Formation and Reactivity of Non‐Stabilized Monomeric Alumoxane Intermediates

Several methods for the oxidation of the beta-diketiminate Al-I complexes (BDI)Al and (BDI*)Al with O-2, Et3P=O or N2O are presented (BDI=HC[C(Me)N(DIPP)](2), DIPP=2,6-diisopropylphenyl; BDI*=HC[C(tBu)N(DIPP)](2)). The selectivity of alumoxane formation depends strongly on the nature of the beta-diketiminate ligand, the experimental procedure and the solvent. The mononuclear alumoxanes (BDI)Al=O and (BDI*)Al=O are highly reactive intermediates that cannot be isolated but either aggregate to give less reactive dimers or decompose by internal C-H activation of a backbone Me group or an iPr group. Ethereal solvents like Et2O react with alumoxanes to give (BDI)Al(OEt)OH and ethylene by beta-H elimination. In case of N2O oxidation, subsequent reaction with a second equivalent of N2O to form the hyponitrite complexes (BDI)Al(N2O2) and (BDI*)Al(N2O2) is observed. Details of the alumoxane formation, reactivity and decomposition have been obtained by DFT calculations. These calculations also underscore their rather bipolar character, represented by the resonance structure (BDI)Al+-O-, and estimate the sigma- and pi-bond character of the Al=O bond at circa 76% and 24%, respectively.