Synthesis, characterization and electrocatalytic activity of a novel binuclear polypyridine nickel complex

Abstract

The pursuit of highly efficient water oxidation catalysts has garnered significant attention due to the central role of the water oxidation reaction in overall water splitting. Herein, we report the synthesis, characterization, and electrocatalytic water oxidation performance of a novel binuclear nickel(II) complex, [Ni₂(BPMAB)(OH₂)₄](ClO₄)₄ (1, BPMAB = 6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine). Single-crystal X-ray diffraction analysis reveals that complex 1 adopts an asymmetric tetragonal bipyramidal geometry, coordinated by nitrogen atoms from the BPMAB ligand and oxygen atoms from water molecules. In a neutral phosphate buffer solution, complex 1 homogeneously catalyzes water oxidation to oxygen, achieving an onset overpotential of 630 mV and a turnover frequency (TOF) of 1.16 s⁻¹. Electrochemical studies suggest that the water oxidation proceeds via a synergistic binuclear mechanism, resulting in a high faradaic efficiency of 96%. To the best of our knowledge, this work presents the first example of a homogeneous Ni-based water oxidation catalyst operating through a binuclear catalytic pathway.

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Introduction

Given the environmental pollution and the greenhouse effect associated with the excessive use of fossil fuels, the large-scale production of clean energy sources, such as hydrogen, has garnered considerable attention in recent decades.^1–4 Overall water splitting, comprising the water oxidation reaction and the proton reduction reaction, is considered a promising pathway for hydrogen generation.^5,6 Water oxidation, however, involves a kinetically sluggish process that requires high activation energy and entails the transfer of four electrons and four protons, making it the bottleneck of overall water splitting.^7,8 Thus, the development of efficient water oxidation catalysts (WOCs) is essential. Inspired by the structure and function of the {Mn₄CaO₅} cluster in the oxygen-evolving center of natural photosystem II, considerable efforts have been devoted to designing polynuclear complexes that mimic the synergistic multinuclear catalytic mechanism of natural water oxidation.^9–12 Such complexes often exhibit remarkable oxygen evolution activity. To date, multinuclear WOCs based on metals such as Mn, Fe, Co, and Cu, particularly binuclear complexes featuring two metal centers bridged by hydroxyl groups or oxygen atoms, have been extensively studied.^13–20 Their biomimetic structures and stepwise oxidation processes, reminiscent of the {Mn₄CaO₅} cluster, make them attractive for chemical, photocatalytic, and electrochemical water oxidation.^21–24 Despite these advances, binuclear Ni complexes remain underexplored as molecular electrochemical WOCs.

In this work, we report for the first time the synthesis, characterization, and electrocatalytic water oxidation activity of a novel binuclear nickel(II) complex, [Ni₂(BPMAB)(OH₂)₄](ClO₄)₄ (1, BPMAB = 6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine, Fig. 1). The BPMAB ligand, featuring a 2,2′-bipyridine backbone, supports the asymmetric binuclear architecture of 1, which adopts a double tetragonal bipyramidal coordination geometry facilitated by coordinated water molecules. Although the two Ni centers in 1 are structurally independent, the flexible azaalkyl chains enable effective communication between them during catalysis, thereby creating a favorable framework for intramolecular bimetallic cooperative water oxidation. In a neutral phosphate buffer solution (PBS), 1 efficiently catalyzes water oxidation to oxygen with a moderate onset overpotential of 630 mV, a high turnover frequency (TOF) of 1.16 s⁻¹, and a faradaic efficiency of 96%. The combination of experimental techniques, including controlled potential electrolysis (CPE) tests, kinetic studies, and scanning electron microscopy (SEM), collectively confirm that 1 functions as a stable molecular WOC without forming heterogeneous active species. Electrochemical analysis further suggests that O–O bond formation is facilitated through intramolecular interaction between the two Ni centers, resulting in enhanced catalytic efficiency, low overpotential, and notable operational stability. This study not only presents the first example of a molecular WOC based on a binuclear Ni complex but also highlights the advantages of multinuclear architectures in catalytic water oxidation. Furthermore, it offers a new structural design paradigm for developing efficient molecular catalysts for energy conversion.

Fig. 1 Ball-and-stick model of the crystal structure of 1 with thermal ellipsoids set at a 30% probability level. H atoms on the ligand, crystal water molecules, and ClO₄⁻ anions are omitted for clarity.

