Disproportionation of O-Benzylhydroxylamine Catalyzed by a Ferric Bis-Picket Fence Porphyrin Complex

Hydroxylamine (NH2OH) is an important molecule in biology that serves as an intermediate in the nitrogen cycle, and that can also be utilized as a nitric oxide donor in mammals under certain conditions. In light of this, the interaction of NH2OH with hemes in proteins and model systems has gained much attention recently. In this study, we use the more stable, oxygen substituted O‐benzylhydroxylamine (NH2OBn) as a model for NH2OH. Here, the reactivity of the ferric bis‐picket fence porphyrin complexes [Fe(3, 5‐Me‐BAFP)(ClO4)] (1) and [Fe(3, 5‐Me‐BAFP)(PF6)] (2) (3,5‐Me‐BAFP2– = dianion of tetra(2, 6‐bis(3,5‐dimethylphenoxy)phenyl)porphyrin)) with NH2OBn is investigated. The product of these reactions is characterized by UV/Vis and EPR spectroscopy and X‐ray crystallography. We found that addition of excess NH2OBn to our ferric porphyrin complexes results in reduction of the heme to the ferrous oxidation state. This is followed by disproportionation of additional NH2OBn to yield the ferrous complex [Fe(3,5‐Me‐BAFP)(NH3)2] (3) as the final product. The crystal structure of (3) constitutes the first structural characterization of a bis‐ammonia complex of a ferrous heme. The stability of this complex may be facilitated by the picket fences of the porphyrin ligand used here.