Formation Mechanism of the Unsubstituted Chlorophosphazene Cl3=NH: A Theoretical Study via Quantum Mechanical Calculations

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Inorganic Chemistry


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(N)1 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 center dot 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.