Kinetic and Mechanistic Studies on the Oxidation of Nitrogen(III) (HNO2/$\rm{{{NO}}_2^ - }$) by the Tris(biguanide)manganese(IV) Ion in Aqueous Acidic Media

Abstract

AbstractThe kinetics of a net two‐electron transfer between an authentic MnIV complex, [Mn(bigH)3]4+ (Fig. 1; bigH = biguanide = C2N5H7), and nitrite in aqueous solution in the pH interval 2.00–3.60 is described. Stoichiometric data for the reaction clearly indicates Δ[MnIV]/Δ[NIII]T = 1.07 ± 0.10, and $\rm{{{NO}}_3^ - }$ is detected as the oxidized product of nitrite ([NIII]T = [HNO2] + [$\rm{{{NO}}_2^ - }$]). Though both HNO2 and $\rm{{{NO}}_2^ - }$ are found to be reactive, the latter is kinetically superior in reducing the fully protonated MnIV complex. Proton‐coupled electron transfer (PCET; 1e, 1H+) reduces the activation barrier for the thermodynamically unfavorable reaction of weakly oxidizing MnIV species. At the end of the redox process, the ligand bigH is released, and the high protonation constants of the ligand carry the overall reaction to completion.