Accepted Article Title: Benchmarking Water Oxidation Catalysts Based on Iridium Complexes: Clues and Doubts on the Nature of Active Species Authors: Alceo Macchioni, Gabriel Menendez Rodriguez, Giordano Gatto, and Cristiano Zuccaccia This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: ChemSusChem 10.1002/cssc.201701818 Link to VoR: http://dx.doi.org/10.1002/cssc.201701818
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FULL PAPER Benchmarking Water Oxidation Catalysts Based on Iridium Complexes: Clues and Doubts on the Nature of Active Species
Abstract: Water Oxidation (WO) is a central reaction in the photoand electro-synthesis of fuels. Iridium complexes have been successfully exploited as water oxidation catalysts (WOCs) with remarkable performances. Herein we report a systematic study aimed at benchmarking well-known Ir WOCs, when NaIO4 is used to drive the reaction. In particular, the following complexes were studied: cis-[Ir(ppy)2(H2O)2]OTf (ppy = 2-phenylpyridine) 1, [Cp*Ir(H2O)3]NO3 (Cp* = cyclopentadienyl anion) 2, [Cp*Ir(bzpy)Cl] (bzpy = 2-benzoylpyridine) 3, [Cp*IrCl2(Me2-NHC)] (NHC = Nheterocyclic carbene) 4, [Cp*Ir(pyalk)Cl] (pyalk = 2-pyridineisopropanoate) 5, [Cp*Ir(pic)NO3] (pic = 2-pyridine-carboxylate) 6, [Cp*Ir{(P(O)(OH)2}3]Na 7, and mer-[IrCl3(pic)(HOMe)]K 8. Their . 2reactivity was compared with that of IrCl3 nH2O 9 and [Ir(OH)6] 10. Most measurements were carried out in phosphate buffer (0.2 M), where 2, 4, 5, 6, 7, and 10 showed very high activity (yield close to -1 100%, TOF up to 554 min with 10, the highest ever observed for a WO driven by NaIO4). The found order of activity is: 10 > 2 ≈ 4 > 6 > 5 > 7 > 1 > 9 > 3 > 8. Clues concerning the molecular nature of the active species were obtained, whereas its exact nature remains doubtfully.
Introduction Water oxidation (WO) to molecular oxygen is considered the ideal reaction for producing electrons and protons to be exploited in the reductive generation of solar fuels.[1],[2] This has caused an explosion of interest for developing efficient water oxidation catalysts (WOCs).[3],[4] Tremendous progresses have been obtained over the last few years with material-based,[5],[6],[7] molecular[8],[9],[10],[11],[12],[13],[14] and heterogenized WOCs.[15],[16],[17],[18] Because WO is an endergonic redox process, the evaluation of WOCs’ performance are carried out by coupling it with a markedly exergonic redox semi-reaction, utilizing chemical and photochemical oxidants,[19] or applying a high electrochemical redox potential. The scales of activity that are deduced from those experiments strongly depend on the nature of the selected oxidant.[20] Even when a single oxidant is considered, a quantitative comparison of WOCs performances
[a]
[b]
are extremely difficult because different research groups carry out the catalytic tests under different conditions, thus simply contrasting TOF (turnover frequency) and TON (turnover number) values may have a little sense. The above considerations convinced us to carry out a comparative evaluation of the performances of molecular WOCs based on iridium, which attracted much attention over the last ten years,[21] due to their remarkable TOF and TON values, under exactly the same experimental conditions. It was decided to test them using NaIO4 as sacrificial oxidant because, being less aggressive than cerium ammonium nitrate (CAN) [22],[23],[24] and not showing by itself any tendency to form nanoparticles, as CAN does, it avoids complications in the kinetic studies. Furthermore, Reek and co-workers have already reported an elegant and comparative study for some Ir WOCs, using CAN as sacrificial oxidant.[25] The following catalysts were selected for this study (Scheme 1): cis-[Ir(ppy)2(H2O)2]OTf (ppy = 2-phenylpyridine) 1, the first Ir molecular WOC, which is also the first organometallic WOC in general, reported by Bernhard in 2008;[26] [Cp*Ir(H2O)3]NO3 (Cp* = cyclopentadienyl anion) 2, the tris-acquo Cp* complex independently reported by Crabtree[27] and our group in 2010;[28] [Cp*Ir(bzpy)Cl] (bzpy = 2-benzoylpyridine) 3,[28],[29],[30] analogous to the first IrCp* WOC reported by Crabtree [Cp*Ir(ppy)Cl] in 2009,[31] but slightly more active and more soluble in water; [Cp*IrCl2(Me2-NHC)] (Me2-NHC = N-dimethylimidazolin-2ylidene) 4,[32],[33],[34] as a representative of the Ir carbene WOCs;[35],[36] [Cp*Ir(pyalk)Cl] (pyalk = 2-pyridine-isopropanoate) 5,[37],[38] perhaps, the most investigated Ir WOC;[39] [Cp*Ir(pic)NO3] (pic = 2-pyridine-carboxylate) 6, apparently the fastest Ir WOC with both CAN[40] and NaIO4;[41] [Cp*Ir{(P(O)(OH)2}3]Na 7,[17] which easily liberates up to three coordination sites by the oxidative expulsion of phosphate; mer[IrCl3(pic)(HOMe)]K 8,[42] bearing the pic ligand as 6, thus potentially able to generate the same active species under the assumption that Cp* completely degrades;[24] IrCl3.nH2O 9[43] and [Ir(OH)6]2- 10, prepared according to the procedure reported by Mallouk.[44],[45],[46],[47] The results of this study, beside to reliably determine the relative activity of the various catalysts, allow also to obtain some clues concerning the active species, which appears to have a molecular nature.
