Isospecific Styrene Polymerization by Chiral Titanium Complexes That Contain a Tetradentate [OSSO]-Type Bis(phenolato) Ligand

Abstract

A series of titanium dichloro and di(isopropoxy) complexes with a 1,4-dithiabutanediyl-linked bis(4,6-disubstituted phenolato) ligand [Ti(OC 6 H 2 -6-R 1 -4-R 2 ) 2 {S(CH 2 ) 2 S}X 2 ] (X = Cl, 2a − i, O i Pr, 3a − i; R 1 = H, Me, i Pr, t Bu, 2-phenyl-2-propyl; R 2 = H, Me, t Bu, OMe, 2-phenyl-2-propyl) were synthesized by reacting the corresponding linked bis(phenol) (HOC 6 H 2 -6-R 1 -4-R 2 ) 2 {S(CH 2 ) 2 S} ( 1a − i ) with the titanium precursor TiX 4 . The NMR spectra of the dichloro complexes Ti(OC 6 H 2 -6-R 1 -4-R 2 ) 2 {S(CH 2 ) 2 S}Cl 2 ] ( 2a − d ) with small ortho substituents R 1 = H, Me, i Pr are in agreement with a C 2 -symmetrical helical structure, but the complexes become fluxional at higher temperatures. The corresponding di(isopropoxy) complexes [Ti(OC 6 H 2 -6-R 1 -4-R 2 ) 2 {S(CH 2 ) 2 S}(O i Pr) 2 ] ( 3a − d ) are fluxional in solution at room temperature due to rapid interconversion between the Δ and Λ isomers. In contrast, both dichloro and di(isopropoxy) complexes 2e − i and 3e − i with bulky ortho substituents R 1 = t Bu and 2-phenyl-2-propyl exhibit a rigid C 2 -symmetrical helical structure in solution up to 100 °C. The helical structure with trans -O,O, cis -S,S, cis- Cl,Cl (“α -cis ”) is confirmed by the single-crystal structure analysis of two dichloro complexes [Ti(OC 6 H 2 -6-R 1 -4-R 2 ) 2 {S(CH 2 ) 2 S}Cl 2 ] ( 2e, R 1 = t Bu, 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(OC 6 H 2 -6-R 1 -4-R 2 ) 2 {S(CH 2 ) 3 S}X 2 ] (R 1 = t Bu; R 2 = Me, t Bu, OMe; X = Cl, 5a − c; O i Pr, 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 (“α -cis ”) to the cis -O,O (“β -cis ”) isomer. A crystal structure determination of ligand [Ti(OC 6 H 2 t Bu 2 -4,6) 2 {S(CH 2 ) 3 S}(O i Pr) 2 ] ( 6b ) revealed the trans -O,O, cis -S,S, cis- Cl,Cl (“α -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.