A Stable Hyponitrite-Bridged Iron Porphyrin Complex (original) (raw)

The coupling of two nitric oxide (NO) molecules in heme active sites is an important contributor to the conversion of NO to nitrous oxide (N 2 O) by heme-containing enzymes. Several formulations for the presumed heme-Fe{N 2 O 2 } n− intermediates have been proposed previously, however, no crystal structures of heme-Fe{N 2 O 2 } n− systems have been reported to date. We report the first isolation and characterization of a stable bimetallic hyponitrite iron porphyrin, [(OEP)Fe] 2 (µ-N 2 O 2), prepared from the reaction of the [(OEP)Fe] 2 (µ-O) with hyponitrous acid. Density functional calculations were performed on the model compound [(porphine)Fe] 2 (µ-N 2 O 2) to characterize its electronic structure and properties. Heme-assisted coupling of nitric oxide (NO) to form Fe{N 2 O 2 } n− intermediates plays an important part in the reduction of NO to N 2 O. 1,2 In transition-metal chemistry, such metal {N 2 O 2 } n− moieties can be generated from attack of NO on a metal-NO group, 3,4 from metalinduced coupling of NO, 5 or from transfer of an intact (N 2 O 2) n− from a diazeniumdiolate to a metal. 6 It is interesting to note that only a small number of metal{N 2 O 2 }-containing compounds have been structurally characterized, and the {N 2 O 2 } binding modes determined to date are sketched below as structures A (M = Pt, Ni), 5,6 B (M = M' = Co), 7 C (M = Ru) 8 and D (M = Co). 9 Enzymes such as the NO reductase (NOR) from Paracoccus denitrificans, cyt ba 3 and caa 3 from Thermus thermophilus, and cyt cbb 3 from Paracoccus stutzeri catalyze the conversion of NO to N 2 O using bimetallic active sites. 1,2 For these biological systems, intermediates resembling structure B have been proposed, where the metal{N 2 O 2 } moiety is N-bound to heme Fe (M), and where M' (non-heme Fe or Cu) may contact both O atoms. 1,10 Collman et al. have proposed, using results from the reaction of a diferrous synthetic model of NOR with NO, that a trans bis-nitrosyl intermediate forms at the active site of NOR followed by NO coupling (structure E) to give N 2 O and a bis-ferric product. 11 Resonance Raman spectroscopy