“In-situ” orbital correlation diagrams for the activation of small molecules by main-group compounds

Traditionally, orbital correlation maps are drawn based on the orbital energy levels of isolated monomers and their complex. But this may be problematic as monomers will be perturbed by each other even without any electron movements (orbital interactions). This “field effect” is missing in current orbital correlation diagrams. Combining the advantages of both MO theory and VB theory, the block-localized wavefunction (BLW) method is the simplest variant of VB theory with the efficiency of MO methods, and the first method that can generate optimal geometries and spectral properties for strictly localized (diabatic) molecular structures and results can be examined by experimental data. With the BLW method, we can precisely consider the “field effect” by generating optimal block-localized “in-situ” frontier orbitals before allowing electrons to move between monomers and orbitals to mix with each other. The generated “in-situ” orbital correlation diagrams are much more informative than traditional orbital correlation diagrams in elucidating bonding natures and chemical reaction mechanisms.
In this talk, I will exemplify the “in-situ” orbital correlation diagrams in the activation of small molecules like CO, N₂, H₂ and CO₂ by main-group compounds. While generally the adsorption and activation of small molecules require transition metals, currently there is a trend to explore multi-bonded and unsaturated main group compounds that can interact with small molecules, in order to find non-metal catalysts. For instance, Braunschweig et al. not only showed that diboryne (B≡B) stabilized by N-heterocyclic carbenes (B₂(NHC^R)₂) can bind and activate CO molecules to forge C-C bonds, but also found that borylene (CAAC)DurB, where CAAC is a cyclic alkyl(amino) carbene and Dur refers to 2,3,5,6-tetramethylphenyl, can bind and activate N₂. We will show how we can use the BLW method to understand the origin of the catalysis in these cases.