Electronic Modification of a Reduced Mononuclear Nonheme Iron Nitrosyl Complex Leads to HNO Release

A new pentadentate-fluorinated N₄S(thiolate) ligand was synthesized. Reaction with Fe(BF₄)₂·6H₂O gives a new thioether complex, [Fe^II(CH₃CN)(N3Py^pFSEtCN)][BF₄]₂ (1), and on-metal deprotection gives the thiolate complex, [Fe^II(CH₃CN)(N3Py^pFS)][BF₄] (2). Reaction of 2 with NO forms a low-spin ground state (S = 1/2) {FeNO}⁷ complex (3). Chemical reduction of 3 with cobaltocene gives a metastable intermediate spin S = 1 {FeNO}⁸ complex (4). Protonation of 4 releases nitroxyl (HNO), as observed by ESI-MS and ³¹P NMR trapping experiments with PPh₃. Complexes 1 and 2 were characterized by single-crystal X-ray crystallography, complexes 2-4 were characterized by EPR and FT-IR spectroscopies, and all iron complexes were characterized by ¹⁹F NMR, UV-vis, and ⁵⁷Fe Mössbauer spectroscopies. These results show that a nonheme iron complex can generate and release HNO, suggesting that nonheme iron centers could be endogenous or exogenous sources of HNO in biological systems. Additionally, the fluorine-substituted N₄S(thiolate) ligand provides a unique spectroscopic handle to monitor the reactivity of the iron center several bonds away from the fluorine substituent.