Protonation, electrochemical properties and molecular structures of halogen-functionalized diiron azadithiolate complexes related to the active site of iron-only hydrogenases

Abstract

Diiron complexes [{(µ-SCH₂)₂NCH₂C₆H₄X}{Fe(CO)₂L}₂] (L = CO, X = 2-Br, 1; 2-F, 2; 3-Br, 3; L = PMe₃, X = 2-Br, 4) were prepared as biomimetic models of the iron-only hydrogenase active site. The N-protonated species [1(NH)]⁺ClO₄⁻, [2(NH)]⁺ClO₄⁻ and the µ-hydride diiron complex [4(FeHFe)]⁺PF₆⁻ were obtained in the presence of proton acids and well characterized. The protonation process of 4 was studied by in-situ IR and NMR spectroscopy, which suggests the formation of the diprotonated species [4(NH)(FeHFe)]²⁺ in the presence of an excess of proton acid. The molecular structures of 1, [1(NH)]⁺ClO₄⁻, 4 and [4(FeHFe)]⁺PF₆⁻ were determined by X-ray crystallography. The single-crystal X-ray analysis reveals that an intramolecular H⋯Br contact (2.82 Å) in the crystalline state of [1(NH)]⁺ClO₄⁻. In the presence of 1–6 equiv of the stronger acid HOTf, complex 1 is readily protonated on the bridged-N atom and can electrochemically catalyze the proton reduction at a relatively mild potential (ca. −1.0 V). Complex 4 is also electrocatalytic active at −1.4 V in the presence of HOTf with formation of the µ-hydride diiron species.