Crystallographic Investigation of Substituted Allyl and Titanacyclobutane Complexes of Bis(2-N,N-dialkylaminoindenyl)titanium. Structure and Reactivity as a Function of the Dialkylamino Substituents

Abstract

Substituted allyl complexes of bis(2-N,N-dialkylaminoindenyl)titanium(III) undergo highly regioselective central carbon alkylation upon treatment with organic free radicals, providing a general synthesis of 2,3-disubstituted titanacyclobutane complexes. This reactivity contrasts that of the corresponding bis(pentamethylcyclopentadienyl)titanium(III) series, for which titanacyclobutane formation is observed only in reactions of the unsubstituted allyl complex. Unexpectedly, the radical alkylation is more general for complexes of bis(2-N,N-dimethylaminoindenyl)titanium than complexes of the closely analogous bis(2-piperidinoindenyl)titanium, despite only subtle differences between the two ancillary ligands. In addition, the 2-methyl-3-alkyltitanacyclobutane complexes derived from alkylation of bis(2-N,N-dimethylaminoindenyl)titanium(η3-crotyl) are more thermally robust than those derived from the 2-piperidinoindenyl ligand system, resisting decomposition via β-hydride elimination from the α-methyl substituent. To gain insight into the function of the dialkylaminoindenyl ligands and to probe the differences between 2-N,N-dimethylaminoindenyl and 2-piperidinoindenyl complexes, crystal structures of bis(2-N,N-dimethylaminoindenyl)titanium(η3-1-phenylallyl) (4a) and bis(2-N,N-dimethylaminoindenyl)titanium(η3-crotyl) (4b) have been determined and compared to the crystal structure of bis(2-piperidinoindenyl)titanium(η3-1-phenylallyl), 3a. The effects of allyl coordination on the disposition of the dialkylaminoindenyl ligands are revealed by a comparison with the crystal structure of the corresponding chloride complex, bis(2-N,N-dimethylaminoindenyl)titanium(III)chloride·lithium chloride·(THF)2, a rare Ti(III)halide/lithium halide adduct. Although it is difficult to control for the effects of crystal packing forces, substantial changes in ancillary ligand structure and orientation are observed as a function of the amino substituent and the coordination environment. The difference in titanacyclobutane stability and the response of the dialkylaminoindenyl ligands to the change in oxidation state have been investigated by comparing the crystal structures of two disubstituted titanacyclobutane complexes, 3-isopropyl-2-phenylbis(2-piperidinoindenyl)titanacyclobutane (7) and 3-isopropyl-2-phenylbis(2-N,N-dimethylaminoindenyl)titanacyclobutane (8), each prepared by free radical alkylation.