy) complexes that contain a 1,5-dithiapentanediyl-linked bis(4,6-disubstituted phenolato) ligand [Ti(OC6H2-6R(1)-4-R-2)(2){S(CH2)(3)S}X-2] (R-1 = Bu-t; R-2 = Me, Bu-t, OMe; X = Cl, 5a-c; (OPr)-Pr-i, 6a-c) were synthesized in an analogous fashion and shown by NMR spectroscopy to be conformationally flexible at room temperature. This fluxionality can be explained by a twisting of one five-membered chelate ring, converting the trans-O,O ("alpha-cis") to the cis-O,O ("beta-cis") isomer. A crystal structure determination of ligand [Ti((OC6H2Bu2)-Bu-t-4,6)(2){S(CH2)(3)S}((OPr)-Pr-i)(2)] (6b) revealed the trans-O,O, cis-S,S, cis-Cl,Cl ("alpha-cis") isomer. Upon activation with methylaluminoxane, the stereorigid derivatives 2e-i efficiently polymerize styrene to give isotactic polystyrene, whereas the conformationally flexible complexes 2a-d produce atactic polystyrene with negligible activity. Under the same conditions, complexes with the 1,5-dithiapentanediyl-linked ligand 5a,b and 6b,c polymerized styrene syndiospecifically with low activity.