Cooperative e ff ects in homogenous water oxidation catalysis by mononuclear ruthenium complexes †

The homogenous water oxidation catalysis by [Ru(terpy)(bipy)Cl] (1) and [Ru(terpy)(Me2bipy)Cl] + (2) (terpy = 2,2’:6’,2’’-terpyridine, bipy = 2,2’-bipyridine, Me2bipy = 4,4’-dimethyl-2,2’-bipyridine) under the influence of two redox mediators [Ru(bipy)3] 2+ (3) and [Ru(phen)2(Me2bipy)] 2+ (4) (phen = 1,10-phenanthroline) was investigated using Ce as sacrificial oxidant. Oxygen evolution experiments revealed that mixtures of both 2–4 and 2–3 produced more molecular oxygen than catalyst 2 alone. In contrast, the combination of mediator 4 and catalyst 1 resulted in a lower catalytic performance of 1. Measurements of the temporal change in the intensity of a UV transition at 261 nm caused by the addition of four equivalents of Ce to 2 revealed three distinctive regions-suggested to correspond to the stepwise processes: (i) [RuvO] → [RuvO]; (ii) [RuvO] → [Ru–(OOH)]; and (iii) [Ru–(OOH)] → [Ru–OH2] . UV-Visible spectrophotometric experiments on the 1–4 and 2–4 mixtures, also carried out with four equivalents of Ce, demonstrated a faster [Ru(phen)2(Me2bipy)] 3+ → [Ru(phen)2(Me2bipy)] 2+ reduction rate in 2–4 than that observed for the 1–4 combination. Cyclic voltammetry data measured for the catalysts and the mixtures revealed a coincidence in the potentials of the Ru/Ru redox process of mediators 3 and 4 and the predicted [RuvO]/[RuvO] potential of catalyst 2. In contrast, the [RuvO]/[RuvO] process for catalyst 1 was found to occur at a higher potential than the Ru/Ru redox process for 4. Both the spectroscopic and electrochemical experiments provide evidence that the interplay between the mediator and the catalyst is an important determinant of the catalytic activity.


