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. 2025 Feb 23;64(9):4267–4274. doi: 10.1021/acs.inorgchem.4c04501

Biomimetic Second Coordination Sphere Effect within Cu-Peptoid Electrocatalyst Enables Homogeneous Water Oxidation at pH 7

Guilin Ruan 1, Suraj Pahar 1, Natalia Fridman 1, Galia Maayan 1,*
PMCID: PMC11898173  PMID: 39987508

Abstract

graphic file with name ic4c04501_0008.jpg

A dinuclear Cu-peptoid, Cu2(BDiE)2, having a diol side chain was developed as a homogeneous electrocatalyst for oxygen evolution reaction (OER) at neutral pH. The molecular structure of Cu2(BDiE)2 was characterized and concluded by ESI-MS, UV–vis, and single-crystal X-ray diffraction. Electrochemical, spectroscopic, and mechanistic studies revealed that borate buffer (the solution medium) has a minor effect during electrocatalysis; however, the diol side chain promotes a second coordination sphere effect via multiple hydrogen bonds which highly stabilize the complex, leading to an OER. Based on these observations and the collected data, we also suggest two different and unique mechanism pathways: a major one, which involves interactions of radical intermediates, which is buffer independent, and a minor one that resembles water nucleophilic attack (WNA) and is assisted by the borate buffer.

Short abstract

A dinuclear Cu-peptoid as a homogeneous electrocatalyst for water oxidation showcases an efficient secondary sphere effect on PECT and O−O bond formation processes, leading to electrocatalytic water oxidation at pH 7. Based on experimental results, two reaction pathways were identified: a unique major one, which is independent of the buffer, and a minor one that resembles a buffer-assisted WNA mechanism.

1. Introduction

Oxygen evolution reaction (OER) via water oxidation is the key challenge for producing green hydrogen fuel due to its high overpotential and slow kinetics.1 In nature, the solar-driven OER is catalyzed by the oxygen evolution complex (OEC) in the enzyme photosystem II near neutral pH, at pH 6.2 It was previously suggested that amino acid residues from the second coordination sphere of OEC, e.g., tyrosine and histidine, help stabilize its redox states and facilitate protons and electrons transfer processes,3,4 thus enabling high activity at near-neutral pH conditions, which is important for a good kinetic balance between OER and hydrogen evolution reaction.5 This motivates the design and development of biomimetic catalytic systems based on earth-abundant metals for OER at near-neutral pH.6−8

Various Cu complexes were developed as OER electrocatalysts, mostly at alkaline pH conditions,9−11 but their utilization for this reaction at neutral pH is still challenging.12−18 Several studies agreed that dinuclear Cu complexes are more reactive at pH 7 than mononuclear ones, due to their charge dispersion capability or synergistic effect,13,19−21 and a few others showed that redox-active ligands (first coordination sphere) participate in water oxidation.22−25 However, the utilization of second coordination sphere effects aimed to enable activity at pH 7 was not demonstrated. Peptoids, N-substituted glycine oligomers, represent an excellent platform for the design and development of second coordination sphere mimics of embedded catalytic centers,26−30 due to their efficient synthesis from primary amines, which allows high sequence versatility and tunability.28,29,31 Nevertheless, reported Cu-peptoid electrocatalysts for OER are only active at alkaline pH (9–11.5),26,27 and are not active at all at pH 7. To enable OER at pH 7 using a metallopeptoid-based electrocatalyst, we set to design a peptoid that incorporates, along with bipyridine (Bipy) and ethanol side chains for Cu(II) coordination, a diol group as the second coordination sphere mimic, for multiple hydrogen bonding stabilization of both the complex and its oxidized intermediates. Such stabilization is anticipated to mitigate the energy barrier and thus facilitate proton transfer processes within OER.30

In this study, we describe the design and synthesis of the peptoid BDiE, having bipyridine (Bpy), ethanol, and propanyl-diol groups as side chains, and its corresponding di-Cu-peptoid Cu2(BDiE)2 upon Cu(II) binding. We discovered that Cu2(BDiE)2 is a highly stable and active electrocatalyst OER at pH 7 and that the diol side chain is out of the first coordination sphere but crucial for stabilization of the metal site during electrolysis and for facilitating proton-coupled electron transfer (PCET) processes that lead to O–O bond formation.

