Trapping and Reactivity of a Molecular Aluminium Oxide Ion

Abstract

Aluminium oxides constitute an important class of inorganic compound that are widely exploited in the chemical industry as catalysts and catalyst supports. Due to the tendency for such systems to aggregate via Al‐O‐Al bridges, the synthesis of well‐defined, soluble, molecular models for these materials is challenging. Here we show that reactions of the potassium aluminyl complex K2[(NON)Al]2 (NON=4,5‐bis(2,6‐diiso‐propylanilido)‐2,7‐di‐tert‐butyl‐9,9‐dimethylxanthene) with CO2, PhNCO and N2O all proceed via a common aluminium oxide intermediate. This highly reactive species can be trapped by coordination of a THF molecule as the anionic oxide complex [(NON)AlO(THF)]−, which features discrete Al−O bonds and dimerizes in the solid state via weak O⋅⋅⋅K interactions. This species reacts with a range of small molecules including N2O (to give a hyponitrite ([N2O2]2−) complex) and H2, the latter offering an unequivocal example of heterolytic E−H bond cleavage across a main group M−O bond.

Carbon Monoxide Activation by a Molecular Aluminium Imide: C−O Bond Cleavage and C−C Bond Formation

“…This strategy has previously been employed by Power, Roesky, and co‐workers (e.g., for IV and V , Figure ), and a related approach has been reported by Coles and co‐workers for the synthesis of an indium imide complex ( VI ) . Similar chemistry has also been employed both by ourselves and by Anker and Coles for the synthesis of molecular aluminium oxide species via the reaction of anionic Al I precursors with N 2 O ( I and II ) …”

“…Within this sphere, the development of well‐defined molecular species containing highly polar Al−E bonds has been shown to offer a route to the activation of small molecules in 1,2‐fashion. Thus, a well‐defined molecular aluminium oxide system ( I , Figure ) has been shown to activate H 2 in a co‐operative (FLP‐like) manner exploiting the Lewis acidic nature of the aluminium centre and the Lewis basic oxide ligand, to give the corresponding aluminium hydroxide hydride . This oxide species is highly reactive and we therefore sought to target related imide species containing a discrete [AlNR] function, which (on the basis of a less polarized and more sterically protected Al−E bond) might be expected to offer greater control of reactivity.…”

“…With the idea of extending this approach to anionic aluminium imide species, we therefore probed the reactivity of the potassium aluminyl complex 1 towards trimethylsilylazide, Me 3 SiN 3 . However, the reaction of 1 with excess azide in toluene results instead in the formation of the (crystallographically characterized) tetrazene complex, 2 , in similar fashion to the formation of a cis ‐hyponitrite complex from 1 and excess N 2 O in the same solvent (Scheme and Supporting Information) . Reasoning that such a transformation proceeds via transient formation of the corresponding trimethylsilyl‐substituted imide complex (which then traps a second equivalent of Me 3 SiN 3 via cycloaddition), we examined the analogous reaction with stoichiometric azide.…”

“…Given the high level of reactivity implied for silyl‐substituted imide species by facile onward conversion to 2 and 3 (Scheme ), and the fact that other highly polarized M−E bonds (such as those found in early transition metal imides, Cui's neutral aluminium imide, and our recently reported aluminium oxide) are known to be reactive towards small molecules and E−H bonds, we set out to probe the corresponding reactivity of 4 . While 4 is indefinitely stable in toluene solution at room temperature and does not undergo further reaction with Me 3 SiN 3 or DippN 3 , it activates dihydrogen in 1,2‐fashion.…”

“…Thus, heating a solution of 4 under an atmosphere of H 2 for 14 h at 80 °C, leads to the formation of (amido)aluminium hydride complex 5 via addition of H 2 across the Al−N bond (Scheme , Figure and Supporting Information). Unlike the reaction with the corresponding aluminium oxide, the reaction in the case of imide 4 requires heating, presumably due to the increased steric shielding of the basic N centre, and partial delocalization of negative charge into the aryl π system of the Dipp substituent.…”