Reduction of Nitrite to Nitric Oxide by Tetranuclear Iron(II)-Polychalcogenides: Direct Reduction by Iron(II) vs the Reaction of Nitrite with Coordinated Polychalcogenides

The complexes, [Fe₂(HPTP)(EPh)₂]¹⁺ (E = S, <b>2a</b>; Se, <b>2b</b>), [Fe₄(HPTP)₂(S₃)(S₆)]²⁺ (<b>3</b>) and [Fe₄(HPTP)₂(Se₃)₂]²⁺ (<b>4</b>), directly reduced nitrite (NO₂^-) to nitric oxide (NO) and generated the DNICs, [Fe(EPh)₂(NO)₂]^1- and [Fe(E₅)(NO)₂]^1- (E = S, Se), respectively. The complexes, <b>2a</b>, <b>2b</b>, <b>3</b>, and <b>4</b>, featured oxidation events of the Fe(II) centers at very negative potentials and could mediate the direct reduction (in the absence of protons) of NO₂^- to NO, where the second "oxo" of NO₂^- was stabilized by the formation of a tetrairon(III) complex, [Fe₄(HPTP)₂(μ-O)₃(μ-OH)]³⁺. The nucleophilic attack of NO₂^- on the "neutral" chalcogen atoms of the coordinated polychalcogenide chains in <b>3</b> or <b>4</b>, however, also generated perthionitrite (SSNO^-) or its selenium analogue and NO, which, in turn, produced [Fe(E₅)(NO)₂]^1- (E = S, Se). Critical analysis of the experimental results, which includes X-ray structural, EPR and IR spectroscopic, and mass spectrometric investigations, including ¹⁵N isotope labeling studies, and comparison with literature reports indicated that the direct reduction of NO₂^- to NO by the Fe(II) centers in <b>3</b> and <b>4</b> was the major reaction pathway while the nucleophilic attack of NO₂^- on the "neutral" chalcogen atoms leading to the generation of [Fe(E₅)(NO)₂]^1- (E = S, Se) was only a minor pathway.