Experimental Determination of an Isolated trans‐Dinitrosyl Manganese(II) Heme Analogue

Abstract The lack of direct proof in either natural or synthetic systems for trans ‐dinitrosyl hemes, a key intermediate in the reactions of heme proteins (e.g. soluble guanylate cyclase (sGC), cytochrome c′ and So H‐NOX) with nitric oxide (NO), has hampered understanding of the exact reaction mechanisms, such as the formation of the five‐coordinate heme complex with NO at the proximal side (5c NO P ). Herein, we report the first isolation of a dinitrosyl metalloporphyrin complex, the six‐coordinate, low‐spin {Mn(NO) 2 } 7 species [Mn(TPP)(NO) 2 ] (TPP 2− = meso ‐tetraphenylporphyrin dianion). The complex shows distinct features, such as an elongated axial bond (1.877(9) vs. 1.641(5) Å), a higher NO stretching bond position (1760 vs. 1735 cm −1 ) and an isotropic resonance at g = 2.0, in sharp contrast to those of five‐coordinate mononitrosyl analogues. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and EPR studies provided deep insight into the reaction processes, demonstrating different responses of porphyrinates to NO.