Tuning the Catalytic Water Oxidation Activity through Structural Modifications of High-Nuclearity Mn-oxo Clusters [Mn₁₈M] (M = Sr²⁺, Mn²⁺)

Abstract

The water oxidation half-reaction is considered the bottleneck in the development of technological advances to replace fossil fuels with sustainable and economically affordable energy sources. In natural photosynthesis, water oxidation occurs in the oxygen evolving complex (OEC), a manganese-oxo cluster {Mn₄CaO₅} with a cubane-like topology that is embedded within a redox-active protein environment located in photosystem II (PS II). Therefore, the preparation of biomimetic manganese-based compounds is appealing for the development of efficient and inexpensive water oxidation catalysts. Here, we present the water oxidation catalytic activity of a high-nuclearity mixed-metal manganese-strontium cluster, [Mn^III₁₂Mn^II₆Sr(μ₄-O₈)(μ₃-Cl)₈(HL^Me)₁₂(MeCN)₆]Cl₂∙15MeOH (Mn₁₈Sr) (HL^Me = 2,6-bis(hydroxymethyl)-p-cresol), in neutral media. This biomimetic mixed-valence cluster features different cubane-like motifs and it is stabilized by redox-active, quinone-like organic ligands. The complex displays a low onset overpotential of 192 mV and overpotentials of 284 and 550 mV at current densities of 1 mA cm⁻² and 10 mA cm⁻², respectively. Direct O₂ evolution measurements under visible light-driven water oxidation conditions demonstrate the catalytic capabilities of this cluster, which exhibits a turnover frequency of 0.48 s⁻¹ and a turnover number of 21.6. This result allows for a direct comparison to be made with the structurally analogous Mn-oxo cluster [Mn^III₁₂Mn^II₇(µ₄-O)₈(µ₃-OCH₃)₂(µ₃-Br)₆(HL^Me)₁₂(MeOH)₅(MeCN)]Br₂·9MeCN·MeOH (Mn₁₉), the water oxidation catalytic activity of which was recently reported by us. This work highlights the potential of this series of compounds towards the water oxidation reaction and their amenability to induce structural changes that modify their reactivity.

1. Introduction

Water oxidation catalysis has become a central focus area in the quest for replacing fossil fuel combustion for energy production by renewable and environmentally friendly energy sources [1,2]. Due to its ubiquity, water represents the ideal source of reducing equivalents to produce H₂ via the water splitting reaction, hence conceptually mimicking natural photosynthesis. This process provides the storage of clean energy in the form of chemical bonds with an intrinsic high gravimetric energy density [3,4,5]. However, the water oxidation half-reaction (Equation (1)) within this scheme is complex and highly energetically demanding. Therefore, it requires the use of water oxidation catalysts (WOCs) that alleviate the energy requirements associated with each of the steps in the reaction mechanism and accelerate the sluggish kinetics of the O−O bond formation [6,7,8]. Indeed, the lack of cost-effective WOCs hampers the generation of economically affordable H₂ that could compete with fossil fuels for energy production [5,9]. In this matter, Ru- and Ir-based materials are highly efficient catalysts; however, their large-scale application is unfeasible due to the scarcity and prohibitive cost of these metals [10,11]. Therefore, earth-abundant-based materials with water oxidation catalytic capabilities have been explored in recent years [12,13]. A myriad of systems have been reported displaying diverse water oxidation activities, which comprise mono- or multinuclear organometallic molecular species [14,15], polyoxometalates [16,17,18,19], colloidal particles [20,21], homo- and heterometallic oxides [22,23,24], metal-organic frameworks [25,26] and even metal-free catalysts [27,28]. Moreover, heterogenization of molecular WOCs through their immobilization into solid supports enables the fabrication of hybrid systems that combine the high catalytic activity of molecular catalysts with the stability and recyclability of heterogeneous materials [29,30].

