The influence of heteroatoms on the aromatic character and the current pathways of B2N2-dibenzo[a,e]pentalenes

Abstract

Four polycyclic molecules have been investigated at the DFT B3LYP/def2-TZVP level of theory using calculated magnetically induced current densities as an indicator of their (anti)aromaticity. Complicated current pathways were found in dibenzo[a,e]pentalene and its three heterocyclic analogues each containing two boron and two nitrogen atoms. The antiaromatic character of the pentalene moiety is weaker in the hydrocarbon molecule and in 5,11-dihydrobenzo[d]benzo[4,5][1,2,3]azadiborolo[3,2-b][1,2,3]azadiborole as compared to the pentalene molecule. The antiaromatic character of the pentalene moiety is completely absent in the other two heterocyclic structures. In 6,12-dihydrobenzo[d]benzo[4,5][1,2,3]diazaborolo[2,1-a][1,2,3]diazaborole all four molecular rings are aromatic according to the ring-current criterion, and in 6,12-dihydrobenzo[d]benzo[3,4][1,2,5]azadiborolo[1,2-a][1,3,2]diazaborole, the diazaborole ring is aromatic, while the azadiborole ring is antiaromatic. In all four molecules the six-membered rings are aromatic sustaining a weaker ring current than benzene does.