Stabilization of [(N₅)₂BX]^2– and [(N₅)₂B₂X₂]^2– (X = H, F, Cl, Br) by Conjugation and Hyperconjugation Effects

The isolation of nucleophilic boron bases has led to a paradigm shift in boron chemistry. Previous studies of the bis(carbene) borylene complexes revealed that these compounds possess strong donor abilities, and their reaction inertness is due to the large steric hindrance between boron reagents and reactant. In the present study, we have theoretically studied the [(N₅)₂BX]^2- and [(N₅)₂B₂X₂]^2- compounds (X = H, F, Cl, Br). Their electronic structures and properties are discussed by using the NBO, LOL, and ELF methods. We found that both π-conjugation and hyperconjugation effects can effectively stabilize the substituted nucleophilic anionic boron compounds [(N₅)₂BX]^2- and [(N₅)₂B₂X₂]^2-. Substituents, especially X = H, stabilize the boron center through highly delocalized π-bonding, involving the formally "empty" in-plane <i>p</i> orbitals of the boron atom. While the halogen substituents have high electron withdrawal ability, leading to systems being less stable, we suggest the borinium anions [(N₅)₂BH]^2- and [(N₅)₂B₂H₂]^2- as possible synthetic targets of novel environmentally friendly catalysts.