Pyrido-annellated diazaphospholenes and phospholenium ions.

Pyrido-annulated 1,3,2-diazaphospholenium ions and P-bis(trimethylsilyl)amino substituted 1,3,2-diazaphospholenes were synthesised and characterised by spectroscopic methods and single-crystal X-ray diffraction studies. The distribution of bond distances provided evidence for pi-electron delocalisation in the fused ring system. This hypothesis was confirmed by calculations of magnetic (NICS, nucleus independent chemical shift) and geometrical (Bird index, bond shortening index) aromaticity indexes which attest aromatic character of the five-membered rings in the cations that is lost upon introduction of a substituent at the phosphorus atom. Computation of isodesmic reaction energies suggest that the reclamation of aromatic stabilisation energies in the products facilitates reaction of P-amino-substituted annulated diazaphospholenes under cleavage of the fused ring system.