$$2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}{\mathrm{E}}^0=1.229\left(\mathrm{V}\right)\mathrm{vs}.\mathrm{RHE}\mathrm{at}25℃$$

(1)

In nature, water oxidation occurs at the oxygen evolving complex (OEC), a {Mn₄CaO₅} oxo-cluster embedded in a protein environment within photosystem II (PS II) [31,32,33]. The efficacy of this biological enzymatic system relies on effective charge separation and accumulation, and electron mobility mechanisms [34]. The {Mn₄CaO₅} cluster uses water as the ultimate source of electrons and transfers them to a tyrosine residue Y161, which locates directly at the periphery of the cluster, and provides an oxidizable side-chain to recover the photoexcited chlorophyll P680* into its ground state through rapid electron injection [35]. Notably, the harsh water oxidation conditions constantly damage the OEC and its surroundings, the structural integrity of which is reconstituted via self-repair mechanisms to avoid the cessation of activity after 30 min [36].

Given the nature of the OEC, it is not surprising that research efforts have been directed to produce bioinspired Mn-based WOCs featuring multiple metallic centers [37,38,39,40,41]. These WOCs are appealing as they can facilitate multiple-electron redox processes by accumulating multiple charge equivalents through different metal centers in a single molecule and within a narrow potential range. In addition, Mn is a highly attractive element due to its high abundance in the Earth’s crust, availability and low toxicity [42]. Unfortunately, the low stability of Mn-based WOCs under operando conditions is a common issue that leads to either the formation of catalytically active heterogeneous manganese oxide, thus precluding the proper characterization of the complex [43], or fast catalytic decay due to catalyst deactivation [44]. A series of dodecanuclear Mn-oxo clusters, [Mn₁₂O₁₂(OAc)_16−xL_x(H₂O)₄] are especially interesting as they show amenability to replace the organic ligand (L) to modulate the WOC activity and solubility in aqueous solutions. For instance, when L = acetate, benzoate, benzenesulfonate, diphenylphosphonate or dichloroacetate, the clusters are water-insoluble and show poor WOC activity in the solid state, displaying high onset overpotentials between 640 and 820 mV. Maayan and co-workers demonstrated that employing redox-active di- or trihydroxybenzoate organic ligands increases the clusters’ solubilities in aqueous solutions, allowing for the study of their WOC properties in the homogeneous phase [45,46]. These clusters displayed good catalytic activity and a reduced onset overpotential thanks to the non-innocent role of the quinone-like ligands. However, the oxygen evolution rate decreases gradually after one hour under an applied bias, indicating that catalyst deactivation is still a problem in these systems. Our group recently reported the activity of a bioinspired, high-nuclearity Mn-oxo cluster, [Mn^III₁₂Mn^II₇(µ₄-O)₈(µ₃-OCH₃)₂(µ₃-Br)₆(HL^Me)₁₂(MeOH)₅(MeCN)]Br₂ (Mn₁₉), which contains 2,6-bis(hydroxymethyl)-p-cresol, a non-innocent, redox-active ligand that resembles the tyrosine residue Y161 in PS II [47]. Importantly, Mn₁₉-modified carbon paste electrodes delivered high catalytic activity in neutral media, displaying 10 mA cm⁻² and even 100 mA cm⁻² at applied overpotentials of 482 and 654 mV, respectively. Despite the high activity shown by this cluster, we identified oxidation of the primary alcohols of the organic ligand as the main deactivation pathway leading to catalyst deactivation. All things considered, Mn-oxo clusters in general have shown potential towards water oxidation catalysis, but their stability issues must be addressed in the near future in order to obtain more durable catalysts before their commercial implementation is possible.

Interestingly, the nonadecanuclear Mn-oxo cluster (Mn₁₉) was initially investigated by Powell et al. due to its unprecedented ground spin state of S = 83/2 [48]. Thereafter, structural variants of this compound where the central Mn^II ion was replaced with various heterometals such Dy^III, Sr^II, Y^III, Cd^II and Lu^III provided a synthetic route for the [Mn₁₈M] family [24]. Additionally, in this work, the authors showed how weak antiferromagnetic interactions between magnetic units can be tuned using different diamagnetic metal centers.

