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-) (2(2-)), [Fe5C(CO)15](2-) (3(2-)), and [Fe6C(CO)18](2-) (4(2-)) 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 2(2-), 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 2(2-) undergoes two successive proton coupled electron transfer events to form H22(2-) which then liberates hydrogen. The higher nuclearity clusters, 3(2-) and 4(2-), do not display the same ability to associate with protons, and accordingly, they produce hydrogen less efficiently.