Adducts of titanium tetrahalides with neutral lewis bases. Part II. Kinetics and reaction mechanism: a variable temperature and variable pressure proton NMR study

Abstract

The ligand exchange reaction cis-TiX 4·2L + *L ai cis-TiX 4·L*L + L has been studied for a series of cis-TiX 4·2L adducts (X=Cl, Br; L=Lewis base) in CH 2Cl 2 and CHCl 3 by variable temperature and variable pressure proton NMR. The exchange reactions are characterized by large, positive values of Δ H* (50.2-61.5 kJ/mol), Δ S* (9.2-103 J/mol K) and Δ V* (17.5-26.1 cm 3/mol) and by first order rate laws. For this series of adducts, Δ G* (213 K) for ligand exchange decreases as the stability of the adduct decreases. It was concluded that this ligand exchange process occurs via a dissociative, D, mechanism. The cis/ trans isomerization of TiCl 4·2TMPA in CHCl 3 was also investigated. In view of the small but positive ΔV* iso (6.2 cm 3/mol), it was concluded that the isomerization process occurs via an intramolecular twist mechanism with an expanded six-coordinate transition state.