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

Abstract We present here a first-generation model and initial reactivity (with O 2 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 + →N 2 O+H 2 O). 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 5 L, 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 [( 5 L)Fe II ⋯Fe II Cl] + ( 2 ) have been synthesized by dithionite reduction of the previously characterized μ-oxo complex [( 5 L)Fe III OFe III Cl] + ( 1 ) and characterized as either a perchlorate (from 2a ; λ max 424 (Soret), 544 nm, tetrahydrofuran (THF)) or tetraarylborate ( 2b ; BAr F ) anion complexes. NMR spectroscopic studies indicates 2 possesses a high-spin heme in non- or weakly coordinating solvents (CH 2 Cl 2 or THF), and the evidence suggests that coordination from one of the pyridyl arms of the TMPA tether is involved. Reaction of 2 with O 2 results in the generation of an intermediate which is relatively stable at −80°C in THF ( λ max ; 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 ( 2a (THF); λ max 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 N 2 O and NO 2 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 O 2 . Discussion includes reference to nitric oxide reductase (NOR) chemistry, and suggestions for the mechanism(s) of the observed reactions and product NO x formation.