Kinetics and mechanisms of the redox reactions of the hydroperoxochromium(III) ion

Abstract

The reactions of the hydroperoxochromium(III) ion, (H[sub 2]O)[sub 5]CrO[sub 2]H[sup 2+] (CrO[sub 2]H[sup 2+]), with Fe[sup 2+], VO[sup 2+], V[sup 2+], Cu[sup +], Ti[sup 3+], Co([14]aneN[sub 4])[sup 2+], Co(Me[sub 6][14]aneN[sub 4])[sup 2+], Co(tim)[sup 2+], and [Ru(NH[sub 3])[sub 6]][sup 2+] have been studied in acidic aqueous solution. The reactions are accompanied by large negative entropies of activation, [minus]110 J mol[sup [minus]1] K[sup [minus]1] for Fe[sup 2+] and [minus]85 J mol[sup [minus]1] K[sup [minus]1] for Ti[sup 3+]. All the reactions studied follow an isokinetic relationship in that [delta]H[sup [double dagger]] is a linear function of [delta]S[sup [double dagger]]. The same is true for the analogous reactions of H[sub 2]O[sub 2]. It is proposed that the reactions of CrO[sub 2]H[sup 2+] take place by an inner-sphere, Fenton-type process yielding pentaaquaoxochromium(IV), (H[sub 2]O)[sub 5]CrO[sup 2+] (CrO[sup 2+]), an an intermediate. The reactivity of CrO[sub 2]H[sup 2+] as an oxygen transfer reagent is about 20 times greater than that of H[sub 2]O[sub 2]. For example, the reactions with (en)[sub 2]CoSCH[sub 2]CH[sub 2]NH[sub 2][sup 2+] to yield (en)[sub 2]CoS(O)CH[sub 2]CH[sub 2]NH[sub 2][sup 2+] have rate constants 20.5 [+-] 0.4 M[sup [minus]1] s[sup [minus]1] (CrO[sub 2]H[sup 2+]) and 1.36 M[sup [minus]1] s[sup [minus]1] (H[sub 2]O[sub 2]), bothmore » in 0.1 M HClO[sub 4] at 25[degrees]C. The chromyl ion, CrO[sup 2+], oxidizes CrO[sub 2]H[sup 2+] to CrO[sub 2][sup 2+] with a rate constant of (1.34 [+-] 0.06) [times] 10[sup 3] M[sup [minus]1] s[sup [minus]1] in 0.10 M HClO[sub 4] in H[sub 2]O and 266 [+-] 10 M[sup [minus]1]s[sup [minus]1] in D[sub 2]O.« less