Tricyclic 1,4-Diphosphinines: Local vs. Global Aromaticity.

1D and 2D (Sigma only model) calculations were performed on recently established tricyclic 1,4-diphosphinines as well as related benzene and pyrazine derivatives. The study was extended to evaluate the effect of the fused rings on the overall aromatic properties with a special focus on functional groups such as carbenes. The effect of non-aromatic heterocycles on the local ring current of the central ring is small, while aromatic heterocycles (e. g. NHC, imidazolium) lead to a global aromaticity. A higher sulfur content of the adjacent five-membered rings reduces the central ring current. The comparison to related tricyclic benzene and pyrazine derivatives showed that the 1,4-diphosphinine systems resemble more closely the situation in the benzene derivatives than the pyrazines. The effect of charged systems was studied using bis(TTF)-fused 1,4-diphosphinines and, according to values, the neutral form doesn't possess significant aromaticity but the tetracation resembles the global aromatic situation observed for other heterocycles in this study.