Dioxygen and nitric oxide reactivity of a reduced heme/non-heme diiron(II) complex [(5L)FeII⋯FeIICl]+. Using a tethered tetraarylporphyrin for the development of an active site reactivity model for bacterial nitric oxide reductase

We present here a first-generation model and initial reactivity (with O2 and NO) study for the heme/non-heme diiron active site chemistry of nitric oxide reductase (NOR), a denitrifying bacterial enzyme which converts nitric oxide to nitrous oxide (2NO+2e−+2H+→N2O+H2O). This research is also pertinent because of the considerable recent biological, chemical and industrial interest in NO and nitrogen oxides. The study employs the binucleating ligand 5L, with tetradentate tris(2-pyridylmethyl)amine (TMPA) chelate tethered to a tetraarylporphyrin (with three 2,6-difluorophenyl meso substituents). The new, reduced, diiron(II) compounds [(5L)FeII⋯FeIICl]+ (2) have been synthesized by dithionite reduction of the previously characterized μ-oxo complex [(5L)FeIIIOFeIIICl]+ (1) and characterized as either a perchlorate (from 2a; λmax 424 (Soret), 544 nm, tetrahydrofuran (THF)) or tetraarylborate (2b; BArF) anion complexes. NMR spectroscopic studies indicates 2 possesses a high-spin heme in non- or weakly coordinating solvents (CH2Cl2 or THF), and the evidence suggests that coordination from one of the pyridyl arms of the TMPA tether is involved. Reaction of 2 with O2 results in the generation of an intermediate which is relatively stable at −80°C in THF (λmax; 416 (Soret), 538 nm), hypothesized to be a peroxo-bridged heme/non-heme diiron(III) complex. Warming of this intermediate gives back 1. The reaction course of 2 with nitric oxide depends on the concentration. On a UV–Vis scale (<10 μM), a low-temperature stable intermediate (from 2a (THF); λmax 414 (Soret), 548 nm) forms, which upon warming gives the μ-oxo complex 1, and presumably produces nitrous oxide. At higher concentrations, gas chromatographic analysis shows that both N2O and NO2 are produced, while UV–Vis, NMR, infrared and resonance Raman spectroscopic evidence indicates that a new red metal complex product obtained contains a iron(II)–nitrosyl moiety. This air-sensitive compound also reverts to 1 upon exposure to O2. Discussion includes reference to nitric oxide reductase (NOR) chemistry, and suggestions for the mechanism(s) of the observed reactions and product NOx formation.