MODEL STUDIES OF NITROSYL INTERMEDIATES IN THE CATALYTIC CYCLE OF DISSIMILATORY NITRITE REDUCTASES

As models for a reaction intermediate in the catalytic cycle of dissim ilatory nitrite reductases, we have examined one-electron oxidation of nitrosyl-iron(II) complexes of octaethylporphyrin (OEP, 1a), oxooctae thylchlorin (oxo-OEC, 1b), and dioxooctaethylisobacteriochlorin (dioxo -OEiBC, 1c). While (OEP)Fe-III(NO) 2a is obtained in the oxidation of 1a, the oxidation products of 1b and 1c afford absorption spectra char acteristic of chlorin and isobacteriochlorin pi-cation radicals. The f ormation of the pi-cation radical complexes (2b and 2c) is confirmed b y a variety of methods including electronic absorption, ESR, NMR and I R spectroscopies. The presence of NO in 2b and 2c as the fifth ligand is evidenced by the observation of nu(15NO) bands at 1686 and 1699 cm( -1), respectively. These results are rationalized by the stabilization of the iron d orbital levels in 2b and 2c. Sixth-ligand effects on th e electronic structures of the oxidation products (2a-c) have also bee n investigated. Ligation of N-methylimidazole (N-MeIm) to 2b and 2c ca uses valence isomerization to give Fe-II(NO+)-(N-MeIm) complexes (3b a nd 3c) as well as (OEP)Fe-II(NO+)(N-MeIm) (3a). Although the six-coord inated imidazole adducts (3a-c) are formulated as Fe-II(NO+)(N-MeIm), the oxo derivatives (3b and 3c) readily release the NO ligand in the p resence of an additional 1 mol equiv of N-MeIm: 3a is relatively stabl e under the same condition.