Trinuclear Ruthenium Macrocycles: Toward Supramolecular Water

Trinuclear Ruthenium Macrocycles: Toward Supramolecular Water Oxidation Catalysis in Pure Water Valentin Kunz,† Marcus Schulze,† David Schmidt,†,‡ and Frank Würthner*,†,‡ †

Institut für Organische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany Center for Nanosystems Chemistry (CNC), Universität Würzburg, Theodor-Boveri-Weg, 97074 Würzburg, Germany

‡

S Supporting Information *

ABSTRACT: The incorporation of a Ru(bda)(bda = 2,2′bipyridine-6,6′-dicarboxylate) water oxidation catalyst into a trinuclear metallosupramolecular macrocycle leads to increased stability and activity compared to the mononuclear reference system. To overcome solubility problems of such large structures in water and the need for large amounts of organic cosolvents, new macrocyclic water oxidation catalysts with improved water solubility become desirable. With triethylene glycol side chains, the required amount of acetonitrile as a cosolvent was halved, whereas the application of charged ammonium side chains allowed catalysis in pure water. The catalytic activity was found to be comparable to the parent compound. Kinetic experiments were performed to explain the rate differences between those new derivatives and showed that Coulombic repulsion between the charged side chains and CeIV leads to slower oxidation processes.

W

which showed increased catalytic activities due to the close proximity of the catalytic centers accelerating the bimolecular reaction mechanism in which two ruthenium oxyl radicals are required to form the O−O bond in the rate-determining step (rds).20 Recently, we reported the metallosupramolecular macrocycle [Ru(bda)(bpb)]3 (MC1), which incorporates three Ru(bda) fragments into a cyclic arrangement by using the bridging ligand bpb (1,4-bis(pyrid-3′-yl)benzene).21 Although a few examples of other macrocyclic WOCs are reported,22,23 it was shown for the first time that the cyclic arrangement of the catalytic units leads to increased stability in comparison to the monomeric reference [Ru(bda)(pic)2] (pic = 4-picoline). This was attributed to prevented self-oxidation and a chelate effect that prohibits ligand dissociation, both of which are considered to be the main degradation pathways.12,24,25 Most interestingly, this catalyst does not operate via the common mechanism established for this catalyst class, namely, the interaction of two metal oxide radicals (I2M).11 By performing kinetic experiments and 18O labeling studies, the mechanism was found to proceed via nucleophilic attack of a solvent water molecule to a ruthenium(V) oxo species with oxidation from a RuIV to a RuV

ater oxidation is a key reaction in the process of using water as a sustainable feed stock for a solarfuel-driven economy.1−3 The splitting of water provides hydrogen that can be used directly or can be converted into methanol and other liquid fuels by the reduction of CO2.4−7 The electrons, which are required to reduce protons to the desired molecular hydrogen, are provided by the oxidative half reaction. For a long time, this oxidation to form oxygen was the bottleneck of the overall water splitting, but tremendous progress in the development of highly active water oxidation catalysts (WOCs) has been achieved since Meyer’s discovery of the “blue dimer” in 1982. 8,9 The best homogeneous catalysts for this reaction are based on the transition metal ruthenium, and especially the [Ru(bda)L2] catalyst family, developed by Sun and co-workers, received a lot of attention recently (bda = 2,2′-bipyridine-6,6′-dicarboxylate, L = N-heterocyclic axial ligand).10,11 Catalysts of this class of compounds were among the first to reach activities comparable to that of the natural oxygen-evolving cluster.12 Furthermore, the easy manipulation of the axial ligands allowed the synthesis of specifically tailored derivatives for all kinds of applications. Direct attachment to a photosensitizer for photodriven applications13 and incorporation into soft materials, such as vesicles,14 nanofibers,15 and polymers,16 were achieved, as well as equipment with binding sites for on-surface deposition to construct efficient anodic devices.17 Even multinuclear structures such as dimers18 and trimers19 were developed, © XXXX American Chemical Society

Received: October 27, 2016 Accepted: December 30, 2016

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DOI: 10.1021/acsenergylett.6b00560 ACS Energy Lett. 2017, 2, 288−293