2. Methods

2.1. Materials and Instrumentations

Materials and chemicals, Rink Amide resin, ethanolamine, 3-amino-1,2-propanediol, N,N′-diisopropylcarbodiimide (DIC), bromoacetic acid, 6-bromo-2,2′-bipyridine, and trifluoroacetic acid (TFA) were purchased from the same companies individually as reported previously from our lab.26,27,32 Reagents and solvents, dichloromethane, HPLC grade water, and acetonitrile were purchased from commercial sources and used without further purification, besides DMF that was dried with molecular sieves. 2-(2,2′-Bipyridine-6-yloxy)ethylamine was synthesized following a previous protocol.33 The −OH/diol group protection of ethanolamine/3-amino-1,2-propanediol was done by a reported procedure.26 Deionized water was obtained by a Milli-Q water purification system. A 0.2 M borate buffer solution (pH 7) was prepared using 0.2 M boric acid and adjusted by adding 0.2 M NaOH solution to achieve a final ionic strength of 0.2 M, which was monitored by an electronic pH meter.

The used instruments for purification and characterization of peptoid ligand and/or Cu-peptoid complex in this article, preparative HPLC, analytical HPLC, ESI and high-resolution-ESI mass spectrometry, UV–vis spectrophotometer, attenuated total reflection (ATR) spectrometer, Zeiss Ultra Plus high-resolution scanning electron microscope (SEM), energy-dispersive X-ray spectrometer (EDS), and Bruker NMR spectrometer AVIII400 have been reported with the same conditions as previously reported.26,27,32,33

2.2. Preparation and Characterization of Peptoid Oligomers

The peptoids BDiE were synthesized manually on Rink amide resin using the submonomer approach.34 Typically, this approach includes two iterated steps after the deprotection of the resin by 20% piperidine DMF solution for 20 min, bromoacetylation, and amine displacement reactions. In each cycle, once bromoacetic acid is incorporated into the resin, the sequential amines are added in the following step for displacing the bromide via SN2 reactions. For peptoid BDiE, 2-(2,2′-bipyridine-6-yloxy)ethylamine, protected 3-amino-1,2-propanediol, and protected ethanolamine were subsequently introduced into the resin in each iterating cycle, with reaction times of 5 h, 20 min, and 20 min, respectively. Following the end of the iteration, the peptoid was cleaved from the resin by 95% TFA in water (40 mL/g resin) for 20 min. The cleavage solution was then dried in a vacuum, and then the residue solid (or gel) was redissolved in HPLC solvent (1:1 HPLC grade acetonitrile/water) and lyophilized overnight. More details of peptoid synthesis have been reported previously.27,33 The peptoid BDiE was further purified to >95% by preparative HPLC and lyophilized overnight, obtaining a yield of 40–50%. The peptoids after purification were characterized by analytical HPLC, ESI-MS, and 1H NMR (Figures S1, S2, and S19).

2.3. Synthesis of Complex Cu2(BDiE)2

Once the peptoid ligand was prepared, 0.1 mmol of it was dissolved in 1 mL of n-propanol, and then this solution was treated with 0.1 mmol of copper perchlorate hexahydrate and stirred for 4 h. A greenish-blue solid precipitate was obtained and further isolated by centrifugation, washed 3 times with n-propanol, and dried in a vacuum. The solid product yielded about 75%. Then the solid compound was redissolved in water in an NMR tube and left in open air. A few days or a week later, a blue crystal was obtained. The isolated crystal was then analyzed in single-crystal X-ray diffraction, ESI-MS, UV–vis, EPR, and bond valence sum (BVS) calculation (Figures S3, S4, S20, S21 and Table S1). More details of the obtained crystal structure are accessed by the Cambridge Crystallographic Data Centre (CCDC number: 2411148).

Note: copper perchlorate hexahydrate has significant safety hazards, including its potential to cause fires or explosions upon contact with reducing agents or combustible substances. It should be tackled with caution in a properly ventilated fume hood, using proper personal protective equipment, including gloves, goggles, and a lab coat.

