Kinetic and mechanistic studies on the reaction of nitric oxide with a water-soluble octa-anionic iron(III) porphyrin complex

doi:10.1021/ic050924t

Kinetic and mechanistic studies on the reaction of ...

Bibliographic description

title:	Kinetic and mechanistic studies on the reaction of nitric oxide with a water-soluble octa-anionic iron(III) porphyrin complex
author:	Jee Joo-Eun, Eigler Siegfried, Hampel Frank, Jux Norbert, Wolak Maria, Zahl Achim, Stochel Grażyna , Eldik Rudi van
Journal title:	Inorganic Chemistry
volume:	44
issue:	22
date of publication:	2005
pages:	7717-7731
ISSN:	0020-1669
eISSN:	1520-510X
DOI:	10.1021/ic050924t
language:	English
Journal language:	English
abstract in english:	The polyanionic water-soluble and non-μ-oxo-dimer-forming iron porphyrin iron(III) 54,104,154,204-tetra-tert-butyl-52,56,152,156-tetrakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin, (P8-)FeIII (1), was synthesized as an octasodium salt by applying well-established porphyrin and organic chemistry procedures to bromomethylated precursor porphyrins and characterized by standard techniques such as UV−vis and 1H NMR spectroscopy. A single pKa1 value of 9.26 was determined for the deprotonation of coordinated water in (P8-)FeIII(H2O)2 (1-H2O) present in aqueous solution at pH <9. The porphyrin complex reversibly binds NO in aqueous solution to give the mononitrosyl adduct, (P8-)FeII(NO+)(L), where L = H2O or OH-. The kinetics of the binding and release of NO was studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques. The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchange mechanism based on the positive values of the activation parameters ΔS⧧ and ΔV⧧ for the “on” and “off” reactions. The rate constant kon = 6.2 × 104 M-1 s-1 (24 °C), determined for NO binding to the monohydroxo-ligated (P8-)FeIII(OH) (1-OH) present in solution at pH >9, is markedly lower than the corresponding value measured for 1-H2O at lower pH (kon = 8.2 × 105 M-1 s-1, 24 °C, pH 7). The observed decrease in the reactivity is contradictory to that expected for the diaqua- and monohydroxo-ligated forms of the iron(III) complex and is accounted for in terms of a mechanistic changeover observed for 1-H2O and 1-OH in their reactions with NO. The mechanistic interpretation offered is further substantiated by the results of water-exchange studies performed on the polyanionic porphyrin complex as a function of pH, temperature, and pressure.
affiliation:	Wydział Chemii : Zakład Chemii Nieorganicznej
type:	journal article
subtype:	academic paper