Operando FT-IR spectroscopy as a useful tool for elucidating the fate of phthalimide-N-oxyl catalytically active species

Abstract

Self-decomposition of phthalimide-N-oxyl radical (PINO), a catalytically active species in N-hydroxyphthalimide (NHPI) catalyzed aerobic oxidation and oxidative coupling reactions, is one of the key reasons limiting its use. However, the literature data do not provide unambiguous information about either the mechanism or the PINO decomposition products. In this work, based on operando IR spectroscopy a new model of PINO self-decomposition was proposed, which in particular explains the contradictions in earlier works on this topic. The kinetics of PINO self-decomposition is complex, consisting of several simultaneous processes characterized by different rates and yielding the different products, making the concept of an effective rate constant impractical for PINO decay. In summary, as the PINO decomposition progresses and its concentration decreases the dominant decomposition pathway switches from the initial formation of the PINO “trimer” to the production of NHPI. The crucial role of medium acidity in determining the path of PINO radical decomposition was discovered. Under highly acidic conditions, the formation of the “trimer” product is suppressed. For the first time, the characteristic vibration frequencies of PINO have been experimentally determined. Operando IR spectroscopy has proven to be a useful tool for studying reactions involving nitroxyl radicals, offering simultaneous monitoring of reactants, intermediates, and products.