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

Substituted allyl complexes of bis(2-N,N-dialkylaminoindenyl)titanium under go highly regioselective central carbon alkylation upon treatment with orga nic free radicals, providing a general synthesis of 2,3-disubstituted titan acyclobutane complexes. This reactivity contrasts that of the corresponding bis (pentamethylcyclopentadienyl)titanium(III) series, for which titanacyc lobutane formation is observed only in reactions of the unsubstituted allyl complex. Unexpectedly, the radical alkylation is more general for complexe s of bis(2-N,N-dimethyl-aminoindenyl)titanium than complexes of the closely analogous bis(2-piperidinoindenyl)titanium, despite only subtle difference s between the two ancillary ligands. In addition, the 2-methyl-3-alkyltitan acyclobutane complexes derived from alkylation of bis(2-N,N-dimethyl-aminoi ndenyl)titanium(eta (3)-crotyl) are more thermally robust than those derive d from the 2-piperidinoindenyl ligand system, resisting decomposition via a -hydride elimination from the a-methyl substituent, To gain insight into th e function of the dialkylaminoindenyl ligands and to probe the differences between 2-N,N-dimethylaminoindenyl and 2-piperidinoindenyl complexes, cryst al structures of bis(2-N,N-dimethylaminoindenyl)titanium(eta (3)-1-phenylal lyl) (4a) and bis(2-N,N-dimethylaminoindenyl)titanium( eta (3)-crotyl) (4b) have been determined and compared to the crystal structure of bis(2-piperi dinoindenyl)titanium(eta (3)-1-phenylallyl), 3a. The effects of allyl coord ination on the disposition of the dialkylaminoindenyl ligands are revealed by a comparison with the crystal structure of the corresponding chloride co mplex, bis( 2-N,N-dimethylaminoindenyl)titanium(III)chloride . lithium chlo ride . (THF)(2), a rare Ti(III)halide/lithium halide adduct. Although it is difficult to control for the effects of crystal packing forces, substantia l changes in ancillary ligand structure and orientation are observed as a f unction of the amino substituent and the coordination environment. The diff erence in titanacyclobutane stability and the response of the dialkylaminoi ndenyl ligands to the change in oxidation state have been investigated by c omparing the crystal structures of two disubstituted titanacyclobutane comp lexes, 3-isopropyl-2-phenylbis(2-piperidinoindenyl)- titanacyclobutane (7) and 3-isopropyl-2-phenylbis(2-N,N-dimethylaminoindenyl)titanacy- clobutane (8), each prepared by free radical alkylation.