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 = tBu,
L = NMe2, 1b; R = tBu, L = NC4H8, 1c; R = tBu, 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 = tBu, L = NMe2, 2b; R = tBu, L = NC4H8, 2c; R = tBu, 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.

将氨基酚类化合物2-(CH₂L)-6-R-C₆H₃OH（其中R=Ph、L=NMe₂记为1a；R=tBu、L=NMe₂记为1b；R=tBu、L=NC₄H₈记为1c；R=tBu、L=NC₅H₁₀记为1d）与1当量的三甲基铝（AlMe₃）反应，可高产率得到对应的二甲基铝配合物{2-(CH₂L)-6-R-C₆H₃O}AlMe₂（R=Ph、L=NMe₂记为2a；R=tBu、L=NMe₂记为2b；R=tBu、L=NC₄H₈记为2c；R=tBu、L=NC₅H₁₀记为2d）。基于配合物2b、2d的X射线衍射分析（X-ray analysis）结果以及2a~2d的核磁共振（NMR）数据，可知化合物2a~2d均为单分子结构，且在四氢呋喃（THF）中性质稳定。{2-(CH₂NMe₂)-6-Ph-C₆H₃O}AlMe₂（2a）可与三(五氟苯基)硼（B(C₆F₅)₃）顺利发生完全反应，生成双核阳离子铝物种3a⁺。X射线衍射分析表明，阳离子3a⁺可被视为三配位阳离子{2-(CH₂NMe₂)-6-Ph-C₆H₃O}AlMe⁺与中性前驱体2a的加合物，两个铝中心通过μ₂-氧代氨基酚盐桥相连。类似地，二甲基铝配合物2b~2d与B(C₆F₅)₃的反应可得到非对映异构混合物形式的双核阳离子铝物种3b~d⁺/3b’~d’⁺（其中3b⁺与3b’⁺的比例为1:1，3c,d⁺与3c’,d’⁺的比例分别为3:1）。通过对3b’⁺的X射线晶体学分析以及对3a⁺和3c,d⁺/3c’,d’⁺的二维核磁共振（2D NMR）研究，可证实3b~d⁺/3b’~d’⁺的结构与3a⁺相似。所有生成的双核阳离子在溶液中均具有良好的稳定性，在氘代溴苯（C₆D₅Br）中升温至80℃时，未观察到任何物种出现流变行为。3b~d⁺/3b’~d’⁺可与路易斯碱（如THF）反应，生成对应的THF加合阳离子4b~d⁺，同时释放1当量的中性前驱体2b~2d。与之相反，3a⁺与THF反应后会生成无法鉴定的产物。3a⁺与3c,d⁺/3c’,d’⁺对乙烯聚合无催化活性，而3b~d⁺/3b’~d’⁺可催化环氧丙烷（PO）聚合，表现出中等活性，产物为低分子量聚环氧丙烷（PPO）。