Experimental section

Materials and characterization

All chemical reagents for ligand synthesis, complex preparation and electrochemical tests were purchased from Energy Chemicals and used without purification. Ultra-pure water (18.25 MΩ cm) used for complex synthesis and electrochemical tests was obtained from a molecular lab water purifier.

Equipment and apparatus

Single X-ray crystal diffraction data were collected at room temperature using a Bruker D8 Venture. Infrared spectra (1 wt% solid sample in KBr pellets) were recorded on a FT-IR spectrometer (Thermo Fisher Scientific Nicolet iS5). UV-vis absorption spectra were measured on a spectrometer equipped with a photomultiplier tube detector (Shimadzu UV-2550). Elemental analysis was conducted on a TJA ICP atomic emission spectrometer (IRIS Advantage ER/S). The microscopic morphology of the ITO electrode was observed via scanning electron microscopy (Carl Zeiss Sigma 300) coupled with energy dispersive X-ray spectroscopy (EDS). Gas measurements of O₂ were carried out on a gas chromatograph (Shimadzu GC-9A). O₂ in the sampled gas was separated by passing through a packed molecular sieve 5A column (2 m × 3 mm) using Ar as a carrier gas and quantified using a thermal conductivity detector.

Electrochemical measurements

All electrochemical measurements in this work were performed in a standard three-electrode system on a CHI660D electrochemical analyzer with glassy carbon (0.071 cm⁻²), Pt wire and Ag/AgCl (saturated KCl) as the working, auxiliary, and reference electrodes, respectively. All tested potentials mentioned in this work were converted to potentials versus the normal hydrogen electrode (NHE) using the equation E(NHE) = E(Ag/AgCl) + 0.197 V. Tin-doped indium oxide (ITO) electrodes (1 cm × 2 cm) were used as working electrodes for long-term controlled potential electrolysis (CPE) tests, which were conducted in a three-electrode two-compartment H-shape electrochemical cell with an ITO electrode (1 cm² immersed in electrolyte) as the working electrode.

Synthesis of the ligand

The 6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine ligand (BPMAB) was synthesized according to the following procedure starting from 2,2′-bipyridine-6,6′-dicarboxaldehyde (Scheme 1). Briefly, to a stirring solution containing 2,2′-bipyridine-6,6′-dicarboxaldehyde (0.42 g, 2.0 mmol) and bis(2-pyridylmethyl)amine (0.80 g, 4.0 mmol) in dichloroethane (30 mL), sodium triacetoxyborohydride (1.27 g, 6.0 mmol) was slowly added. The mixture was stirred overnight under an inert atmosphere. The reaction was then quenched with aqueous ammonia (1 M, 100 mL), and extracted with dichloromethane (3 × 30 mL). The combined organic extracts were dried over anhydrous Na₄SO₄. After removal of the solvent by rotary evaporation, the crude product was recrystallized from acetonitrile to afford BPMAB as a white powder (0.92 g, 80% yield). ¹H NMR (DMSO-d6, 400 MHz): 8.50 (d, 4H, Py-H), 8.25 (d, 2H, Py-Py-H), 7.93 (t, 2H, Py-Py-H), 7.77 (t, 4H, Py-H), 7.59–7.66 (t, 6H, Py-H + Py-Py-H), 7.25 (t, 4H, Py-H), 3.84–3.92 (d + d, 12H, Py-Py-CH₂N + Py-CH₂N) ppm. ¹³C-NMR (DMSO-d6, 400 MHz): 159.51, 159.01, 155.09, 149.33, 138.16, 137.04, 123.35, 123.07, 122.64, 119.27, 55.91 and 55.81 ppm.

Scheme 1 The synthesis procedure of the ligand BPMAB.

Synthesis of the complex

To a stirring solution of the BPMAB ligand (0.59 g, 1.0 mmol) in methanol (20 mL), Ni(ClO₄)₂·6H₂O (0.73 g, 2.0 mmol) was added. The resulting mixed solution was stirred overnight at room temperature. After removal of the solvent by rotary evaporation, the residue was redissolved in water (20 mL). Slow evaporation of the aqueous solution at room temperature afforded blue block crystals, which were isolated via filtration and air-dried. 1 was obtained in 81% yield (0.97 g). Mol. formula, C₃₆H₄₆Cl₄N₈Ni₂O₂₂: calc. C, 35.97; H, 3.86; N, 9.32%. Found, C, 35.85; H, 3.93; N, 9.52% (Scheme 2).

