High Turnover Frequency H2 Evolution Electrocatalysis at Low Acid Concentration Promoted by Bioinspired (NCS2)Ni(II) Complexes

Abstract

Electrochemical proton reduction to produce hydrogen is considered a sustainable approach to shift the fossil fuel-based energy production toward renewable energy sources. Although the development of molecular electrocatalysts for the hydrogen evolution reaction (HER) has gained significant attention, most of these molecular catalysts require either strong acids or often operate at high proton concentration to achieve high turnover. Herein, we report the synthesis and charcterization of two Ni<sup>II</sup> complexes, [(N2S2)Ni(MeCN)<sub>2</sub>](OTf)<sub>2</sub> (<b>1</b><b>•(OTf)<sub>2</sub></b>) and (NCHS2)Ni(OTf)<sub>2</sub> (<b>2</b>) bearing bioinspired 3,7-dithia-1,5(2,6)-dipyridinacyclooctaphane (N2S2) and 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane (NCHS2) ligands, respectively, along with their electrochemical HER in a non-aqueous electrolyte. Our Ni complexes show high turnover frequencies greater than 200,000 s<sup>–1</sup> in the presence of 0.043 M of trifluoroacetic acid with ≥1 M of water present. Under these electrochemical conditions, <b>2</b> exhibited 2.5-fold faster kinetics at 240 mV lower overpotential than that of <b>1</b><sup>2+</sup>. Furthermore, <b>2</b> initiates electrochemical proton reduction at the potential where Ni<sup>II/I</sup> redox couple occurs, whereas the similar HER electrocatalysis carried out by <b>1</b><sup>2+</sup><sub> </sub>was observed at the potential for the Ni<sup>I/0</sup> redox couple. The electrochemical analysis revealed that <b>2</b> undergoes an uncommon HER mechanism proposed to involve a Ni<sup>III</sup>–hydride species – a typical pathway followed by [NiFe] hydrogenase enzymes, upon activating the C–H bond of the coordinating NCHS2 ligand, and the resulting organometallic Ni complex is proposed to be the active HER electrocatalyst. This organometallic Ni complex derivative, [(NCS2)Ni(MeCN)<sub>2</sub>]<sup>2+</sup> (<b>5</b>) was synthesized independently and its performance for the HER supports the proposed HER mechanism for <b>2</b>. Additionally, electron paramagnetic resonance (EPR) spectroscopy was employed to probe the accessibility to Ni<sup>I</sup> and Ni<sup>III</sup> species proposed as intermediates in the described HER mechanisms. Importantly, comparative catalytic Tafel plots were constructed to benchmark the HER activity of <b>1</b><sup>2+</sup> and <b>2</b> versus previously reported known Ni-based HER electrocatalysts. Overall, the organometallic (NCS2)Ni system reported below represents a novel bioinspired molecular HER electrocatalyst that exhibits a high turnover frequency and more closely resembles the Ni<sup>I</sup>/Ni<sup>III</sup> HER mechanism proposed to pe operative in [NiFe] hydrogenases.