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ACS Energy Letters species being the rds. This monomolecular mechanism is advantageous for any kind of application with diffusion limitations, as, for example, on electrode surfaces.16 To the best of our knowledge, it is the first catalyst that combines very high activity toward chemically driven catalytic water oxidation (TON = 7400, TOF = 150 s−1) with this monomolecular water nucleophilic attack mechanism.10 However, incorporating the Ru(bda) fragment into such large metallosupramolecular structures significantly decreases the solubility in water. Thus, at least 50% (v/v) acetonitrile is required as organic cosolvent to perform catalytic water oxidation studies. From a green chemistry perspective, this is highly undesired because acetonitrile is regarded to be a noneco-friendly solvent, especially compared to water.26 Therefore, we intended to develop fully water-soluble derivatives of this macrocyclic system to enable oxidative splitting of water completely without any cosolvents. Furthermore, it is known that acetonitrile acts as a competitive binder to the seventh coordination site of the Ru(bda) fragment, preventing the coordination of water, thereby creating an off-pathway that slows down the overall catalytic rate.27 By avoiding the use of acetonitrile, an increase in catalytic activity can therefore be anticipated. Here, we report a comprehensive study on two analogues of the macrocycle [Ru(bda)(bpb)]3 (MC1) bearing additional water-solubilizing groups such as triethylene glycol chains (MC2) or protonable tertiary amines (MC3) (Scheme 1). Those solubilizing groups were incorporated into the bridging ligands L1−L3 to minimize the alteration of electrochemical and catalytic properties of the Ru(bda) system. Detailed synthetic procedures and structural characterization are provided in the Supporting Information. To confirm the macrocyclic arrangement and to prove the stability of the new catalysts, we performed temperaturedependent 1H NMR experiments in different solvents. Figure 1a a shows exemplarily the aromatic region of the 1H NMR spectrum of MC3 in CD2Cl2/MeOD 5:1. Only three signals for the bda ligand (blue) and four signals for the bridging ligand L3 (red) can be observed, proving the D3h symmetry of the cyclic system. Although the macrocycle MC3 is not fully soluble in neutral water, the addition of a few equivalents of ascorbic acid, commonly added to NMR samples to reduce traces of paramagnetic RuIII to diamagnetic RuII, is sufficient to bring the macrocycle into solution due to concomitant protonation of the amine groups. As can be seen from Figure 1b, most NMR signals in the aromatic region are significantly shifted to lower fields due to the different solvent environment, but the amount, multiplicity, and sharpness of the signals are not affected and the cyclic structure stays intact even after heating the sample to 355 K (Figures S18 and S19). Also after acidification with trifluoromethanesulfonic acid to a pH of 2, no decomposition was observed over 1 week (Figures 1c and S20). However, when the pH was further lowered to a value of 1, a severe signal broadening was observed. This broadening can be overcome by heating the sample up to 355 K (Figure S21) and can be explained by the reduced symmetry upon protonation of one of the carboxylate arms and the resulting dynamic coordinating/ decoordinating equilibrium between the two carboxylates of one bda ligand, which is a known phenomenon for Ru(bda) complexes under highly acidic conditions.27,28 All in all, these NMR experiments clearly prove the high stability of the cyclic arrangement in water, even under strongly acidic conditions and elevated temperatures.

Scheme 1. Synthesis of the Trinuclear Macrocycles MC1− MC3

In the RuII state, with an 18 valence electron configuration, always one of the carboxylate functionalities has first to dissociate for the accommodation of a new incoming ligand. Although electrochemical measurements performed on related Ru(bda)-based WOCs in aqueous solutions suggest the coordination of water in the RuII state,11 this cannot be seen by NMR spectroscopy because the ligand exchange between the water molecule and the carboxylate group is very fast.29 In contrast, for the stronger σ-donor acetonitrile, which exchanges only slowly with the carboxylate ligands, strongly broadened resonances can be observed for related mononuclear complexes.27−29 Therefore, no sharp 1H NMR spectrum for the triethylene glycol macrocycle MC2 could be obtained because always some acetonitrile is required for solubility. Accordingly, also the spectrum of MC3 broadens upon addition of acetonitrile into the solvent mixture (Figure S22). The broadening is stronger than that for the mononuclear reference systems due to the formation of many isomers upon desymmetrization. 289