Scheme 2 The synthesis procedure of complex 1.

Caution! Perchlorate salts are inflammable, potentially explosive, and harmful when swallowed. When using related compounds, safety precautions should be taken, caution is advised and handling of only small quantities is recommended.

Results and discussion

General characterization of the ligand and complex

The structure and purity of the BPMAB ligand were verified by ¹H and ¹³C NMR spectroscopy (Fig. S1 and S2). The molecular structure of 1 was unambiguously determined by single-crystal X-ray diffraction analysis. An enlarged view of the crystal structure in the ac plane (Fig. S3) reveals that each cationic unit of 1 contains four coordinated water molecules, two lattice water molecules, and four perchlorate counter anions. The asymmetric unit comprises two Ni atoms with distinct coordination environments. One Ni center is coordinated by five nitrogen atoms from the BPMAB ligand and one oxygen atom from a water molecule, adopting a distorted tetragonal bipyramidal geometry. The axial positions are occupied by atom N4 and the oxygen atom O1 (∠N4–Ni–O1 = 172.3°), while the equatorial plane consists of four nitrogen atoms. The other Ni center is coordinated by three nitrogen atoms from the ligand and three oxygen atoms from water molecules, also forming a distorted tetragonal bipyramidal configuration, with N6 and O4 as axial ligands (∠N6–Ni–O4 = 172.1°) and two nitrogen and two oxygen atoms in the equatorial plane (Fig. S4). The presence of four coordinated water molecules provides multiple potential reactive sites for catalytic water oxidation. The IR spectrum of 1 exhibits a broad band at 3415 cm⁻¹, attributed to O–H stretching vibrations of water molecules, which is absent in the spectrum of the free BPMAB ligand, confirming the incorporation of water in the complex structure (Fig. S5). In the UV-vis absorption spectrum of 1 in neutral PBS, the bands at 564 nm and 895 nm are assigned to the ³A_2g(F) → ³T_1g(F) and ³A_2g(F) → ³T_2g(F) transitions, respectively. The third expected transition, ³A_2g(F) → ³T_1g(P), cannot be observed in the UV region due to overlap with the charge-transfer band of the ligand.^25,26 Furthermore, the linear relationship between the absorbance and concentration of 1 indicates its good hydrolytic stability and solubility in aqueous solution (Fig. S6).²⁷ The mass spectrum analysis of the novel Ni complex dissolved in aqueous solution was conducted using a high resolution mass spectrometer. As shown in Fig. S7, the mass peak at m/z = 346.0704 is assigned to the mass signal of {[Ni₂(BPMAB)]-2H⁺}²⁺ (m/z = 346.0728), indicating that the binuclear structure of 1 can be stabilized by the BPMAB ligand in aqueous solution.

Electrochemical tests

Consecutive cyclic voltammetry (CV) scans were performed to preliminarily assess the redox behaviors and catalytic activity of 1. During the 10 continuous CV cycles in the positive potential region, the maximum catalytic current initially decreases but gradually stabilizes at 1.12 mA cm⁻², displaying the features of homogeneous electrocatalysts for water oxidation (Fig. S8a). Finally, a stable catalytic current emerged at 1.45 V (defined as the catalytic current density of 0.1 mA cm⁻²),²¹ corresponding to an onset overpotential of 630 mV (Fig. 2 and Fig. S8b). In contrast, no significant catalytic current was observed in the absence of 1, confirming its essential role in catalyzing water oxidation. Besides, an obvious irreversible redox event with E_pa at 1.23 V and E_pc at 1.11 V are observed from the CV curves. However, the further oxidation process of 1 at higher potential cannot be observed, probably because the further redox event was masked by the significant catalytic current. After the above consecutive CV measurements, the GC electrode was rinsed with pure water without polishing and then retested in a fresh 1-free PBS. The resulting CV test showed negligible catalytic current, comparable to the background response, indicating that 1 remains intact under catalytic conditions and that no active heterogeneous deposits formed on the electrode surface during the consecutive scans.²⁸ Besides, after the consecutive CV scan, the electrolyte was retested using the CV method. And the resulted CV curve is almost the same as the stabilized CV curve, suggesting the catalytic stability of 1 (Fig. S8b).

