Synthesis and electrochemical properties of a nickel(II) thiacalix[4]arene-based electrocatalyst for the hydrogen evolution reaction

Abstract

Thiacalix[4]arene with four sulfur-bridging atoms and four hydroxy coordinating groups in the lower rim is a promising candidate for powerful electrocatalytic hydrogen production. However, there are only a few examples reported in the literature where conjugated thiacalix[4]arene is used as a pre-catalyst for the hydrogen evolution reaction. The present work evaluates the electrocatalytic performance of a nickel(II) coordination complex based on generic thiacalix[4]arene. The coordination complex is characterized by cyclic voltammetry, where two reduction peaks are observed at −0.57 and −1.33 V for the two redox couples Ni²⁺/Ni⁺ and Ni⁺/Ni, respectively. The proton reduction occurs at the second peak potential with a slight shift at about −1.0 V, with increasing peak currents directly related to the number of acetic acid equivalents. The maximum peak current was observed to be at about 67 µA for 20 equiv. of acetic acid.

Introduction

The discovery of eco-friendly and sustainable energy sources is an important target for researchers because of the depleting levels of fossil fuels and environmental concerns. ¹ Electrochemically generated hydrogen is considered as a promising alternative energy source. However, it is necessary to use a catalyst for its production, either by proton reduction or water splitting. In attempting to improve the performance of catalysts for the hydrogen evolution reaction (HER) using low-cost transition metals, many researchers have developed nanostructured compounds.^2–8 In addition, transition metal-based catalysts with macrocyclic organic frameworks such as porphyrin have become the subject of intensive studies by many scientists in recent years. ⁹ Among the many macrocyclic ligands, calix[n]arenes have emerged as a unique core because of their tunable bowl-shaped supramolecular cavity and easy variation/derivatization.^10–12 They constitute chemically and thermally stable circular platforms that can accept up to four coordinating units containing heteroatoms such as phosphorus, sulfur, nitrogen, and oxygen. Besides, these macrocycles can be easily obtained by condensation between para-tert-butyl phenol and formaldehyde. Depending on the base, temperature, and reaction time, they can be prepared with different sizes ranging from 4 to 20 phenol groups connected through methylene bridges.^13–15 Calix[4]arenes 1 (C4A) and their sulfur-containing analogues, thiacalix[4]arenes 2 (TC4A) possess wide (upper) and narrow (lower) rims that can be judiciously functionalized to achieve an extensive range of physical and chemical properties (Figure 1). ¹⁶

Figure 1.

The chemical structures of calix[4]arene (1) (C4A) and thiacalix[4]arene (2) (TC4A).

Recently, the electrocatalytic activities of calix[n]arenes have started to attract more attention in HERs, glucose oxidation reactions (GORs), hydrogen oxidation reactions (HORs), oxygen evolution reactions (OERs), oxygen reduction reactions (ORRs), and carbon dioxide reduction.^17–26 Interestingly, TC4As require further studies, and to the best of our knowledge, only a very few papers report their use in photocatalytic and electrocatalytic energy production.^19–23 Wang et al. ¹⁹ reported an ultrafine Pt nanocluster confined in a calixarene-based {Ni₂₄} coordination cage, which catalyzes the electrochemical production of hydrogen at low overpotential with a high current density. In addition, glassy carbon electrodes modified by calixarene-functionalized graphene oxide composites for HER and OER have been reported by Zhang et al. ²⁴ Genorio et al. ²⁵ have demonstrated good selectivity toward hydrogen oxidation and oxygen reduction by patterning of platinum with C4A molecules. In all the above-mentioned examples, researchers have used noble metals as electrocatalysts, even if they are confined within the calixarene at very low concentrations.

Very recently, Liao and co-workers ²⁰ reported the electrocatalytic oxidation of glucose using a thiacalixarene-based {Ni₁₈} coordination wheel with onset and peak potentials of 0.3 and 0.46 V, respectively, in an alkaline medium. Besides, the encapsulation of a metallic cluster such as AuPd inside the cavity led to a higher current density than the commercially available Pt/C electrode toward the HER. Recently, the same authors reported four nickel-thiacalixarene complexes, an {Ni₈} cluster, an {Ni₂₀} coordination wheel, an {Ni₂₄}coordination bowl, and a twisted double-bowl-shaped {Ni₃₂}, in the presence of 2,5-dihydro-1,3,4-thiadiazole-2,5-dithiol. Efficient electrocatalytic activity toward glucose oxidation was apparent for the {Ni₂₀} coordination wheel, which was attributed to its accessible active nickel sites and a through-hull channel at an appropriate size. ²¹ Earlier, we demonstrated the electrocatalytic activity of a polyoxothiomolybdate derivative toward hydrogen evolution while the oxygen-containing equivalent, a polyoxometalate derivative, could not promote the electrocatalytic reaction. ²⁷

Sulfur-embedded ligands and the corresponding complexes show better electrocatalytic performances than the oxygen analogues. We herein present the electrocatalytic performance of a nickel(II) TC4A complex toward hydrogen evolution on a glassy carbon electrode.

