Inhibitory Effect of Water on the Oxygen Reduction Catalyzed by Cobalt(II) Tetraphenylporphyrin

Abstract

Stopped-flow kinetic measurements, UV-vis spectroscopy, rotating disk voltammetry, and quantum chemical calculations are used to clarify the role of water in the homogeneous two-electron reduction of O2 to H2O2 in 1,2-dichloroethane (DCE) using ferrocene (Fc) as an electron donor, tetrakis(pentafluorophenyl)boric acid (HTB) as a proton donor, and [5,10,15,20-tetraphenyl-21H,23H-porphine]cobalt(II) (Co(II)TTP) as a catalyst. Kinetic analysis suggests that the reaction is controlled by the intramolecular proton coupled electron transfer to the O2 molecule coordinated to the metal center producing the O2H(•) radical. This rate-determining step is common to both the O2 reduction by Fc catalyzed by Co(II)TPP and the O2 reduction by Co(II)TPP itself. Experimental data point to the competitive coordination of water to the metal center leading to a strong inhibition of the catalytic reaction. In agreement with this finding, quantum chemical calculations indicate that water is bound to the metal center much more strongly than triplet O2. A similar effect is demonstrated also for the O2 reduction catalyzed by the porphyrin free base (H2TPP), though its rate is lower by 2 orders of magnitude.