Fig. 2 CV scan of 0.5 mM of 1 in 0.1 M PBS at pH 7.0, fresh 0.1 M PBS without 1 and that of the as-used GC electrode washed with pure water in fresh PBS without 1.

To further evaluate the long-term catalytic performance, stability, and faradaic efficiency of 1 toward electrochemical water oxidation, a CPE test was conducted at a constant applied potential of 1.65 V (Fig. S9). Over the course of the 4 h CPE test, the catalytic current density initially increased and eventually stabilized at approximately 0.61 mA cm⁻² (Fig. 3a). The initial rise in current is attributed to the gradual accumulation of active molecular intermediates, rather than to the formation of heterogeneous deposits on the ITO electrode.^29,30 After the CPE experiment, the maximum charge was 7.54 C during the CPE test with the presence of 1 (Fig. S10). The amount of O₂ was quantified by gas chromatography and determined to be 18.6 µmol, giving the faradaic efficiency of 96% for 1 (Fig. 3b).

Fig. 3 (a) Catalytic current obtained using the CPE test of 0.5 mM of 1 in neutral PBS at 1.65 V using ITO electrode as the working electrode. (b) Theoretical and measured amounts of oxygen.

The CPE results further confirm the homogeneous nature and catalytic stability of complex 1. It has been reported that under neutral conditions, Ni oxide films can be electrodeposited on electrode surfaces through the decomposition of certain Ni complexes, and such films often act as efficient catalysts for oxygen evolution. However, after the 4-hour CPE test, the used ITO electrode (denoted as-used CPE ITO) was thoroughly rinsed and then subjected to CPE again in a fresh buffer solution without 1. As shown in Fig. 3a, the post-CPE ITO electrode exhibited only negligible current, comparable to the background level. To further verify that the initial increase in catalytic current arises from the accumulation of molecularly active intermediates rather than heterogeneous deposits, the morphology of the post-CPE ITO electrode was examined by scanning electron microscopy (SEM). The SEM images revealed that the surface of the as-used CPE ITO (named post-CPE ITO-1) remains nearly identical to that of a clean ITO electrode, with no observable deposited films or particles (Fig. S11). Additionally, EDS analysis detected no Ni signal on the surface of post-CPE ITO-1 (Fig. S12), confirming the absence of Ni-containing deposits. Therefore, it is concluded that 1 maintains its molecular structure during the long-term CPE test.

This binuclear nickel complex also exhibits fundamentally distinct catalytic behaviour compared to Ni²⁺. As shown in Fig. S13, during the consecutive CV scan under identical conditions, the catalytic current density of Ni(ClO₄)₂·6H₂O increases gradually. The increasing currents for Ni(ClO₄)₂·6H₂O are ascribed to the gradual electrodeposition of certain active catalytic species on the surface of GC electrodes, e.g. NiO_x (a known electrocatalytic heterogeneous WOC).^31–33 Besides, after the consecutive CV test, the used GC electrode was washed but not polished, and then retested by the CV method in fresh PBS free of 1. The substantial catalytic current further confirms the deposition of heterogeneous active species on the surface of the GC electrode (Fig. S13).^34–36 Subsequently, the CPE test of Ni(ClO₄)₂·6H₂O was conducted (Fig. S14). During the electrolysis test, the catalytic current gradually increased, indicating the accumulation of heterogeneous active species on the ITO electrode surface during the CPE test. The image of ITO electrodes used for the electrolysis tests of 1 and Ni(ClO₄)₂·6H₂O (named post-CPE ITO-Ni²⁺) are shown in Fig. S15. The post-CPE ITO-1 is almost the same as the clean ITO electrode, while significant brown deposition can be observed on the surface of the post-CPE ITO-Ni²⁺, indicating that 1 is a molecular WOC that exhibits catalytic properties entirely distinct from the Ni²⁺. Besides, the microscopic surface morphology and EDS analysis of the post-CPE ITO-Ni²⁺ are displayed in Fig. S11c and S12c, revealing the significant deposition of heterogeneous Ni-containing species on the surface of post-CPE ITO-Ni²⁺. Therefore, the catalytic behavior of 1 is distinct from that of Ni²⁺, supporting that 1 functions as a genuine molecular catalyst rather than pre-catalyst for electrochemical water oxidation. Collectively, these control experiments and characterization studies demonstrate that 1 is a homogeneous molecular WOC without decomposition into heterogeneous active phases.

