Synthesis and characterization of a model complex for flavodiiron NO reductases that stabilizes a diiron mononitrosyl complex.

Flavodiiron NO reductases (FNORs) are important enzymes in microbial pathogenesis, as they equip microbes with resistance to the human immune defense agent nitric oxide (NO). DFT calculations predict that a network of second coordination sphere (SCS) hydrogen bonds is critical for the key NN coupling step in the NO reduction reaction catalyzed by FNORs. In this study, we report the synthesis of a model complex of FNORs with pendant hydrogen bond donors. For this purpose, the ligand H[BPMP] (= 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol) was modified with two amide groups in the SCS. Reaction of the precursor complex [Fe2(BPMP(NHCOtBu)2)(OAc)](OTf)2 (1) (OTf- = triflate anion) with NO in the presence of base led to the surprising isolation of a diiron mononitrosyl complex, [Fe2(BPMP(NHCOtBu)(NCOtBu))(OAc)(NO)](OTf) (2) and a triiron decomposition product, [Fe3(BPMP(NHCOtBu)2)(OAc)2(μ-O)2(ONO)](OTf) (3), which were both structurally characterized. Complex 2 models the corresponding mononitrosyl adduct in FNORs. This result points towards a strategy that can be used to stabilize mononitrosyl diiron complexes, using the SCS.