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system MC1.21 For the macrocycle MC3, pH-dependent measurements were performed under acidic conditions (pH 1−3) to ensure full protonation of the amino groups over the whole pH range. From those measurements, a Pourbaix (E vs pH) diagram can be constructed, which provides information about proton coupling of the individual redox events (Figure S25).32 The first oxidation from [RuII|RuII|RuII] to [RuIII−OH2| RuIII−OH2|RuIII−OH2]3+ is found to be proton-uncoupled, whereas the other two oxidation events are characterized by a slope of −44 mV/pH. According to the Nernst equation, this correlates with a 3e−/2H+ proton-coupled electron transfer process, which is in line with the results previously reported for MC1.21 To determine the optimal reaction conditions for each catalyst, the catalytic activity was screened in different solvent compositions for all catalysts under otherwise fixed conditions (Figures S27−S29). Cerium(IV) ammonium nitrate (CAN) was used as the sacrificial oxidant dissolved in CH3CN/H2O (pH 1, acid: CF3SO3H) mixtures (v/v) with different mixing ratios.33 After injection of the catalyst solution, the evolution of molecular oxygen was monitored using pressure transducers, and the gas composition was analyzed with gas chromatography (GC) at the end of the reaction. All three catalysts show different optimal solvent compositions as it has been determined by the initial rate of catalysis. For optimal performance, MC1 requires approximately 60% acetonitrile, whereas the amount of cosolvent can be lowered to 30% for MC2 and to 20% for MC3 (Figure 2a). If the amount of cosolvent is too low, precipitation occurs during the reaction, leading to slower catalysis and thus lower turnover numbers. At higher acetonitrile contents than required for optimal solubility, the catalytic performance decreases due to the inhibiting role of acetonitrile as a competitive binder, which has also been demonstrated for [Ru(bda)(pic)2] as a reference (Figure S30).27 Interestingly, the initial rates of MC3 reach a plateau for low acetonitrile contents, and only after some time, partial precipitation of apparently less soluble intermediates occurs, which leads to slower catalysis. However, in contrast to the other two catalysts, the water oxidation reaction never ceases completely, even under completely acetonitrile-free conditions. Therefore, catalyst MC3 was studied both in pure aqueous trifluoromethanesulfonic acid at pH 1 as well as under conditions showing the optimal performance using 20% acetonitrile as a cosolvent. Concentration-dependent measurements under optimized reaction conditions were used to determine the turnover number (TON, Figure 2b) and the turnover frequency (TOF, Figure 2c) and to get further information about the kinetics of the reaction. In pure water (pH 1), a high TOF of 72 ± 6 s−1 and a maximal TON of 2.2 × 103 can be achieved for MC3 (Figure S33). Analysis of the reaction head space composition using GC confirmed that the increase in pressure is solely due to the formation of oxygen as the only gaseous product (Figure S37). The addition of 20% (v/v) acetonitrile leads to an increased TOF of 98 ± 9 s−1 and a TON of 2.8 × 103 (Figure S34), which can be attributed to improved solubility and thus faster reaction. The catalytic activity is significantly higher for MC2 with a TOF of 147 ± 9 s−1 and a TON of 5.2 × 103 using 30% acetonitrile (Figure S32). Comparison of the catalytic activities of these new derivatives with the already reported parent compound MC1 (TOF = 150 ± 7 s−1 and TON = 7.4 × 103, Figure S31) shows similar catalytic activities for all macrocycles.21 The slightly lower TONs of the new supra-

Figure 1. Aromatic region of the 1H NMR (400 MHz) spectra of MC3 in different solvents (few equiv of ascorbic acid were added as reductant): (a) CD2Cl2/MeOD 5:1, (b) D2O (pH 7), and (c) D2O (pH 2, acid: CF3SO3H).

The UV/vis absorption spectra of all [Ru(bda)L]3 macrocycles in the [RuII|RuII|RuII] state show a strong absorption band at around 300 nm for the ligand-centered L-π → L-π* transitions and bands at 360, 450, and 490 nm, which can be attributed to metal-to-ligand charge transfer (MLCT) absorptions. The band at around 360 nm can be attributed to the Ru-d → L-π* transition, whereas the 450−490 nm absorption can be assigned to the Ru-d → bda-π* transitions.30 For MC3, those low-energy MLCT bands are slightly blue-shifted, whereas the high-energy MLCT band at 360 nm is bathochromically shifted, meaning that the axial ligand L3 is slightly less electrondonating compared to L1 and L2 (Figure S23).31 Differential pulse voltammetry was used to verify that the electronic features of the ruthenium centers are not strongly perturbed by exchange of the bridging ligand. The macrocycle MC3 was measured in pure water at pH 1, whereas macrocycles MC1 and MC2 were investigated in the presence of the noncoordinating CF3CH2OH as a cosolvent, which is known to have no influence on the redox potentials (in contrast to the coordinating solvent acetonitrile).21,28 At pH 1, all voltammograms exhibit three oxidation events at potentials around 0.75, 1.1, and 1.4 V vs NHE (NHE = normal hydrogen electrode), which can be assigned to the [RuII|RuII|RuII]/[RuIII−OH2| Ru III −OH 2 |Ru III −OH 2 ] 3+ , [Ru III −OH 2 |Ru III −OH 2 |Ru III − OH2]3+/[RuIV−OH|RuIV−OH|RuIV−OH2]4+, and [RuIV−OH| RuIV−OH|RuIV−OH2]4+/[RuVO|RuV−OH|RuV−OH]5+ oxidations, respectively (Figure S24). After the last oxidation, a strong increase in current is observed due to the catalytic oxidation of water. For all macrocyclic WOCs, this onset potential is centered at around 1.5 V vs NHE, meaning that all catalysts are capable of oxidizing water with an overpotential of ∼270 mV. Additionally, the very similar oxidation potentials of all macrocycles prove that the redox properties of the new WOCs are not changed much compared to that of the parent 290


