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

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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.

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