The Dialuminene Ar <sup>iPr8</sup> AlAlAr <sup>iPr8</sup> (Ar <sup>iPr8</sup> =C <sub>6</sub> H‐2,6‐(C <sub>6</sub> H <sub>2</sub> ‐2,4,6‐ <sup>i</sup> Pr <sub>3</sub> ) <sub>2</sub> ‐3,5‐ <sup>i</sup> Pr <sub>2</sub> )
Annika Lehmann, Joshua D. Queen, Christopher J. Roberts, Kari Rissanen, Heikki M. Tuononen, Philip P. Power
Abstract
Abstract Careful analysis of the crystals formed in the reduction of Ar iPr8 AlI 2 (Ar iPr8 =C 6 H‐2,6‐(C 6 H 2 ‐2,4,6‐ i Pr 3 ) 2 ‐3,5‐ i Pr 2 ) with sodium on sodium chloride showed them to contain the long sought‐after dialuminene Ar iPr8 AlAlAr 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 iPr8 Al(C 2 H 4 ) 2 AlAr iPr8 ( 2 ) and the aluminacyclopentane Ar iPr8 Al(C 4 H 8 ) ( 3 ), that can both form from the aluminacyclopropane intermediate Ar iPr8 Al(C 2 H 4 ). 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 2 [AlAr iPr6 ] 3 ( 4 ), instead of Ar iPr6 AlAlAr iPr6 .