Formation of 1,10-Phenanthroline‑<i>N</i>,<i>N</i>′‑dioxide under Mild Conditions: The Kinetics and Mechanism of the Oxidation of 1,10-Phenanthroline by Peroxomonosulfate Ion (Oxone)

Abstract

This paper confirms the unexpected formation of 1,10-phenanthroline-<i>N</i>,<i>N</i>′<i>-</i>dioxide (phenO₂) when 1,10-phenanthroline (phen) is oxidized by peroxomonosulfate ion (PMS) in a neutral aqueous solution. The kinetics of oxidation of phen by PMS features a complex pH dependence. In 1.00 M H₂SO₄, 1,10-phenanthroline-mono-<i>N</i>-oxide (phenO) is the sole product of the reaction. The rate of the N-oxidation is highly dependent on pH with a maximum at pH ∼6.7. The formation of phenO occurs via two parallel pathways: the rate constant of the oxidation of phen (<i>k</i> = 3.1 ± 0.1 M^–1 s^–1) is significantly larger than that of Hphen⁺ [<i>k</i> = (4.1 ± 0.3) × 10^–3 M^–1 s^–1] because the two N atoms are open to oxidative attack in the deprotonated substrate while an internal hydrogen bond hinders the oxidation of the protonated form. With an excess of PMS, four consecutive oxidation steps were found in nearly neutral solutions. In the early stage of the reaction, the stepwise oxidation results in the formation of phenO, which is converted into phenO₂ in the second step. The formation of phenO₂ was confirmed by ¹H NMR and ESI-MS methods. The results presented here offer the possibility of designing an experimental protocol for preparing phenO₂