Steric effects on the structures, reactivity and coordination chemistry of tris(2-pyridyl)aluminates

Abstract

Introducing substituents in the 6-position of the 2-pyridyl rings of tris-pyridyl aluminate anions, of the type [EtAl(2-py´)3]- (py´ = a substituted 2-pyridyl group), has a large impact on their metal coordination characteristics. This is seen most remarkably in the desolvation of the THF solvate [EtAl(6-Me-2-py)3Li•THF] to give the monomer [EtAl(6-Me-2-py)3Li] (1), containing a pyramidal, three-coordinate Li+ cation. Similar monomeric complexes are observed for [EtAl(6-CF3-2-py)3Li] (2) and [EtAl(6-Br-2-py)3Li] (3), containing CF3 and Br substituents (R). This steric influence can be exploited in the synthesis of a new class of terminal Al-OH complexes, as is seen in the controlled hydrolysis of 2 and 3 to give [EtAl(OH)(6-R-2-py)2]- anions, as in the dimer [EtAl(OH)(6-Br-2-py)2Li]2 (5). Attempts to deprotonate the Al-OH group of 5 using Et2Zn led only to the formation of the zincate complex [LiZn(6-Br-py)3]2 (6), while reactions of the 6-Br substituted 3 and the unsubstituted complex [EtAl(2-py)3Li] with MeOH give [EtAl(OMe)(6-Br-2-py)2Li]2 (7) and [EtAl(OMe)(2-py)2Li]2 (8), respectively, having similar dimeric arrangements to 5. The combined studies presented here provide key synthetic tools for the functionalization and elaboration of tris-pyridyl aluminate ligands.