Equilibria and kinetics of the reactions between hydrogen peroxide and methyltrioxorhenium in aqueous perchloric acid solutions

Abstract

In aqueous solutions the colorless compounds CH[sub 3]ReO[sub 3] (=MTO) and H[sub 2]O[sub 2] form 1:1 and 1:2 adducts. The latter is yellow, with [epsilon][sub 360] = 1.1 x 10[sup 3] L mol[sup [minus]1] cm[sup [minus]1]. Peroxide binding to MTO shows cooperativity, as shown by the inversion of the usual order of binding constants. The stepwise equilibrium constants are K[sub 1] = 7.7 L mol[sup [minus]1] and K[sub 2] = 145 L mol[sup [minus]1] at 25[degrees]C. The buildup of product, which occurs on the stopped-flow time scale at 9-680 mM H[sub 2]O[sub 2], is fit by biexponential kinetics. The equilibria and rates are independent of [H[sub 3]O[sup +]] in the range 10[sup [minus]1]-10[sup [minus]3] M. The peroxide complexes decompose more rapidly at lower [H[sub 3]O[sup +]], particularly at pH > 3. Possible structures for 1:1 and 1:2 MOT-H[sub 2]O[sub 2] adducts are presented and discussed.