O-H Bond Energies of 1-Hydroxy-2,2,6,6-tetramethylpiperidine and Its Derivatives

Redox sequences of 1-hydroxy-2,2,6,6-tetramethylpiperidine/oxoammonium (TEMPOH/TEMPO⁺) play crucial roles in catalysis, energy storage, and biological metabolism, primarily through the reorganization of their O-H bonds. Understanding the thermodynamics of these bonds is essential for predicting reaction outcomes and resolving mechanistic ambiguities that arise from competing pathways, such as direct hydride transfer (H^-T), electron transfer (ET), and hydrogen atom transfer (HAT). However, systematic investigations into the O-H bond energies of hydroxylamines remain elusive. Here, we synthesized 18 structurally diverse hydroxylamines, including sterically hindered TEMPO analogs, bicyclic nitroxides, and diarylnitroxides. Through calorimetric and electrochemical analyses, we determined the bond energy associated with hydride, proton, and hydrogen atom transfer, as well as redox potentials in acetonitrile. Based on this, the structure-reactivity relationship was analyzed. These thermodynamic benchmarks were further used to rationalize literature-reported reaction mechanisms.