A Monodisperse, End‐Capped Ru(bda) Oligomer with Outstanding Performance in Heterogeneous Electrochemical Water Oxidation

Water oxidation catalysis is a key step for sustainable fuel production by water splitting into hydrogen and oxygen. The synthesis of a novel coordination oligomer based on four Ru(bda) (bda = 2,2′‐bipyridine‐6,6′‐dicarboxylate) centers, three 4,4′‐bipyridine (4,4′‐bpy) linkers, and two 4‐picoline (4‐pic) end caps is reported. The monodispersity of this tetranuclear compound is characterized by NMR techniques. Heterogeneous electrochemical water oxidation after immobilization on multi‐walled carbon nanotubes (MWCNTs) shows catalytic performance unprecedented for this compound class, with a turnover frequency (TOF) of 133 s−1 and a turnover number (TON) of 4.89 × 106, at a current density of 43.8 mA cm−2 and a potential of 1.45 V versus normal hydrogen electrode (NHE).


Introduction
The development of sustainable and carbon-neutral energy sources is essential for the transformation of the worldwide energy system in response to man-made climate change. [1]owever, the usage of renewable energy harvested by solar cells or wind turbines is currently still hampered by limited storage capacities for electricity. [2]One promising alternative is chemical energy storage, e.g., the production of green hydrogen by electrochemical water splitting. [3]In order to facilitate this process, the search for efficient and stable catalysts is ongoing. [4]For direct water splitting, the O 2 -evolving oxidative half-reaction is the most challenging part because the energetically uphill four-electron process requires rigorous DOI: 10.1002/admt.202301721operation conditions. [1,5]Organometallic ruthenium complexes offer a broad variety of mono-and multinuclear structures with intriguing spectroscopic and electronic properties, [6] giving rise to a plethora of potential applications in homogeneous and heterogeneous environments. [7]Consequently, molecular Ru-based water oxidation catalysts (WOCs) emerged as the most promising class of catalysts with outstanding activity and stability. [5]Pioneering work in this area was already done in the 1980s, e.g., with the introduction of the first RuWOC by Meyer and co-workers. [8]The whole field was however rejuvenated in the 2000s and has continuously advanced ever since, with mononuclear, [9] dinuclear, [10] macrocyclic, [11] and polymeric [12] complexes being applied in photo- [13] or electrochemical [14] variants.
In this regard, heterogeneous electrochemical water oxidation is particularly interesting due to straightforward setups, high stability of the catalysts, and the low amount of required active materials. [15]14a] In order to facilitate charge transport and injection into electrodes, immobilization of molecular WOCs on multi-walled carbon nanotubes (MWCNTs) is particularly promising due to the high conductivity of the nanotubes and the possibility for highly stable anchoring of the WOCs via CH- and - interactions. [16]hereby, high loading, and thus, current density, but also excellent catalyst stability was achieved with both molecular [14b,16a,17] and polyoxometalate [18] catalysts.

Synthesis and Characterization
Our previous research [14b,17a] suggested that sufficient extension of Ru(bda) oligomers is required for stable binding to the  surface by van der Waals interactions.Accordingly, we were not sure if a Ru(bda) trimer formed by a modular approach from the literature-known end cap unit Ru(bda)(dmso)(pic) (2) [19] and the central connection unit Ru(bda)(4,4′-bpy) 2 (3) [20] would form sufficiently stable linkage to MWCNTs.However, much to our surprise, the reaction of the two building blocks in 2:1 ratio in 2,2,2trifluoroethanol (TFE) at 80 °C[21] for 19 h (Figure 1) afforded the tetranuclear Ru(bda) oligomer 1 in 94% yield (based on starting material 3).
Thanks to the poor solubility of Ru oligomer 1, facile purification of the crude product was achieved by evaporation of the reaction mixture and sequential washing with MeOH, CH 2 Cl 2 , Et 2 O, and n-pentane.The chemical composition of the oligomer was confirmed by 1 H NMR end-group analysis (Figure 1).Integer ratios between protons from the pic, bda, and linker moieties indicate a pure tetranuclear complex with sum formula [Ru(bda)] 4 (4,4′-bpy) 3 pic 2 .To determine the molecular size and to further validate the chemical composition, diffusion-ordered NMR spectroscopy (DOSY) was measured.The well-defined trace in the 2D plot (Figure S1, Supporting Information) and the excellent agreement of the experimental decay curves with a monoexponential fit (diffusion coefficient D = 3.84 × 10 −10 m 2 s −1 ) suggest that the sample consists of only one molecular species.By contrast, the application of the log-normal fitting model [22] for polydisperse samples was not feasible (see example in Figure S3, Supporting Information).Due to the prolate shape of this structure, the spherical particle approach of the Stokes-Einstein equation is not applicable, [23] which is why we used a cylindrical model introduced by Tirado et al. [24] to estimate the aspect ratio and molecular dimensions of 1.Based on molecular modeling (Figures S5 and S6, Supporting Information) and literature data, [17a] the width of the oligomer chain was estimated based on the width of one bda unit and fixed at 0.91 nm.A length of 5.60 nm was thus calculated for oligomer 1 (Figure S4, Supporting Information), which is in good agreement with a molecular structure obtained from force field calculations (UFF, Materials Studio). [25]Based on this combined analytical data, axial ligand scrambling appears to occur under our reaction conditions, leading to monodisperse tetranuclear Ru complex 1 as the only isolable product.
Whereas solid samples of 1 can be stored for prolonged times, 1 H NMR spectroscopy in organic solvents indicated decomposition of the Ru complexes within a few hours.Limited stability of the coordinative Ru─N bonds in solution was also observed in ESI mass spectrometry, as only fragments with up to two Ru(bda) units were detected (Figure S11, Supporting Information).

