Hydrogen Evolution by Homoleptic Ni-Tetrathiolato Electrocatalysts

In combating the global warming crisis, there is a particular interest in mitigating fossil fuel use as it remains a significant source of anthropogenic CO₂ emissions. One solution for reducing fossil fuel emissions is the development of alternative fuel sources that are clean and renewable such as hydrogen gas (H₂). Natural systems have evolved efficient machinery utilizing cheap, earth-abundant metals that can provide sources of alternative fuel. One enzyme of interest contains a redox-active cofactor that relies on nickel and iron ions, namely the NiFe-hydrogenases ([NiFe]-H2ases) that catalyze the reversible oxidation of H₂.¹ Although both metals play a role in H₂ activation, only Ni participates in the redox process to interconvert H⁺ and H₂. In turn, construction of low molecular weight structural and functional analogs of the Ni site is of interest to better understand enzyme mechanism and develop simpler systems capable of performing the hydrogen evolution reaction (HER). In this work, we synthesized homoleptic Ni-tetrathiolato electrocatalysts that are capable of performing HER evidenced by a catalytic current response in cyclic voltammograms upon H⁺ titration. Isolation of the Ni-tetrathiolato complexes require rigorous aprotic and anerobic conditions in order to control speciation towards nucleation to higher order {Ni_n(SR)_3n} polymeric species, as indicated by NMR studies. We show that modification of the second coordination sphere of these Ni-tetrathiolato complexes with H-bond donors impedes degradation and impedes formation of higher ordered oligomeric species in protic environments. The stability of a simple Ni-tetrathiolato complex under electrocatalytic conditions is only achieved under excess thiolate conditions limiting the HER activity. However, the robustness imparted by the H-bond donors ultimately improves the HER electrocatalytic activity of these Ni tetrathiolato complexes.

[1] Lubitz, W., Ogata, H., Rudiger, O., and Reijerse, E. Chem. Rev. 2014, 11, 4081.