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.