Mechanistic investigations of ruthenium(III) catalyzed oxidation of pentoxifylline by copper(III) periodate complex in aqueous alkaline medium

Abstract

The kinetics of the oxidation of ruthenium(III)-catalyzed oxidation of pentoxifylline (PTX) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.30 mol dm⁻³ was studied spectrophotometrically. The reaction between PTX and DPC in alkaline medium in the presence of Ru(III) exhibits 1:2 stoichiometry (PTX:DPC). The reaction was of first order in DPC, less than the unit order in [PTX] and [OH⁻] and negative fractional order in [IO₄ ⁻]. The order in [Ru(III)] was unity. Intervention of free radicals was observed in the reaction. The main products were identified by TLC and spectral studies including LC-MS. The oxidation reaction in alkaline medium has been shown to proceed via a Ru(III)-PTX complex, which reacts with monoperiodatocuprate(III) to decompose in a rate determining step followed by a fast step to give the products. The reaction constants involved in different steps of the mechanism were calculated. The activation parameters with respect to the slow step of the mechanism were computed and discussed, and thermodynamic quantities were also determined. The active species of catalyst and oxidant have been identified.

Graphical abstract

Appendix

According to Scheme 1,

$$ {\text{Rate = }}{\frac{{ - {\text{d}}\left[ {\text{DPC}} \right]}}{\text{dt}}} = k\left[ {\text{C}} \right]\left[ {{\text{Cu}}\left( {{\text{H}}_{2} {\text{IO}}_{6} } \right)\left( {{\text{H}}_{2} {\text{O}}} \right)_{2} } \right] = {\frac{{kK_{1} K_{2} K_{3} \left[ {\text{DPC}} \right]\left[ {\text{PTX}} \right]\left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]}}{{\left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right]}}} $$

(11)

The total concentration of DPC, [DPC]_T, is given by (subscripts T and f stand for total and free, respectively)

$$ \left[ {\text{DPC}} \right]_{\text{T}} = \left[ {\text{DPC}} \right]_{\text{f}} + \left[ {{\text{Cu}}\left( {{\text{H}}_{2} {\text{IO}}_{6} } \right)\left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)} \right]^{2 - } + \left[ {{\text{Cu}}\left( {{\text{H}}_{2} {\text{IO}}_{6} } \right)\left( {{\text{H}}_{2} {\text{O}}} \right)_{2} } \right] $$

$$ \left[ {\text{DPC}} \right]_{\text{T}} = \left[ {\text{DPC}} \right]_{\text{f}} + K_{1} \left[ {{\text{OH}}^{ - } } \right]\left[ {\text{DPC}} \right] + {\frac{{K_{2} \left[ {{\text{Cu}}\left( {{\text{H}}_{2} {\text{IO}}_{6} } \right)\left( {{\text{H}}_{3} {\text{IO}}_{6} } \right)} \right]^{2 - } }}{{\left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right]}}} $$

$$ \left[ {\text{DPC}} \right]_{\text{T}} = \left[ {\text{DPC}} \right]_{\text{f}} \left[ {1 + K_{1} \left[ {{\text{OH}}^{ - } } \right] + {\frac{{K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right]}}{{\left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right]}}}} \right] $$

$$ \left[ {\text{DPC}} \right]_{\text{f}} = {\frac{{\left[ {\text{DPC}} \right]_{\text{T}} \left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right]}}{{\left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} \left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right]}}} $$

(12)

Similarly,

$$ \left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]_{\text{T}} = \left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]_{\text{f}} + \left[ {\text{C}} \right] = \left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]_{\text{f}} + K_{3} \left[ {\text{PTX}} \right]\left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]_{\text{f}} $$

$$ \left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]_{\text{f}} = {\frac{{\left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]_{\text{T}} }}{{1 + K_{3} \left[ {\text{PTX}} \right]}}} $$

(13)

$$ \left[ {\text{PTX}} \right]_{\text{f}} = {\frac{{\left[ {\text{PTX}} \right]_{\text{T}} }}{{1 + K_{3} \left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]}}} $$

In view of the low concentrations of Ru(III) used, we have

$$ \left[ {\text{PTX}} \right]_{\text{T}} = \left[ {\text{PTX}} \right]_{\text{f}} $$

(14)

Similarly,

$$ \left[ {{\text{OH}}^{ - } } \right]_{\text{T}} = \left[ {{\text{OH}}^{ - } } \right]_{\text{f}} $$

(15)

Substituting Eqs. (12), (13), (14), and (15) in Eq. (11) and omitting the T and f subscripts, we get

$$ {\text{Rate = }}{\frac{{ - {\text{d}}\left[ {\text{DPC}} \right]}}{\text{dt}}} = {\frac{{kK_{1} K_{2} K_{3} \left[ {\text{DPC}} \right]\left[ {\text{PTX}} \right]\left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]}}{\begin{gathered} \left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} \left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right] + K_{1} K_{2} K_{3} \left[ {{\text{OH}}^{ - } } \right]\left[ {\text{PTX}} \right] \hfill \\ + K_{3} \left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right]\left[ {\text{PTX}} \right] + K_{1} K_{2} \left[ {\text{PTX}} \right]\left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right] \hfill \\ \end{gathered} }} $$

The terms (K ₃[H₃IO₆ ²⁻][PTX]) and (K ₁ K ₃[PTX][OH⁻][H₃IO₆ ²⁻]) of the denominator are neglected in view of their lower values as compared to the periodate used in the study. Therefore,

$$ {\text{Rate = }}{\frac{{ - {\text{d}}\left[ {\text{DPC}} \right]}}{\text{dt}}} = {\frac{{kK_{1} K_{2} K_{3} \left[ {\text{DPC}} \right]\left[ {\text{PTX}} \right]\left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{Ru}}\left( {\text{III}} \right)} \right]}}{{\left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} \left[ {{\text{OH}}^{ - } } \right]\left[ {{\text{H}}_{3} {\text{IO}}_{6}^{2 - } } \right] + K_{1} K_{2} \left[ {{\text{OH}}^{ - } } \right] + K_{1} K_{2} K_{3} \left[ {{\text{OH}}^{ - } } \right]\left[ {\text{PTX}} \right]}}} $$

(16)