Abstract
Water-soluble cobalt tetrakis(N-methyl-2-pyridy1)porphine (Co(2-TMPyP)) has been investigated for their catalytic behavior toward nitric oxide reduction. The acid dissociation constants (p&) for [CoIn(2-TMPyP)(H2O)2l5+ are 5.5 and 9.6. Nitrite anion coordinates to the Co(II1) center in pH 4-12 buffer solutions. The nitrite complex [CO~~I (~ -TMP~P) (NO~) ]~+ exhibits IR bands at 1401, 1306 and 830 cm-', indicating a cobalt-nitro complex. The 5-coordinated cobaltous porphyrin, [CO~I(~-TMP~P)(H~O)]~+, has been identified by ESR and has pKa = 13.0. This is the first case that the pKa of a Co(I1) porphyrin is presented. Nitric oxide from the disproportionation of nitrite anion in acidic solution can coordinate to the Co(I1) center. Resonance Raman spectrum of [CoI1(2TMPyP)(N0)l4+ exhibits a peak at 1722 cm-', which disappears upon exposure to air. Electrocatalytic reduction of [Con(2-TMPyP)(N0)l4+ at -0.3 V in pH 4.0 buffer solution generates nitrous oxide in a slow rate. Bulk electrolysis was also conducted at -0.7 V, at which [Co1(2-TMPyP)I3+ reacts with nitrite anion and nitric oxide. The reduction products are mainly ammonia and hydroxylamine.
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