Metalloenzyme, such as hydrogenases (H₂ases), acetyl coenzyme A synthase (ACS), nitrogenases, speed up a variety of reactions that are attractive to theoretical study and industrial application [1]. The mimic of the active sites in various metalloenzyme is an attractive topic in the field of science (Scheme 1). The chemists can be inspired by the natural metabolic processes, and the biologists can learn from the research of the same reactions catalyzed by synthetic model complexes.

In the reported works about mimicking the active site of [FeFe]-H₂ases, most of model complexes were synthesized by introducing functional ligands into the classical diiron dithiolate complexes[(μ-SR)₂Fe₂(CO)₆] [2]. However, the mimic of [NiFe]-H₂ases is more challenging due to the different coordination environments for the two metal centers [3]. Firstly, the structural analogues of these two distinct metal units were synthesized, respectively. Then, the two model complexes were bridged through suitable linkages. Due to the close resemblance with Ni(Scystein)₄ site of [NiFe]-H₂ases, a various of NiN₂S₂ and NiS₄ metallothiolate complexes were choose as S-donor ligands to bind with the other mononuclear organometallic complexes, such as M(CO)_n (M = Mn, Ru), Cp^*M (M = Fe, Co, Ru, Rh, Ir), (C₆Me₆)Ru, Fe[P(OR)₃]₃ [4]. The synthesized model complexes not only possess the structure characters of active sites in [NiFe]-H₂ases, but also reappear their fascinating catalytic abilities [5].

In this work, we reported the synthesis of an air-stable dinickel complex [Ni(phma)(μ-S, S')Ni(dppp)] (1) bearing a NiN₂S₂ metallothiolate ligand. The thiolate group link two nickel atoms as a bridge, and this structure mimic the principal feature of the active site of [NiFe]-H₂ases. In addition, the redox property and particularly electrocatalytic H₂-evoluting activity of 1 were investigated.

Reaction of [NEt₄]₂[Ni(phma)] with one equivalent of [NiCl₂(dppp)] in acetonitrile at room temperature gave the desired dinuclear complex [Ni(phma)(μ-S, S')Ni(dppp)] (1) in good yield. The crystal structure of 1 (Fig. 1) shows that both nickel centers display a slightly distorted square planar geometry. The two coordination planes are folded along the μS-μS vector. The dihedral angle defined by the two Ni(μ-S)₂ planes is 103.57°. Ni(2) is ligated by the two phosphorus atoms from the dppp ligand and the two sulfur atoms of the phma ligand. And Ni(2) lies 0.012 Å from the P₂S₂ plane. The Ni(2)PC₃P cyclohexane ring is in the chair conformation. Ni(1) ligated by the two nitrogen and two sulfur atoms of the phma ligand. The NiN₂S₂ unit is essentially planar with a 5-5-5 ring pattern, while Ni(1) lies 0.134 Å from the plane. The Ni(1)–S (2.13 Å) are shorter Ni(2)–S (2.28 Å). The terminal N–Ni–N angle is constrained in the present structure to the square 86.65(13)°. The Ni…Ni distance is 2.6793(7) Å. It has been reported that such a distance is indicative of a Ni–Ni interaction [6].

The reduction of 1 was firstly investigated with cyclic voltammetry in DMF. The cyclic voltammogram (CV) exhibits one redox couple at -0.95 V vs. ferrocene (Fc) followed by an irreversible peak at -1.93 V (Fig. 2a). When the cathodic scan is limited to -1.35 V to avoid the second reduction event, the reversibility of thefirst couple was obviously enhanced [7]. The peak-to-peak separation (ΔE_p) of the first redox couple is 72 mV (i_pa/i_pc ≈ 1) at a scan rate of 100 mV/s, indicating a reversible one-electron transfer process [8]. This redox process was ascribed tothe one-electron reduction of Ni^II/Ni^I couple in NiP₂S₂ site [9]. Whatever the scan rate used in the 25–2500 mV/s range, this process indeed appears always chemically reversible (Fig. 2b and Fig. S2 in Supporting information). The variable-scan rate studies prove that this Ni^II/Ni^I reduction event is a diffusioncontrolled process. The irreversible process at -1.93 V cannot be explicitly assigned to the reduction of Ni^I/Ni⁰ at NiP₂S₂ moiety or Ni^II/Ni^I at NiN₂S₂ moiety. Hegg reported that the redox of Ni^II/Ni^I couple for Ni[N, N'-ethylenebis(2-methylmercaptoacetamide)] located at -1.95 V as a quasireversible wave [10]. More likely, the irreversible peak would be ascribed to the reduction of Ni^II/Ni^I at NiN₂S₂ moiety.