Electrochemical Water Oxidation
Due to a-compared to our trinuclear macrocycles [14b] extended size of the tetranuclear oligomer 1 and its poor solubility, we envisioned good adhesion and high performance as active electrode material in electrochemical water oxidation.For better conductivity and charge injection, [16a] 1 was initially immobilized on MWCNTs by adding a TFE solution (100 μL, 1 mg mL −1 ) of the oligomeric WOC to a THF dispersion of MWC-NTs (1 mL, 1 mg mL −1 ) prepared by ultrasonication (Figure 2).Instant decolorization of the supernatant solution indicated successful anchoring of the WOC on the MWCNTs to form hybrid material 1@CNT, which was further analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) (Figure S9, Supporting Information).In contrast, immobilization of the monomeric precursors 2 or 3 on MWCNTs was not possible, as the solution was still intensely colored after any deposition attempts.This observation clearly highlights the importance of a multinuclear structure for stable anchoring.Electrodes 1@CNT@GC were prepared by dropcasting a 1@CNT dispersion (4 × 20 μL in THF/TFE) on GC electrodes until the electrode surface was covered by a continuous layer of 1@CNT (see Figure 2 and Supporting Information for details).
Subsequently, the electrochemical performance of 1@CNT@GC electrodes was analyzed by various analytical techniques.The setup consisted of a glass vial filled with phosphate buffer (phbf) solution (1 m ionic strength) as electrolyte, 1@CNT@GC working electrode, Hg/HgSO 4 reference electrode, and platinum mesh counter electrode.Noncatalytic waves for the Ru III/II , Ru IV/III, and Ru V/IV oxidations were observed at around 700, 820, and 1070 mV versus normal hydrogen electrode (NHE), respectively (Figure 3a).The current density of the catalytic wave was initially quite low but increased significantly after activation by 100 consecutive cyclic voltammetry (CV) cycles (Figure 3b,c).During activation, the Ru III/II wave remains around 710 mV but drops significantly in intensity, while the Ru IV/III wave shifts to around 900-950 mV, and the Ru V/IV wave disappears below the intensifying catalytic wave.Based on previous observations, [17a] we assume that the catalytically active species [14b,17] is only formed after partial dissociation of the bda ligand upon electrochemical activation.This allows the alignment of one carboxypyridine unit with the MWCNT surface, thereby enhancing the surface binding by replacing weaker CH- with stronger - interactions.17a] While the activated state of 1@CNT@GC is stable for some time, controlled potential electrolysis (CPE) measurements of activated electrodes that were stored at ambient conditions for several hours before the measurement, again displayed a slight increase in current density for the initial cycles, which indicates that the activated WOCs slowly convert back to the original state, but can be reactivated.
In Table 1, the averaged electrocatalytic properties of 1 are summarized and compared with benchmark examples from the literature.Intriguingly, oligomeric WOC 1 showed unprecedented activity and stability even at low catalyst loading.
The obtained overpotential of 358 mV approaches the theoretical minimum of around 300 mV for catalysts following the water nucleophilic attack (WNA) mechanism [26] and compares well with reported values for similar catalysts (entries 2, 4, and 7 in Table 1).
Surface coverage of 1@CNT@GC was determined by the integration of a noncatalytic CV wave.On average, 0.8 nmol cm −2 of active Ru sites were found, which is lower than for longer oligomers or rigid macrocycles (entries 2, 3, 7 in Table 1) but comparable to electrodes modified with molecular WOCs (entries 4, 5, 6 in Table 1).
To estimate the catalytic activity, several methods have been proposed for the calculation of catalytic rates [16a,27] and one must carefully distinguish the information that is conveyed.27c] For a better comparison with benchmark examples from the literature, both methods have been applied to 1@CNT@GC.While FOWA (Figure 4a) gave apparent rate constants k WNA in the range of 6.46 × 10 3 s −1 per Ru center, a TOF of 133 s −1 per Ru center was obtained as an average from several measurements.For both parameters, the activity of 1 is much higher compared to mononuclear bda complexes or macrocycle MC3 (entries 2-4 in Table 1), possibly due to higher accessibility of the water binding sites in comparison with the somewhat more restricted macrocyclic structure of MC3.9c] Faraday efficiency was determined by quantitative O 2 detection during CPE experiments, for which an airtight two-compartment cell was used to separate the working and counter electrodes.To produce sufficient amounts of O 2 , a catalyst-coated GC plate (1 cm 2 ) was used as the working electrode.The evolved gaseous O 2 in the cell headspace was detected by a Clark electrode sensor and an excellent Faraday efficiency of 95% was calculated in relation to the theoretical maximum yield predicted from CPE measurements (Figure 4b).The following TON determination was carried out assuming a charge-to-O 2 conversion efficiency of 100%, which was justified by the previous findings.CPE experiments revealed a current density of around 35 mA cm −2 , which remained constant for over 12 h (Figure 4c), while even higher peak current densities (on average 44 mA cm −2 ) were observed for short durations in CV.Therefore, a remarkably high TON of 4.89 × 10 6 per Ru(bda) unit was counted before the cutoff criterion of 90% current density related to the starting value was reached.To the best of our knowledge, this TON value surpasses any molecular Ru-based electrocatalyst known from literature so far.The typical mononuclear RuWOCs and most multinuclear structures are outperformed by two to three orders of magnitude, [16a,28] with only Ru(tda)(Pyr-Py) 2 (entry 4 in Table 1) [27b] being in the same range as 1, presumably due to strong - interactions between its pyrene moieties and MWCNTs.We attribute this superior performance to a stabilizing effect of the picoline end caps.While literature-known oligomers/polymers possess potentially reactive free Ru(bda) binding sites at the ends, [17a] 1 is terminated by strongly coordinated monodentate picoline ligands.Thereby, one potential catalyst deactivation pathway via oligomer decomposition is suppressed.14b] Based on the molecular structures, we propose that the conformational rigidity of MC3 limits the stability under operating conditions.Presumably, the restricted geometry of macrocyclic MC3 hampers any conformational changes, which, however, are necessary to reach the activated state of the catalyst.In contrast, steric hindrance is less critical in the rather flexible oligomer 1, which therefore exhibits both stronger binding to the MWCNTs and better accessibility of the reactive Ru sites.This assumption is in accordance with the experimental findings, as 1 surpasses MC3 in both k WNA and TON.

