Stability and Three-Dimensional Aromaticity ofcloso-NBn-1HnAzaboranes,n= 5−12

Abstract

Computations on all the possible positional isomers of the closo-azaboranes NB_n-1H_n (n = 5−12) reveal substantial differences in the relative energies. Data at the B3LYP/6-311+G** level of density functional theory (DFT) agree well with expectations based on the topological charge stabilization, with the qualitative connectivity preferences of Williams, and with the Jemmis−Schleyer six interstitial electron rules. The energetic relationship involving each of the most stable positional isomers, 1-NB₄H₅, NB₅H₆, 2-NB₆H₇, 1-NB₇H₈, 4-NB₈H₉, 1-NB₉H₁₀, 2-NB₁₀H₁₁, NB₁₁H₁₂, was based on the energies (ΔH) of the model reaction: NBH₂ + (n−1)BH_increment → NB_nH_n+1 (n = 4−11). This evaluation shows that the stabilities of closo-azaboranes NB_n-1H_n (n = 5−12) increase with increasing cluster size from 5 to 12 vertexes. The “three-dimensional aromaticity” of these closo-azaboranes NB_n-1H_n (n = 5−12) is demonstrated by their the nucleus-independent chemical shifts (NICS) and their magnetic susceptibilities (χ), which match one another well. However, there is no direct relationship between these magnetic properties and the relative stabilities of the positional isomers of each cluster. As expected, other energy contributions such as topological charge stabilization and connectivity can be equally important.