Aromaticity of Diazaborines and Their Protonated Forms

Substitution of a CH group in benzene with nitrogen has a little effect on its aromaticity (Wang et al., Org. Lett. 2010, 12, 4824). How does the same type of substitution affect aromatic character of the three isomeric azaborines? Does further protonation change aromaticity of diazaborines? This work is aimed at answering these questions. Such a knowledge should be of interest for further exploration and application of BN/CC isosterism. Aromaticity of diazaborines and their protonated forms is studied with the aid of four aromaticity indices, HOMA, NICS(0)_πzz, PDI and ECRE. Generally, NICS(0)_πzz and PDI point to similar aromaticity of diazaborines and their parent azaborines, while HOMA and ECRE indicate some changes. Thus, aromaticity of 1,2-azaborine slightly decreases/increases when CH meta/ortho,para to B is substituted with nitrogen. Aromaticity of the most aromatic 1,3-azaborine remains almost unchanged when CH meta to B and N is replaced with nitrogen, and becomes slightly weaker when any other CH group is substituted with nitrogen. Replacement of the CH ortho to N in 1,4-azaborine does not change much its cyclic delocalization, while replacement of the CH ortho to B leads to smaller cyclic delocalization. Protonated forms are either of similar or decreased aromaticity compared with neutral molecules.