Conclusion
We have presented a straightforward synthetic protocol for a new monodisperse tetranuclear coordination oligomer 1 with composition [(Ru(bda)) 4 (4,4′-bpy) 3 pic 2 ] by reaction between state-of-theart Ru(bda) precursors 2 and 3 in TFE.Excellent performance in heterogeneous electrochemical water oxidation catalysis at neutral pH was observed even at low catalyst loading, with a very high k WNA of around 6500 s −1 , as well as new record TOF of 133 s −1 and TON of more than 4.8 million turnovers per active metal site.This outstanding performance is achieved by a combination of several beneficial properties.Strong adhesion to the electrode surface is maintained by the oligomeric structure with multiple anchoring sites and the poor solubility prohibits creeping detachment.The durability of the system is enhanced by the inert 4picoline end caps, which serve as protective groups to diminish deactivation pathways via gradual axial ligand dissociation.
The combination of high catalytic performance and unprecedented stability for oligomeric WOC 1 as electrode support in electrochemical water oxidation paves the way for device integration and further investigations under true operating conditions.These exciting initial findings will help to identify and further optimize well-defined molecular WOCs that may compete with and outperform metal oxides in large-scale applications.

Experimental Section
Materials: All reactions were carried out in standard glass equipment.All chemicals were purchased from usual commercial suppliers and used without further purification, unless stated otherwise.Conventional solvents were distilled prior to use.

Figure 2 .
Figure 2. Preparation process of the glassy carbon electrodes.

Table 1 .
Comparison of electrochemical data for 1 (average values obtained from several measurements) with state-of-the-art materials from literature.

bda)(H 2 O) 2
in the course of this work from available literature data as value was not reported in in original publication.