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Abstract
Development of cheap and efficient electrocatalyst is essential for realizing hydrogen gas as a clean alternative fuel source. Nature employs earth-abundant metals such as Ni and Fe in hydrogenases enzymes to reversibly catalyze proton reduction to H2. Among the three classes of hydrogenases ([NiFe], [FeFe], and [Fe]-only), [NiFe]-hydrogenase is the most well-studied due to its ability to repair itself upon oxidative damage. Its active site features a pseudo-tetrahedral, see-saw like, Ni(CysS)4 unit bridged to a low-spin Fe(II) coordinated by CO and CN- ligands, forming a butterfly-like structure. Experimental and computational studies suggest Ni serves as the redox-active center mediating formation of the metal-hydride intermediate. Synthetic analogs of the Ni site have provided valuable insights but fail to capture the unique thiolate-rich tetrahedral Ni(II) center found in the native enzyme. To address this, we developed synthetic analogs utilizing aryl-thiolates to gain a deeper insight into the structure-to-function relationships that underpin hydrogenases activity using [Ni(S-p-CF3-Ph)4]2- as a model system. These complexes form putative [Ni(SR)3-solv]- intermediates that result in the formation of higher ordered complexes under protic conditions. However, this behavior is suppressed by addition of excess thiolate allowing the electrocatalytic activity to be assessed and its mechanism to be determined. To improve protic stability and catalytic activity of these systems, we incorporated ortho-carboxamide moieties to engage the coordinated thiolates in intramolecular hydrogen-bonding. Although we found improved protic stability, under electrocatalytic conditions the complex decomposes into active deposits on the electrode surface. Interestingly, this modification destabilizes the tetrahedral geometry resulting in a mixture of tetrahedral and square-planar homoleptic Ni(II) tetrathiolates in their solution-state. To increase the stability of these complexes under electrocatalytic conditions, we utilized a pentacoordinate PyPS-p-CF3 ligand scaffold for a non-heme Fe-tetrathiolate complex which displays instability under protic conditions in MeCN resulting in active heterogenous deposits. However, in DMF deposition is mitigated, affording the activity of homogenous material to be assessed. Experimental and computational work suggests the important role the ligand platforms serve to catalyze proton reduction. This work provides a foundation for developing thiolate-rich Ni and Fe synthetic mimics that more accurately reflect the structural features of native enzyme active