G. Menendez Rodriguez, G. Gatto, Prof. Dr. Cristiano Zuccaccia and Prof. Dr. Alceo Macchioni Department of Chemistry, Biology and Biotechnology University of Perugia Via Elce di Sotto 8, 06123 – Perugia (Italy) E-mail: [email protected] Prof. Dr. Alceo Macchioni Department of Chemistry and Applied Biosciences ETH Zürich Vladimir-Prelog-Weg 2, CH-8093 – Zürich (Switzerland) Supporting information for this article is given via a link at the end of the document.
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Gabriel Menendez Rodriguez,[a] Giordano Gatto,[a] Cristiano Zuccaccia[a] and Alceo Macchioni*[a][b]
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FULL PAPER the complex activation and speciation of WOCs. Two fitting examples are illustrated in Figure 1. The first refers to catalyst 1 and shows that, even when an almost ideal and monoexponential TON trend is observed, the derivative (TOF) exhibits some noise that is perfectly averaged by the PB fitting (Figure 1, left). When a rather slow catalyst as 8 is considered (Figure 1, right), the derivative of the experimental TON versus t trend exhibits a lot of oscillations and an extended regime of linearity, where the TOF remains constant.
Results and Discussion Catalytic tests were prevalently performed in water at pH 7 by 0.200 M phosphate buffer at 25 °C. Kinetics was carried out using differential manometry, which allows obtaining reliable TON and TOF parameters. For each catalyst, the orders in catalyst and NaIO4 were determined by varying the catalyst concentration (1 µM, 2.5 µM, 5 µM and 10 µM) at [NaIO4] = 20 mM and the NaIO4 concentration (5 mM, 10 mM, 20 mM, 40 mM) at [cat] = 5 µM, respectively. Under those conditions of micromolar catalyst concentration and millimolar sacrificial oxidant concentration, all previous DLS measurements did not show any evidence of nanoparticles formation, for some catalysts considered in this paper[41] and even for catalysts that, in principle should be me more prone to form them.[48] Some kinetic measurements were duplicate and triplicate to evaluate the error in TOF and TON values that was found to be approximately equal to 10 %. Data are reported in Table 1S.
Fitting methodology and analysis of kinetic data Rate of O2 production as a function of time (t) [and TOF versus reaction conversion (X)] trends were obtained by the derivative of experimental O2 versus t trends. Due to extreme sensitivity of manometry to very small ambient alterations, the O2 versus t experimental profiles exhibited some apparently irrelevant oscillations that, nevertheless, makes their numerical derivative noisy (Figure 1). It was thought that an obvious solution to this problem was to fit O2 versus t trends with whatever mathematical function, capable of perfectly following the trend but averaging oscillations, and perform the derivative of the resulting analytical fitting equation. After many unsuccessful attempts, using the mathematical functions implemented in standard software, excellent fittings of the measured O2 versus t trends by manometry were obtained with a composite mathematical function developed by Peters and Baskin (PB) for distinguishing sigmoidal and bi-linear growth profiles of plant roots (Supporting Information).[49] It has the big advantage to perfectly follow the trends of O2 productions even when asymmetric sigmoidal profiles, with extended linear regions, are present. These trends are often observed in WO catalysis due to
Figure 1. Experimental (grey) and PB fitting (red) of TON (top) and TOF (bottpm) versus t trends for catalyst 1 (left, [cat] = 5 µM, [NaIO4] = 40 mM) and 8 (right, [cat] = 2.5 µM, [NaIO4] = 20 mM).
