The Fe₂(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 N2 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 N2 O formation is observed. Instead, the dimer, [Fe2 (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.