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molecular WOCs might be due to a somehow decreased stability caused by the newly introduced side chains that make the compounds more prone to oxidative decomposition (e.g., benzylic position).25 Indeed, ligand oxidation could be demonstrated only for L2 and L3 by differential pulse and cyclic voltammetry with an obviously irreversible oxidation process for L3 (Figure S26). Under comparable conditions, MC2 reaches the same TOF as MC1, however in a different solvent composition. Although MC3 exhibits a lower activity compared to the other two catalysts, the TOF of 98 s−1 (33 s−1 per ruthenium center) is still in the same range compared to the monomeric reference complex [Ru(bda)(pic)2] (TOF = 33 s−1).12 However, it has to be noted that the high performance of [Ru(bda)pic2] is achieved with only 1% (v/v) of acetonitrile as the cosolvent and by the usually much faster I2M mechanism, whereas the macrocycles reported here work via nucleophilic attack of a water molecule to a ruthenium oxide species. Thus, all macrocyclic catalysts show the expected linear dependency of the reaction rate on the catalyst concentration, meaning that the reaction is pseudo-first-order in catalyst concentration with a monomolecular rds (the CAN concentration can be neglected due to the large excess). This also explains the high activity of MC3 considering its highly charged nature under the reaction conditions noting that for a mononuclear [Ru(bda)L2] catalyst operating via the I2M mechanism only negligible catalytic activity was observed when equipped with cationic axial ligands.34 To finally prove the macrocyclic integrity of the newly synthesized WOCs during catalysis, MC3 was exemplarily analyzed by MALDI-ToF mass spectrometry after 30 catalytic cycles. The mass spectrum of MC3 thus obtained is nearly identical to the high-resolution ESI-ToF mass spectrum of the analytical pure sample exhibiting the same mass spectrometric fragmentation behavior (Figure S35). Thus, breakup of the trimetallic macrocycle and formation of monometallic catalytic species seem to be highly unlikely for the initial phase of the catalytic reaction. The difference in catalytic activity between the highly active MC1 and MC2 and the somewhat slower MC3 can be rationalized by the fact that the rds was previously found to be the oxidation from a RuIV to a RuV species,21 presumably proceeding via inner-sphere electron transfer.35 Although the oxidation potentials of this processes are almost the same for all compounds (Figure S24), this oxidation event should be slowed down considerably for MC3 using CAN as the oxidant due to Coulombic repulsion between the highly charged catalyst and the CeIV cation. To demonstrate that the charge of the catalyst really influences the rate of oxidation, a UV/vis experiment was performed monitoring the bleach of the MLCT band upon oxidation of [RuII|RuII|RuII] to [RuIII−OH2|RuIII− OH2|RuIII−OH2]3+ by adding 1 equiv of CAN per ruthenium center to a 0.1 mM solution of the catalyst. This was done for both catalysts under the same reaction conditions and under previously optimized solvent compositions, namely, 30% acetonitrile for MC2 and 20% for MC3. This one-electron oxidation is a second-order reaction with the reaction rate depending on the concentration of both the reductant (RuII centers) and the oxidant (CeIV). In the special case that the concentrations of the oxidant and reductant are the same, the rate eq 1 can be simplified to

Figure 2. (a) Variation of the solvent composition for the catalytic water oxidation reaction using catalysts MC1−MC3 at otherwise fixed conditions ([CAN] = 536 mM, [MC1−MC3] = 24.3 μM, V = 3.4 mL, pH 1, acid: CF3SO3H) and the resulting initial rates. (b) Concentration-dependent amount of evolved oxygen that was used to determine the TONs for MC1−MC3. (Note that at high catalyst concentrations the TON is limited due to full consumption of the CAN.) (c) Concentration-dependent initial rates and linear regression to determine the corresponding TOFs for MC1−MC3. Concentration-dependent measurements have been performed under optimized solvent conditions (CH3CN/H2O (pH 1) 6:4 for MC1, 3:7 for MC2, and 2:8 for MC3; V = 3.4 mL, [CAN] = 536 mM).