Introduction
The splitting of water into oxygen and hydrogen using sunlight has a great potential for developing an alternative renewable energy source that will help to overcome the looming shortfall in fossil fuels.However, despite the promise, to achieve water splitting (2H 2 O → O 2 + 2H 2 ) a substantial energy input additional to the thermodynamic requirement (1.23 V or 267 kJ mol −1 per mole of H 2 produced) is needed to compensate for losses arising from, for example, cell resistance and various activation barriers.In this regard, the mechanistically complex water oxidation half reaction (2H 2 O → O 2 + 4H + + 4e − ) presents particular challenges as it involves the extraction of four electrons and four protons from two water molecules and the formation of a stable diatomic O-O bond.Thus, efficient catalysts need to be developed such that the energy required to drive the water-splitting reaction is as close as possible to the thermodynamic value (1.23 V).There has been much interest in this field of research and efforts have been made to develop heterogeneous photocatalysts as well as photo-electrocatalysts for water splitting. 1ingle semiconductors or combinations of semiconductors with appropriate valence and conduction band positioning (Z-scheme) have been used to split water directly and more active catalysts have been introduced to enhance performance (these include metals and a variety of transition metal oxides). 1,2In parallel with these studies, there has been a longstanding interest in molecular catalysts that oxidise water or reduce water, with a focus on the former being driven in part by the fact that a tetramanganese cluster in water oxidation complex (WOC) of photosystem II is the only catalyst known to catalyse water oxidation in nature. 36][7][8][9] Initially, the focus was on dinuclear complexes owing to the discovery that the ruthenium blue dimer (cis,cis-[(bipy) 2 (H 2 O)Ru III (µ-O)Ru III (OH 2 )(bipy) 2 ] 4+ ; bipy = 2,2′-bipyridine) catalysed this reaction, 6 and a mechanistic understanding of the pathways leading to oxygen formation was developed from these studies. 7][10][11] They do so by forming a reactive [Ru V vO] intermediate, generated through successive oxidation processes 5,8 The formation of the [Ru V vO] moiety is central to the catalytic evolution of oxygen, which has been proposed to occur through either the attack of [Ru V vO] by water or, alternatively, through intra-or inter-molecular coupling of two [Ru V vO] species. 5,8hese mononuclear ruthenium complexes have largely been investigated using Ce 4+ as a sacrificial oxidant, 5,8,9,11 and although investigations of the interactions between potential photosensitisers (or dyes) and the ruthenium catalysts is very important for the development of a light-induced water-splitting system, to date only a handful of reports have focussed on this aspect in homogenous systems. 9,10For example, the catalytic rate of Ce 4+ -assisted water oxidation by the blue dimer in homogenous solution was enhanced by a factor of ∼30 on addition of mediators such as [Ru(bipy) 2 L] 2+ , where L = 2,2′-bipyridine, 2,2′-bipyrimidine and 2,2′-bipyrazine. 9In a more recent report, a covalently-linked mediator/catalyst dinuclear complex
A series of oxygen evolution experiments using catalysts 1 and 2 as a 1 : 1 mixture with either 3 or 4 at the concentration 73 µM was carried out in an air-tight vessel using 0.1 M HClO 4 solution as the reaction medium and (NH 4 ) 2 [Ce(NO 3 ) 6 ] (0.073 M or 1000 equivalents) as the oxidant.The molecular oxygen evolved from the reaction was detected in the headspace over the course of several hours by a Clark-type micro-sensor.The oxygen measurement for the individual catalysts revealed that 2 produced approximately 16 μmol oxygen after six hours whereas 1 produced twice as much (30 μmol).In order to test the effect of the redox mediators 3 and 4 on the catalytic performance of 1 and 2, the oxygen evolution of each catalyst in the presence of the mediators was investigated.Upon mixing with 4 in a 1 : 1 ratio, oxygen production catalysed by 1 was found to decrease slightly to 23 μmol over the six hours of testing.In contrast, the addition of mediator 4 to catalyst 2 resulted in a significant increase in oxygen production from 16 μmol to 38 μmol over the same period.No molecular oxygen could be detected from a control experiment using only mediator 4 (Fig. 1).The enhancement of the catalytic properties of 2 affected by the redox mediator prompted us to perform further analyses in order to elucidate the mode of interaction between the tris(diimine)ruthenium(II) complexes and the catalyst.Our initial investigations using UV-Visible, 1 H NMR and electrochemical techniques are discussed in the following sections.
The changes in the UV-Visible spectrum of the individual catalysts and the mixtures, before and after the addition of four equivalents of Ce 4+ , were studied in 0.1 M HClO 4 .The initial UV-Visible spectrum of catalyst 2 (before Ce 4+ was added) showed three absorption bands (480 nm, 313 nm and 281 nm).The absorption at 480 nm, assigned as MLCT (metalto-ligand charge transfer) band, was found to disappear upon addition of four equivalents of Ce 4+ (Fig. 2, panel a).This spectral assignment has been reported earlier for the reaction of the closely-related [Ru(terpy)(bpm)(OH 2 )] 2+ complex with Ce 4+ in 0.1 M HClO 4 . 8The addition of two equivalents of Ce 4+ to the solution of [Ru(terpy)(bpm)(OH 2 )] 2+ resulted in the two oxidation steps that could be followed spectroscopically: Further addition of one equivalent of Ce 4+ to the [Ru IV vO] 2+ species (or three equivalents to [Ru II -OH 2 ] 2+ ) triggered three stepwise redox processes.Three distinctive regions based on the absorbance changes at 283 nm were observed and attributed to the reactions given in eqn (3)-( 5). 8u The transient [Ru V vO] 3+ ion has been proposed to react further with water to give what is tentatively assigned as the [Ru III -OOH] 2+ species which slowly decays to [Ru II -OH 2 ] 2+  possibly by releasing O 2 . 8In the present investigation, three regions were indeed observed at the slightly lower wavelength (261 nm), compared with 283 nm in the previous study, suggesting that catalyst 2 also undergoes the stepwise redox processes described by eqn (3)-( 5) after the addition in this case of four equivalents of Ce 4+ (see Fig. 3, top panel).Oxygen can be released if further Ce 4+ is added to the peroxido species [Ru III -OOH] 2+ according to eqn ( 6) and (7). 8This was demonstrated in the present study by using an excess (1000 equivalents) of Ce 4+ in the oxygen evolution experiment mentioned earlier.
