The Fe 2 (NO) 2 Diamond Core: A Unique Structural Motif In Non‐Heme Iron–NO Chemistry

Non‐heme high‐spin (hs) {FeNO} 8 complexes have been proposed as important intermediates towards N 2 O formation in flavodiiron NO reductases (FNORs). Many hs‐{FeNO} 8 complexes disproportionate by forming dinitrosyl iron complexes (DNICs), but the mechanism of this reaction is not understood. While investigating this process, we isolated a new type of non‐heme iron nitrosyl complex that is stabilized by an unexpected spin‐state change. Upon reduction of the hs‐{FeNO} 7 complex, [Fe(TPA)(NO)(OTf)](OTf) ( 1 ), the N‐O stretching band vanishes, but no sign of DNIC or N 2 O formation is observed. Instead, the dimer, [Fe 2 (TPA) 2 (NO) 2 ](OTf) 2 ( 2 ) could be isolated and structurally characterized. We propose that 2 is formed from dimerization of the hs‐{FeNO} 8 intermediate, followed by a spin state change of the iron centers to low‐spin (ls), and speculate that 2 models intermediates in hs‐{FeNO} 8 complexes that precede the disproportionation reaction.