Reactivity of methane mono-oxygenase, insights from quantum mechanic studies on synthetic iron model complexes

Methane mono-oxygenase (MMO) and deoxyhemerythrin (DHr) are examples of di-Iron enzymes that catalyze the dissociative and non-dissociative binding of molecular oxygen. To mimic the MMO active site with a finite cluster, we chose to study the binuclear heptapodate coordinated iron(III)-complexes of N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diamino- propane (HPTB) and N,N,N′,N′-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-diamino-propane (HPTP). These have active sites of the form [Fe₂(HPTP)(μ-OH)]⁴⁺ (1) and [Fe₂(HPTB)(μ-OH)]⁴⁺ (2). Quantum mechanics structures are compared with the experimental data obtained from the EXAFS analysis. For the O₂ binding on the reduced active site, the μ-η¹:η¹-O₂ mode seems the slightly more stable precursor to the O=Fe-O-Fe=O bis-ferryl (re)active site. The nature of the ferryl groups are these of a reactive two center three electron bond.