A Cyclic Iminoborane-NHC Adduct: Synthesis, Reactivity, and Bonding Analysis

Lewis-base coordinated iminoborane adducts, in contrast to their isoelectronic analogue imines, remain largely unexplored given the lack of efficient synthetic strategies for generating robust compounds. Herein, we report the preparation of a cyclic amino iminoborane carbene complex 2 obtained in quantitative yield by adding NHC to the 1,8-(trimethylsilyl)-aminonaphthalene complex of boron 1 to induce the elimination of trimethylsilyl chloride (TMSCl). The iminoborane-NHC adduct 2 shows unprecedented thermal stability both in the solid and solution phases, due to the rigid, pre-established geometry of the 1,8-diaminonaphthalene scaffold. Theoretical calculations reveal an exceptionally strong iminoborane-NHC bond as a consequence of the enhanced boron-center acidity in combination with the lower steric and electronic shielding. We show that the chemical bond can be understood as donor-acceptor interaction, leading to a different kind of electronic situation of the B=N pi-bond. The high conjugation between the p(z)-lone pair of the tricoordinated sp(2) hybridized N atom and the B=N pi-system results in a particularly long B=N double bond distance. Taking advantage of the pendant lone pair of the dicoordinated sp(2) hybridized N atom, the iminoborane-NHC adduct gives access to NHC-stabilized borenium cation 3 through the reaction with trimethylsilyl triflate (Me3SiOTf) or to the gallium adduct 4 by reacting with GaCl3. Incorporating an iminoborane functional group into a pi-conjugated system brings a new bonding situation for broadening the scope of BN-containing polyaromatic systems.

Automated summary

A new synthetic strategy was developed to prepare a highly stable cyclic amino iminoborane carbene complex, showing unprecedented thermal stability. Furthermore, the formation of the chemical bond can be explained as a donor-acceptor interaction, resulting in a different electronic structure of the B=N pi-bond.