Formation Mechanism of the Unsubstituted Chlorophosphazene Cl₃PNH: A Theoretical Study via Quantum Mechanical Calculations

Although the synthesis of chlorophosphazene polymers has been explored for more than 100 years, the shortest yet most illusive monomer, Cl₃PNH, has never been isolated and fully characterized. Here we investigate the formation of Cl₃PNH from PCl₅ and NH₃ in chlorobenzene through quantum mechanical calculations. The potential energy surface was mapped using the MP2 Hamiltonian in conjunction with Dunning’s correlation-consistent basis sets (aug-cc-pVXZ, where X = D and T). Along with HOMO/LUMO frontier molecular orbitals and natural bond orbital analyses, we found that instead of following the S_N1 path proposed in the literature, the reaction proceeds via an addition–elimination mechanism. Our results also indicate that due to the low-lying stable intermediates (IM), most steps are exothermic such that the production of Cl₃PNH·2HCl can be completed once the energy barrier for the formation of [PCl₄-NH₃]⁺Cl^– is overcome. Therefore, our theoretical work might explain the challenges in isolating any of the IMs in a typical chlorophosphazene reaction in chlorobenzene.