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+ (1), with PhE– and NO2– produced [Fe(6-COO–-tpa)(EPh)] (E = S, 2a; Se, 3) and [Fe(6-COO–-tpa)(κ2-O,O′-NO2)] (4), respectively (6-COOH-tpa is bis(2-pyridylmethyl)(6-carboxyl-2-pyridylmethyl)amine). Treatment of 4 with 2 equiv of PhEH (E = S, Se) produced NO in ∼40% yields, respectively, along with 1 and the DNICs, [Fe(EPh)2(NO)2]1– (E = S, Se). Treatment of 4 with excess PhEH produced NO in similar yields, while 4 was converted to the same DNICs and 2a/3 (instead of 1). 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+ (6), which has also been synthesized separately. Compound 6 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-p-COOH)] (7) and [Fe(6-COO–-tpa)(S-C6H4-o-OH)] (8), with 0.5 and 1 equiv of NO2– could produce NO only in 8–26% yields, treatment of 4 with HS-C6H4-p-COOH and HS-C6H4-o-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.