Dioxygen and nitric oxide reactivity of a reduced heme/non-heme diiron(II)complex [(L-5)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 O-2 a nd NO) study for the heme/non-heme diiron active site chemistry of nitric o xide 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 a nd industrial interest in NO and nitrogen oxides. The study employs the bin ucleating ligand L-5, with tetradentate tris(2-pyridylmethyl)amine (TMPA) c helate tethered to a tetraarylporphyrin (with three 2,6-difluorophenyl meso substituents). The new, reduced, diiron(II) compounds [(L-5)Fe-II... Fe-II -Cl](+) (2) have been synthesized by dithionite reduction of the previously characterized mu-oxo complex [(L-5)Fe-III-O-Fe-III-Cl](+) (1) and characte rized as either a perchlorate (from 2a; lambda(max) 424 (Soret), 544 nm, te trahydrofuran (THF)) or tetraarylborate (2b; BArF) anion complexes. NMR spe ctroscopic studies indicates 2 possesses a high-spin heme in non- or weakly coordinating solvents (CH2Cl2 or THF), and the evidence suggests that coor dination from one of the pyridyl arms of the TMPA tether is involved. React ion of 2 with O-2 results in the generation of an intermediate which is rel atively stable at -80 degrees C in THF (lambda(max); 416 (Soret), 538 nm), hypothesized to be a peroxo-bridged heme/non-heme diiron(III) complex. Warm ing of this intermediate gives back 1. The reaction course of 2 with nitric oxide depends on the concentration. On a UV-Vis scale (< 10 mu M), a low-t emperature stable intermediate (from 2a (THF); lambda(max) 414 (Soret), 548 nm) forms, which upon warming gives the mu-oxo complex 1, and presumably p roduces nitrous oxide. At higher concentrations, gas chromatographic analys is shows that both N2O and NO2 are produced, while UV-Vis, NMR, infrared an d resonance Raman spectroscopic evidence indicates that a new red metal com plex product obtained contains a iron(II)-nitrosyl moiety, This air-sensiti ve compound also reverts to 1 upon exposure to O-2. Discussion includes ref erence to nitric oxide reductase (NOR) chemistry, and suggestions for the m echanism(s) of the observed reactions and product NOx formation. (C) 2000 E lsevier Science S.A. All rights reserved.