Here, we report the heterogeneous water oxidation catalytic activity at pH = 7.2 of [Mn^III₁₂Mn^II₆Sr(μ₄-O₈)(μ₃-Cl)₈(HL^Me)₁₂(MeCN)₆]Cl₂∙15MeOH (Mn₁₈Sr), a bioinspired, high-nuclearity Mn-oxo cluster which is analogous to the previously reported Mn₁₉ complex. As part of a modified carbon paste electrode, Mn₁₈Sr displays an onset overpotential of 192 mV and achieves a current density of 10 mA cm⁻² at 550 mV. Post-catalytic characterization indicates that Mn₁₈Sr is the true catalyst under the examined experimental conditions and that it follows the same, or a very similar, deactivation pathway than that found for the Mn₁₉ complex. Additionally, light-induced water oxidation experiments confirm O₂ evolution promoted by Mn₁₈Sr, revealing a TOF of 0.48 s⁻¹ and a maximum TON of 21.6. Mn₁₈Sr is a mixed-valence, polynuclear Mn complex stabilized by redox-active, tridentate 2,6-bis(hydroxymethyl)-p-cresol (H₃L^Me) ligands featuring different cubane motifs, which are organized to form a supertetrahedral structure (Figure 1). Within this structure, Mn₁₈Sr displays cubane-like {Mn₇Cl₂O₇} clusters containing 7-coordinated ‘dangling’ Mn^II atoms that are assumed to act as the catalytically active centers. Additionally, the presence of Sr^II is also intriguing, as this is the only metal known which can replace Ca^II in the OEC without rendering the cluster catalytically inactive [49]. Moreover, in a recent study by Mameri et al., Mn₁₈Sr and Mn₁₉ were successfully employed in the electrocatalytic oxidation of benzyl alcohol into benzaldehyde using O₂ as an oxidant and TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as a co-catalyst [50]. Therefore, given the previously reported high WOC activity for the homologous Mn₁₉ cluster, its resemblance with the OEC, its insolubility in aqueous solutions and facile preparation from inexpensive and environmentally benign starting materials, Mn₁₈Sr is an attractive candidate for heterogeneous H₂O oxidation catalysis.

2. Materials and Methods

All chemicals and solvents were used as purchased without further purification. The Mn and Sr salts precursors were purchased from abcr GmbH (Karlsruhe, Germany), whereas the rest of precursors were purchased from Sigma-Aldrich Ltd. (Gillingham, United Kingdom). The modified carbon paste electrodes were prepared using ALS Co. Ltd. (Tokyo, Japan), CPO Carbon Paste Oil.

Synthesis of [Mn^III₁₂Mn^II₆Sr(μ₄-O)₈(μ₃-Cl)₈(HL^Me)₁₂(MeCN)₆]Cl₂∙15MeOH (Mn₁₈Sr):

The synthesis of Mn₁₈Sr was performed following a previously reported procedure [50]. MnCl₂·4H₂O (0.2 g, 1 mmol), Et₃N (0.1 g, 1 mmol) and 2,6-bis(hydroxymethyl)-4-methylphenol (H₃L^Me) (0.17 g, 1 mmol) were added into a solvent mixture of MeCN and MeOH (25 mL/5 mL) and stirred for 30 min at room temperature. Then, Sr(NO₃)₂ (0.11 g, 0.5 mmol, 0.5 equiv.) was added. The resulting mixture was stirred for an additional 1 h and then refluxed for 2 h. The dark brown mixture was cooled, filtered and left to crystallize at room temperature. Dark brown crystals of Mn₁₈Sr were obtained after 24 h of slow evaporation. The title compound was characterized by FT-IR (Figure S1a), which matched that of the previously reported Mn₁₈Sr, and by Raman spectroscopy (Figure S2a).

