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 = t Bu, L = NMe2, 1b; R = t Bu, L = NC4H8, 1c; R = t Bu, 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 = t Bu, L = NMe2, 2b; R = t Bu, L = NC4H8, 2c; R = t Bu, 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 μ2-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 °C 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.