Synthesis and structure of trialkyltantalum complexes stabilized by aminopyridinato ligands

(4-Methylpyridin-2-yl)(trimethylsilyl)amine (1), (6-methylpyridin-2-yl)(trimethylsilyl)amine (2), and (2,6-diisopropylphenyl)(pyridin-2-yl)amine (3) were deprotonated and used as ligands to synthesize trialkyltantalum complexes. The reaction of 2 equiv. of 1 or 2 with pentabenzyltantalum afforded tribenzyltantalum(v) complexes by toluene elimination. Analogous reaction using 3 failed. Lithiation of 3 followed by the reaction with tribenzyltantalum dichloride gave rise to the corresponding tribenzyl complex. Other alkyltantalum complexes stabilized by this ligand environment can be prepared by treating tantalum pentachloride with 2 equiv. of lithiated 3 to form a bis(aminopyridinato)tantalum trichloride. The reaction of this trichloride with 3 equiv. of alkyllithium compounds like methyllithium affords the corresponding trialkyltantalum complexes. X-ray diffraction studies of four of the synthesized complexes were carried out. They adopt two different coordination environments, either slightly distorted capped octahedrons (sterically less demanding aminopyridinato ligands) or pentagonal bipyramids (bulkier aminopyridinato ligands). The alkyl species were surprisingly stable at elevated temperatures and no formation of mixed alkyl/alkyhdene complexes was observed. Alkyl cation formation and the behaviour of a selection of these compounds in olefin polymerization were explored.