To further evaluate the catalytic stability and activity of 1, kinetic studies of water oxidation were conducted. Cyclic voltammetry (CV) measurements were performed at varying concentrations of 1. The steady-state catalytic current density (j_cat) exhibited linear dependence on the catalyst concentration (Fig. S16), consistent with the equation j_cat = n′F[1](D_catk_cat)^1/2 (Fig. S17), where n′ = 4 is the number of total transferred electrons in water oxidation reaction, F = 96 485 C mol⁻¹ is the Faraday constant, [1] is the concentration of complex 1, D_cat is the diffusion coefficient of 1, and k_cat is the estimated turnover frequency (TOF) of 1.^37,38 This behavior indicates that water oxidation catalyzed by 1 follows the homogeneous monomolecular catalytic mechanism. Additionally, the current density (j_d) of the anodic peak current of the redox process at E_pa at 1.23 V, which is a non-catalytic redox process, also showed a linear relationship with the concentration of 1 (Fig. S18), displaying the characteristic of a diffusion-controlled electrode process. This relationship can be described by the Randles–Sevcik relation j_d = 0.446nF[1](αFυD_cat/RT)^1/2,^39,40 where α = 0.5 is the transfer coefficient for the irreversible redox couple, R = 8.314 J mol⁻¹ K⁻¹ is the ideal gas constant, n = 2 is the number of transferred electrons for this non-catalytic irreversible redox couple, υ is the scan rate of the CV test, and T = 298 K, respectively. Therefore, the TOF for electrochemical water oxidation mediated by 1 can be calculated by a simplified equation j_cat/j_d = 0.718 (k_cat/υ)^1/2. According the CV of 1 at different scan rates, the k_cat of 1 is estimated to be 1.16 s⁻¹ (Fig. S19 and S20). This value is higher than those reported TOF for some mononuclear Ni-based WOCs (Table 1),^29,41–45 highlighting the beneficial role of the binuclear structure in achieving the high catalytic efficiency of 1.

Table 1 Onset overpotential and TOF of electrochemical water oxidation catalyzed 1 and some reported Ni based complexes

Catalyst^a	pH	η/mV^b	k cat/s^−1	Ref.
a TAML = tetraamido macrocyclic tetra-anionic, mcp = (1R,2R)-N^1,N^2-dimethyl-N^1,N^2-bis(pyridin-2-ylmethyl)cyclohexane, L1 = (2-pyridylmethyl)[6-hydroxy-2-pyridine)methyl)][(6-mesityl-2-pyridyl)methyl]amine, mabpy = 6,6′-bis(methylaminomethyl) 2,2′-bipyridine, L = bis(2-pyridylmethyl-imidazolyl-idene) methane, tmbptu = 2,6,10-trimethyl-1,11-bis(2-pyridyl)-2,6,10-triazaundecanone. b η = onset overpotential.
[Ni(TAML)]^2−	7.0	680	0.32	38
[Ni(mcp)(H2O)2]^2+	7.0	480	0.19	40
[Ni^II-L1]^2+	7.0	580	—	41
[Ni(mabpy)]^2+	7.0	573	—	44
[NiL]^2+	9.0	550	0.15	45
[Ni(tmbptu)(H2O)]^2+	9.0	561	0.033	25
1	7.0	630	1.16	This work

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To further investigate the redox properties of 1, differential pulse voltammetry (DPV) tests were carried out. As illustrated in Fig. S21, two DPV single peaks reveals two consecutive oxidation processes, whose potential shows dependence on the pH values of the electrolyte. The relationship between the redox potential of each redox event of 1 and the pH (6.0–8.0) of PBS is illustrated by the Pourbaix diagram (Fig. S22). Specifically, the potential of the first and the second redox process decreases dramatically with the increase of the PBS with the almost constant negative slope of −29 mV pH⁻¹ and −32 mV pH⁻¹, respectively, confirming that the two oxidation processes of 1 are proton-coupled electron transfer (PCET) processes involving two electrons and one proton.^46,47