The UV-Visible spectrum of the mixture of catalyst 2 and 4 before the Ce 4+ addition showed the MLCT band at 455 nm and three other bands in the UV regions (313, 281 and 264 nm).As observed for the individual catalyst, the MLCT band also disappeared upon adding the oxidant (Fig. 2, panel b).In addition to the three redox processes observed for the individual catalyst, oxidation of the mixture with four equivalents of Ce 4+ is also expected to trigger the formation of [Ru( phen) 2 (Me 2 bipy)] 3+ according to eqn (8).
As in the study with 2 alone, the absorbance changes at 261 nm was also monitored for the 2-4 mixture to see whether the features associated with (3), ( 4) and ( 5) could be identified on the same time scale.These features were absent (Fig. 3, bottom) partly because of the coincidental rise in a strong ligand-centred absorption at 264 nm as is typically observed in this region for similar tris(diimine)ruthenium(II) complexes. 14lternatively, it is possible that the oxidation step (3) is greatly accelerated by [Ru( phen) 2 (Me 2 bipy)] 3+ such that it could not be detected on the minute time scale.The acceleration of reaction (3) by [Ru( phen) 2 (Me 2 bipy)] 3+ (see eqn ( 9)) during the oxidation of 2 by Ce 4+ is consistent with the enhanced oxygen production by 2 in the presence of 4, as shown in Fig. 1.
A 1 H NMR experiment was carried out on the mixture of 2-4 in D 2 O-CD 3 CN to further probe the processes occurring on addition of Ce 4+ (Fig. 4).The characteristic C6 proton resonance of the bipy ligand, located between 9 to 10 ppm, is affected by the adjacent chlorido or aqua ligand in [Ru(terpy)-(xbipy)Cl] + or [Ru(terpy)(xbipy)(OH 2 )] 2+ is a useful tool to identify the presence of [Ru II -Cl] + or [Ru II -OH 2 ] 2+ species. 11,13s shown in Fig. 4, the C6 proton at 9.45 ppm of 2 disappears on addition of Ce 4+ to the mixture because of the formation of paramagnetic higher-valency ruthenium species.Over a period of 180 minutes, [Ru II -OD 2 ] 2+ was clearly generated as indicated by the reappearance of the C6 proton signal.
Although the stepwise processes (3) to (5) in the 2-4 mixture could not be identified from the UV-Visible spectral data, process (5) or the regeneration of [Ru II -OH 2 ] 2+ (or the deuterated forms) was evident from the time-course 1 H NMR data.
The cyclic voltammograms of 1, 2, 3 and 4 as individual complexes and in mixtures were measured in CH 3 CN-0.1 M HClO 4 (1 : 6) at the concentration 0.48 mM and the data are presented in Fig. 6, 7 and Table 1.The Ru II /Ru III oxidation potentials (E pa vs. NHE) of 1 and 2 of 0.97 V and 0.93 V, respectively, lie within the range of those reported for closelyrelated complexes. 11The Ru II /Ru III oxidation potentials (E pa vs. NHE) for 3 and 4 were observed at 1.37 V and 1.39 V, respectively.There were no significant differences in these potentials when measured as mixtures of complexes.
A close analysis of the cyclic voltammetry of the individual catalysts reveals a significant increase in current starting from 1.37 V (Fig. 6, bottom) in 2 which, as described in eqn ( 2) and (3), may be attributed to the formation of [Ru IV vO] 2+ and [Ru V vO] 3+ leading to the onset of water oxidation.The redox waves of the [Ru IV vO] 2+ and [Ru V vO] 3+ are generally not welldefined as they coincide with the onset of water oxidation.On the other hand, the cyclic voltammetry of 1 reveals that the oxidation processes occurs at much higher potential (1.55 V).The predicted [Ru IV vO] 2+ and [Ru V vO] 3+ species that are accessible at a reasonably lower potential in 2 may be 'switched on' by both redox mediators 3 and 4 as the Ru II /Ru III potentials of the two complexes occur in the same range.In contrast, this may be more difficult for catalyst 1 because the higher potential results in a slight increase in the reaction driving force.This postulate is consistent with both the oxygen evolution and the UV-Visible spectrophotometric data presented earlier.
The significant difference in the oxygen evolution performance of 2 in the presence of 3 and 4 is interesting (Fig. 1), and worthy of further comment.From the electrochemistry point of view, although the anodic potentials (E pa ) of both 3 and 4 are identical, the cathodic potential (E pc ) of 4 was significantly lower (see Table 1).When compared with [Ru(bipy) 3 ] 2+ , it is feasible that the [Ru( phen) 2 (Me 2 bipy)] 2+/3+ redox process in the acidified, and predominantly aqueous, solution may be accompanied by structural changes that lead to electro- by Ce 4+ (eqn (10)), an intra-molecular electron transfer between the two ruthenium centres leads to a redox equilibrium described in eqn (11). 10Further reactions (eqn ( 12)-( 14)) follow what was described earlier in eqn ( 4), ( 6) and ( 7). 10 Ru II /Ru III (1) 0.97 0.90 0.94 70 a An increase in current starting from 1.37 V for catalyst 2 (Fig. 6, top) reflects the Ru III /Ru IV and Ru IV /Ru V oxidation processes.b Ru III / Ru IV and Ru IV /Ru V oxidation processes in 1 predicted to start at 1.55 V.
In keeping with this study, an (inter-molecular) electron transfer between [Ru( phen) 2 (Me 2 bipy)] 3+ in 4 and [Ru IV vO] 2+ in 2 was demonstrated in the present study suggesting a similar mechanism of the cooperative effect proposed in Fig. 8.