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d[Ru II] = k·[Ru II]· [Ce IV ] = k·[Ru II]2 dt

(1)


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Although the other oxidation processes could not be further analyzed due to a lack of significant changes in the UV/vis spectra, it is a valid assumption that this behavior also strongly influences them. Because the oxidation from a RuIV to a RuV species is assumed to be rate-determining, the Coulombic repulsion between the positively charged side chains of MC3 and the CeIV cation seems to be accountable for slowing down the water oxidation catalysis in comparison to MC1 and MC2. For MC2, also a beneficial effect of the triethylene glycol chains cannot fully be excluded, which are known to bind metal ions and thus can bring the cerium(IV) into close contact with the ruthenium center, possibly leading to faster oxidation processes.39 However, this effect would not explain almost identical TOF values that were observed for the two catalysts MC1 and MC2. In conclusion, we equipped a trinuclear macrocyclic [Ru(bda)L]3 WOC with triethylene glycol and protonable tertiary amine side chains to improve the water solubility of such supramolecular systems. Accordingly, it was possible to perform the water oxidation catalysis under much more ecofriendly reaction conditions in pure water. By the introduction of triethylene glycol side chains, the required amount of acetonitrile was halved. Complete water solubility under acidic conditions could be achieved by the introduction of charged ammonium side chains, however at the expense of reduced catalytic activity in chemically driven water oxidation experiments presumably due to Coulombic repulsions with the oxidant cerium(IV). Because MC3 is soluble in pure water, the typical line broadening in NMR experiments originating from acetonitrile coordination could be avoided. Thus, it was possible to prove the high stability of the cyclic arrangement in aqueous solutions with 1H NMR studies under highly acidic conditions at pH 1 and at elevated temperatures up to 355 K.

This rate law can be used to determine the second-order rate constant k of the oxidation process by fitting absorption spectral changes at a specific wavelength according to eq 2 (with A0 being the initial and A∞ the final absorption, t the time, and [RuII]0 the initial ruthenium concentration)36,37 A=

A 0 + A∞·[Ru II]0 ·k·t [Ru II]0 ·k·t + 1

(2)

By fitting the decay of the MLCT absorption band at 450 nm, which is characteristic for the RuII state,38 the average rate constant for the oxidation of all ruthenium centers from the oxidation state +II to the oxidation state +III can readily be estimated. This oxidation process is roughly three times faster for MC2 (k = 1687 M−1 s−1) compared to that for MC3 (504 M−1 s−1) under the particularly optimized reaction conditions (Figure 3). The difference becomes even more evident if the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00560. Detailed information about synthesis, characterization, electrochemistry, and catalytical measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Frank Würthner: 0000-0001-7245-0471

Figure 3. UV/vis absorption spectra of MC2 in CH3CN/H2O (pH 1) 3:7 (a) and MC3 in 2:8 (b) before (black) and after (red) the addition of 1 equiv of CAN (V = 2.0 mL, [cat] = 0.1 mM, [Ru centers] = 0.3 mM, [CAN] = 0.3 mM) and the time-resolved decay of the absorption at 450 nm as insets.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Bavarian Research Program “Solar Technologies Go Hybrid”. M.S. thanks the Fonds der Chemischen Industrie for a Kekulé fellowship.

oxidation is performed under exactly the same reaction conditions (30% acetonitrile each). The rate of the RuII/III oxidation of MC3 decreases to 221 M−1 s−1, becoming almost 8 times slower than that for MC2 (Figure S38). This clearly demonstrates that the oxidation processes of MC3 are dramatically slowed down by introducing charged side chains. Obviously, a Coulombic repulsion accounts for this effect because the oxidation potentials were not affected by the side chains, as has been proven with differential pulse voltammetry.

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Trinuclear Ruthenium Macrocycles: Toward Supramolecular Water

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