METAL IONS AND HYDROGEN PEROXIDE XXIX. On the Kinetics and Mechanism of the Catalase-like Activity of Nickel(II) and Nickel(II)-Amine Complexes

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Abstract
The disproportionation of H2O2, catalyzed by Ni2+ aq and several Ni2+−amine complexes, was investigated. (i) With Ni2+ aq in the pH range 6.6–8.2 and without buffer the following rate law holds: μ0 = d[O2/dt = k[Ni2+] [H2O2]2/[H+] (k = (1.77 ± 0.23) × 10−9 mol−1 1. sec−1). In the presence of phosphate buffer and in the pH range 6–7 the initial rate, μ0, is proportional to 1/[H+]. (ii) For the buffer-free Ni2+−2,2′-bipyridyl 1:1 system in the pH range 4.5–8 μ0 = ((k 1/[H+]) + k 2)[H2O2]([Ni2+]tot = [Bipy]tot)0.5 where k 1 = (1.35 ± 0.20) × 10−12 mol0.5 l.−0.5 sec−1 and k 2 = (2.5 ± 0.5) × 10−6 mol−0.5 l.0.5 sec−1. With borate buffer μ0 is increased, but in the pH-dependent part the given proportionalities between μ0 and [reactants] are valid. For these conditions the Ni2+−2,2′-bipyridyl 1:1 complex is the catalytically most active species. (iii) In the buffer-free system with Ni(ethylenediamine)2+ 2 as catalyst in the pH range 8.2–10 it appears that an analog rate law holds as given above for Ni2+ aq (cf. i). Borate buffer has a rate enhancing effect on the catalytic activity of Ni(En)2+ 2 Also there is evidence for μ0 being proportional to 1/[H+]2 and this means two HOO− (or corresponding borate derivatives) are participating in the reaction before the rate determining step. (iv) With 2-picolylamine the 1:1 complex reaches about the activity of Ni2+, while with 4-aminomethylimidazole or histamine none of the complex species is as active as Ni2+. - As far as possible the reaction laws are given for the mentioned systems. Reaction mechanisms and the structure of the active species, as well as the rate enhancing effect of borate, are discussed.

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