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A comparative mechanistic study of the reversible binding of NO to a water-soluble octa-cationic Fe(III) porphyrin complex
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dc.type
JournalArticle
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dc.description.physical
1326-1337
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dc.abstract.en
The water-soluble, non-mu-oxo dimer-forming porphyrin, [5,10,15,20-tetrakis-4'-t-butylphenyl-2',6'-bis-(N-methylene-(4''-t-butylpyridinium))porphyrinato]iron(III) octabromide, $(P^{8+})Fe^{III}$, with eight positively charged substituents in the ortho positions of the phenyl rings, was characterized by UV-vis and $^{1}\textrm{H}$ NMR spectroscopy and $^{17}\textrm{O}$ NMR water-exchange studies in aqueous solution. Spectrophotometric titrations of $(P^{8+})Fe^{III}$ indicated a $pK_{a1}$ value of 5.0 for coordinated water in $(P^{8+})Fe^{III}(H_{2}O)_{2}$. The monohydroxo-ligated $(P^{8+})Fe^{III}(OH)(H_{2}O)$ formed at 5 < pH < 12 has a weakly bound water molecule that undergoes an exchange reaction, $k_{ex}=5.5 x 10^{4}s^{-1}$, significantly faster than water exchange on $(P^{8+})Fe^{III}(H_{2}O)_{2}$, viz. k_{ex}=5.5 x 10^{4}s^{-1} at $25^{o}\textrm{C}$. The porphyrin complex reacts with nitric oxide to yield the nitrosyl adduct, $(P^{8+})Fe^{II}(NO^{+})(L)(L=H_{2}O or OH^{-})$. The diaqua-ligated $(P^{8+})Fe^{III}(H_{2}O)_{2}$ binds and releases NO according to a dissociatively activated mechanism, analogous to that reported earlier for other $(P)Fe^{III}(H_{2}O)_{2}$ complexes. Coordination of NO to $(P^{8+})Fe^{III}(OH)(H_{2}O)$ at high pH follows an associative mode, as evidenced by negative $\Delta$S⧧ and $\Delta$V⧧ values measured for this reaction. The observed ca. 10-fold decrease in the NO binding rate on going from six-coordinate $(P^{8+})Fe^{III}(H_{2}O)_{2}$ $(k_{on}=15.1 x 10^{3}M^{-1}s^{-1})$ to $(P^{8+})Fe^{III}(OH)(H_{2}O)$ $(k_{on}=1.56 x 10^{3}M^{-1}s^{-1} at 25^{o}\textrm{C})$ is ascribed to the different nature of the rate-limiting step for NO binding at low and high pH, respectively. The results are compared with data reported for other water-soluble iron(III) porphyrins with positively and negatively charged meso substituents. Influence of the porphyrin periphery on the dynamics of reversible NO binding to these $(P)Fe^{III}$ complexes as a function of pH is discussed on the basis of available experimental data.
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dc.description.volume
45
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dc.description.number
3
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dc.identifier.doi
10.1021/ic051339v
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dc.identifier.eissn
1520-510X
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dc.title.journal
Inorganic Chemistry
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dc.language.container
eng
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dc.affiliation
Wydział Chemii : Zakład Chemii Nieorganicznej
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dc.subtype
Article
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dc.rights.original
bez licencji
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