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 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.

Keywords: aluminum; density functional calculations; main group elements; multiple bonds; reaction mechanisms.

Figure 1

Structurally characterized terphenyl‐substituted group 13 dimetallynes (I), dimetalleneds (II), and metallanediyls (III) along with selected examples of cycloaddition products of dialuminenes with arene solvents (IV and V). Ar^iPr4=C₆H₃‐2,6‐(C₆H₃‐2,6‐ⁱPr₂)₂, Ar^iPr6=C₆H₃‐2,6‐(C₆H₂‐2,4,6‐ⁱPr₃)₂, and Ar^iPr8=C₆H‐2,6‐(C₆H₂‐2,4,6‐ⁱPr₃)₂‐3,5‐ⁱPr₂.

Figure 2

Compounds synthetized and characterized in this work.

Figure 3

Single crystal X‐ray structure of Ar^iPr8AlAlAr^iPr8 (1) with carbon atoms of the isopropyl groups shown in wireframe format and hydrogen atoms not shown. Thermal ellipsoids are drawn at 50 % probability level. Selected bond lengths (Å) and angles (°) with calculated values in square brackets: Al1−Al2=2.648(2) [2.680]; Al1−C1=1.996(3) [2.009], Al2−C43=2.013(3) [2.015], Al1−Al2−C43=133.6(1) [128.3], Al2−Al1−C1=129.5(1) [127.2], C1−Al1−Al2−C43=−173.8(2) [−174.0].

Figure 4

Isosurface (±0.04) plots of frontier Kohn‐Sham orbitals of Ar^iPr8AlAlAr^iPr8 (1) with orbital energies given in atomic units. For clarity, hydrogen atoms are not shown.

Figure 5

Calculated mechanism for the reaction of two equivalents of H₂ with Ar^iPr8AlAlAr^iPr8. Relative Gibbs energies refer to gas phase calculations at room temperature. For clarity, the dissociation of Ar^iPr8Al(H)₂AlAr^iPr8 is not shown.

Figure 6

Single crystal X‐ray structures of Ar^iPr8Al(C₂H₄)₂AlAr^iPr8 (2, top) and Na₂[AlAr^iPr6]₃ (4, bottom) with carbon atoms of the isopropyl groups shown in wireframe format and hydrogen atoms not shown. Thermal ellipsoids are drawn at 50 % probability level.

Figure 7

Calculated mechanism for the reaction of two equivalents of C₂H₄ with :AlAr^iPr8. Relative Gibbs energies refer to gas phase calculations at room temperature.