The Electrochemistry of Iron Porphyrin Nitrosyls in the Presence of Pyridines and Amines
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Inorganica Chimica Acta
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)−. There were also no significant differences in the Fe(P)(NO)–ligand formation constants between P = TPP and TPC. The formation constants for the Fe(P)(NO)–ligand complex, KNO-L, varied linearly with the pKa of the ligand. The slopes of the pKa versus log KNO-L curves were 0.22, 0.19 and 0.20 for P equal to TPP, TPC and PPDME, respectively. These slopes were significantly smaller than the values previously observed for FeII(P) and FeIII(P), but only slightly smaller than the complexes where NO was replaced by CS or CSe (0.31 and 0.23, respectively). 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 to generate the bis-ligand complex, Fe(P)(L)2.