Kinetic study of hydrogen peroxide reaction with hydroxonitrilotri(methylenephosphonato)iron(III) complex

The decomposition of hydrogen peroxide in the presence of hydroxonitrilotri(methylenephosphonato)iron(III), [Fe(NTMP)(OH)4−], was studied in nitrate media (μ=0.10–0.26 M) over the 0.2–0.5 mM concentration range for the iron complex and the temperature range 26–40°C. The rate law; $$\begin{gathered} {{ - d[H_2 O_2 ]} \mathord{\left/ {\vphantom {{ - d[H_2 O_2 ]} {dt}}} \right. \kern-\nulldelimiterspace} {dt}} = \left( {\frac{{k_i k_1 k_3 }}{{k_4 }} \cdot K_a } \right)^{0.5} \hfill \\ \times \frac{{[Fe(NTMP)(OH)^{4 - } ][H_2 O_2 ]}}{{[H^ + ]^{0.5} }} \hfill \\ \end{gathered} $$ holds over the 9.3–11.0 pH range. At 26°C, the specific rate constants for the reactions; $$\begin{gathered} [Fe(NTMP)(OH)]^{4 - } + HO_2^ - \mathop \to \limits^{k_i } [Fe(NTMP)(OH)]^{5 - } \hfill \\ + O_2^{ - \cdot } + H^ + \hfill \\ and \hfill \\ [Fe(NTMP)(OH)]^{4 - } + O_2^{ - \cdot } \hfill \\ \mathop \to \limits^{k_1 } [Fe(NTMP)(OH)]^{5 - } + O_2 \hfill \\ \end{gathered} $$ were estimated as 11 and 100 M−1 s−1 respectively. The corresponding activation enthalpies and activation entropies were calculated from a least-squares fit of the temperature-dependence of the specific rate constants. The results support a mechanism where electron-transfer step is the rate-determining step.