Mono-, di-, tri- and tetraphosphatriafulvenes: Electronic structure and aromaticity

Aromaticity and electronic structure of triafulvenes containing 1, 2, 3 and 4 phosphorus atoms were studied using Density Functional Theory computations. It was found that the donor-acceptor interaction between the “filled” ring- and “empty” exo-Lewis-π orbitals is a good criterion for the aromaticity of triafulvenes. Although phosphorus is less electronegative than carbon, the exo-phosphorus increases, whereas the ring-phosphorus decreases the aromaticity. We have shown that the aromatic stabilization effect of the exo-phosphorus is larger than the destabilization of the ring-phosphorus. The final order of the aromaticity, compared with some reference compounds is cyclopropenone > 4-silatriafulvene ≈ 4-phosphatriafulvene > 1,4-diphosphatriafulvene > methylenecyclopropene > 1-phosphatriafulvene > 1,2,4-triphosphatriafulvene > 1,2-diphosphatriafulvene > 3,4-diphosphatriafulvene > 1,2,3,4-tetraphosphatriafulvene. The two last compounds are non-aromatic, ylidic structures.