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 N-benzyl derivative. Of specific interest are the causes of the low reactivity of oxidized models toward H₂, which contrasts with the high activity of these enzymes for H₂ oxidation. The redox and acid−base properties of the model complexes [Fe₂[(SCH₂)₂NR](CO)₃(dppv)(PMe₃)]⁺ ([2]⁺ for R = H and [2′]⁺ for R = CH₂C₆H₅, dppv = cis-1,2-bis(diphenylphosphino)ethylene)) indicate that addition of H₂ followed by deprotonation are (i) endothermic for the mixed valence (Fe^IIFe^I) state and (ii) exothermic for the diferrous (Fe^IIFe^II) 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 [2′]⁺ induces disproportionation to a 1:1 mixture of the ammonium [H2′]⁺ (Fe^IFe^I) and the dication [2′]²⁺ (Fe^IIFe^II). This effect is consistent with substantial enhancement of the basicity of the amine in the Fe^IFe^I state vs the Fe^IIFe^I state. The Fe^IFe^I 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 H₂ activation by the [FeFe]-hydrogenases.