2.4. Electrochemical Methods

The electrochemical properties of the prepared Cu complex were measured using an IVIUMSTAT.XRe or PalmSens potentiostat/galvanostat. The Cu complex was initially characterized at RT by cyclic voltammetry (CV), as well as differential pulse voltammetry (DPV), to understand the working potential. These measurements were carried out in a beaker cell with a three-electrode system. Glassy carbon (GC) was used as a working electrode (WE) (0.07 cm2) unless mentioned specifically otherwise, Ag/AgCl as a reference electrode, and Pt as a counter electrode. The controlled potential electrolysis (CPE) experiment according to applied potential was done to evaluate the Faradaic efficiency (FE%), as well as electrochemical stability. Oxygen evolution was monitored in the gas phase with a Fixed-Needle-Type Oxygen Minisensor (from PyroScience) placed in the headspace of the reaction vassal (working electrode side). The Faraday efficiency was calculated by the total charge passed during CPE and the total amount of generated oxygen as a four-electron oxidation process: FE% = n(O2, exp.)/(Q/nF) × 100%, where n(O2, exp.) is the measured oxygen by the sensor from the CPE experiment, mol; Q is the accumulated charge from the CPE experiment, C; n is the number of electrons transferred, which is 4; and F is the Faraday constant, which is 96,485 C/mol. All reported potentials in the present work are reported vs NHE by adding 0.20 V to the measured potential unless specified otherwise. The spectroelectrochemistry experiment was done in a special cuvette where 1 mL of catalyst solution was placed and it contained three electrode chambers: Pt mesh as working, Pt as counter, and Ag/AgCl as reference electrodes, and spectral were recorded in Agilent Technologies Cary 60 UV–vis spectrophotometer.

3. Results and Discussion

3.1. Synthesis and Analysis of Molecular Structure

The peptoid BDiE (Figure 1a) was synthesized on a solid support, cleaved, and purified by high-performance liquid chromatography to >99% purity (Figure S1).34 Its molecular weight, determined by ESI-MS, was consistent with the expected mass for its sequence (Figure S2). BDiE was treated with 1 equiv of [Cu(H2O)6](ClO4)2 in n-propanol, forming a greenish-blue precipitate, which was isolated and recrystallized from water and characterized by single-crystal X-ray analysis (Figure 1b). The bond valence sum (BVS) calculation from the bond lengths was obtained as 2.02 for both Cu ions, indicating +2 oxidation state (Table S1).35 Therefore, the obtained complex was [Cu2(BDiE)2(H2O)](ClO4)4 (abbreviated as Cu2(BDiE)2). The distance between two Cu ions is 4.467 Å and they are bridged by a H2O molecule with a Cu–O–Cu angle of 128.65° and a symmetric Cu–O bond length of 2.478 Å (Figure 1). Each Cu is coordinated to three N atoms (two from Bpy of one peptoid and one from the secondary amine of the other peptoid) and three O atoms (one from the H2O bridge, one from the −OH side chain, and 1 from a backbone carbonyl). The ESI-MS spectrum (Figure S3) showcases a dominant m/z peak of 1433.2173 consistent with [Cu2(BDiE)2(ClO4)3]+, suggesting that the complex exists as the dinuclear form also in the liquid phase and that the H2O bridge is labile.27 In addition, using UV–vis spectroscopy, the absorbance band at 323 nm assigned to Cu2(BDiE)2 increased linearly with the increase in the concentration of the complex in borate buffer at pH 7 (Figure S4), indicating that Cu2(BDiE)2 is maintained as one single species in these conditions, which is stable for at least 24 h. The EPR spectrum of Cu2(BDiE)2 showed a signal at half-field region (ΔMS = ±2, Figure S20), further supporting that it is a dinuclear complex.36,37

Figure 1.

Figure 1

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(a) Molecular design of ligand BDiE; (b) dinuclear Cu centers with atom and bond distance in the crystal structure (ellipsoid style of 50% probability level) of Cu2(BDiE)2; (c, d) molecular and crystal structures of Cu2(BDiE)2, hydrogen atoms, and guest molecules are excluded to enhance clarity.

3.2. Electrochemical Properties and Oxygen Evolution

The electrochemical properties of Cu2(BDiE)2 were measured in 0.2 M borate buffer at pH 7. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were obtained using glassy carbon (GC) as a working electrode (WE), Ag/AgCl as a reference electrode, and platinum wire as a counter electrode. An oxidation wave with high current intensity was displayed in the CV measurement of Cu2(BDiE)2 at Ep = +1.37 V vs NHE (defined as peak potential value in the corresponding DPV, Figure 2a). Together with a reversed CV scan for the detection of O2 from the reduction at −0.2 V (vs NHE, unless specified otherwise), where the reduction is defined as CuII/CuI and it can further reduce oxygen, this oxidation Ep is confirmed to be a catalytic event of OER (Figure S5). CV of [Cu(H2O)6](ClO4)2 in the same buffer resulted in a crossover indicating heterogeneous catalysis behavior (Figure 2b). These distinct results between Cu2(BDiE)2 and [Cu(H2O)6](ClO4)2 indicate that free Cu ion impurity does not exist in the solution of Cu2(BDiE)2. Therefore, the catalytic activity is attributed to added Cu2(BDiE)2.