Synthesis of [Mn^III₁₂Mn^II₇(µ₄-O)₈(µ₃-OCH₃)₂(µ₃-Br)₆(HL^Me)₁₂(MeOH)₅(MeCN)]Br₂·9MeCN·MeOH (Mn₁₉):

The synthesis of Mn₁₉ was performed following a previously reported procedure [47]. MnBr₂∙4H₂O (0.3 g, 1.4 mmol), Et₃N (0.18 g, 1.79 mmol) and 2,6-bis(hydroxymethyl)-4-methylphenol (H₃L^Me) (0.17 g, 1 mmol) were added into a solvent mixture of MeCN and MeOH (30 mL/5 mL). The reaction mixture was then stirred for 2 h at 30 °C and then refluxed for 1 h. Next, the reaction mixture was cooled at room temperature, filtered to remove insoluble components and left to crystallize by slow evaporation. Uniform single-crystals of Mn₁₉ formed within a few days. The title compound was characterized by FT-IR (Figure S1b), which matched that of the previously reported Mn₁₉, and by Raman spectroscopy (Figure S2c).

Synthesis ofthe PS Complex [Ru(bpy)₂(deeb)](PF₆)₂:

The PS [Ru(bpy)₂(deeb)](PF₆)₂ was prepared in a three-step synthesis (summarized in Scheme 1) according to adapted literature protocols outlined by Ma et al. [51] and Meyer et al. [52].

Diethyl 2,2′-bipyridine-4,4′-dicarboxylate (deeb) was synthesized according to a modified literature procedure outlined by Ma et al. [51]. Initially, 2,2′-bipyridine-4,4′-dicarboxylic acid (0.30 g, 1.228 mmol) and concentrated H₂SO₄ (1 mL) were refluxed in EtOH (10 mL) for 4 h. The reaction mixture was then allowed to cool to room temperature and distilled water (30 mL) was added. The resulting white precipitate was then collected by filtration and washed with MeOH before drying under vacuum. The yield was 0.28 g, 1.02 mmol, 82%.

The ruthenium polypyridine precursor [Ru(bpy)₂]Cl₂ was synthesized according to an adapted literature procedure outlined by Meyer et al. [52]. RuCl₃·3H₂O (2.00 g, 7.649 mmol), bpy (2.39 g, 15.298 mmol) and LiCl (2.15 g, 50.715 mmol) were refluxed together in reagent grade DMF (14 mL) for 8 h. The reaction mixture was then cooled to 0 °C, causing the product to precipitate which was subsequently collected by filtration. The black crystalline product obtained was then washed with distilled H₂O (3 × 10 mL) followed by diethyl ether (3 × 10 mL) before allowing the product to dry in air. The yield was 1.593 g, 3.290 mmol, 43%.

The polypyridyl PS [Ru(bpy)₂(deeb)](PF₆)₂ was synthesized according to an adapted literature procedure described by Ma et al. [51]. Ru(bpy)₂Cl₂ (38.0 mg, 0.08 mmol) and deeb (50.0 mg, 0.16 mmol) were refluxed in EtOH (10 mL) under an atmosphere of nitrogen for 48 h. The solvent was then evaporated under reduced pressure, and the resulting residue was recrystallized from an aqueous saturated KPF₆ solution to afford the formation [Ru(bpy)₂(deeb)](PF₆)₂ as a red powder. The yield was 60.1 mg, 0.057 mmol, 69%.

