Electrocatalytic Hydrogen Evolution from Water by a Series of Iron Carbonyl Clusters

The development of efficient hydrogen evolving electrocatalysts that operate near neutral pH in aqueous solution remains of significant interest. A series of low-valent iron clusters have been investigated to provide insight into the structure–function relationships affecting their ability to promote formation of cluster-hydride intermediates and to promote electrocatalytic hydrogen evolution from water. Each of the metal carbonyl anions, [Fe4N­(CO)12]− (1–), [Fe4C­(CO)12]2– (22–), [Fe5C­(CO)15]2– (32–), and [Fe6C­(CO)18]2– (42–) were isolated as their sodium salt to provide the necessary solubility in water. At pH 5 and −1.25 V vs SCE the clusters afford hydrogen with Faradaic efficiencies ranging from 53–98%. pH dependent cyclic voltammetry measurements provide insight into catalytic intermediates. Both of the butterfly shaped clusters, 1– and 22–, stabilize protonated adducts and are effective catalysts. Initial reduction of butterfly shaped 1– is pH-independent and subsequently, successive protonation events afford H1–, and then hydrogen. In contrast, butterfly shaped 22– undergoes two successive proton coupled electron transfer events to form H222– which then liberates hydrogen. The higher nuclearity clusters, 32– and 42–, do not display the same ability to associate with protons, and accordingly, they produce hydrogen less efficiently.