Figure 2.

Figure 2

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(a) CV with and without 0.5 mM Cu2(BDiE)2; DPV of Cu2(BDiE)2 is shown as a dashed line; (b) CV with 1 mM [Cu(H2O)6](ClO4)2 salt; the arrows show the direction of scanning; all experiments were performed in 0.2 M borate buffer at pH 7 with a scan rate of 100 mV/s.

O2 evolution was investigated using a controlled potential electrolysis (CPE) experiment at +1.5 V with ITO as WE in 0.2 M borate buffer at pH 7 (Figure 3). In 4 h, the oxygen increment was 4.9 μmol with 0.25 mM of Cu2(BDiE)2 and 1.3 μmol without it. It resulted in a turnover number (TON) of 3, proving that oxygen evolution was produced in an electrocatalytic behavior instead of a stoichiometric reaction. This electrolysis afforded a charge accumulation of 1.6 or 0.1 C in the presence or absence of Cu2(BDiE)2. Based on 4e– transfer water oxidation, the Faradaic efficiency (FE%) was calculated to be 85%. As OER can last for 4 h, we expected that Cu2(BDiE)2 is stable in these conditions. Indeed, a 40 min CPE spectroelectrochemical study (Figure 4) showed that when OER started, the absorbance at 323 nm started to decrease. At the end of the reaction, the UV–vis spectrum was measured again after the three electrodes were removed from the solution, and it showed that the reduced absorbance recovered to the original level within 20 h, with an isosbestic point near 303 nm. The long time of absorbance recovery might imply the high stability of intermediates, indicating that peptoid ligand assists the stabilization of high oxidation state species.26

Figure 3.

Figure 3

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(a) O2 evolution and (b) charge accumulation during 4 h CPE with and without Cu2(BDiE)2 (0.25 mM). The experiments were conducted at +1.5 V using ITO as the working electrode, Ag/AgCl as the reference electrode, and Pt wire as the counter electrode in an H-cell.

Figure 4.

Figure 4

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(a) UV–vis spectra during the 40 min spectroelectrochemical experiment with applied potential +1.5 V using Pt net as WE, Ag/AgCl as RE, and Pt wire as counter electrode in 0.2 M borate buffer at pH 7; (b) consecutive UV–vis spectra after 40 min spectroelectrochemical experiment when three electrodes were removed; (c) absorbance plot at 323 nm vs time (min).

3.3. Homogeneity and Kinetics Studies

Additionally, ITO WE from the 4 h CPE experiment was examined with HR-SEM and EDS before and after the 4 h OER (Figures S8 and S9). The results showed that no particles were found on the electrode’s surface in HR-SEM, and no signal assigned as Cu element was found attached during EDS after electrolysis. A common concern of homogeneity is that the dissolved catalyst may form in situ solid oxide on WE during electrocatalysis and redissolve back into solution after the applied potential is stopped. This phenomenon might be detected on the surface of WE. However, since the absorbance at 323 nm defined as Cu2(BDiE)2 recovered back to the same level after electrocatalysis without placing WE in the solution, the absorbance change is by homogeneous species and there is no Cu ion loss in the solution. Therefore, this concern can be eliminated. Moreover, the CV scans at different scan rates (Figure S10) showed that the noncatalytic peak current id from CuII/I oxidation of Cu2(BDiE)2 varies linearly with the square root of the scan rate, ν1/2. Considering the irreversibility, id, and ν1/2 follow the below relation

3.3. 1

The value of the diffusion coefficient DCu was calculated to be 9 × 10–6 cm2/s, fitting in the diffusion-controlled process.38 Furthermore, CVs before and after the 4 h CPE are identical without any drastic change (Figure S11). Collectively, they indicate that the catalytic process is truly homogeneous.

CVs at different scan rates and concentrations of Cu2(BDiE)2 were performed to gain insights into the kinetics (Figure S12). The linear dependence of the catalytic current icat on the concentration of Cu2(BDiE)2 points out that the kinetics of OER is performed by single-molecule catalysis with first-order kinetics. Therefore, the catalytic process obeys the relationship displayed in eq 2

3.3. 2

The correlation and slope value between icat/id and ν–1/2 is obtained by the division of eqs 1 and 2

3.3. 3

Subsequently, TOF (kcat) is calculated to be 0.2 s–1 by the linear slope value.9 TOF was also determined by foot-of-the-wave analysis (FOWA) as 76 s–1 (Figure S12). Although Cu2(BDiE)2 follows an activity–stability trade-off,39 the high stability should lead to a higher amount of oxygen over time, thus compensating, in the long run, for the moderate kinetics.