Electrocatalytic Water Oxidation: Electrochemical measurements were performed with a Biologic VSP potentiostat. Ohmic drop was compensated prior to each experiment using the positive feedback compensation method as implemented in the instrumental setup. A three-electrode set-up was used comprising a Saturated Calomel Electrode (SCE) reference electrode, carbon paste (CP) working electrode and a Pt wire counter electrode (alternatively, a Pt mesh was employed for bulk water electrolysis experiments). The CP working electrodes employed for cyclic voltammetry (CV) and bulk water electrolysis had a surface area of 0.02 cm², whereas those employed for linear sweep voltammetry (LSV) had a surface area of 0.07 cm². These surface areas were employed to calculate the current densities. To prepare Mn₁₈Sr-modified CP working electrodes (Mn₁₈Sr/CP), Mn₁₈Sr was blended with CP at a 30% weight ratio (30 wt %). Then, the resulting mixture was inserted inside the CP electrode pocket. All the measurements were carried out in a 50 mM potassium phosphate (KP_i) buffer solution with KNO₃ (1 M) as electrolyte at pH 7.2. An ALS RRDE-3A set-up with a CP rotating disc electrode was used for the LSV measurements at 1600 r.p.m., and at a scan rate of 10 mV s⁻¹. Tafel data was extracted from these LSV experiments and the onset overpotential was estimated from the intersection point between the tangent line of the Faradaic current at 1 mA cm⁻² and the non-Faradaic current. Chronopotentiometry at applied current densities of 1 mA cm⁻² and 10 mA cm⁻² were performed in an H-cell where the working and reference electrodes were separated from the counter electrode by a glass frit (P0). For these chronopotentiometric experiments, the CP working electrode surface was covered with 4 μL of Nafion 117 solution using a micropipette and dried at room temperature.

All the applied potentials (E_app) were converted to the NHE reference scale using:

$${\mathrm{E}}_{\mathrm{NHE}}={\mathrm{E}}_{\mathrm{SCE}}+0.241\left(\mathrm{V}\right)$$

(2)

The overpotentials (η) were calculated by subtracting the thermodynamic water oxidation potential ${\mathrm{E}}_{{\mathrm{H}}_2\mathrm{O}/{\mathrm{O}}_2}^0$ from E_app as:

$$\mathsf{\eta }={\mathrm{E}}_{\mathrm{app}}-{\mathrm{E}}_{{\mathrm{H}}_2\mathrm{O}/{\mathrm{O}}_2}^0$$

(3)

where ${\mathrm{E}}_{{\mathrm{H}}_2\mathrm{O}/{\mathrm{O}}_2}^0$ was pH-corrected employing the Nernst equation:

$${\mathrm{E}}_{{\mathrm{H}}_2\mathrm{O}/{\mathrm{O}}_2}^0=1.229-\left(0.059\times \mathrm{pH}\right)\left(\mathrm{V}\right)\mathrm{vs}.\mathrm{NHE}\mathrm{at}25℃$$

(4)

Light-Induced Water Oxidation Catalysis: Oxygen evolution was measured using a Unisense oxygen sensor (Clark electrode, OX-NP model). The microsensor was connected to a monomer and a computer operating the SensorTrace Suite software package. A two-point calibration method was employed prior to the measurements, using (a) a zero-oxygen determination of an anoxic aqueous solution (prepared using a mixture of NaOH and sodium ascorbate) and (b) air-saturated distilled water at a known temperature (prepared by vigorously bubbling air for 10 min into the Unisense calibration chamber (CAL300). Light-driven O₂ evolution experiments were carried out using a Prizmatix LED light (λ = 470 nm), connected to a fiber optic cable. The light intensity of the incident radiation at the center of the reactor was determined to be 10 mW cm⁻². The operating distance between the fiber optic tip and the reaction vessel was 1 cm.

