Cobalt(II) Nitrosyl Cation Radicals of Porphyrins, Chlorins, and Isobacteriochlorins. Models for Nitrite and Sulfite Reductases and Implications for A_1u Heme Radicals

Oxidation of cobalt(II) nitrosyl complexes of porphyrins (P), chlorins (C), and isobacteriochlorins (iBC) yields Co^IINO π-cation radicals. The radicals are stable and recyclable to the parent compound without loss of NO. The oxidized species have been characterized by electrochemistry, visible and infrared spectroscopy, and electron spin resonance (ESR). Interest in these intermediates is prompted by the presence of iron Cs or iBCs in enzymes that catalyze the reductions of nitrite to ammonia or nitric oxide (nitrite reductases) and the recent identification of cobalt iBC in sulfite reductases, enzymes which are also capable of nitrite reduction. The optical spectra of the Co^IINO radicals resemble those previously obtained on oxidation of Fe^IINOC and iBC and lend support to the assignment of those ESR-silent species as Fe^IINOC⁺ and iBC⁺ π-cation radicals. As found in other series with the same macrocycles (Fe^IINO, Fe^IIICl, Zn, H₂), the Co^IINO complexes become progressively easier to oxidize as the porphyrin is saturated (P <C <iBC, easiest). In all three species, the Co-NO bonds remain bent, suggestive of some Co-NO^- character. The C and iBC radicals exhibit ESR features characteristic of “a_1u” radicals that include spin delocalization onto the metal. Similar spin profiles may rationalize the ESR spectra of presumed “a_1u” porphyrin radicals in oxidized chloroperoxidase, catalase, and cytochrome P450.