Reversible binding of nitric oxide and carbon-carbon bond formation in a meso-hydroxylated heme

Exposure of a pyridine solution of (py)2Fe(OEPO) (1) (OEPO is the trianion of octaethyloxophlorin, py is pyridine) at 22 degrees C to nitric oxide under the strict exclusion of dioxygen results in a color change from green to red-brown and the formation of {(py)(ON)Fe(OEPO)}2 (2). Monitoring the reaction by 1H NMR spectroscopy shows that the reaction is reversible. The air-sensitive product {(py)(ON)Fe(OEPO)}2 has been isolated and characterized by X-ray crystallography. NO binding is accompanied by association of two hemes through the formation of a new C-C bond between two meso carbon atoms on the porphyrin periphery. The macrocyclic ligand has a ruffled distortion that positions the C5-O1 group with its short C=O bond distance (1.237(3) A) in close proximity to the NO ligand. Since further attack upon 1 by O2 during heme degradation involves reaction in the vicinity of the oxygenated meso position, the positioning of the NO ligand so that the O2...C5 distance is only 3.100(3) A presents a highly suggestive model for the next stage of attack upon the heme periphery.