Two Push-Pull Channels Enhance the Dinitrogen Activation by Borylene Compounds.

Recently, Braunschweig et al. found that borylene (CAAC)DurB, in which CAAC is a cyclic alkyl(amino) carbene and Dur refers to 2,3,5,6-tetramethylphenyl, can bind and activate N-2, and the resulting [(CAAC)DurB](2)N-2 is of a bent BNNB core. The N-2 ligand in transition metal complexes is generally linear, so herein, the bonding nature of both terminal end-on and end-on bridging borylene-N-2 complexes is investigated with valence bond (VB) theory. In the terminal end-on (CAAC)HBN2 the bonding follows the mechanism in transition metals with a sigma donation and a pi back-donation, but in the end-on bridging borylene-N-2 complex, the sigma donation comes from the pi orbitals of N-2, and thus, there are two opposite and perpendicular push-pull channels. It is the push-pull interaction that governs the enhanced activation of N-2 and the BNNB bent geometry. It is expected that the substituents bonded to B can modulate the bent angle and the strength of the push-pull interaction. Indeed, (CAAC)FB exhibits enhanced catalytic capacity for the activation of N-2.