Reactions of a heme-superoxo complex toward a cuprous chelate and •NO(g): C c O and NOD chemistry

Abstract

Following up on the characterization of a new (heme)Fe^III-superoxide species formed from the cryogenic oxygenation of a ferrous-heme (P^Py)Fe^II (1) (P^Py = a tetraarylporphyrinate with a covalently tethered pyridine group as a potential axial base), giving (P^Py)Fe^III-O₂^•- (2) (Li Y et al., Polyhedron 2013; 58: 60-64), we report here on (i) its use in forming a cytochrome c oxidase (CcO) model compound, or (ii) in a reaction with nitrogen monoxide (•NO; nitric oxide) to mimic nitric oxide dioxygenase (NOD) chemistry. Reaction of (2) with the cuprous chelate [Cu^I(AN)][B(C₆F₅)₄] (AN = bis[3-(dimethylamino) propyl]amine) gives a meta-stable product [(P^Py)Fe^III-([Formula: see text])-Cu^II(AN)][B(C₆F₅)4] (3a), possessing a high-spin iron(III) and Cu(II) side-on bridged peroxo moiety with a μ-η²:η²-binding motif. This complex thermally decays to a corresponding μ-oxo complex [(P^Py)Fe^III-(O^2-)-Cu^II(AN)][B(C₆F₅)4] (3). Both (3) and (3a) have been characterized by UV-vis, ²H NMR and EPR spectroscopies. When (2) is exposed to •NO_(g), a ferric heme nitrato compound forms; if 2,4-di-tert-butylphenol is added prior to •NO_(g) exposure, phenol ortho-nitration occurs with the iron product being the ferric hydroxide complex (P^Py) Fe^III(OH) (5). The latter reactions mimic the action of NOD's.

Keywords: heme-copper-μ-oxo; heme-superoxo; high spin heme-copper peroxo; nitrate; peroxynitrite.

Fig. 1

UV-vis spectra of (1, black) a reduced (P^Py)Fe^II + [Cu^I(AN)]⁺ 1:1 mixture; (3a, red) high spin peroxo complex [(P^Py)Fe^III-(${\mathrm{O}}_2^{2-}$) Cu^II(AN)]⁺; (3, green) μ-oxo complex [(P^Py)Fe^III-(O²-)-Cu^II(AN)]⁺

Fig. 2

²H NMR spectra at −80 °C in THF (1) (d₈-P^Py)Fe^II-THF, (2) (d₈-P^Py)Fe^III-(${\mathrm{O}}_2^{2-}$) (3a) [(d₈-P^Py)Fe^III-(O²)-Cu^II(AN)]⁺ and (3) [(d₈-P^Py)Fe^III-(O₂-)-Cu^II(AN)]⁺

Fig. 3

UV-vis spectra showing superoxo (2, red) formed from reduced (P^Py)FeII (1, black) by bubbling O_2(g) at −80 °C; nitrato complex (P^Py)Fe^III-ONO₂ (4, grey) generated immediately after addition of •NO_(g)

Fig. 4

UV-vis spectroscopy in THF at −80 °C. The black spectrum is reduced (P^Py)Fe^II (1); red is (2 + DTBP), and green is (5)

Scheme 1

Formation of (P^Py)Fe^III-(O₂^•-) (2) via oxygenation of (P^Py)Fe^II (1) and subsequent reaction with [Cu^I(AN)]⁺ to give the meta-stable intermediate, the high-spin heme-peroxo-copper complex (3a), which decays to give the m-oxo complex [(P^Py)Fe^III-(O₂^-)- Cu^II(AN)]⁺ (3)

Scheme 2

Reaction sequence where •NO_(g) is added to superoxo complex (2) to give nitrato complex (4). In the presence of a phenolic substrate, the same reaction gives (5) as a final product along with the ortho-nitrated phenol