Is Hexachloro-cyclo-triphosphazene Aromatic? Evidence from Experimental Charge Density Analysis.

Experimental charge density studies of hexachloro-cyclo-triphosphazene (1) and the boat conformation of octachloro-cyclo-tetraphosphazene (2 a) were performed to unambiguously describe the origin of the electron delocalization in the P N ring in 1. The obtained results were compared to DFT studies in the solid state and the gas phase. Electron density analysis revealed a highly polarized nature of the P-N bonds and a modular structure of the P N and P N rings, which can be separated into independent Cl PN units with a perfect transferability between the compounds. Further analysis of the source function experimentally proves the presence of negative hyperconjugation involving both out-of-plane and in-plane nitrogen electrons as well as electrons of the chlorine atoms. Finally, these results discard the presence of pseudoaromatic delocalization in the nearly planar P N ring.