Osmium(VIII)/Ruthenium(III) Catalysed Oxidation of l-lysine by Diperiodatocuprate(III) in Aqueous Alkaline Medium: A Comparative Mechanistic Approach by Stopped Flow Technique

Abstract

The kinetics of osmium(VIII) and ruthenium(III) catalysed oxidation of l-lysine (l-lys) by diperiodatocuprate(III) (DPC) in alkaline medium at a constant ionic strength of 0.15 mol dm⁻³ was studied spectrophotometrically. The reaction between l-lys and DPC in alkaline medium exhibits 1:2 stoichiometry in both catalysed reaction (l-lys: DPC). The reaction is first order in [DPC] and has less than unit order both in [l-lys] and [alkali]. Increase in periodate concentration decreases the rate. Intervention of free radicals was observed in the reaction. The main products were identified by spot test, IR and GC-MS studies. Probable mechanisms are proposed and discussed. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to the slow step of the mechanism are computed and discussed and thermodynamic quantities are also determined. It has been observed that the catalytic efficiency for the present reaction is in the order of Os(VIII)  >  Ru(III). The active species of catalyst and oxidant have been identified.

Graphical Abstract

The kinetic and mechanistic investigations of the reaction between DPC and l-lysine has been studied in presence of microamounts of ruthenium(III) and osmium(VIII) in alkaline medium. The monoperiodatoargentate(III), [Ru(H₂O)₅OH]²⁺ and [OsO₄(OH)₂]²⁻ are considered as the active species of oxidant, DPC, ruthenium(III) and osmium(VIII) respectively.

Appendix A

According to Scheme 2

$$ \begin{aligned} \hbox{Rate} =& \hbox{k[C]} [\hbox{Cu}(\hbox{H}_{3}\hbox{IO}_{6})(\hbox{OH})_{2}]^-\\ =&\frac{\hbox{kK}_{1}K_{2}K_{3}\hbox{[DPC][Ru(III)]} [\hbox{OH}^-][{\sc l}\hbox{-Lys]}}{[\hbox{H}_{3}\hbox{IO}_{6}^{2-}]}\\ \end{aligned} $$

(I)

The total concentration of DPC is given by (Where T and f stands for total and free)

$$ \begin{aligned} \hbox{[DPC]}_{\rm T} =& \hbox{[DPC]}_{\rm f} + [\hbox{Cu}(\hbox{OH})_{2}(\hbox{H}_{3}\hbox{IO}_{6}) (\hbox{H}_{2}\hbox{IO}_{6})]^{4-} + [\hbox{Cu}(\hbox{H}_{3}\hbox{IO}_{6})(\hbox{OH})_{2}]^-\\ =&\hbox{[DPC]}_{\rm f}\left\{\frac{[\hbox{H}_{3}\hbox{IO}_{6}^{2-}] + K_{1} [\hbox{OH}^-][\hbox{H}_{3}\hbox{IO}_{6}^{2-}] + K_1K_2[\hbox{OH}^-]}{[\hbox{H}_3\hbox{IO}_6^{2-}]}\right\}\\ \end{aligned} $$

Therefore,

$$ \hbox{[DPC]}_{\rm f} =\frac{\hbox{[DPC]}_{\rm T}[\hbox{H}_{2}\hbox{IO}_{6}^{3-}]}{ [\hbox{H}_{3}\hbox{IO}_{6}^{2-}] + K_{1} [\hbox{OH}^-] [\hbox{H}_{3}\hbox{IO}_{6}^{2-}] + K_1K_2[\hbox{OH}^-]} $$

(II)

Similarly,

$$ \begin{aligned} \hbox{[OH]}_{\rm T} =& \hbox{[OH]}_{\rm f} + [\hbox{Cu}(\hbox{OH})_{2}(\hbox{H}_{3}\hbox{IO}_{6}) (\hbox{H}_{2}\hbox{IO}_{6})]^{4-} + [\hbox{Cu}(\hbox{H}_{3}\hbox{IO}_{6})(\hbox{OH})_{2}]^-\\ =& \hbox{[OH]}_{\rm f} + K_{1}\hbox{[DPC]}[\hbox{OH}^-] + \frac{K_1K_2\hbox{[PDC]}[\hbox{OH}^-]}{ [\hbox{H}_{3}\hbox{IO}_{6}^{2-}]}\\ \end{aligned} $$

In view of the low concentration of [DPC] and [H₃IO ²⁻₆ ] used,

$$ \hbox{[OH]}_{\rm T}= \hbox{[OH]}^{\rm f} $$

(III)

Similarly,

$$ \begin{aligned}[{\sc l}\hbox{-lys}]_{\rm T} =& [{\sc l}\hbox{-lys}]_{\rm f} +\hbox{[C]}\\ =& [{\sc l}\hbox{-lys}]_{\rm f} (1 + K_{3}\hbox{[Ru(III)]})\\ \end{aligned} $$

In view of the low concentration of [Ru(III)]) used,

$$ [{\sc l}\hbox{-lys}]_{\rm T} = [{\sc l}\hbox{-lys}]_{\rm f} $$

(IV)

$$ \begin{aligned} \hbox{[Ru(III)]}_{\rm T}=& \hbox{[Ru(III)]}_{\rm f} + \hbox{C}\\ =& \hbox{[Ru(III)]}_{\rm f} \{1+ K_{3} [{\sc l}\hbox{-lys}]\}\\ \end{aligned} $$

$$ \hbox{[Ru(III)]}_{\rm f} \frac{\hbox{[Ru(III)]}_{\rm T}} {1+ K_{3} [{\sc l}\hbox{-Lys}]} $$

(V)

Substituting (II), (III), (IV) and (V) in (I) and omitting the subscripts T and f, we get

$$ \frac{\hbox{Rate}}{\hbox{[DPC]}}= K_{\rm C}= k_{\rm T}-k_{\rm U} =\frac{\hbox{kK}_{1}{K}_{2}K_{3} [{\sc l}\hbox{-Lys}][\hbox{OH}^-][\hbox{Ru(III)}]} {[\hbox{H}_{3}\hbox{IO}_{6}^{2-}] + K_{1} [\hbox{OH}^-] [\hbox{H}_{3}\hbox{IO}_{6}^{2-}] + K_{1}K_{2}[\hbox{OH}^-] + K_1K_2 K_3[\hbox{OH}^-][{\sc l}\hbox{-Lys}]} $$

Similarly for Os(VIII) catalysis, the rate law can be derived.