Free and Complexed Fluoropentazoles: Anomalous Ring Charge and Appreciable Kinetic Stability.

Fluoronitrogens are strongly related to high-energy density materials. Fluoropentazole ( cyclo -FN 5 ) with hitherto the highest N/F ratio at ambient pressure belongs to the family of well-known and 119-year-old pentazoles. Due to the presence of the overwhelmingly electronegative F element, cyclo -FN 5 is the only pentazole to date that contains a cationic N 5 ring. However, also due to F's large electronegativity, the reported ring-destruction barrier of cyclo -FN 5 is nearly the lowest (6.7 kcal/mol), which has made its synthesis and characterization quite difficult. Here, cyclo -FN 5 was re-predicted to bear an almost doubled ring-destruction barrier (∼12-14 kcal/mol) at the CBS-QB3, G4, W1BD, CCSD(T)/CBS//CCSD/cc-pVTZ, and CCSD(T)/CBS//CCSD(T)/aug-cc-pVTZ levels. Promisingly, upon further complexation with Lewis acid(s), the ring-destruction barrier of cyclo -FN 5 might reach 23 kcal/mol, which is comparable to that of the well-known and already synthesized arylpentazoles (∼20 kcal/mol), and the positive charge on the N 5 ring can be increased to 0.66 |e|.