Redox and Spectroscopic Properties of Protonated Species of Iron Porphyrin Nitroxlys and Their Analogues

Nitrite reduction to ammonia or nitrous oxide involves a series of electron transfer and protonation steps which are carried out by assimilatory or dissimilatory nitrite reductases. In the assimilatory process, nitrite incorporated into the biomass while in the dissimilatory process, it is excreted from the cell and the reaction is a source of energy. The complexes that will be studied in this work are models for assimilatory (siroheme) and dissimilatory (heme d1 ) nitrite reductases. Iron porphyrin nitrosyls were reduced in the presence of weak acids such as phenol and substituted phenols. Voltammetric techniques such as cyclic and rotating ring disk electrode (RRDE) voltammetry were employed to elucidate the reduction/protonation reaction mechanism and kinetics. Cyclic voltammograms showed two closely spaced waves for the reduction of Fe(OEP)(NO) (OEP= octaethylporphyrin) in the presence of substituted phenols. The first wave corresponded to a single electron reduction and the second wave was a multielectron process which was kinetically controlled. To determine the kinetics of the first protonation, RRDE voltammetry on the first wave was studied. UV-visible spectroelectrochemistry was carried out to identify the protonated species that were formed. The results indicated that the two-protonated species were present, which were identified as Fe(OEP)(HNO) and Fe(OEP)(NH2OH). The formation of these two species was suppressed by adding the conjugate base of the substituted phenol to the solution. FTIR spectroelectrochemistry was also employed to confirm those protonated species. Fe(OEP)(HNO) and Fe(OEP)(NH2OH) were generated chemically and verified by UV-visible, FTIR and NMR spectroscopy. Other models showed similar behavior with slight differences. A series of iron corrole nitrosyl complexes were also studied. Even though the one electron reduction species were stable in both the voltammetric and spectroelectrochemical timescale, the protonation of reduced species was not observed in the experimental time scale