Functional Nonheme Diiron(II) Complexes Catalyze the Direct Reduction of Nitrite to Nitric Oxide in Relevance to the Diiron Protein YtfE

The present work reports the functional modeling chemistry of YtfE, which features a nonheme diiron active site and mediates the direct reduction of NO₂^- to NO. The model complex, [Fe₂(HPTP)Cl₂]¹⁺ (<b>1</b>), reduces NO₂^- to NO in a 100% yield within 12 h and generates [Fe₄(HPTP)₂(μ-O)₃(μ-OH)]³⁺ (<b>2</b>). Similar to YtfE, the reaction involves stepwise oxidation of two Fe(II) centers and product (NO) inhibition, of which the latter produces [Fe₂(HPTP)(NO)₂Cl₂]¹⁺ (<b>3</b>). Complex <b>3</b> could also be synthesized by the reaction of [Fe₂(HPTP)(NO)₂(ClO₄)]²⁺ (<b>4</b>) and chloride. Complex <b>1</b> catalyzes the reduction of NO₂^- to NO in the presence of PhS^-, albeit with a low TON of 5, due to the formation of an insoluble product, [Fe₂(HPTP)(μ-SPh)Cl₂] (<b>5</b>). Another model complex [Fe₂(HPTP)(OPr)]¹⁺ (<b>6</b>), reduced NO₂^- to NO in an 80% yield after 24 h, generated [Fe₂(HPTP)(OPr)(NO)₂]¹⁺ (<b>7</b>), and offered a TON of 19. The third model complex, [Fe₂(HPTP)(ClO₄)₂]¹⁺ (<b>8</b>), could reduce NO₂^- to NO in a 100% yield but only after 48 h. A comparison of these results establishes that easy oxidation of the Fe(II) centers, easy accessibility of the Fe(II) centers for the coordination of NO₂^-, and easy release of NO from the in situ generated dinitrosyl diiron complex increase the efficiency of the functional model complexes of YtfE.