Results and discussion

Compared to C4A, the S-containing counterpart (TC4A) possesses delocalized electrons, making it a powerful candidate for electrochemical applications, including HER. The TC4A molecule has hydrophobic cavities formed by benzene rings and tert-butyl groups, capable of hosting organic molecules. In addition, the TC4A molecule contains free electron pairs on sulfur and the oxygen atoms of hydroxy groups, which can form chelates with metal ions through coordinate–covalent bonds. Metal ions, for example, nickel, in complexes with the thiacalix[4]arene are necessary for electrocatalytic reactions. As shown in Figure 2, the prepared nickel complex, obtained from mixing TC4A and [Ni(DMSO)₆](ClO₄)₂ in DMF in the presence of triethylamine (see section “Experimental section”), contains two parts interconnected by an Ni^part1–S^part2 bond. ²⁸ The first moiety is formed from two TC4A units connected through four nickel ions using three sulfur and four oxygen donor atoms. The second part is composed of two TC4A units, where the first is coordinated to two nickel ions through its four oxygens and two sulfur atoms. The nickel ions are then coordinated to three oxygens and two sulfurs of the second TC4A units. The presence of the six nickel ions close to the lower rims of the TC4As and the small sizes of both H⁺ and H₂ gas indicated that the nickel ions catalyze the proton reduction reaction in a confined space close to the hydrophobic cavity.

Figure 2.

Structure of 3 (hydrogen and nitrogen atoms are not shown). The complete structure is presented at the top (the Ni^part1–S^part2 bond is highlighted in cyan). The tetranuclear and the binuclear moieties are shown at the bottom after removing the Ni–S bond interconnecting them (CCDC-662598).

The complex was characterized by cyclic voltammetry in DMF solution containing 0.2 M tetrabutylammonium tetrafluoroborate ([NBu₄][BF₄]) and complex 3 (2.5 mM) (see Figure 2). At negative potential, the Ni(II) complex exhibits two reduction peaks with E_p^red = −0.57 and −1.33 V for Ni²⁺/Ni⁺ and Ni⁺/Ni, respectively. The corresponding oxidation peaks are retarded because of the resistivity; they appear at −1.25 and −0.38 V for Ni⁺/Ni and Ni²⁺/Ni⁺, respectively (Figure 3). The cyclic voltammograms at 20–100 mV scan rates were also recorded (Figure 4). An increase in the scan rate significantly enhances the height of the redox peaks at the same temperature. The capacitive currents that follow a linear dependence on the scan rate are investigated in the work of Dylla et al. ²⁹ as

Figure 3.

Cyclic voltammetry of the Ni(II) complex (2.5 mM) in DMF solution containing [NBu₄][BF₄] (0.2 M) at the carbon electrode versus Ag/AgCl at a scan rate of 100 mV.

Figure 4.

Cyclic voltammetry of Ni(II) complex 3 (2.5 mM) in DMF solution containing [NBu₄][BF₄] (0.2 M) at the carbon electrode versus Ag/AgCl at different scan rates, and the root square of the peak potential versus the peak current.

$$I_{\mathrm{cap}}=A·C_f·v$$

where A is the surface area exposed to the electrolyte in cm², C_f is the capacitance in Farads, including both double-layer capacitance, and v is the scan rate. The anodic peak currents for the electrode in the investigated electrolyte are shown in Figure 4 as a function of the square root of the scan rate. Linear dependence was obtained, which indicates a non-absorption process at the electrode.

Complex 3 was evaluated in electrocatalysis to transform a proton into hydrogen by reduction. It is worth noting that the direct reduction of a CH₃COOH proton at the carbon electrode in DMF containing [NBu₄][BF₄] (0.2 M) occurs at −1.85 V under catalyst-free conditions (Figure 5). Interestingly, the presence of complex 3 shifts the reduction potential to about 850 mV. The proton reduction occurs at the second reduction wave with a potential close to −1.25 V in the presence of 2.5 equiv. of CH₃COOH (Figure 6(a)). Note that, the peak potential shifts positively on increasing the number of acid equivalents until it reaches saturation at 20 equiv. at a potential of −1.0 V (Figure 6(b)). Similarly, the anodic peak current increases with the number of acetic acid equivalents until a maximum of 67 µA (Figure 6(c)).

Figure 5.

Cyclic voltammetry obtained from the catalyst-free solution of DMF containing CH₃COOH (20 mM) and [Bu₄N][BF₄] (0.2 M), scan rate 100 mVs⁻¹ at a glassy carbon electrode under N₂.

