Reactions of the Tris(3,4,7,8-trimethylphenanthroline)iron(II,III) Redox Couple in Nitrous Acid

Article abstract of DOI:10.1021/ja00338a022

The kinetics and mechanisms of the redox reactions of ^2+/3+ (TMP = 3,4,7,8-tetramethylphenanthroline) with nitrous acid have been investigated in aqueous solution at 25.0 deg C in sulfate media.With a large excess of nitrite, ³⁺ at pH >2 is reduced quantitatively to ²⁺ with non-pseudo-first-order kinetics; the reaction is strongly inhibited by ²⁺.Acid also inhibits the reaction, but there is a direct dependence on ->.The proposed mechanism involves protonation of NO2^- to form uncrative HNO2; NO2^- is oxidized quasi-reversibly by ³⁺ to form NO2 with a rate constant, k₁, of 3.9E3 M^-1 s^-1, and then NO2 disproportionates to from NO3^- and NO2^-.At pH ²⁺ in quantitatively oxidized in nitrous acid with pseudo-first-order kinetics; the reaction is strongly inhibited by NO.In the presence of added NO the rate law shows one term first order in and another term approximately second order in .The path first order , which is undetectably slow in the absence of added NO, has a rate constant of 95 M^-1 s^-1, and it is interpreted as the direct reduction of HNO2.The path second order in is inverse in , and it is interpreted as the rapid equilibrium formation of NO2 and NO by disproportionation of HNO2, followed by rate limiting (k_-1=2E6 M^-1 s^-1) reduction of NO2 by ²⁺.This last step is the microscopic reverse of the rate-limiting step for the reaction of NO2^- with ³⁺.The equivalence of the ratio k₁/k_-1 and the thermodynamically determined equilibrium constant confirms the mechanistic assignments.The combined effects of reversible reduction of HNO2 to NO and irreversible oxidation of HNO2 to NO3^- lead to biphasic kinetics in the pH range 1-2.