As illustrated in Figure 1 (right), the PB mathematical equation nicely averages the oscillations, even in this extreme case, and perfectly follows the experimental trend. All O2 versus t data were fitted with the PB function. The derivative of the PB fits provided reliable reaction rate trends as function of time. Reaction rate over catalyst concentration led to TOF, which was plotted versus X [= 2nO2/n(NaIO4)0]. A summary of the most important kinetic and catalytic data is reported in Table 1. Particularly, Table 1 shows the ranges of TOFmax and yield, and the order in NaIO4 (n) and catalyst (m) for each catalyst. Yield was evaluated by the ratio between the observed TON over that expected, based on the concentrations of NaIO4 and catalyst.
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Scheme 1. Ir WOCs considered in this study.
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FULL PAPER
Entry
Cat
TOFmax -1 (min )
Yield (%)
Xmax
1
1
78 ± 6 215 ± 19
86/100
2
2
180 ± 16 444 ± 37
3
3
4
[a]
m
n
0.05/0.25
0.89
0.49
96/100
0.07/0.25
0.91
0.45
25 ± 2 68 ± 6
70/100
0.10/0.34
0.78
0.49
4
145 ± 14 394 ± 35
97/100
0.05/0.27
0.87
0.46
5
5
179 ± 16 369 ± 33
85/100
0.04/0.31
0.89
0.35
6
6
165 ± 15 465 ± 41
92/100
0.06/0.16
0.98
0.48
7
7
129 ± 11 408 ± 36
89/100
0.07/0.28
0.91
0.49
8
8
6 ± 0.8 10 ± 0.9
17 /92
[b]
0.04/0.12
0.85
-0.15
9
9
60 ± 6 116 ± 10
80/100
0.07/0.25
0.98
0.47
10
10
204 ± 20 554 ± 55
92/100
0.04/0.26
0.87
0.26
[a]
Xmax is defined as the reaction conversion at which TOFmax is observed. Due to the low TOF, kinetic experiments with a large excess of NaIO4 were stopped before the end (it is presumable that they go to completion as observed for all other catalysts). [b]
Six graphs are reported in SI for each catalyst, showing: 1) O2 versus t at different catalyst concentration; 2) rate of O2 production at different catalyst concentration; 3) TOF versus X at different catalyst concentration; 4) O2 versus t at different NaIO4 concentration; 5) rate of O2 production at different NaIO4 concentration; 6) TOF versus X at different NaIO4 concentration. An example of such graphs is reported in Figure 2 for catalyst 6. Looking at the plots of Figure 2 and analogous ones reported in SI, it is immediately clear that catalysts 1-10 are extremely efficient. From O2 versus t and rate versus t trends, it is evident that all moles of O2, expected based on the amount of NaIO4 used, are formed at all catalyst and NaIO4 concentrations. TOF versus X plots are instead useful to graphically evaluate the order in catalyst according to reaction progress kinetic analysis (RPKA) proposed by Blackmond.[50],[51] For instance, because the TOF versus X plots at different catalyst concentration values show a substantial overlapping (Figure 2 and SI) it can be immediately concluded that reaction order in catalyst is 1. TOF versus X plots can also provide precious information about the tendency of catalysts to activate (vide infra).
Figure 2. Kinetic trends for catalyst 6 (water solution at pH 7 by phosphate buffer, 25 °C) at different catalyst (top: black, 1 mM; red, 2.5 mM; blue, 5 mM; green, 10 mM) and NaIO4 (bottom: grey, 5 mM; orange, 10 mM; blue, 20 mM; brown, 40 mM) concentration.
The order in catalyst, m, approaches 1 for all catalysts, as mention above (Table 1). It is interesting to notice that m = 1.6/1.7 was found for catalysts 2, 4 and 5, when WO reaction is driven by CAN.[25] This was attributed to the possibility of WOCs to form multimetallic species exhibiting a higher activity than the single site precursors.[25] Apparently, the formation of such multimetallic species in unlikely when NaIO4 is used under our experimental conditions. The order in NaIO4, evaluated at the time of TOFmax, is close to 0.5 for all catalysts except 8. For the latter an apparent zero order is observed. Because the maximum activity is reached at different X and t values on varying both catalyst and NaIO4 concentration, such an order cannot be used to deduce reliable mechanistic information. As a matter of fact, logkobs/log[NaIO4] plots carried out at low X values, for which the initial rate approximation can be considered valid, provide an order ranging from 0.7 to 0.9 for all catalysts (SI).
