Role of the Azadithiolate Cofactor in Models for [FeFe]-Hydrogenase: Novel Structures and Catalytic Implications

This paper summarizes studies on the redox behavior of synthetic models for the [FeFe]-hydrogenases, consisting of diiron dithiolato carbonyl complexes bearing the amine cofactor and its <i>N</i>-benzyl derivative. Of specific interest are the causes of the low reactivity of oxidized models toward H<sub>2</sub>, which contrasts with the high activity of these enzymes for H<sub>2</sub> oxidation. The redox and acid−base properties of the model complexes [Fe<sub>2</sub>[(SCH<sub>2</sub>)<sub>2</sub>NR](CO)<sub>3</sub>(dppv)(PMe<sub>3</sub>)]<sup>+</sup> ([<b>2</b>]<sup>+</sup> for R = H and [<b>2′</b>]<sup>+</sup> for R = CH<sub>2</sub>C<sub>6</sub>H<sub>5</sub>, dppv = <i>cis</i>-1,2-bis(diphenylphosphino)ethylene)) indicate that addition of H<sub>2</sub> followed by deprotonation are (i) endothermic for the mixed valence (Fe<sup>II</sup>Fe<sup>I</sup>) state and (ii) exothermic for the diferrous (Fe<sup>II</sup>Fe<sup>II</sup>) state. The diferrous state is shown to be unstable with respect to coordination of the amine to Fe, a derivative of which was characterized crystallographically. The redox and acid−base properties for the mixed valence models differ strongly for those containing the amine cofactor versus those derived from propanedithiolate. Protonation of [<b>2</b>′]<sup>+</sup> induces disproportionation to a 1:1 mixture of the ammonium [H<b>2</b>′]<sup>+</sup> (Fe<sup>I</sup>Fe<sup>I</sup>) and the dication [<b>2′</b>]<sup>2+</sup> (Fe<sup>II</sup>Fe<sup>II</sup>). This effect is consistent with substantial enhancement of the basicity of the amine in the Fe<sup>I</sup>Fe<sup>I</sup> state vs the Fe<sup>II</sup>Fe<sup>I</sup> state. The Fe<sup>I</sup>Fe<sup>I</sup> ammonium compounds are rapid and efficient H-atom donors toward the nitroxyl compound TEMPO. The atom transfer is proposed to proceed via the hydride. Collectively, the results suggest that proton-coupled electron-transfer pathways should be considered for H<sub>2</sub> activation by the [FeFe]-hydrogenases.