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

Abstract

Abstract. Hydroxylamine (NH 2 OH) 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 NH 2 OH with hemes in proteins and model systems has gained much attention recently. In this study, we use the more stable, oxygen substituted O-benzylhydroxylamine (NH 2 OBn) as a model for NH 2 OH. Here, the reactivity of the ferric bis-picket fence porphyrin complexes [Fe(3,5-Me-BAFP)(ClO 4 )] (1) and [Fe(3,5-Me-BAFP)(PF 6 )] (2) (3,5-Me-BAFP 2-= dianion of tetra(2,6-bis(3,5-dimethylphenoxy)phenyl)porphyrin)) with NH 2 OBn

Insight into Solvent Coordination of an Iron Porphyrin Hydroxylamine Complex from Spectroscopy and DFT Calculations

“…The results are summarized in Table . Further reduction of hydroxylamine to ammonia, as was observed by McQuarters et al, was not consistent with the data. First, two resonances are observed for the ligand in the NMR spectrum, as was observed for the cobalt complex.…”

“…Stable ferrous porphyrin N ‐alkyl hydroxylamine complexes have been reported . On the other hand, ferric “picket‐fence” porphyrins will react with O ‐alkyl hydroxylamine to form a bis(ammonia) ferrous complex rather than a ferrous‐nitrosyl species . Hydroxylamine itself was found to disproportionate in the presence of water‐soluble ferric porphyrins to form NH 3 and N 2 O …”

“…[7] On the other hand, ferric "picket-fence" porphyrins will react with O-alkyl hydroxylamine to form a bis(ammonia) ferrous complex rather than a ferrous-nitrosyl species. [8] Hydroxylamine itself was found to disproportionate in the presence of water-soluble ferric porphyrins to form NH 3 and N 2 O. [9] The reduction of Fe(OEP)(NO) in THF occurs in two one-electron steps with the formation of Fe(OEP)(NO)and Fe(OEP)(NO) 2-.…”

“…Mechanisms for the decomposition of the ferrous-hydroxylamine complexes generally involve the presence of excess hydroxylamine with a bis(hydroxylamine) complex. [8,9] The excess hydroxylamine initiates a radical reaction on the complex, which is followed by an internal disproportionation to eventually lead to the ferrous nitrosyl. [8] This mechanism cannot occur with the complex described in this report, as only a mono complex was formed and there was no excess hydroxylamine.…”

Controlling a burn: outer-sphere gating of hydroxylamine oxidation by a distal base in cytochrome P460

“…Nature has evolved Fe-based enzymes bearing heme P460 cofactors that effect the controlled release of electrons from NH 2 OH 12,13. This is remarkable given that more commonly reported reaction outcomes between NH 2 OH and Fe comprise disproportionation or uncontrolled production of ill-defined mixtures bearing a variety of gaseous oxynitrogen species 14–17. Consequently, P460 cofactors represent a valuable system from which to glean insights into controlled redox chemistry.…”