Typically, [Ru(bpy)₂(deeb)](PF₆)₂ (deeb = 4,4-di(ethoxycarbonyl)-2,2-bipyridine) (2.00 mg, 0.47 mM) as a photosensitizer (PS) and Na₂S₂O₈ (11.9 mg, 10 mM) as sacrificial electron acceptor (SEA) were loaded into a crimp vial, which was then sealed with a butyl-rubber septum and covered with aluminum foil to prevent early reaction. Two 0.1 mg/mL catalysts stock solution were prepared by dispersing either Mn₁₈Sr or Mn₁₉ in a 10 mM NaP_i buffer aqueous solution at pH 7.0. For comparison of the water oxidation catalytic activity of these two catalysts, we employed 5 nmol of either Mn₁₈Sr or Mn₁₉ in the experiments by injecting the corresponding volume of the stock solution into the vial. The final volume of the mixture was adjusted to 5 mL by the addition of NaP_i buffer solution. The OX-NP Clark electrode (Unisense) and grounding cable were inserted into the solution phase through the septum and the real-time O₂ quantities were monitored thereafter. The aqueous mixture was then de-aerated by bubbling N₂ in the vial for several minutes, before removing the N₂ inlet and waiting for a consistent, steady O₂ reading. At this point, the content of the vial was illuminated whilst the O₂ concentration in solution was monitored in real time.

The turnover numbers (TON) and initial turnover frequencies (TOF) were calculated as follows:

$$\mathrm{TON}=\frac{{\mathrm{n}}_{{\mathrm{O}}_2}}{{\mathrm{n}}_{\mathrm{catalyst}}}$$

(5)

$$\mathrm{TOF}=\frac{{\mathrm{n}}_{{\mathrm{O}}_2}}{{\mathrm{n}}_{\mathrm{catalyst}}\times time}$$

(6)

where ${\mathrm{n}}_{{\mathrm{O}}_2}$ is the number of moles of O₂ produced, ${\mathrm{n}}_{\mathrm{catalyst}}$ is the number of moles of catalyst employed and the time period considered was 10 s after the onset of the O₂ evolution.

3. Results

Heterogeneous water oxidation electrocatalytic experiments were performed using Mn₁₈Sr-modified carbon paste (CP) working electrodes, where the Mn₁₈Sr weight ratio (wt %) in the CP blend was kept at 30% (Mn₁₈Sr/CP). An SCE reference electrode and a Pt wire (or mesh) counter electrode completed the three-electrode set-up. These measurements were conducted in a 50 mM phosphate (KP_i) buffer at pH 7.2 using KNO₃ (1 M) as electrolyte.

Initial cyclic voltammetry (CV) using the Mn₁₈Sr/CP electrodes shows increasing current densities at oxidative potentials that clearly contrast with the catalyst-free CP electrodes (blank), indicating the appearance of a multielectron transfer catalytic process (Figure 2a). Is worth mentioning that a crossover loop process appears at the firsts CV cycles. This behavior corresponds to the redox activity of the organic p-cresol type (HL^Me)²⁻ ligand stabilizing the Mn₁₈Sr cluster as we previously demonstrated, and it arises from the partial ligand oxidation to form semihydroquinone, semiquinone or carboxylate derivatives [47]. The electrocatalytic activity and kinetics of the Mn₁₈Sr/CP electrodes were further investigated by linear sweep voltammetry (LSV). Figure 2b shows the performance of Mn₁₈Sr/CP, which reaches current densities of 1 mA cm⁻² and 10 mA cm⁻² at 284 and 550 mV of overpotential, respectively. Moreover, these electrodes show a remarkably low onset overpotential (η_onset) of 192 mV (Figure S3). The LSV data were employed to study the kinetics of the electrode using a Tafel plot (Figure 2c). Mn₁₈Sr/CP shows a slope of 220 mV dec⁻¹ at low overpotentials, which increases to 260 mV dec⁻¹ at >300 mV overpotentials. These slope values are higher than the typical Tafel slopes, suggesting that the kinetics of the electrode are limited by a diffusion process and electron transfer. Moreover, organic ligand oxidation may be competing with the water oxidation reaction.