As indicated by the DPV results, two oxidation events are observed for 1, with the first attributed to the Ni^IVNi^II/Ni^II₂ couple and the second to the Ni^IV₂/Ni^IVNi^II redox process. Although Ni-centered oxidation is a prerequisite for electrocatalytic water oxidation, Fig. 2 and Scheme 3 suggest that the two-electron oxidation of a single Ni center at E_1/2 = 1.17 V is insufficient to initiate catalysis. Instead, oxidation of both Ni centers is required to drive water oxidation, supporting the operation of an intramolecular binuclear catalytic mechanism. As the Pourbaix diagram provides definitive evidence of the two consecutive 2e⁻/H⁺ PCET processes of 1, the first two-electron oxidation process may occur either at the Ni1 center or the Ni2 center to form the hydroxyl coordinated Ni^IV species Int 1 or Int 1′, respectively (Scheme 3).

Scheme 3 Proposed two step two-electron oxidation process of 1via the 2e⁻/H⁺ PCET process.

Although the crystal structure of 1 indicates a binuclear distance of 5.903(3) Å, which might appear too long for direct metal–metal interaction, the flexibility of the ligand framework enables adaptive conformational changes. The 2,2′-bipyridine unit is coordinated to one Ni center, while the other is bound to a bis(2-pyridylmethyl)amine group connected via a rotatable methylene linker. Rotation around the C–C σ bond allows the two Ni centers to approach each other during catalysis, enabling the intramolecular binuclear synergistic catalysis process. Considering the initial water-coordinated state of the Ni center of 1, the consecutive 2e⁻/H⁺ PCET process occurring on the two Ni centers should cause the generation of two Ni^IV–OH moieties in 1. Therefore, it is supposed that the O–O bond formation process, which is regarded as the rate-determining step for water oxidation,^48–50 can be achieved via the coupling between two Ni^IV–OH unites. However, attempts to capture signals of relevant intermediates through spectroscopic methods were failed, probably due to the extremely short lifetime of related catalytic intermediate species and the limited temporal resolution of our current instrumentation. Therefore, theoretical calculations were employed to investigate the mechanism of water oxidation catalyzed by 1 (Fig. 4). Theoretical calculations reveal the energy change of −9.7 kcal mol⁻¹ for the formation of Int 1 from 1, whereas the energy change of the formation of Int 1′ is −6.6 kcal mol⁻¹, indicating that while both pathways are feasible, oxidation initiating at the Ni1 center is more favorable. The difference should be attributed to their different electronic properties caused by the distinct coordination environments, where Ni1 is surrounded by five N atoms, and one O atom, resulting in a higher electron density compared to Ni2, which is coordinated by three N atoms and three O atoms. Therefore, it is inferred that in the binuclear complex, the two consecutive oxidation steps proceed first at Ni1, followed by Ni2, ultimately yielding species Int 2 containing two hydroxyl-coordinated Ni^IV centers with an energy change of −1.6 kcal mol⁻¹. Subsequently, the formation of O–O bond containing species Int 3, which is widely regarded as the rate-determining step for the water oxidation cycle, can be achieved by the coupling of two Ni^IV–OH units with the energy demand of 14.1 kcal mol⁻¹, suggesting that the intramolecular coupling of two Ni^IV-OH units in Int 2 to form the O–O bond is indeed feasible. In summary, theoretical calculation results prove the feasibility of the proposed intramolecular binuclear synergistic catalysis mechanism of 1.

Fig. 4 Calculated relative free energy diagram of the oxidation process of 1 and the O–O bond formation during water oxidation.

Conclusions

In this work, we report the synthesis, structural characterization, and electrocatalytic water oxidation properties of a novel binuclear Ni complex, [Ni₂(BPMAB)(OH₂)₄](ClO₄)₄, for the first time. Comprehensive experimental evidence confirms that this complex acts as an efficient homogeneous molecular WOC, exhibiting a moderate onset overpotential of 6300 mV and a high faradaic efficiency of 96%. This study not only expands the family of Ni-based molecular WOCs but also demonstrates that the rational design of multinuclear architectures represents a promising strategy for developing highly active electrocatalysts for water oxidation.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data, including the structural characterization results and electrochemical test data of 1 have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03685b.

CCDC 2488032 for 1 contains the supplementary crystallographic data for this paper.⁵¹

Acknowledgements

This work was financially supported by the Science and Technology Innovation Team Plan for the Youths in Universities of Hubei Province (T2023026) and the Research Foundation of Huanggang Normal University (2042020043).

Notes and references

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