Mixed Low-Valent Alanes from the Bis(4-methyl-benzoxazol-2-yl)methanide Ligand

Within this work, an aluminum dihydride complex ([(^4-MeBox₂CH)AlH₂]) (1) based on the bis(4-methyl-benzoxazol-2-yl)methanide ligand was synthesized and characterized by spectroscopic methods (NMR, ATR-IR, and fluorescence), DSC (differential scanning calorimetry), mass spectrometry (LIFDI), and single crystal X-ray diffraction. The reactivity of alane 1 was investigated toward the reducing agents [^DippNacNacAl^I] and [(^MesNacNacMg^I)₂], which gave the dialane compounds [(^4-MeBox₂CH)HAl^II-Al^IIH(^DippNacNac)] (2) and [{(^4-MeBox₂CH)Al^IIH}₂] (4a), respectively. Furthermore, dialuminoxanes [{(^4-MeBox₂CH)AlH}₂(μ-O)] (4b) and [({(^MesNacNac)Mg}₂(μ-H)){H₃Al^II-Al^IIH(^4-MeBox₂CH)}] (4c) were isolated as byproducts, with 4b co-crystallizing with 4a. The hydricity of both hydrides in the mixed-ligated dialane 2 were examined by a reaction with 1 equiv of trityl borate ([Ph₃C][B(C₆F₅)₄]), which resulted in [(^4-MeBox₂CH)HAl^II-Al^II(^DippNacNac)][B(C₆F₅)₄] (3). Due to the formation of 4b, complex 1 was reacted with 0.5 equiv of water, which causes the likely synthesis of insoluble oligomeric alumoxanes. To prevent this reaction and support the formation of well-defined dialumoxanes, 1 was initially converted to [(^4-MeBox₂CH)(DippO)AlH] (5) by the deprotonation of 2,6-diisopropylphenol (propofol). This sterically encumbered compound 5 was subsequently reacted with 0.5 equiv of water, which resulted in defined molecules of [{(^4-MeBox₂CH)(DippO)Al}₂(μ-O)] (6). All these compounds exemplify the versatility of the ^4-MeBox₂CH ligand in low-valent aluminum chemistry.