Dioxygen and nitric oxide reactivity of a reduced heme/non-heme diiron(II) complex [(⁵L)Fe(II)···Fe(II)-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-pyridyl-methyl)amine (TMPA) chelate tethered to a tetraarylporphyrin (with three 2,6-difluorophenyl meso substituents). The new, reduced, diiron(II) compounds [(⁵L)Fe(II...)Fe(II)-Cl]⁺ (2) have been synthesized by dithionite reduction of the previously characterized μ-oxo complex [(⁵L)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 (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 (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 (< 10 μM), a low-temperature stable intermediate (from 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 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 NO(x) formation. (C) 2000 Elsevier Science S.A.