In situ monitoring of the degradation of iron porphyrins by dioxygen with hydrazine as sacrificial reductant.: Detection of paramagnetic intermediates in the coupled oxidation process by 1H NMR spectroscopy

The effects of using hydrazine rather than ascorbic acid on the coupled oxidation of (OEP)Fe-II(py)(2) (OEP is the dianion of octaethylporphyrin, py is pyridine) have been investigated with the goal of directly detecting reactive intermediates during the process of heme degradation by dioxygen. The reaction products, [(OEOP)Fe-II(py)(2)]Cl and (OEB)Fe-III(py)(2) (OEOP is the monoanion of octaethyl-5-oxaporphyrin and OEB is the trianion of octaethylbilindione), and their yields are similar to those of the standard coupled oxidation process. The reaction has been monitored in situ in pyridine/dichloromethane mixtures by H-1 NMR spectroscopy. The recently isolated and crystallographically characterized complex, (OEPO)Fe(py)(2) (OEOP is the trianion of octaethyloxaphlorin), has been identified as a key intermediate. Addition of dioxygen to (OEP)Fe-II(py)(2) in pyridine with hydrazine present also produces two new transient species: (OEPO)Fe(py)(N2D4) and (OEPO)Fe(N2D4)(2). These complexes have also be produced independently by low-temperature titration of hydrazine into a solution of {(OEPO)Fe}(2). Thus, hydrazine acts as an axial ligand during the early stages of the coupled oxidation process. However, the two hydrazine-containing complexes eventually are converted into (OEPO)Fe(py)(2) before [(OEOP)Fe-II(py)(2)]Cl and (OEB)Fe-III(py)(2) are formed. The observations reported here suggest that the coupled oxidation process can be divided into two stages. The first stage involves the meso C-H bond and results in introduction of oxygen at that site with the formation of the three intermediates: (OEPO)Fe(N2H4)(2), (OEPO)Fe(N2H2)(py), and (OEPO)Fe(py)(2). The second stage of the coupled oxidation process involves C-C bond breaking and the conversion of the hydroxylated heme, (OEPO)Fe(py)(2), into the final products, [(OEOP)Fe-II(py)(2)](+) and (OEB)Fe-III(py)(2).