Structural diversity and versatility for organoaluminum complexes supported by mono- and di-anionic aminophenolate bidentate ligands

The present contribution describes the synthesis and structural characterization of structurally diverse organoaluminum species supported by variously substituted aminophenolate-type ligands: these Al complexes are all derived from the reaction of AlMe3 with aminophenols 2-CH2NH(R)-C6H3OH (1a, R = mesityl (Mes); 1b, R = 2,6-di-isopropylphenyl (Diip)) and 2-CH2NH(R)-4,6-Bu-t(2)-C6H2OH (1c, R = Mes; 1d, R = Diip). The low temperature reaction of AlMe3 with 1a-b readily affords the corresponding Al dimeric species [mu-eta(1), eta(1)-N,O-{2-CH2NH(R)-C6H4O}](2)Al2Me4 (2a-b), consisting of twelve-membered ring aluminacycles with two mu-eta(1), eta(1)-N,O-aminophenolate units, as determined by X-ray crystallographic studies. Heating a toluene solution of 2a (80 degrees C, 3 h) affords the quantitative and direct formation of the dinuclear aluminium complex Al[eta(2)-N; mu, eta(2)-O-{2-CH2N(Mes)-C6H4O}](AlMe2) (4a) while species 2b, under the aforementioned conditions, affords the formation of the Al dimeric species [eta(2)-N,O-{2-CH2N(Dipp)-C6H4O}AlMe](2) (3b), as deduced from X-ray crystallography for both 3b and 4a. In contrast, the reaction of bulky aminophenol pro-ligands 1c-d with AlMe3 afford the corresponding monomeric Al aminophenolate chelate complexes eta(2)-N,O-{2-CH2NH(R)-4,6-Bu-t(2)-C6H2O}AlMe2 (5c-d; R = Mes, Diip; Scheme 3) as confirmed by X-ray crystallographic analysis in the case of 5d. Subsequent heating of species 5c-d yields, via a methane elimination route, the corresponding Al-THF amido species eta(2)-N,O-{2-CH2N(R)-4,6-Bu-t(2)-C6H2O}Al(Me)(THF) (6c-d; R = Mes, Diip). Compounds 6c-6d, which are of the type {X-2}Al(R)(L) (L labile), may well be useful as novel well-defined Lewis acid species of potential use for various chemical transformations. Overall, the sterics of the aminophenol backbone and, to a lesser extent, the reaction conditions that are used for a given ligand/AlMe3 set essentially govern the rather diverse "structural" outcome in these reactions, with a preference toward the formation of mononuclear Al species (i.e. species 5c-d and 6c-d) as the steric demand of the chelating N,O-ligand increases. (C) 2011 Elsevier B.V. All rights reserved.