Mechanism of Solid-State Thermolysis of Ammonia Borane: A ~(15)N NMR Study Using Fast Magic-Angle Spinning and Dynamic Nuclear Polarization

摘要

The solid-state thermolysis of ammonia borane (NH3BH3, AB) was explored using state-of-the-art ~(15)N solid-state NMR spectroscopy, including 2D indirectly detected ~1H{~(15)N} heteronuclear correlation and dynamic nuclear polarization (DNP)-enhanced ~1N{~1H} cross-polarization experiments as well as ~(11)B NMR. The complementary use of ~(15)N and ~(11)B NMR experiments, supported by density functional theory calculations of the chemical shift tensors, provided insights into the dehydrogenation mechanism of AB-insights that have not been available by ~(11)B NMR alone. Specifically, highly branched polyaminoborane derivatives were shown to form from AB via oligomerization in the "head-to-tail" manner, which then transform directly into hexagonal boron nitride analog through the dehydrocycllzation reaction, bypassing the formation of polyiminobor-ane.