Heme and non-heme iron in biology mediate the storage/release
of NO<sup>•</sup> from <i>S</i>-nitrosothiols as a means to control the
biological concentration of NO<sup>•</sup>. Despite their importance in many
physiological processes, the mechanisms of N-S bond formation/cleavage at Fe
centers have been controversial. Herein, we report
the interconversion of NO<sup>•</sup> and <i>S</i>-nitrosothiols mediated
by Fe<sup>II</sup>/Fe<sup>III</sup> chloride complexes. The reaction of two
equivalents of <i>S</i>-nitrosothiol (Ph<sub>3</sub>CSNO) with [Cl<sub>6</sub>Fe<sup>II</sup><sub>2</sub>]<sup>2</sup><sup>-</sup>
results in facile release of NO<sup>•</sup> and formation of iron(III)
halothiolate. Detailed spectroscopic studies, including in situ UV-vis, IR, and
Mössbauer spectroscopy, support the interaction of the S−atom with the Fe<sup>II</sup> center. This is in
contrast to the proposed mechanism of NO<sup>•</sup> release from the
well-studied “red product” <i>k</i><sup>1</sup>-N
bound <i>S</i>-nitrosothiol Fe<sup>II</sup> complex,
[(CN)<sub>5</sub>Fe(<i>k</i><sup>1</sup>-N-RSNO)]<sup>3</sup><sup>-</sup>.
Additionally, Fe<sup>III</sup> chloride can mediate NO<sup>•</sup> storage
through the formation of <i>S</i>-nitrosothiols. Treatment of iron(III)
halothiolate with two equivalents of NO<sup>•</sup> regenerates Ph<sub>3</sub>CSNO
with the Fe<sup>II</sup> source
trapped as the <i>S</i> = 3/2 {FeNO}<sup>7</sup> species [Cl<sub>3</sub>FeNO]<sup>-</sup>,
which is inert towards further coordination and activation of <i>S</i>-nitrosothiols.
Our work demonstrates how labile iron can mediate the interconversion of NO<sup>•</sup>/thiolate
and <i>S</i>-nitrosothiol, which has important implications for how Nature
manages the biological concentration of free NO<sup>•</sup>.
In situ FT-IR Spectroscopic Studies on the Mechanism of the Catalytic Oxidation of Carbon Monoxide over Supported Cobalt Catalysts