3.4. Role of Borate Buffer

We anticipated that the borate buffer does not have a major role in OER in these conditions because the main species of the buffer at pH 7 is the neutral B(OH)3 rather than the charged B(OH)4–, which is present in pH > 9.2,27 and B(OH)3 is not capable of a nucleophilic attack on the metal-oxo intermediate for O–O bond formation,40 which is a rate-determining step for OER. Indeed, CVs and DPVs with Cu2(BDiE)2 done in borate and phosphate buffers displayed similar oxidation potential and current intensity (Figure 5a), indicating that the borate buffer is not superior to phosphate buffer in these conditions, probably contributing to transfer proton instead of critically transfer oxygen-atom.41 Furthermore, the kinetic value kB, assigning the partial rate of OER determined by the B(OH)3, was calculated as 0.88 M–1 s–1 (Figure 5b and Table S2), which is similar to that of H2O at pH 7,41 and about 900 times slower than that of B(OH)4– at pH > 9, indicating that B(OH)3 has no superior role over H2O (as a proton acceptor) for OER in these conditions.

Figure 5.

Figure 5

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(a) CVs and DPVs of 0.4 mM Cu2(BDiE)2 in 0.2 M borate/phosphate buffer at pH 7; (b) CVs of 0.25 mM Cu2(BDiE)2 in different concentrations of borate buffer.

3.5. Mechanistic Study

In the lack of a major buffer effect on the catalytic OER, we suggest that the diol side chain in BDiE, which is in the second coordination sphere of the metal centers within Cu2(BDiE)2, has an important role in the catalytic activity at pH 7. First, a similar dinuclear Cu-peptoid having only one −OH group on the dangling side chain27 showed lower catalytic activity and higher onset potential compared with those of Cu2(BDiE)2 (Figures 6a and S14). Second, to further understand the role of the diol side chain, the OER mechanism using Cu2(BDiE)2 as an electrocatalyst was investigated. A Pourbaix diagram was generated by plotting data from DPVs measured under different pH conditions, from pH 6.6 to 8.0 (Figure 6b). This diagram shows that the oxidation wave of CuII/CuI at +0.25 V is independent of the pH and is nonrelated to the catalytic event. At high potential, in which water oxidation takes place, a linear slope value of 0.055 was obtained at +1.37 V. This slope value is similar to the theoretical slope value of 0.059, which is in line with a PCET (nH+/ne–) process.42 Based on the Pourbaix diagram, and taking into account that the H2O bridge between the two Cu centers of Cu2(BDiE)2 is labile, we propose that Cu2(BDiE)2 is oxidized from CuII-(H2O)-CuII to CuIII(OH)CuIII(OH) (int1, Figure 7), through subsequent PCET processes, where the bridging water molecule is deprotonated and dissociated from one of the Cu centers and a water molecule from the bulk solution coordinates to this Cu center and deprotonated with coupled-electron transfer.27 As the two Cu centers are symmetrical, we can suggest that in the next step, one of the CuIII–OH centers in int1 is further oxidized by a PCET reaction to form CuIII(O•)CuIII(OH), int2.40 Once a reactive Cu-oxyl radical species is formed, it is stabilized by coupling with the coordinated hydroxide of the unreacted CuIII–OH center, and in turn, the high oxidation state CuIII is reduced by pulling an electron from the OH ligand, and the Cu–OH bond is cleaved, yielding the hydroperoxide species int3.43 To support the formation of the O–O bond, we wished to generate the peroxo intermediate first by chemical oxidation with H2O2, characterize the product by infrared spectroscopy (IR) analysis, and then compare this spectrum to the one obtained upon the electrochemical (CPE) oxidation of Cu2(BDiE)2.26 The complex was thus oxidized by either H2O2 or CPE and in both cases the IR spectra exhibited a transmittance at 883–889 cm–1, assigned to the O–O bond of Cu–OOH. A control IR spectrum of unoxidized Cu2(BDiE)2 did not show this signal (Figure S16).44−48 These results support the formation of hydroperoxo intermediates during electrocatalytic water oxidation, which is consistent with the proposed int3. Finally, int3 undergoes a further PCET reaction, forming and releasing O2, completing the OER cycle, and regenerating the initial species Cu2(BDiE)2. This pathway is further supported by the kinetic isotope effect (KIE) study in both H2O and D2O. The KIE value remains below 1.5 (from ∼1.1 to ∼1.4), regardless of the concentration of the catalyst (Figure S15), indicating that no atomic proton transfer process occurs during O–O bond formation.19 This proposed mechanism suggests that the diol side chains assist the H+ transfer during PCET processes, by doubly stabilizing them both in the first and second steps of the reaction, therefore facilitating the formation of int2 for O–O bond formation.49

Figure 6.

