Selective oxygen capture by lithium aluminates: a solid state and theoretical structural study

Abstract

Reaction of PhC(O)N(Me)H with AlMe₃ in toluene results in facile CH₄ evolution and formation of the amidoalane PhC(O)N(Me)AlMe₂, 6. The addition of 1 eq. Bu^tLi affords the lithium aluminate [PhC(O)N(Me)Al(Me)₂Bu^t]Li, 7, which on treatment with oxygen yields the mixed-anion species [PhC(O)N(Me)Al(Me)(Bu^t)OMe]Li·[PhC(O)N(Me)Al(Me)(OBu^t)OMe]Li, 8. In the solid state 8 forms a dimer based on a tetranuclear (LiO)₄ ladder structure in which terminal mono-oxygenated aluminate ligands and tripodal bis-oxygenated aluminate ligands span end and central Li⁺ cations. Replacement of PhC(O)N(Me)H in the above reaction sequence with the more sterically congested amide PhC(O)N(Ph)H results in the formation of the amidoalane PhC(O)N(Ph)AlMe₂, 9, which in turn affords the lithium aluminate [PhC(O)N(Ph)Al(Me)₂Bu^t]Li, 10, and upon treatment of this with oxygen, the 70∶30 11a∶11b mixture [PhC(O)N(Ph)Al(Me)(OR)R′]Li, 11 (R = Bu^t, R′ = Me, 11a; R′ = Bu^t, R = Me, 11b). Both 10 and 11 are dimeric in the solid state, suggesting that the selective oxygenation process, and therefore the character of the oxygenated product, is templated by the structure of the precursor aluminate complex. Calculations are presented which corroborate the competitive nature of the inclusion of oxygen atoms into Al–Me and Al–Bu^t groups in species of the type reported here.