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 N2 , and the resulting [(CAAC)DurB]2 N2 is of a bent BNNB core. The N2 ligand in transition metal complexes is generally linear, so herein, the bonding nature of both terminal end-on and end-on bridging borylene-N2 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 σ donation and a π back-donation, but in the end-on bridging borylene-N2 complex, the σ donation comes from the π orbitals of N2 , and thus, there are two opposite and perpendicular push-pull channels. It is the push-pull interaction that governs the enhanced activation of N2 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 N2 .