Synthesis and structural characterization of neutral and cationic alkylaluminum complexes based on bidentate aminophenolate ligands

The reaction of the aminophenols 2-(CH2L)-6-R-C6H3OH (R = Ph, L = NMe2, 1a; R = Bu-t, L = NMe2, 1b; R = Bu-t, L = NC4H8, 1c; R = Bu-t, L = NC5H10, 1d) with 1 equiv of AlMe3 affords the corresponding dimethyl Al complexes {2-(CH2L)-6-R-C6H3O}AlMe2 (R = Ph, L = NMe2, 2a; R = Bu-t, L = NMe2, 2b; R = Bu-t, L = NC4H8, 2c; R = Bu-t, L = NC5H10, 2d) in high yields. Compounds 2a-d appear to be monomeric, on the basis of X-ray analysis for 2b,d and NMR data for 2a-d, and are stable in the presence of THF. {2-(CH2NMe2)-6-Ph-C6H3O}AlMe2 (2a) cleanly reacts with B(C6F5)(3) to yield the dinuclear cationic Al species 3a(+). X-ray diffraction analysis shows that the cation 3a(+) can be seen as an adduct of the three-coordinate cation {2-(CH2NMe2)-6-Ph-C6H3O}AlMe+ and the neutral precursor 2a, in which the two Al centers are connected via a mu(2)-O aminophenolate bridge. Similarly, the reaction of Al dimethyl complexes 2b-d with B(C6F5)(3) yields dinuclear cationic Al species 3b-d(+)/3b'-d'+ (3b+/3b'(+) in a 1/1 ratio; 3c,d+/3c',d'(+) in a 3/1 ratio, respectively) as diastereomeric mixtures. Cations 3b-d(+)/3b'-d'(+) adopt a structure similar to that of cation 3a(+), as determined by X-ray crystallography analysis for 3b'(+) and 2D NMR studies for 3a(+) and 3c,d+/3c',d'+. All the formed dinuclear cations are quite stable and robust in solution, and no fluxional behavior for any of them was observed up to 80 degreesC in C6D5Br. Cations 3b-d(+)/3b'-d'(+) react with a Lewis base such as THF to afford the corresponding THF adduct cation 4b-d(+) along with 1 equiv of the corresponding neutral precursor 2b-d. In contrast, 3a(+) reacts with THF to yield unidentified species. 3a(+) and 3c,d(+)/3c',d'(+) are inactive in ethylene polymerization, but cations 3b-d(+)/3b'-d'(+) polymerize PO with moderate activity to yield low-molecular-weight PPO.