Spectroelectrochemical Study of Iron Porphyrin Nitrosyls in the Presence of Weak Acids

The infrared spectroelectrochemistry of Fe(OEP)(NO) and Fe(OEPone)(NO), where OEP = octaethylporphyrin and OEPone = octaethylporphinone, was studied in the presence of phenol and substituted phenols. THF-d₈ was used as the solvent in order to have the widest transparent spectral region. Previous work in our laboratory [1] has shown that the first reduction product, Fe(OEP)(NO)^-, was stable on the spectroelectrochemical timescale. Because the first electron is added to a SOMO orbital, the protonated nitrosyl was not reduced at the potential that it was generated, and kinetic evidence has indicated that a second proton can be added, prior to further reduction.

                            Fe(OEP)(NO)^- + PhOH →  Fe(OEP)(HNO)  +  PhO^-

                            Fe(OEP)(HNO) + PhOH →  Fe(OEP)(H₂NO)⁺  +  PhO^-

Previous work has shown that the Fe(OEP)(H₂NO)⁺ is reduced by three electrons at a potential significantly negative of the Fe(OEP)(NO)^0/- wave. The FTIR spectroelectrochemical reduction of Fe(OEP)(¹⁵NO) in the presence of 4 mM phenol is shown in figure below (the ¹⁵N isotope was used because it moved the ν_NO band for the reduction product away from the residual bands for protonated THF). The spectra showed the disappearance of the ν_NO band at 1640 cm^-1 and the appearance of the ν_NO band for Fe(OEP)(NO)^- at 1420 cm^-1. Other bands were observed for the disappearance of phenol and the appearance of phenolate, either generated by the reactions above or by electrolysis. The band at 1343 cm^-1 was a porphyrin band for Fe(OEP)(NO)^-. If the potential is scanned to more negative potentials, the 1420  and 1343 cm^-1 disappeared as the Fe(OEP)(NO)^- species was reduced. At this concentration of phenol, no significant reaction was observed between Fe(OEP)(NO)^- and PhOH (phenol). At higher concentrations of phenol, reaction of the Fe(OEP)(NO)^- species was observed as the 1420 and 1343 cm^-1 disappeared. Under these conditions, detailed analysis was difficult because the phenolate bands due to the direct reduction of phenol obscured the spectrum. The catalytic reduction of H⁺ by the reduced iron nitrosyl was not observed, as has been seen when stronger acids (such as acetic acid) were used as a proton source [2].

In order to observe the reaction more clearly, two important problems needed to be solved in order to obtain good spectroelectrochemical data. First, the direct reduction of phenol/substituted phenols had to be minimized so that their bands did not overwhelm the spectrum. In addition, the analysis of the data was simplified when the changes in phenol/phenolate bands were only related to the reduction of the iron nitrosyl. The direct reduction was minimized by modifying the platinum surface to raise the overpotential for hydrogen ion reduction. Second, as the reaction proceeded, the solution will naturally become more basic as phenolate ion was generated, raising the pH of the solution. This problem was resolved by buffering the solution with phenolate salts so that the phenol/phenolate ratio remains relatively constant during the reaction. This was also an important reason to minimize the direct reduction of phenol at the electrode surface. Under these conditions, the FTIR spectra were clearer and the related N-O vibration and porphyrin modes could be observed. The spectral results will be presented.

In order to observe the oxidation state of the porphyrin itself during the reduction, iron porphinones nitrosyls were studied. In the infrared, the ν_NO and ν_CO bands could be readily observed and the FTIR study of these complexes in the absence of weak acids has been reported [3]. Reduction of the Fe(OEPone)(NO) caused downshifts in both the ν_NO and ν_CO bands. The electronic structures of the reduced iron porphinone nitrosyl species were interpreted from the comparision of the observed infrared bands and DFT calculations.

[1] Y. M. Liu and M. D. Ryan, J. Electroanal. Chem.(1994) 368, 209.

[2] L. E. Goodrich, S. Roy, E. E. Alp, J. Zhao, M. Y. Hu and N. Lehnert, Inorg. Chem.(2013) 52, 7766.

[3] Z. Wei and M. D. Ryan, Inorg. Chem. (2010) 49, 6948.