Electrocatalytic reactions of dioxygen and nitric oxide with reduced (nitrosyl) cobalt porphyrins — cyclic voltammetry and computational chemistry

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Water-soluble cationic cobalt(tetra(N-methyl-pyridyl)porphyrins, CoTMpyP(4) and CoTMpyP(2) and the β-pyrrole octabrominated derivative CoTMpyP(4)Br8 have been studied by cyclic voltammetry under aqueous aerobic conditions at pH of 4.0 with and without addition of nitrite to explore their ability to catalyze the reduction of dioxygen. The porphyrins and their nitro/nitrosyl derivatives were studied in aqueous acetate buffer solution at a solid silver electrode and then also as they were immobilized in Nafion® film loaded with silver nanoparticles which provided electrical conductivity. Under these reducing conditions with nitrite in solution, coordinated nitrite appears to undergo an oxo-transfer/reduction reaction resulting in the formation of the nitrosyl ligand. The reaction of the nitrosyl complex allows a short-lived electrocatalytic reduction reaction with dioxygen before NO is lost from the complex by dissociation. The thermodynamics of the reactions and possible catalytic intermediates in the reduction of dioxygen were studied by DFT (BP/6-31G*) computations. An unusual N -bound cyclic NO3- structure was obtained in the optimized geometries for the product of reduced nitrosyl porphyrins and dioxygen in the model complexes and for [CoTMpyP(2)(NO-O2)]- . These structures are tentatively proposed to represent an intermediate in the mechanism of activation of dioxygen in catalytic reduction. DFT (BP/6-31G*) computations were also applied to probe thermodynamics and intermediates in the known catalytic reduction of NO to N2O by the reduced porphyrin under anaerobic conditions. Thermodynamics estimates suggest that reduction occurs through coordinated NO in the reduced porphyrin species similar to cytochrome P450nor (nitric oxide reductase), but the detailed mechanism is not clear.