The Dialuminene ArAlAlAr (Ar=CH-2,6-(CH-2,4,6-Pr)-3,5-Pr)

Careful analysis of the crystals formed in the reduction of ArAlI (Ar=CH-2,6-(CH-2,4,6-Pr)-3,5-Pr) with sodium on sodium chloride showed them to contain the long sought-after dialuminene ArAlAlAr (1) that forms alongside the previously characterized alanediyl :AlAr. 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 :AlAr with dihydrogen involves 1, not :AlAr, as the reactive species. In contrast, the reaction of :AlAr with ethylene gave two products, the 1,4-dialuminacyclohexane ArAl(CH)AlAr (2) and the aluminacyclopentane ArAl(CH) (3), that can both form from the aluminacyclopropane intermediate ArAl(CH). 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, Na[AlAr] (4), instead of ArAlAlAr.