Figure 6

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(a) CVs with Cu2(BDiE)2 and reported catalyst17 and without catalyst in 0.2 M borate buffer at pH 7; (b) pH study using Cu2(BDiE)2 in 0.2 M borate buffer, inset is the plotted Pourbaix diagram.

Figure 7.

Figure 7

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Proposed mechanism of using Cu2(BDiE)2 for homogeneous electrocatalytic OER at pH 7. PBL: peptoid backbone linker, B: borate buffer species.

Although our results suggest the above pathway as the major mechanism of the reaction, they also indicate that kB of borate buffer is not so low to be considered negligible but 0.88 M–1 s–1 and the KIE value is not 1 but ranges from 1.1 to 1.4. Hence, we propose that a minor pathway of the formation of the O–O bond involving borate buffer for proton transfer coexists within this reaction. In this pathway, instead of a H2O molecule from the blank solution participating in subsequent PCET reactions in the first step of the reaction, the Cu2(BDiE)2 undergoes the PCET processes followed by the formation of int4. In the next step, water nucleophilic attack (WNA) on int4, assisted by borate buffer, facilitates the proton transfer, leading to the formation of hydroperoxo intermediate int5. This intermediate is similar to int3 from the major pathway, proven to be a hydroperoxo species, but in int5, the Cu ion bound to the peroxo group is in the 2+ state while in int3, it is in the 3+ state. Eventually, int5 releases dioxygen and reforms back to Cu2(BDiE)2.

To support the suggestion that int5, which consists of CuII–OOH, is a minor intermediate, the UV–vis spectrum was measured during the oxidation reaction both with H2O2 and by CPE. The UV–vis spectrum measured after the reaction with H2O2 showed that the absorbance at 320 nm is almost unchanged and only slightly increased (Figure S17). This result is different from the spectrum recorded during the CPE experiment showing a large decrease in the intensity at the same wavelength (Figure 4a).26,27 Our group has previously studied changes in similar absorbance spectra and concluded that such a decrease in absorbance is due to the oxidation of CuII to CuIII. As the oxidation in the reaction catalyzed here occurs electrochemically, it is reasonable that int3, which contains CuIII–OOH, is more dominant than int5 having CuII–OOH.50 These results further support the pathway involving hydroperoxo intermediate int5 (CuII–OOH) as a minor mechanism for the catalytic reaction, which coexists with the major mechanism where int3 (CuIII–OOH) is dominant.

4. Conclusions

We introduced here the highly stable dinuclear Cu-peptoid, Cu2(BDiE)2, as an electrocatalytic OER at pH 7. The electrochemical properties, the OER performance, the buffer effect, and the benefits of peptoid ligand design have been investigated and discussed. Cu2(BDiE)2 can activate OER at pH 7 without buffer assistance, and more importantly, the diol side chains in the second coordination sphere accelerate the PCET reactions that lead to O–O bond formation and oxygen evolution. Along with this mechanism, a minor pathway involving a weak borate buffer effect for proton transfer during water nucleophilic attack is also proposed. Our study highlights the advancement in ligand design with the second coordination sphere effect for OER at neutral pH conditions and offers insights into protein mimics to address the oxidative transformation of small molecules.

Acknowledgments

The authors thank Mrs. Larisa Panz from the Schulich Faculty of Chemistry for her assistance with ESI-MS measurements and simulations.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c04501.

  • Analytical HPLCs, ESI-MS, and 1H NMR spectra for ligands and complexes; cyclic voltammetry, UV–vis spectroscopies, IR spectra, EPR, and details of crystal structures for complexes; SEM and EDS for the surface of the electrode (PDF)

Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

The research was funded by the Israel Ministry of Energy, grant number 2033945.

The authors declare no competing financial interest.

Supplementary Material

ic4c04501_si_001.pdf (3.7MB, pdf)

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