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 alanediylAlAriPr8. 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 a Al-Al double dative interaction augmented with diradical character and stabilized by dispersion interactions. Density functional theory calculations showed that the reactivity ofAlAriPr8 with dihydrogen involves 1, notAlAriPr8, as the reactive species. In contrast, the reaction ofAlAriPr8 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.