N-N bridged pyrazoles from nitrogen-centered radicals: A comparative study of spin densities, molecular structures, and reactivities

Nitrogen-centered radicals are versatile synthetic intermediates with the ability to undergo diverse reactions. However, the utility of these reactive species in N-N coupling reactions, especially in nitrogen-rich heterocycles, remains a significant challenge due to the high electronegativity of the nitrogen atoms and the poor molecular stability of high-nitrogen compounds. In this work, the possibility of N-H bond cleavage in different functionalized pyrazoles to produce corresponding nitrogen-centered radicals was investigated through density functional theory calculations. Based on the calculated results, the N-N bridged 4c and 4e were synthesized with expected reaction efficiencies, which were further analyzed by frontier molecular orbital theory. Furthermore, the molecular stability of the synthesized N-N bridged bipyrazoles was also studied.