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-dithiabutanediyllinked bis(4,6-disubstituted phenolato) ligand [Ti(OC6H2-6-R1-4-R2)2{S(CH2)2S}X2] (X ) Cl, 2a-i, OiPr, 3a-i; R1 ) H, Me, iPr, tBu, 2-phenyl-2-propyl; R2 ) H, Me, tBu, OMe, 2-phenyl-2-propyl) were synthesized by reacting the corresponding linked bis(phenol) (HOC6H2-6-R1-4-R2)2{S(CH2)2S} (1a-i) with the titanium precursor TiX4. The NMR spectra of the dichloro complexes Ti(OC6H2-6-R1-4-R2)2{S(CH2)2S}Cl2] (2a-d) with small ortho substituents R1 ) H, Me, iPr are in agreement with a C2-symmetrical helical structure, but the complexes become fluxional at higher temperatures. The corresponding di(isopropoxy) complexes [Ti(OC6H2-6-R1-4-R2)2{S(CH2)2S}(OiPr)2] (3a-d) are fluxional in solution at room temperature due to rapid interconversion between the delta and lambda isomers. In contrast, both dichloro and di(isopropoxy) complexes 2e-i and 3e-i with bulky ortho substituents R1 ) tBu 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-R1-4-R2)2{S(CH2)2S}Cl2] (2e, R1 ) tBu, R2 ) Me; 2i, R1, R2 ) 2-phenyl-2-propyl). Related dichloro and di(isopropoxy) complexes that contain a 1,5-dithiapentanediyl-linked bis(4,6-disubstituted phenolato) ligand [Ti(OC6H2-6-R1-4-R2)2{S(CH2)3S}X2] (R1 ) tBu; R2 ) Me, tBu, OMe; X ) Cl, 5a-c; OiPr, 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 fivemembered chelate ring, converting the trans-O,O (“alpha-cis”) to the cis-O,O (“beta-cis”) isomer. A crystal structure determination of ligand [Ti(OC6H2tBu2-4,6)2{S(CH2)3S}(OiPr)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-dithiapentanediyllinked ligand 5a,b and 6b,c polymerized styrene syndiospecifically with low activity.