Reversible Dissociation of a Dialumene*

Abstract

Dialumenes are neutral Al^I compounds with Al=Al multiple bonds. We report the isolation of an amidophosphine-supported dialumene. Our X-ray crystallographic, spectroscopic, and computational DFT analyses reveal a long and extreme trans-bent Al=Al bond with a low dissociation energy and bond order. In solution, the dialumene can dissociate into monomeric Al^I species. Reactivity studies reveal two modes of reaction: as dialumene or as aluminyl monomers.

Keywords: aluminium; aluminium(I) compounds; dialumene; low-valent atoms; multiple bonds.

Figure 1

a) Base‐coordinated dialumenes (R₃Si=Me^tBu₂Si; Tip=2,4,6‐triisopropylphenyl). b) “masked” dialumenes (Ar*=2,6‐(2,6‐diisopropylphenyl)phenyl; Bbp=2,6‐(bis(trimethylsilyl)methyl)phenyl). c) Reversible reductive elimination in VI (Mes=2,4,6‐trimethylphenyl).

Scheme 1

Preparation of dialumene 1.

Figure 2

X‐ray crystal structure of dialumene 1 (H atoms omitted for clarity). Thermal ellipsoids at 50 % probability. Major component of disordered Al/ligand displayed (1A). Selected bond distances [Å] and angles [°]: Al1‐Al1′ 2.5190(14); N1‐Al1 1.909(2); P1‐Al1 2.4816(9); N1‐Al1‐P1 84.86(7); θ=48.8; τ=0.

Figure 3

Electronic structure analysis of 1. a) NLMOs (isovalue= 0.036) of the Al=Al bond. b) ELF localisation domains (isovalue= 0.795) of the Al=Al core. M _n indicates attractors. c) Laplacian of the electron density in the P−Al=Al−P plane. Areas of charge concentration and depletion (blue/pink), bond paths (black) and bond critical points shown.

Figure 4

a) ³¹P{¹H} NMR spectra of 1 (161 MHz, [D₈]toluene) recorded at 203–300 K. b) Inversion at aluminium exchanges 1A and 1B, but is degenerate for 1C. c) Intermolecular dissociation/recombination of 1 exchanges all diastereomers. d) Reaction energy profile for the “trans‐flip” in diastereomers 1A‐C at T=298.15 K (geometries optimised at M062X‐D3/def2SVP, energies calculated at B3LYP‐D3/6‐311G(2d,2p) corrected for C₆H₆ solvent).

Scheme 2

Reactivity of dialumene 1 with alkenes/alkynes.

Figure 5

X‐ray crystal structure of aluminacyclopropene 6 (H atoms omitted). Thermal ellipsoids at 50 % probability. The asymmetric unit comprises two molecules; just one is shown.