The activation of small molecules play an important role to towards more sustainable chemical processes. Renewable solar and wind resources can provide the energetic driving for such reactions, leading to electrocatalytic transformations. Here, I will present our research on the cathodic, reductive activation of O₂, N₂ and CO₂ using trimetal substituted polyoxometalates as active site functional mimics of redox metalloenymes. Second, I will present the use of metal guest-Keplerate host supramolecules as inorganic analogs of redox metalloenyme assemblies.

Cathodic activation of O₂: Paradoxically, nature’s monooxygenase enzymes activate O₂ typically via a two-electron reductive pathway. Both Fe and Cu-based catalysis using reducing agents under protic conditions is known, but surprisingly, cathodic electrocatalysis using H₂O as a proton and electron source is almost unreported. Recently, we found that iron Keplerates, {Fe₃₀W₇₂} can be used as electrocatalysts for the oxidation of light alkanes and alkenes in water.[1] Mechanistic studies have revealed that reaction intermediates have reactivity profiles similar to those observed for Compound I of cytochrome P-450.[2] More recently, we have also found that tetra-Cu Weakley polyoxometalates also are also very efficient electrocatalysts for the cathodic activation of O₂ and show reactivity profiles similar to those of the iron Keplerates.

Reduction of CO₂ to CO: The removal of CO₂ from the atmosphere through is capture or sequestration is a feasible technology, however, is not sustainable due to the high cost of the process and the low value of captured CO₂requiring the transformation of CO₂ to a higher valued products. Therefore, we have prepared a series of trimetallo substituted polyoxometalates that on the one hand can catalyze the reversible reduction of CO₂ and oxidation of CO,and on the other hand can be tuned to reduce CO₂ with very low overpotentials.[3]

Reduction of N₂ to NH₃: The electrification of ammonia synthesis is a key target for its decentralization and toward lowering the impact of chemical processes on atmospheric carbon dioxide concentrations. Using catalyst a tri-iron substituted polyoxotungstate, {SiFe₃W₉} in the presence of either Li⁺ or Na⁺ cations as promotors through their binding to {SiFe₃W₉} we show that in an undivided cell electrolyzer, rates of NH₃ formation was at up to 1.15 nmol sec^–1 cm^–2 with moderate faradaic efficiencies of ~25%. Based on an assumption of arbitrary 10% catalyst coverage on a Cu foil cathode, a TOF of 64 sec^–1 was calculated. 

Iron-nickel guest-{Mo₆₀W₇₂} host supramolecules as an inorganic functional mimic of a hydrogenase enzyme: Can soluble inorganic metal oxides and the related guest-host complexes with encapsulated transition metals incorporated through assembly reactions, act as functional analogues of redox metalloenzymes that carry out multielectron transformations of small molecules? Here we show, that Fe-Ni assemblies bound to mercaptopropionate lignads within {Mo₆₀W₇₂} acts as a hydrogenase enzyme complex electro- and photoelectrochemically (PEC) forming hydrogen from protons and electrons. Reactions rate comparable to those found in the wild type enzyme are observed with very high faradaic efficiency under PEC conditions.