Reduction of Nitrite in an Iron(II)-Nitrito Compound by Thiols and Selenol Produces Dinitrosyl Iron Complexes via an {FeNO}⁷ Intermediate

Reaction of an Fe(II) complex, [Fe(6-COO^--tpa)]¹⁺ (<b>1</b>), with PhE^- and NO₂^- produced [Fe(6-COO^--tpa)(EPh)] (E = S, <b>2a</b>; Se, <b>3</b>) and [Fe(6-COO^--tpa)<i>(</i>κ²<i>-</i>O,O'-NO₂)] (<b>4</b>), respectively (6-COOH-tpa is bis(2-pyridylmethyl)(6-carboxyl-2-pyridylmethyl)amine). Treatment of <b>4</b> with 2 equiv of PhEH (E = S, Se) produced NO in ∼40% yields, respectively, along with <b>1</b> and the DNICs, [Fe(EPh)₂(NO)₂]^1- (E = S, Se). Treatment of <b>4</b> with excess PhEH produced NO in similar yields, while <b>4</b> was converted to the same DNICs and <b>2a</b>/<b>3</b> (instead of <b>1</b>). The DNICs have been proposed to be generated via the reaction of PhE^- with an in situ generated, unstable {FeNO}⁷ intermediate, [Fe(6-COO^--tpa)(NO)]¹⁺ (<b>6</b>), which has also been synthesized separately. Compound <b>6</b> reacts with PhS^- to generate [Fe(SPh)₂(NO)₂]^1-, thus supporting the proposed reaction pathway. Finally, while the treatment of two unique compounds, featuring inbuilt proton sources, [Fe(6-COO^--tpa)(S-C₆H₄-<i>p</i>-COOH)] (<b>7</b>) and [Fe(6-COO^--tpa)(S-C₆H₄-<i>o</i>-OH)] (<b>8</b>), with 0.5 and 1 equiv of NO₂^- could produce NO only in 8-26% yields, treatment of <b>4</b> with HS-C₆H₄-<i>p</i>-COOH and HS-C₆H₄-<i>o</i>-OH produced NO in much higher yields (65-77%). The combined results delineated the importance of coordination of NO₂^- for the proton-assisted reduction of NO₂^- to generate NO.