# Synthesis, Properties, and Electrochemical Proton Reduction of a Homoleptic Tetrathiolato Ni-Site Model of [NiFe]-Hydrogenase

## TL;DR

This paper reports the synthesis and electrochemical evaluation of a nickel-based model compound for [NiFe]-hydrogenase, showing modest hydrogen production activity.

## Contribution

A tetrathiolato Ni2+ complex is shown to act as a modest electrocatalyst for the hydrogen evolution reaction.

## Key findings

- The compound (Et4N)2[Ni(S-p-CF3–Ph)4] exhibits a distorted tetrahedral geometry confirmed by X-ray crystallography.
- The complex shows electrochemical HER activity with a turnover frequency of 14.5 ± 3.6 s–1 and an overpotential of 0.72 ± 0.02 V.
- An ECCE-type mechanism is proposed for the HER based on experiments and DFT computations.

## Abstract

[NiFe]-hydrogenase
enzymes process H2 at a
nonplanar
tetracysteinato-Ni site, the sole participator in proton binding/redox
chemistry during turnover. With the objective of assessing whether
a simple tetrahedral/tetrathiolato-Ni2+ core could promote
H2 evolution reaction (HER), we synthesized (Et4N)2[Ni­(S-p-CF3–Ph)4] (1) employing para-trifluoromethylbenzenethiolate
(−S-p-CF3–Ph)
as a Ni-site analog of [NiFe]-hydrogenase. Spectroscopic measurements
and X-ray crystallography confirm the distorted tetrahedral geometry
of 1. Dissolution of 1 results in partial
thiolate dissociation and formation of S,S-bridged complexes such as (Et4N)2[Ni2(S-p-CF3–Ph)6] (3) among other ill-defined species. Dissociation
is further accelerated in the presence of Brønsted acids, complicating
the assessment of 1 for proton reduction. However, this
dissociation/proton instability is suppressed in the presence of additional
thiolate ligand to ensure tetrahedral/tetrathiolato 1 persists in solution. Electrochemical HER activity was evaluated
by monitoring the current response of an MeCN solution of 1/excess thiolate after sequential titration with a weak Brønsted
acid (acetic acid). The results suggest that 1 is a modest
electrocatalyst for the HER with a turnover frequency of 14.5 ±
3.6 s–1 and an overpotential of 0.72 ± 0.02
V. Control experiments and supplementary DFT computations indicate
that 1, or a species derived from 1, is
responsible for the HER and suggest an ECCE-type mechanism.