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

Abstract

The interaction of dioxygen with various tetrahedral aluminum alkyls, (tBu)3Al.OEt2 (1), tBu2Al(mu-OtBu)2AltBu2 (6), (tBu)2Al(mesal) (2) [mesal=methyl salicylate anion], R2Al(mu-pz)2AlR2 [pz=deprotonated pyrazole, R= Me (3a), Et (3b), and tBu (3c)], R2Al(mu-3,5-Me2pz)2AIR2[3,5-Me2pz = deprotonated 3,5-dimethylpyrazole, R= Me (4a), and Et (4b)], and Et2B(mu-pz)2AlEt2 (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 O2, only the formation of the monoalkoxide compound6 was observed. The latter di-tert-butyl compound as well as all planar aluminapyrazoles, that is, the tert-butyl derivative 3c and lower alkylaluminum derivatives with the more demanding 3,5-dimethylpyrazoyl ligands 4a and 4b, 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, 3a and 3b, 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 O2 to form the alkyl(alkoxy)aluminum complexes. In the reaction of 5 with O2 for example, the Et-B bonds remained intact, and the dimeric five-coordinate compound [Et2B(mu-pz)2 Al(mu-OEt)Et]2 (9) was isolated in good yield. The interaction of mononuclear di-tert-butyl chelate complex 2 with O2 at -15 degrees C gives (tBuOO)(tBuO)Al(mu-OtBu)2Al(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 3c, 4a, 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 O2 molecule to the four-coordinate metal center. This analysis and the observed apparent dissimilarity in the reactions of model four-coordinate aluminum alkyls with O2 clearly show that the stereoelectronic prerequisites are responsible for the fundamentally different reactivity.

Reaching Milestones in the Oxygenation Chemistry of Magnesium Alkyls: towards Intimate States of O2 Activation and the First Monomeric Well‐Defined Magnesium Alkylperoxide

“…The crucial step of the ISET mechanism involves a non‐specific activation of an O 2 molecule on the metal centre followed by single electron transfer (SET) from the M−C bond to the O 2 molecule (we note that the ISET mechanism nicely collaborates with our systematic studies on both the mentioned above controlled oxygenation of organometallics with non‐redox‐active metal centre as well as the reactivity of organozincs incorporating non‐innocent ligands, that is, stable nitroxide radicals, α ‐diimines and α ‐diketones). Moreover, we have also emphasized the essential role of the coordination state of the oxygenated organoaluminum and organozinc compounds, as well as the geometric requirements of an O 2 molecule approaching the metal centre . Apart from the fundamental aspects, these systematic studies have advanced rational design of metal alkyl/air systems as radical initiators for organic reactions, stoichiometric and catalytic asymmetric epoxidation of enones and pave the way to well‐defined metal alkoxides (not accessible through the classical alcoholysis path) as potential initiators of cyclic esters polymerization as well as to a new approach for the preparation of semiconductor zinc oxide quantum dots based on organozinc/air systems …”

First Structurally Authenticated Zinc Alkylperoxide: A Model System for the Epoxidation of Enones

“…Our recent studies have been directed towards designing main‐group complexes as model species for investigations of the relationship between the molecular geometry and reactivity or other property of interest 7a,b. 8 Thus, investigations on the reactivity of aluminum alkyl species with molecular oxygen have shown that the key feature in the oxygenation reaction is the O 2 attack on the four‐coordinate metal center, followed by an insertion of O 2 into the AlC bond to generate an AlOOR moiety 7a,b. As an extension of these studies we turned our attention to the design of zinc alkyl complexes as model species with which a better understanding of dioxygen activation by organometallic compounds could be gained.…”

Synthesis and structure of diethylaluminum malonate: An unusual hexanuclear aluminum complex with malonate trianion ligands

“…The central Al 2 O 2 ring in the molecule of 1 is typical for monoalkoxide aluminum compounds. However, in comparison with AleO bond lengths in dimethylaluminum 2,6-diphenylphenolate [1.879(1) Å] [12] and di-tert-butyl tert-butoxide aluminum [1.880(1) and 1.878(1) Å] [13], the Al(1)eO(1) distances [1.901(1) and 1.891(1) Å] are slightly longer. This is probably caused by an electronwithdrawing effect of carboxylate groups.…”

“…Chemical shifts were referenced to the residual proton signals of CDCl 3 (7.26 ppm). 13 C NMR spectra were acquired at 100.60 MHz (standard: chloroform 13 CDCl 3 , 77.20 ppm). IR spectra were obtained using a Nicolet 6700 FT-IR (ATR) infrared spectrometer.…”