Synthesis and Structural Characterization of Neutral and Cationic Alkylaluminum Complexes Based on Bidentate Aminophenolate Ligands

The reaction of the aminophenols 2-(CH 2 L)-6-R-C 6 H 3 OH (R = Ph, L = NMe 2, 1a; R = t Bu, L = NMe 2, 1b; R = t Bu, L = NC 4 H 8, 1c; R = t Bu, L = NC 5 H 10, 1d ) with 1 equiv of AlMe 3 affords the corresponding dimethyl Al complexes {2-(CH 2 L)-6-R-C 6 H 3 O}AlMe 2 (R = Ph, L = NMe 2, 2a; R = t Bu, L = NMe 2, 2b; R = t Bu, L = NC 4 H 8, 2c; R = t Bu, L = NC 5 H 10, 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-(CH 2 NMe 2 )-6-Ph-C 6 H 3 O}AlMe 2 ( 2a ) cleanly reacts with B(C 6 F 5 ) 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-(CH 2 NMe 2 )-6-Ph-C 6 H 3 O}AlMe + and the neutral precursor 2a, in which the two Al centers are connected via a μ 2 -O aminophenolate bridge. Similarly, the reaction of Al dimethyl complexes 2b − d with B(C 6 F 5 ) 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 C 6 D 5 Br. 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.