A Low-Potential π‑Extended Viologen Electron Donor Results in Increased H₂ Production by CpI [FeFe]-Hydrogenase

[FeFe]-hydrogenases are known for their exceptional dihydrogen production activity, which opens a pathway to green hydrogen formation. To achieve the high turnover numbers found in natural environments, researchers aim to mimic biological electron donors using artificial electron delivery systems. In this sense, the use of an externally applied electrochemical potential is of particular interest, for which redox mediators often have to be included to effectively shuttle electrons between the electrode and the enzyme. The class of viologens, with methylviologen being the most prominent example, has been shown to perform efficient electron transfer. However, the reduced forms of such viologens are restricted in terms of stability and accessibility of the more reduced, low-potential species. Herein, we report the electrochemical and spectroscopic characterization of a viologen-based derivative, which is capable of storing two electrons at a low potential of -0.75 V vs. SHE. In combination with the [FeFe]-hydrogenase CpI, an activity of up to 10,480 μmol_H2 mg _CpI ^-1 min^-1 (molar activity of 11,179 μmol_H2 μmol _CpI ^-1 s^-1) could be achieved, which is 10 times greater than the commonly used dithionite-driven methylviologen assay under the same conditions.