In comparison with the catalytic activity previously reported by the Mn₁₉ cluster, Mn₁₈Sr shows a reduction of ca. 60 mV in the onset overpotential. We also observe a slight decrease of ca. 10 mV in the overpotential needed to reach 1 mA cm⁻². However, due to its increased Tafel slope, Mn₁₈Sr displays current densities of 10 mA cm⁻² and 100 mA cm⁻² at higher overpotentials than Mn₁₉ (see

Table 1. Comparison of the electrocatalytic water oxidation performed by 30% Mn₁₈Sr/CP ¹ and 30% Mn₁₉/CP electrodes.

	30% Mn18Sr/CP	30% Mn19/CP 2
ηonset 3 (mV)	192	255
η 4 (mV) @ j = 1 mA cm−2	284	296
η (mV) @ j = 10 mA cm−2	550	482
η (mV) @ j = 100 mA cm−2	827	654
Tafel slope (mV dec−1)	220/260	205

¹ Carbon Paste; ² Data obtained from reference [47]; ³ Onset overpotential; ⁴ Overpotential at the given current density.

and Figure 2d).

The stability of the Mn₁₈Sr/CP electrodes was investigated with repetitive CV and chronopotentiometric experiments. Repetitive CV measurements show that after 10 CV cycles, the activity of the Mn₁₈Sr/CP decays, suggesting that the catalyst slowly decomposes to a catalytically inactive species (Figure 3a). Moreover, the water oxidation catalytic activity decay is accompanied by the appearance of a pale brown precipitate on the electrode surface. We already observed this same behavior when employing Mn₁₉/CP electrodes and identified the new inactive species as amorphous manganese-potassium carbonate-phosphate species. We characterized the pale brown precipitate obtained with the Mn₁₈Sr/CP electrodes by Raman spectroscopy. Figure S2b shows the Raman spectra of this precipitate features only two bands that originate from carbon vibrations at 712 and 1049 cm⁻¹. This result is analogous to the one found for Mn₁₉/CP electrodes (Figure S2d), where we identify the Mn₁₉ cluster as the true active species whereas oxidation of the primary alcohols of the p-cresol type ligand was identified as the main catalyst deactivation pathway. Therefore, given the similar behavior to Mn₁₉/CP electrodes, we can conclude that under the tested conditions, Mn₁₈Sr, or a species closely related, must act as the true catalytically active species before it undergoes deactivation.

Consistent with the repetitive CV study, we observe a decrease in the WOC activity over time during the chronopotentiometric experiments (Figure 3b). It is important to note that for these experiments, we followed a common procedure to cover the electrode surface with a Nafion film [53]. The Nafion polymer acts as a proton shuttle transferring the protons generated at the electrode during the proton-coupled electron transfer (PCET) [54] events to the bulk and increases the mechanical stability of the CP electrodes. The Mn₁₈Sr/CP electrodes show relatively good stability for one hour at applied current densities of 1 mA cm⁻² and 10 mA cm⁻², with initial overpotentials of 280 and 550 mV, respectively. The displayed overpotentials slightly increase over time, suggesting catalyst deactivation, which is not surprising given the low stability performed by Mn-oxo clusters under operando conditions. In fact, restarting the chronopotentiometry at an applied current density of 1 mA cm⁻² after ca. 80 min does not recover the initial performance of the electrode. Therefore, the applied overpotentials needed to reach current densities of 1 mA cm⁻² and 10 mA cm⁻² increase ca. 100 mV after 10 and 3 h, respectively (see Figure S4).

Finally, the oxygen evolution promoted by Mn₁₈Sr was confirmed under light-driven water oxidation conditions by employing a well-known protocol [55]. In this system, [Ru(bpy)₂(deeb)](PF₆)₂ was used as a PS and Na₂S₂O₈ as SEA within a 5 nmol Mn₁₈Sr suspension in 5 mL of 10 mM NaP_i buffer aqueous solution at pH = 7. The O₂ evolved upon light irradiation (LED lamp, λ = 470 nm) was monitored employing a Clark electrode and compared with that of the analogous Mn₁₉ under the same experimental conditions.

