Influence of an Extremely Negatively Charged Porphyrin on the Reversible Binding Kinetics of NO to Fe(III) and the Subsequent Reductive Nitrosylation

The polyanionic, water-soluble, and non-μ-oxo dimer-forming iron porphyrin (hexadecasodium iron 5⁴,10⁴,15⁴,20⁴-tetra-t-butyl-5²,5⁶,10²,10⁶,15²,15⁶,20²,20⁶-octakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin), (P^16-)Fe^III, with 16 negatively charged meso substituents on the porphyrin was synthesized and fully characterized by UV−vis and ¹H NMR spectroscopy. A single pK_a1 value of 9.90 ± 0.01 was determined for the deprotonation of coordinated water in the six-coordinate (P^16-)Fe^III(H₂O)₂ and as attributed to the formation of the five-coordinate monohydroxo-ligated form, (P^16-)Fe^III(OH). The porphyrin complex reversibly binds NO in aqueous solution to yield the nitric oxide adduct, (P^16-)Fe^II(NO⁺)(L), where L = H₂O or OH^-. The kinetics for the reversible binding of NO were studied as a function of pH, temperature, and pressure using the stopped-flow technique. The data for the binding of NO to the diaqua complex are consistent with the operation of a dissociative mechanism on the basis of the significantly positive values of ΔS^⧧ and ΔV^⧧, whereas the monohydroxo complex favors an associatively activated mechanism as determined from the corresponding negative activation parameters. The rate constant, k_on = 3.1 × 10⁴ M^-1 s^-1 at 25 °C, determined for the NO binding to (P^16-)Fe^III(OH) at higher pH, is significantly lower than the corresponding value measured for (P^16-)Fe^III(H₂O)₂ at lower pH, namely, k_on = 11.3 × 10⁵ M^-1 s^-1 at 25 °C. This decrease in the reactivity is analogous to that reported for other diaqua- and monohydroxo-ligated ferric porphyrin complexes, and is accounted for in terms of a mechanistic changeover observed for (P^16-)Fe^III(H₂O)₂ and (P^16-)Fe^III(OH). The formed nitrosyl complex, (P^16-)Fe^II(NO⁺)(H₂O), undergoes subsequent reductive nitrosylation to produce (P^16-)Fe^II(NO), which is catalyzed by nitrite produced during the reaction. Concentration-, pH-, temperature-, and pressure-dependent kinetic data are reported for this reaction. Data for the reversible binding of NO and the subsequent reductive nitrosylation reaction are discussed in reference to that available for other iron(III) porphyrins in terms of the influence of the porphyrin periphery.