Electrochemistry and spectroelectrochemistry of nitrite reductase model complexes
The electrochemistry of Fe(P)(NO), where P was tetraphenylporphyrin (TPP), tetraphenylchlorin (TPC) and protoporphyrin dimethyl ester (PPDME), was studied in the presence of substituted pyridines and various amines. Both in the presence and absence of the ligand, the first reduction wave of Fe(P)(NO) was reversible. Weak complexes between the iron porphyrin nitrosyls and the pyridines or amines were observed. Upon reduction, the pyridine or amine was lost, and there was no evidence of complexation of the ligand with Fe(P)(NO)$\sp-$. There were also no significant differences in the Fe(P)(NO)-ligand formation constants between P = TPP and TPC. The visible spectra of Fe(P)(NO) in the presence of the nitrogenous bases were also obtained. With the addition of ligand, the Soret band shifted to longer wavelengths (405 to 419 nm for pyridine), while the long wavelength region shifted to shorter wavelengths (532 to 520 nm for pyridine). Spectra of Fe(P)(NO)(L) at high concentrations of L were not stable indefinitely, but slowly lost NO generate the bis-ligand complex, Fe(P)(L)$\sb2$. Resonance Raman and NMR spectroscopic measurements were performed to elucidate the structure of the reduced product of Fe(TPP)(NO). While there were small changes in some of the resonance Raman frequencies upon reduction, the spectrum was characteristic of a low-spin ferrous porphyrin. The spectrum of Fe(TPP)(NO) could be regenerated by returning the potential to the initial potential, indicating that the NO ligand was not lost during the reduction. The $\sp2$H NMR spectra of Fe(TPP)(NO)$\sp-$ were also indicative of little change in the iron oxidation state. The pyrrole resonance of Fe(TPP-d$\sb8$)(NO) shifted from 6.2 ppm (room temperature, temperature dependent) to 7.4 ppm (temperature independent) upon reduction. The electrochemistry and spectroelectrochemistry of Fe(P)Cl, where P = TPP, TPC and tetraphenylisobacteriochlorin (TPiBC), were studied in methylene chloride and tetrahydrofuran. While the reduction of Fe(P)Cl showed no distinctive difference due to the saturation of porphyrin macrocycles, the oxidation of Fe(TPiBC)Cl was easier than those of Fe(TPC)Cl and Fe(TPP)Cl by 300 mV and 500 mV, respectively. The spectroelectrochemical results showed that the second reduction product of Fe(P)Cl in ethylene chloride could react with this solvent to give a $\sigma$-alkyl iron porphyrin.
In Kyu Choi,
"Electrochemistry and spectroelectrochemistry of nitrite reductase model complexes"
(January 1, 1989).
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