The Dialuminene AriPr8AlAlAriPr8 (AriPr8=C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2)

Abstract

Careful analysis of the crystals formed in the reduction of AriPr8AlI2 (AriPr8=C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2) with sodium on sodium chloride showed them to contain the long sought-after dialuminene AriPr8AlAlAriPr8 (1) that forms alongside the previously characterized alanediyl :AlAriPr8. The single crystal X-ray structure of 1 revealed a nearly planar, trans-bent C(ipso)AlAlC(ipso) core with an Al−Al distance of 2.648(2) Å. The molecular and electronic structure of 1 are consistent with an Al−Al double dative interaction augmented with diradical character and stabilized by dispersion interactions. Density functional theory calculations showed that the reactivity of :AlAriPr8 with dihydrogen involves 1, not :AlAriPr8, as the reactive species. In contrast, the reaction of :AlAriPr8 with ethylene gave two products, the 1,4-dialuminacyclohexane AriPr8Al(C2H4)2AlAriPr8 (2) and the aluminacyclopentane AriPr8Al(C4H8) (3), that can both form from the aluminacyclopropane intermediate AriPr8Al(C2H4). Although the [2+2+2] cycloaddition of 1 with two equivalents of ethylene was also calculated to be exergonic, it is likely to be kinetically blocked by the numerous isopropyl substituents surrounding the Al−Al bond. Attempts to fine-tune the steric bulk of the terphenyl ligand to allow stronger Al−Al bonding were unsuccessful, leading to the isolation of the sodium salt of a cyclotrialuminene, Na2[AlAriPr6]3 (4), instead of AriPr6AlAlAriPr6.