Figure 6.

(a) Cyclic voltammograms obtained from using solutions of DMF containing Ni(II) complex 3 (2.5 mM), different concentrations of CH₃COOH and [Bu₄N][BF₄] (0.2 M), scan rate 100 mVs⁻¹ at a glassy carbon electrode under N₂. (b) The peak potential shift versus the number of acetic acid equivalents. (c) The number of acetic acid equivalents versus the peak current.

In the past, researchers have also investigated the electrocatalytic potential of Ni(II)-based coordination complexes for proton reduction into hydrogen. For example, Ni(II) coordination complexes 5 and 6 (Figure 7) based on hangman porphyrin were evaluated for HER using benzoic acid as the proton source at a glassy carbon electrode. In the presence of the Ni(II) porphyrin complex 5, bearing a carboxylic acid group, reduction of the proton in acetonitrile took place at −1.76 V versus Fc⁺/Fc. For compound 6 bearing a bromo group, the reduction potential shifted slightly to a less positive value as compared to 5. ³⁰ Using acetic acid as the proton source, Han and co-workers ³¹ studied the electrocatalytic performance of the Ni(II) porphyrin 4 bearing four pentafluorophenyls at meso positions. The potential for the reduction of a proton was observed at −1.72 V, being close to the second reduction peak at −1.85 V. Even though the potential for the reduction of a proton is measured versus ferrocene, it is quite clear that our complex 3 gave better results, as the peak potential for reduction of the proton occurred at −1.0 V. Overall, these results attest to the promising electrocatalytic performance of thiacalixarene ligands.

Figure 7.

The chemical structures of different porphyrin derivatives used for HER in the previous studies.

Conclusion

A nickel complex based on the TC4A ligand has been synthesized. Its electrochemical activity for proton reduction into hydrogen has been evaluated. Cyclic voltammetry studies at negative potentials show two reduction peaks for Ni²⁺/Ni⁺ and Ni⁺/Ni at E_p^red = −0.57 and −1.33 V. The presence of a catalyst results in a potential shift of about 850 mV to a more positive potential for the second peak potential. In addition, the peak current increases based on the number of acetic acid equivalents until a maximum of 67 µA for 20 equiv. of acid. The studied nickel(II) complex gave better electrocatalytic results compared to some well-known nickel(II)-based porphyrin derivatives. The presence of cavitands in the complex structure with a hydrophobic interior suggests a catalytic process in a confined space where the hydrogen is formed on nickel sites close to the lower rim of the TC4A ligand.

Experimental section

Materials

All starting materials, including dry solvents, were purchased from Afaq Sada Trading Est and used as received. Chemicals including 4-tert-butylphenol, sulfur, tetraethylene glycol dimethyl ether, sodium hydroxide, hydrochloric acid, 37% triethylamine, and solvents were obtained from Merck or Acros Organics. The TC4A ligand was synthesized according to a protocol described in the literature. ³² All analytical data were consistent with the literature data and confirmed the structure of the synthesized complex.

Synthesis

Ni₆(TC4A)(TC4A-H)₂(TC4A-H₂)·4CH₃CN·6H₂O (3) was prepared according to a reported procedure. ²⁸ To a solution of TC4A (1.40 g, 1.67 mmol in 40 mL DMF) containing triethylamine (2 mL) was added a solution of [Ni(DMSO)₆](ClO₄)₂ (3.00 g, 5.44 mmol in 20 mL DMF). The resulting mixture is refluxed for 1 h. The solution was then diluted with CH₂Cl₂ (ca. 500 mL) and washed thoroughly with water. The organic solvent was dried over MgSO₄ and concentrated under vacuum to about 50 mL. At room temperature, an equal volume of ethanol was added to the CH₂Cl₂ solution resulting in the formation of a precipitate containing crystalline and fine amorphous solids. The latter was removed by slow shaking and decantation. During the preparation of the complex, the unit cell of one single crystal was determined by single-crystal X-ray diffraction (XRD), which confirmed the consistency of the parameters with those in the literature. ²⁸ The collected crystalline solid was dried and analyzed by elemental analysis. Anal. calcd for C₁₆₀H₁₈₀Ni₆O₁₆S₁₆ (3224.5): C, 59.60; H, 5.63; S, 15.9; found: C, 59.72; H, 5.70; S, 16.00.

Electrochemical measurements

Electrochemical measurements were performed at 25 °C using a standard three-compartment cell with a working glassy carbon electrode (surface = 0.07 cm²), a reference Ag/AgCl electrode and platinum wire as the counter electrode. Before use, the working electrode was polished using emery papers of different grades, then washed with the used solvent and dried. A potentiostat (Autolab PGSTAT 128N) and NOVA 1.10 software were used for recording and processing the electrochemical data.