The electrocatalytic performance of 1 for the H₂ evolution was evaluated in DMF using ClCH₂COOH (pK_a^DMF = 10.0; E°_HA = -1.36 V vs. Fc) as proton source [11]. To emphasize an important point, complex 1 is quite stable in the presence of excess ClCH₂COOH, as monitored by UV–vis spectrometry (Fig. S9 in Supporting information). Upon the addition of 0.3 equiv. acid, a new reduction event appears at about -1.86 V (Fig. 2c), which is ascribed to a prewave of total catalysis (zone KT2, pure kinetic conditions and total catalysis) [12]. In this case, complex 1 consumes all acids local to the electrode surface resulting in a CV with two peaks. An irreversible catalytic peak is initially observed at positive potential of 1^-/1^2-. As the potential is scanned more negative, the reduction of 1^-/1^2- redox is still seen in the same position with no acid present. Subsequently, the addition of 0.6 equiv. ClCH₂COOH leads an overlap of these two peaks indicating the transition from KT2 into K zone [12]. Between 0.7 equiv. and 6 equiv. of ClCH₂COOH, the catalytic current (i_cat1) slowly grows with increasing of acid (Fig. S3 in Suppporting information). We propose this is an inefficient H₂ evolution reaction. The peak potential and peak current of total catalysis waveforms notonly depend on the substrate concentration, but also relate to the scan rate. Increase of scan rate could also push the CV response into zone K. In the present of 0.4 equiv. ClCH₂COOH, when the scan rate is above 900 mV/s, the prewave shifts to more negative potentials erasing the distinction of these two peaks (Fig. 2d).

When the potential is scanned more negative, a remarkable enhancement of catalytic current (i_cat2) are observed at about -2.25 V indicating another H₂-evoluting process (Fig. 3a). The standard rinse experiments excluded that the electrocatalysis is initiated by the electrodeposits of an ill-defined nanoparticulate material at the electrode surface (Fig. S7 in Suppporting information). The linear correlation between i_cat2 and the square root of acid concentration indicates that the observed rate constant for catalysis is first-order in acid (Fig. 3b). Furthermore, i_cat2 also shows varied linearly with the square root of scan rate (Fig. S6 in Suppporting information), implying that the electrocatalytic process is diffusion controlled. The overpotential (η) of 0.800.97 V for electrocatalytic proton reduction is given by calculating the difference between the catalytic half-wave potential (E_cat/2) and the standard potential for the formation of H₂ from ClCH₂COOH (E°_HA) [13]. On the basis of the electrochemical data, the mechanism of the electrocatalytic H₂ evolution is proposed to be an ECCE process starting from 1^- [14]. But with side phenomena, such as consumption of the substrate, deactivation of the catalyst, inhibition of the current by H₂ adsorption to the electrode, and background H₂ evolution, no matter how to increase the scan rate or acid concentration, the catalytic response currents of complex 1 does not show idealized S-type with fixed plateau current [15]. Therefore, we cannot get convincing kinetic information for this electrocatalytic H₂-evoluting process via the ratio of i_cat/i_p and the foot-of-the-wave method [16]. As emphasized by Dempsey et al., it should be careful to use the ratio of i_cat/i_p to calculate the rate constant (k_obs) of electrocatalytic H₂-evoluting reaction [13]. The applications of eqs. 2 and 5 are restricted to the S-shaped catalytic waves of zones KS and KD. Before the calculation, it also needs to ensure that an appropriate scan rate is being utilized. With the precondition Eq. 2, i_cat should be independence on the scan rate [17].

(1)

(2)

(3)

(4)

(5)

Controlled potential electrolysis (CPE) experiments were performed at -1.83 V and -2.43 V (vs. Fc), respectively, which provide further information for the electrocatalytic H₂ evolution with 1. After electrolysis over a 3 h period at -2.43 V, 7.44 μmol H₂ was detected by gas chromatography, thus giving a Faradaic efficiency of 72% and a turnover number (TON) of 1.2. UV–vis spectroscopy showed that 65% of 1 was still in the solution. (Fig. S10 in Supporting information) Electrolyzing the identical solution at -1.83 V resulted in the formation of 1.80 μmol H₂, giving a reduced TON of only 0.3.

In summary, the metallothiolate [Ni(phma)]^2- was used as cisdithiolate sulfur donor to react with [NiCl₂(dppp)], affording an airstable dinickel complex 1, which not only possess the principal feature of the active site of [NiFe]-H₂ases, but also reappear their fascinating catalytic abilities. When the redox property was investigated via CV and CPE experiments, 1 was verified to be a moderate electrocatalysts of H₂-evoluting reaction using ClCH₂COOH as the proton source.

Acknowledgments

This work was supported by National Natural Science Foundation of China (Nos. 21773184, 21671158 and 21601164), Natural Science Foundation of Henan (No.162300410052), Key Science and Technology Project of Henan (No. 172102310137).

Appendix A. Supplementary data

CCDC 1942436 and 1942438 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336, 033. Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.01.033.