UC Davis

Careful analysis of the crystals formed in the reduction of Ar^iPr8AlI₂ (Ar^iPr8=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 Ar^iPr8AlAlAr^iPr8 (1) that forms alongside the previously characterized alanediyl :AlAr^iPr8. 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^iPr8 with dihydrogen involves 1, not :AlAr^iPr8, as the reactive species. In contrast, the reaction of :AlAr^iPr8 with ethylene gave two products, the 1,4-dialuminacyclohexane Ar^iPr8Al(C₂H₄)₂AlAr^iPr8 (2) and the aluminacyclopentane Ar^iPr8Al(C₄H₈) (3), that can both form from the aluminacyclopropane intermediate Ar^iPr8Al(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[AlAr^iPr6]₃ (4), instead of Ar^iPr6AlAlAr^iPr6.