Alumazene adducts with triphenylphosphine oxide

Abstract

1,3,5-trimethyl-2,4,6-tris(2,6-diisopropylphenyl)alumazene [2,6-(i-Pr)₂C₆H₃NAlMe]₃, 1, is a benzene-like iminoalane with a planar hexagonal (Al–N)₃ core. The aluminum centers are highly Lewis acidic due to the low delocalization of nitrogen lone pairs. We examined the alumazene ability to form acid-base adducts with triphenylphosphine oxide. Both monoadduct, 1(OPPh₃), 2, and bisadduct 1(OPPh₃)₂, 3, were prepared, isolated, and spectroscopically characterized. Their molecular structures were determined by the single-crystal X-ray diffraction method. The monoadduct 2 displays a boat conformation of the central ring with very short Al–N bonds (1.779, 1.769(2) Å ). The bisadduct 3 features the expected trans geometry in contrast to the previously reported bisadduct cis-1[OP(OMe)₃]₂. A different behavior of 2 and 3 in solution was observed by NMR spectroscopy. The monoadduct 2 is stable to dissociation while 3 shows signal broadening by chemical exchange because of the weakening of the second coordination Al–O bond and/or internal rotation.