Lanthanum Tribenzyl Complexes as Convenient Starting Materials for Organolanthanum Chemistry

Abstract

Simple tribenzyl complexes of lanthanum, [La(CH2C6H4-4-R)3(THF)3] (R = H (1a), Me (1b)), were prepared in a remarkably straightforward fashion from LaBr3(THF)4 and potassium benzyl reagents. Single-crystal X-ray diffraction revealed a fac arrangement of the three THF ligands and η2 binding of the benzyl groups. These compounds are convenient precursors to other organolanthanum complexes. Reaction of 1a with the amidine ArNCPhNHAr (Ar = 2,6-Pri2C6H3) affords the corresponding mono(amidinate) dibenzyl derivative 2. Complex 1b reacts with LiCH2C6H4-4-Me to give the THF-free anion [La(CH2C6H4-4-Me)4]- (3). Reactions of 1 with 1 or 2 equiv of [PhNMe2H][B(C6X5)4] (X = H, F) generate the corresponding mono- and dicationic benzyl species [La(CH2C6H4-4-R)2(THF)4]+ (4) and [La(CH2C6H4-4-R)(THF)6]2+ (5), which were structurally characterized. Scouting ethylene polymerization experiments indicate that these species are only modestly active catalysts but suggest that the monocationic dibenzyl species is more efficient. Both neutral and cationic lanthanum benzyl complexes effect the catalytic intramolecular hydroamination/cyclization of 2,2-dimethyl-4-pentenylamine. It was also observed that polycationic La species without ancillary ligands effectively catalyze the isomerization of the substrate to (E)-2,2-dimethyl-3-pentenylamine.