Electrochemistry of Nitrite Reductase Model Compounds. 1. Electrochemistry of Iron Tetraphenylchlorin
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American Chemical Society
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The electrochemistry of iron tetraphenylchlorin was studied in several solvents and in the presence of substituted pyridines and amines. Iron tetraphenylchlorin was found to be both quantitatively and qualitatively similar to iron tetraphenylporphyrin. The variations in the cumulative formation constants of substituted pyridines with ferrous and ferric chlorin were measured with differential pulse polarography and cyclic voltammetry. The formation constants for pyridines with ferric chlorin, β2III, were found to vary linearly with the pKa of the pyridine with a slope of 1.22 ± 0.14. By contrast, the β2II values for the ferrous chlorin-pyridine complex were not linearly related to the pKa of the pyridine. These results appeared to indicate that significant π-interactions (back-bonding) are involved in the ferrous complexes, which are not significant for the ferric species. When amines are used that cannot participate in π-bonding, a much greater sensitivity of the β2II values to the pKa of the amine was observed than was previously assumed. In addition to the electrochemical results, the UV-visible spectra of chloro(tetraphenylchlorinato)iron(III), Fe(TPC)Cl, and its µ-oxo complex, (FeTPC)2O, were reported. Fe(TPC)CI had bands at 385, 415, 600, 646, and 740 nm, while (FeTPC)2O had bands at 406, 537, 605, and 646 nm.