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

[FeFe]-hydrogenases are known for their bias towards proton reduction and the formation of dihydrogen with exceptional reaction rates, which opens a pathway to green hydrogen formation. To achieve the high turnover numbers found in the natural environments, researchers have to mimic the biological electron donors by 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(s) and the enzyme(s). The class of viologens, with methylviologen (MV) as the most prominent candidate, has shown to perform efficient electron transfer. However, the reduced forms of MV are restricted in terms of stability and accessibility of higher reduced, low-potential species. Herein, we report the electrochemical and spectroscopic characterization of a modified 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 μmolH2 mgCpI −1 min−1 (TOF = 11’179 s−1) could be achieved, 10 times higher than the parent methylviologen system under the same conditions.