Conclusions
This study has provided evidence of the enhancement of the water oxidation catalytic properties of 2 under the influence of the redox mediator 4, [Ru( phen) 2 (Me 2 bipy)] 3+ .The ability of 4 to accelerate the critical oxidation of the catalyst, [Ru IV vO] 2+ → [Ru V vO] 3+ , may be due in part to the slightly better potential match between the two redox events when compared to [Ru(bpy) 3 ] 2+ (3).Work is currently underway to further probe such electron transfer processes using ruthenium catalysts and mediators appropriate for immobilisation on electrode surfaces.This accessibility of the electron transfer between the redox mediator (dye) and the catalyst is significant for the design of catalytic assemblies for the photo-induced oxidation of water.For example, mediator-catalyst assemblies can be attached to, for example, dyed-titania semiconductor films to construct photoanodes capable of splitting water with visible light. 15

Materials and synthesis
[Ru(bipy) 3 ]Cl 2 and (NH 4 ) 2 [Ce(NO 3 ) 6 ] were purchased from Sigma Aldrich and used as received.All other chemicals were sourced from commercial suppliers and used without further purification except where otherwise indicated.

Oxygen evolution
Oxygen measurements were performed using a Unisense OXY-500 microsensor connected to a Unisense OXY Meter.The sensor was calibrated using air and argon for 100% O 2 and 0% O 2 , respectively.The signal was processed using Sensor Trace software.To measure oxygen evolution by the individual complexes and the mixtures (1 : 1 complex to redox mediator), each complex and redox mediator (0.51 μmol) was suspended in 0.1 M HClO 4 (6 ml) in an air-tight vessel.Argon was then purged to obtain zero oxygen reading.A solution of (NH 4 ) 2 [Ce(NO 3 ) 6 ] (0.28 g, 0.51 mmol) in 0.1 M HClO 4 (1 ml) was charged through a septum after the reading had stabilised for 20 min.The final concentrations of the complexes and the Ce 4+ in the vessel were 73 µM and 0.073 M, respectively.The oxygen sensor was measured in the headspace and the signal was collected for 6 to 10 h.

