Six-coordinate aluminium cations: characterization, catalysis, and theory

The new thf supported cations, [Salen(tBu)Al(thf)2]+ (2), [Salpen(tBu)Al(thf)2]+ (3), [Salben(tBu)Al(thf)2]+ (4) [Salophen(tBu)Al(thf)2]+ (5) and [Salomphen(tBu)Al(thf)2]+ (6), with BPh4− as the counter anion, have been prepared from salt elimination reactions with the respective Salen(tBu)AlCl reagent, including one that is new, Salben(tBu)AlCl (1). The cations were observed to polymerize propylene oxide. Based upon the results of experimental and theoretical work the mechanism appears to be one in which a carbocation is the propagating species although the PDI values are remarkably low, ≈1.2 in some cases. All of the potential catalysts were characterized spectroscopically and, in the case of 3, by X-ray crystallography.