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

Abstract

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 [DippNacNacAlI] and [(MesNacNacMgI)2], which gave the dialane compounds [(4-MeBox2CH)HAlII-AlIIH(DippNacNac)] (2) and [{(4-MeBox2CH)AlIIH}2] (4a), respectively. Furthermore, dialuminoxanes [{(4-MeBox2CH)AlH}2(μ-O)] (4b) and [({(MesNacNac)Mg}2(μ-H)){H3AlII-AlIIH(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)HAlII-AlII(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.