Figure 4 shows the O₂ evolution profiles displayed by both catalysts where the generated O₂ quantity continuously rises after the mixture is irradiated and reaches a plateau after ca. 120 s. The Mn₁₉ cluster delivers an overall better catalytic activity with slightly superior kinetics than Mn₁₈Sr, which is in good agreement with the electrocatalytic experiments shown above. Hence, Mn₁₉ reaches a turnover number (TON) of 25.8 and a turnover frequency (TOF) of 0.53 s⁻¹, whereas Mn₁₈Sr shows a TON of 21.6 and a TOF of 0.48 s⁻¹.

4. Discussion

The exceptional OER activities observed using Mn₁₈Sr and Mn₁₉ could stem from several shared structural features. For example, both compounds comprise high-nuclearity cluster cores, allowing the four electrons which must be abstracted from H₂O to promote O₂ evolution to be distributed between either eighteen or nineteen redox-active Mn centers in Mn₁₈Sr or Mn₁₉, respectively. This difference could underlie the marginally superior O₂ evolution activity displayed by Mn₁₉ in comparison to Mn₁₈Sr. In addition, each cluster contains twelve Mn^III centers, which are integral to catalytic H₂O oxidation in many WOCs including the OEC of PS II. Furthermore, the geometric configurations of the Mn-oxo clusters Mn₁₈Sr and Mn₁₉ resemble the OEC: two pseudocubane motifs appended by ‘dangling’ Mn^II centers, which are coordinated by terminal, labile solvent molecules, constitute a total of six substrate-accessible active sites per cluster. Finally, the presence of redox-active O-donor ligands which stabilize the cores structures of Mn₁₈Sr and Mn₁₉ represents an additional parallel between these clusters and the OEC. Further research directions may include the replacement of the redox-active ligand to increase the stability of the clusters, which could also improve their water oxidation catalytic performance. Moreover, DFT calculations will help elucidate the WOC reaction mechanisms performed by these clusters to pinpoint key structure–reactivity relationships, and to disclose the role and stability of the redox-active quinone-like organic ligands.

5. Conclusions

In summary, we have shown the WOC activity in neutral media of a mixed-valence, high-nuclearity Mn-oxo cluster containing redox-active ligands, [Mn^III₁₂Mn^II₆Sr(μ₄-O₈)(μ₃-Cl)₈(HL^Me)₁₂(MeCN)₆]Cl₂∙15MeOH (Mn₁₈Sr), and compared with that of the analogous [Mn^III₁₂Mn^II₇(μ₄-O)₈(μ₃-OCH₃)₂(μ₃-Br)₆(HL^Me)₁₂(MeOH)₅(MeCN)]Br₂·9MeCN·MeOH (Mn₁₉) complex under the same working conditions. As part of a modified carbon paste working electrode, Mn₁₈Sr shows good water oxidation electrocatalytic activity with a low onset overpotential of 192 mV. Moreover, the Mn₁₈Sr-modified electrodes display relatively low overpotentials of 284 and 550 mV at current densities of 1 mA cm⁻² and 10 mA cm⁻², respectively. Unsurprisingly, the catalytic activity of this Mn-oxo cluster decays over time, where the organic ligand oxidation is identified as the main catalyst deactivation pathway. Moreover, the catalytically inactive species that appear after catalyst deactivation has been identified as a manganese-potassium carbonate-phosphate species. Importantly, these results are in good agreement with our previous investigations employing the Mn₁₉ compound. Under light-induced water oxidation conditions, Mn₁₈Sr promotes O₂ evolution upon light irradiation, displaying an initial TOF of 0.48 s⁻¹ and a maximum TON of 21.6. Overall, the Mn₁₉ cluster displays an improved WOC activity than Mn₁₈Sr due to its faster kinetics.

Our results confirm the potential of this family of high-nuclearity Mn-oxo clusters as WOCs and highlights their amenability to structural modifications to tune their electronic properties and modulate their reactivity.