Reactivity of Various Four‐Coordinate Aluminum Alkyls towards Dioxygen: Evidence for Spatial Requirements in the Insertion of an Oxygen Molecule into the Al−C Bond

The interaction of dioxygen with various tetrahedral aluminum alkyls, ( t Bu) 3 Al⋅OEt 2 ( 1 ), t Bu 2 Al(μ‐ O t Bu) 2 Al t Bu 2 ( 6 ), ( t Bu) 2 Al(mesal) ( 2 ) [mesal=methyl salicylate anion], R 2 Al(μ‐pz) 2 AlR 2 [pz=deprotonated pyrazole, R=Me ( 3 a ), Et ( 3 b ), and t Bu ( 3 c )], R 2 Al(μ‐3,5‐Me 2 pz) 2 AlR 2 [3,5‐Me 2 pz=deprotonated 3,5‐dimethylpyrazole, R=Me ( 4 a ), and Et ( 4 b )], and Et 2 B(μ‐pz) 2 AlEt 2 ( 5 ), has been investigated. We were particularly interested in the effect of steric hindrances both caused by the metal‐bonded substituents and those that result from the nature of the bifunctional ligand used in the oxygenation reaction. In the reaction of 1 with O 2 , only the formation of the monoalkoxide compound 6 was observed. The latter di‐ tert ‐butyl compound as well as all planar aluminapyrazoles, that is, the tert ‐butyl derivative 3 c and lower alkylaluminum derivatives with the more demanding 3,5‐dimethylpyrazoyl ligands 4 a and 4 b , are stable under an atmosphere of dry oxygen and ambient conditions. Inspection of the space‐filling representation of these compounds has undoubtedly shown that the bulky tert ‐butyl groups or pyrazolyles ligands, respectively, provide steric protection for the metal center from the dioxygen attack. In contrast, the dialkylaluminum derivatives of pyrazole, 3 a and 3 b , and the diethylaluminum bis(1‐pyrazolyl)borate complex 5 , all with the metal center eclipsed with respect to the plane defined by the four nitrogen atoms, react smoothly with O 2 to form the alkyl(alkoxy)aluminum complexes. In the reaction of 5 with O 2 for example, the Et−B bonds remained intact, and the dimeric five‐coordinate compound [Et 2 B(μ‐pz) 2 Al(μ‐OEt)Et] 2 ( 9 ) was isolated in good yield. The interaction of mononuclear di‐ tert ‐butyl chelate complex 2 with O 2 at −15 °C gives ( t BuOO)( t BuO)Al(μ‐O t Bu) 2 Al(mesal) 2 ( 7 ) in high yield, and the presence of the alkylperoxo moiety is a particularly significant point in the resulting product. All the compounds have been characterized spectroscopically, and the structures of 3 c, 4 a, 6, 7 , and 9 have been confirmed by X‐ray crystallography. Structural features of 1 – 6 are discussed and are considered in relation to the possible approach pathways of the O 2 molecule to the four‐coordinate metal center. This analysis and the observed apparent dissimilarity in the reactions of model four‐coordinate aluminum alkyls with O 2 clearly show that the stereoelectronic prerequisites are responsible for the fundamentally different reactivity.