Naphthylaminoborane: from structural switches to frustrated Lewis pair reactivity†
A series of naphthyl-bridged amino-borane derivatives, namely 1-(dimethylamino)-8-naphthylboranes (1, 3, 5, 7) and 5-(dimethylamino)-6-acenaphthylboranes (2, 4, 6, 8, 10, 11), differing in the steric and electronic properties of the boryl moiety, have been synthesized and fully characterized by spectroscopic and crystallographic means. Structural X-ray analysis of the peri-atom displacement and ring torsion angles served to experimentally assess the presence and magnitude of the B–N interactions. The reversible quaternarization of nitrogen has been explored and was found to provide an efficient switch corresponding to different molecular organizations. The electronic characteristics of the nature of B–N interactions were further studied by Natural Bonding Orbital analysis derived from the theoretically calculated electron densities. This real-space bonding indicator discriminates the bonding B–N contact in 5 from the nonbonding in 8, which correlates with the flexibility of the naphthyl scaffold to respond to the Lewis acidity of boron allowing shorter peri interactions. Whereas, the steric shielding imposed by the two mesityl groups, and/or the rigidity of the acenaphthene framework disrupt B–N interaction. Thus, this communication reports on the modulation of the B–N bonding continuum by means of structural tuning leading to a molecular switch, as well as its implications towards revealing FLP reactivities through the isolation of intermediates of a stepwise mechanism.