ELECTROCHEMICAL AND SPECTROSCOPIC STUDIES OF IRON PORPHYRIN NITROSYLS AND THEIR REDUCTION PRODUCTS

Fe(P)(NO), where P = TPP, TPC, or OEP, is reduced in three one-electron steps in nonaqueous solvents. The products of the first two waves (Fe(P)(NO)- and Fe(P)(NO)2-) were stable, and the visible spectra were obtained by using OTTLE spectroelectrochemistry. The vibrational spectra of Fe(P)(NO) and its first reduction product obtained coulometrically were recorded. The porphyrin vibrations for both species were consistent with low-spin ferrous complexes. The nu-NO and nu-Fe-N bands could also be observed for both complexes, though nu-NO for Fe(TPP)(NO)- was quite weak. For Fe(TPP)(NO), nu-NO (N-15 values in parentheses) was 1681 cm-1 (1647 cm-1) and nu-Fe-N was 525 cm-1 (517 cm-1). Upon reduction, nu-NO decreased to 146 cm-1 (1475 cm-1) while nu-Fe-N increased to 549 cm-1 (538 cm-1). These results were consistent with the addition of the electron to the half-filled d(z)2 + sigma-N orbital, which is formed from pi*NO and iron d(z)2 orbitals. Therefore, addition of an electron to this orbital would lead to a strengthening of the Fe-N bond and a weakening of the N-O bond. Dc polarography of Fe(TPP)(NO) and Fe(TPC)(NO) was carried out in the presence of several substituted phenols. The limiting current and half-wave potential of the first wave were unaffected by the presence of the phenols, except at high phenol concentrations. A new second wave appeared, though, in the presence of phenols, and the limiting current and half-wave potential for this wave depended strongly on the concentration and identity of the weak acid. The overall reduction appeared to involve three electrons on the polarographic time scale, to yield Fe(P)- and hydroxylamine. Further reduction to ammonia was observed on the coulometric time scale. Exhaustive electrolysis gave ammonia in nearly quantitative yield for 2-chlorophenol concentrations greater than 20 mM. No differences were observed in the polarographic behavior of Fe(TPP)(NO) and Fe(TPC)(NO), but somewhat higher concentrations of 2-chlorophenol were needed to generate ammonia coulometrically.