UV-Visible spectrophotometry
UV-Visible spectra were recorded on a Varian Cary 300 spectrophotometer.To measure the spectra of either individual complexes or the mixture of complexes, each compound (0.51 μmol) was dissolved in distilled water (0.5 ml) and 0.1 M HClO 4 (6.5 ml) was then added.A 1 ml aliquot was taken from Fig. 8 (Top) Proposed mechanism of the mediator/catalyst cooperative effect observed in the 2-4 mixture. 9,10The catalytic mechanism involving only Ce 4+ is given in the bottom diagram. 8The green colour indicates the catalyst and the red indicates the redox mediator.
this stock into a 1 cm path length cuvette and 1.95 of 0.1 M HClO 4 was added.The initial spectrum was recorded prior to adding 0.05 ml (NH 4 ) 2 [Ce(NO 3 ) 6 ] in 0.1 M HClO 4 .The concentration of the complex and (NH 4 ) 2 [Ce(NO 3 ) 6 ] in the cuvette were 24 µM and 97 µM, respectively.The spectra were then recorded in 2, 5, 10 and 15 min intervals for 15 h.The spectrum was then recorded every 10 min for the first 30 min then every 30 min for additional 2.5 h.

Electrochemistry
Electrochemical measurements were performed on a VSP Bio-Logic potentiostat connected to a three-electrode system.Cyclic voltammetry of the individual complexes and the mixtures were measured in CH 3 CN-0.1 M HClO 4 (1 : 6) at the scan rate 50 mV s −1 on a glassy carbon working electrode.A platinum wire was used as an auxiliary electrode and Ag/AgCl electrode was used the reference.The concentration of each complex in all cases was 0.48 mM.The observed potentials were corrected relative to NHE.

Fig. 1 (
Fig. 1 (Top) Oxygen evolution from the reaction between the complexes and Ce 4+ in 0.1 M HClO 4 detected in the headspace.(Bottom) Turnover numbers (TONs).Blue = 2-4 mixture; green = 1 only; black = 1-4 mixture; orange = 2-3 mixture; purple = 2 only; red = 4 only.The error bars of the full and crossed lines are in grey.In all cases, the concentration of each complex and Ce 4+ were 73 µM and 0.073 M, respectively.

Fig. 2
Fig. 2 Spectral changes before and after addition of four equivalent Ce 4+ in (a) 2 only; (b) 2-4 mixture and (c) 1-4 mixture.In all cases the complex concentration was 24 µM.

Fig. 4 Fig. 3 (
Fig. 4 Time dependence of the evolution of the 1 H NMR spectrum of a 1 : 1 mixture of 2 and 4 in CD 3 CN-0.1 M HClO 4 in D 2 O (1 : 4) (4.22 mM) before and after addition of four equivalent Ce 4+ .

Fig. 7 (
Fig. 7 (Top) Comparison of the cyclic voltammetry of mixtures of 1-3 (green) and 1-4 (red) in top panel and of 2-3 (green) and 2-4 (red) in bottom panel.Complex concentrations, electrode configuration and CV measurement conditions as indicated in caption to Fig. 6.

Fig. 6 (
Fig. 6 (Top) Comparison of the cyclic voltammetry (CV) of 1 (black) and 2 (red) showing the Ru II /Ru III redox wave and the predicted Ru III to Ru IV and Ru IV to Ru V oxidations leading to the onset of water oxidation.(Bottom) Comparison of the CV traces showing the [Ru(bipy) 3 ] 2+/3+ redox wave for 3 (green) and [Ru( phen) 2 (Me 2 bipy)] 2+/3+ in 4 (red).The traces were recorded at 0.48 mM complex concentrations in CH 3 CN-0.1 M HClO 4 (1 : 6) using a three electrode system comprising a glassy carbon working electrode, platinum auxiliary electrode and Ag/AgCl reference electrode at the scan rate 50 mV s −1 at 23 °C.

Table 1
Assignment of the redox potentials obtained from the CV experiments of the individual complexes and the mixtures, at 0.48 mM concentrations, carried out in CH 3 CN-0.1 M HClO 4 (1 : 6) on a glassy carbon electrode at the scan rate 50 mV s −1 .E 1/2 = (E pa − E pc )/2 in volts relative to NHE; ΔE = |E pa − E pc | in mV Complexes and mixtures Assignment E pa (V) E pc (V) E 1/2 (V) ΔE (mV)