Kinetics of electrocatalysis of dibromoalkyl reductions using electrodes with covalently immobilized metallotetraphenylporphyrins

The reduction of PhCHBrCH2Br, PhCHBrCHBrPh, and CH2BrCHBrCH3 at the surfaces of electrodes to which cobalt (II) or copper (II) tetra(p-aminophenyl)porphyrin has been covalently attached is strongly catalyzed by reduction of the metalloporphyrin. The rate of electrocatalytic reduction was measured using rotated disk electrode voltammetry, and was independent of the amount of metalloporphyrin on the electrode above an estimated monomolecular coverage level. The results are consistent with theory which assumes that the rate of diffusion of electrochemical charge through the porphyrin layer is faster than the rate of diffusion of catalytic substrate through the layer. Comparison of the electrocatalytic rates for the different substrates indicates the electron transfer mediation involves specific interactions between substrate and metalloporphyrin rather than being a simple outer sphere electron transfer event. Potential step chronoamperometry is introduced as an alternative method for electrocatalytic measurements at modified electrodes. (Author)