GAS-PHASE CHEMISTRY OF THE DIMETHYLALUMINUM OXIDE ION AND RELATED ALUMINUM-OXIDE IONS - COMPARISON OF REACTIVITY WITH SILOXIDE IONS

The anion [(CH3)2AlO]- (1) has been prepared in a flowing afterglow selected ion flow tube (FA-SIFT) by ion selection from the reaction mixture of trimethylaluminum dimer and hydroxide. This unusual ion is quite reactive and has been extensively studied. It reacts with halogen-containing compounds of several types, including fluorobenzene, chlorobenzene, bromofluorobenzene, acetyl chloride, and HF, to give [(CH3)2Al(OH)X]-, where X = F, Cl, and Br. Its chemistry with hexafluorobenzene and silicon tetrafluoride has also been detailed as have a number of reactions with carboxylic acids, esters, anhydrides, sulfur-containing neutrals, alcohols, water, and ammonia. The dominant reaction pattern of 1 involves six-centered processes which require a relatively acidic hydrogen and a lone pair donor atom in the neutral reactant. In these reactions, ion 1 abstracts a proton as the lone pair atom adds to aluminum, often with concomitant extrusion of a small, neutral molecule. We have compared the reactivity of 1 with that of [(CH3)3SiO]- and briefly examined other aluminum oxide ions. The kinetics of several reactions as well the thermochemical relationships among some of these ions have also been studied. Acidity studies coupled with thermochemical analysis establish that the conjugate acid of [AlO]- is AlOH, not HAlO. Computational studies have been carried out to probe both the reaction of trimethylaluminum and hydroxide, and the acidities and stabilities of AlOH and HAlO.