Revisiting the π-Back-Donation in the NHC-B≡B-NHC Molecule.

As the first thermal stable molecule with a B≡B bond, the diboryne complex protected by N-heterocyclic carbene ligands (NHC-B≡B-NHC) has attracted much interest. Researchers point out that π-back-donation highly stabilizes the B≡B bond besides σ-donation, both of which are induced by NHC ligands. In this work, details of the π-back-donation are revisited by using DFT calculations. There are two delocalized π* orbitals in NHC, and the symmetry of one π* orbital is highly adaptive to the π orbitals in B≡B bond, whereas the other cannot be involved in the π-back-donation. In staggered configuration, two orthogonal π orbitals of B≡B interact with this π* orbital in each NHC ligand, respectively, to form π-back-donations in both sides. This interaction has proven to be more intensive than π-conjunction, resulting in the lower energy of the staggered isomer compared with the eclipsed one containing greater π-conjunction. Moreover, intensity of the π-back-donation can be enhanced by reducing the energy levels of the matched π* orbitals in ligands, which gives references for the design of stable diborynes.