Fluxional behaviour of tricyclo[2.2.1.0^2,6]heptaphosphide trisanion: a DFT investigation

Abstract

The structures of the tricyclo[2.2.1.02,6]heptaphosphide trisanion, tricyclo[2.2.1.02,6]heptaphosphane triradical and their carbocyclic analogues have been investigated theoretically at the density functional theory (DFT) (B3LYP/6-31 + G*) level. The existence of negative hyperconjugation in these molecules could be established by natural bond orbital analysis. The weakening of the σ bonds combined with the possibility of the valence-shell expansion at the anionic charge carrying phosphorus atoms in the tricyclo[2.2.1.02,6]heptaphosphide trisanion induces a degenerate [2,2]sigmatropic rearrangement with a low energy barrier making all the phosphorus atoms equivalent, as detected by 31P NMR experimentally. This energy barrier is enhanced in the presence of counterions. Its carbocyclic analogue trisanion fails to undergo a [2,2]sigmatropic rearrangement due to the inability of the anionic carbon centres to expand their valence shell. The tricyclo[2.2.1.02,6]heptaphosphane triradical and its carbocyclic analogue undergo a [2,2]sigmatropic rearrangement, as valence-shell expansion at the carbon atom is not required in this case. A lower activation energy barrier for the [2,2]sigmatropic rearrangement of the tricyclo[2.2.1.02,6]heptane triradical as compared to that for its phospha-analogue can be rationalised on the basis of the higher ring strain of cyclopropane than triphosphirane.