The stoichiometry and kinetics of oxidation of 1,4-benzenediol by diaquotetrakis( 2,2'-bipyridine)- μ-oxodiruthenium(III) cation in perchlorate medium

Abstract

The stoichiometry and kinetics of oxidation of 1,4-benzenediol (hydroquinone, H₂O) by the oxobridged dimer, diaquotetrakis(2,2'-bipyridine-,u-oxodiruthenium(III) cation ([RU₂O]⁴⁺) has been investigated in aqueous perchloric acid. At [H⁺] = 0.05 mol dm^-3, <em>1= </em>1.0 mol dm^-3 (NaCIO₄) and T= 25.0 ± 0.1°C, the reaction conforms to an overall equation,<br><br> 2[RuP]⁴⁺ + H₂O = 2[Ru₂O]³⁺ + Q + 2H⁺<br><br> The experimental result is consistent with a second-order rate law,<br><br> - 1/2<em>d</em>[Ru₂O⁴⁺]/ <em>dt = </em>[Ru₂O⁴⁺][H₂Q]<br><br> with <em>k</em>₂= (12.4± 1.1) x 10^-2 mol^-1 dm³ S^-1. Added Li⁺, Na⁺, Pb²⁺, Mg²⁺ and Mn²⁺ have no effect on the rate of the reaction while Cl^-, Br^- and NO catalyze the reaction. Rate of the reaction is independent of change in (acid) of the reaction medium in the range 0.05 ≤ 1≤ 2.0 mol dm^-3 and is not affected by changes in ionic strength in the range 0.25 ≤ l ≤ 2.0 mol dm^-3. On the basis of the results of this investigation, the absence of both spectroscopic and kinetic evidence of complex formation prior to electron-transfer and the non-conformity of our results with the Michaelis-Menten equation, it is suggested that the oxidation of hydroquinone by [Ru₂O]⁴⁺ most probably proceed by an outer-sphere mechanism.