Revisiting the electronic structure of N₂-bound cAAC-borylene at the CASSCF level: a detailed bonding picture of...

A base-trapped borylene species featuring a cyclic-(alkyl)(amino)carbene (cAAC) has shown unique bonding interactions with dinitrogen, thereby, opening a new avenue for N₂ activation by main-group compounds. The detailed electronic structure and qualitative bonding picture between cAAC-trapped borylene and N₂ remain to be fully understood. This work presents a multiconfigurational complete active space self-consistent field (CASSCF)-based electronic structure investigation on the N₂-bound cAAC-borylene species (1) isolated by Braunschweig et al. Specifically, the synergistic bonding between the borylene units and N₂ involving the donation from the N-N σ to the unoccupied orbital of borylene and back-donation from the occupied orbital of borylene to the N-N π* has been unequivocally established using CASSCF-derived natural orbitals and electronic configuration. Bonding interactions between the HOMO of the borylene units and the N-N π* (HOMO_cAAC-B + π*_NN) and the LUMO of the borylene units and the N-N σ (LUMO_cAAC-B + σ_NN) in 1 were apparent through the CASSCF-derived natural orbitals. The unique bonding of the B-N-N-B core in 1 and the resulting geometry have also been compared with the M-N-N-M core of a prototypical transition metal(M)-N₂ complex. Finally, the change in the electronic structure and geometry of the N₂-bound borylene species 1 on two-electron reduction has been investigated in the context of N₂ activation.