Comparative analysis of catalysts performance 2, 4, 5, 6, 7 and 10 catalysts exhibit extremely high TOFmax values (> 300 min-1, Figure 3), whereas the order of reactivity of the other catalysts is the following: 1>9>3>8 Interestingly, compound 8, bearing the pic-ligand, but missing the Cp*-one, is the slowest catalyst (TOFmax ≤ 10 min-1), whereas 6 having both pic- and Cp*-ligands is (one of) the fastest (TOFmax ≤ 465 min-1). 8 is also slower than IrCl3.nH2O (9) that, in principle, could be generated by 8 after pic and MeOH degradation/detachment from the metal center. Complex 1 is more than three times faster than 3, both bearing a pyridinecyclometallated ligand. In strict contrast, when CAN is used as sacrificial oxidant 3 shows a TOF 8.4-12.7 min-1,[29] which is 4771 times higher than that of 1 (0.18 min-1),[26] confirming that the scale of activity can be strongly dependent on the choice of the oxidant. Coming to the fastest catalysts, the best performance is observed for [Ir(OH)6]2- (10), which reaches the highest ever
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Table 1. Summary of kinetic and catalytic performances of catalysts 1-10 (water solution at pH 7 by phosphate buffer, 25 °C).
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FULL PAPER As mentioned above, all catalysts, except 8 at very high [NaIO4]/[cat] ratio, are able to accomplish all expected cycles based on the concentration of NaIO4.
Figure 4. TOF versus X (left) and t (right) for catalytic experiments performed with [cat] = 5 µM and [NaIO4] = 5 mM (water solution at pH 7 by phosphate buffer, 25 °C).
Figure 3. TOF values for the studied catalysts as a function of catalyst (left, [NaIO4] = 20 mM) and NaIO4 (right, [cat] = 5 µM) concentration (water solution at pH 7 by phosphate buffer, 25 °C).
Complexes 2, 4, 5, 6 and 7 also exhibit remarkable TOF values not that far from that of 10 (Figure 3). For instance, at [NaIO4] = 20 mM; [cat] = 5 µM, which are the typical conditions in our lab, the order of activity is the following: 10 (402 min-1) > 2 (320 min-1) ≈ 4 (316 min-1) > 6 (288 min-1) ≈ 5 (286 min-1) > 7 (255 min-1) Attempts to evaluate the performance of IrOx.nH2O nanoparticles 11 (mean diameter of 2 nm), prepared according to the procedure reported by Mallouk,[52] were conducted (SI), despite we were aware that 11 are stable only at acidic pH, whereas at neutral/basic pH they are in equilibrium with 10. The 10/11 solutions thus obtained show TOF values that are significantly lower than those of solutions containing only 10, at the same iridium and NaIO4 concentrations.
It is worth noticing that catalyst 6, bearing the pic-ligand is easier to activate than the other fast catalysts. 0.16 is the highest Xvalue at which TOFmax is reached for 6, whereas it is >0.25 for 2, 4, 5, 7 and 10 (Table 1). This phenomenon is even more evident when experiments with the lowest [NaIO4]/[cat] ratio are considered (Figure 4, top); 6 shows a TOFmax when X = 0.08, whereas TOFmax occurs at X > 0.20 for all other catalysts, except 8 (X = 0.08), which is extremely slow but it also bears the picligand (Figure 4). Also in terms of time of activation, 6 is the fastest, followed by 4, 2, 5, and 7, focusing on the most active catalysts (Figure 4, bottom). It is not surprising that 7 needs more time than other catalysts to reach TOFmax because it must undergo at least an oxidative expulsion of H3PO4 before becoming active.[17] Some catalytic experiments were carried out using 20 mM of NaIO4 and 5 µM of catalysts at pH 7, regulated by the addition of KOH, for 2, 4, 5, 6 and 10. Results are reported in SI and graphically illustrated in Figure 5. A decrease of activity is observed for all catalysts but the same scale than in buffered solution is found. It is presumable that the presence of phosphate bases facilitates the proton removal from the water molecule that is attacking the Ir=O moiety in the rate determining
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reported TOF = 554 min-1 ([NaIO4] = 40 mM; [cat] = 5 µM), when NaIO4 is used as electron acceptor. Because 10 is known to be an intermediate of the formation of IrOx.nH2O nanoparticles (11),[52] its monomeric nature was checked before each single catalytic experiment by recording the UV-Vis spectrum. Differently from 11, which has an absorption band at 580 nm, 10 has no visible absorbance but a strong UV-band at 313 nm.[44]
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step, as proposed by Meyer for ruthenium complexes.[15] Interestingly, at pH 7, but in the absence of the phosphate buffer, IrOx.nH2O nanoparticles 11 exhibit a much smaller TOF (ca 200 min-1, [cat] = 5 mM, [NaIO4] = 20 mM), which might be due to their higher difficulty to disaggregate into 10.
Figure 5. TOF of some fastest catalysts for catalytic experiments performed at pH 7 at 25 °C in buffered (red) and unbuffered (black) solutions with [cat] = 5 µM and [NaIO4] = 20 mM.
On the nature of active species Soon after the first reports on the activity of Ir organometalliccomplexes (IrOMCs) in WO catalysis, a hot controversy about the nature of active species raised in the literature, stimulated by some papers[22],[43],[53],[54] reporting evidence on the transformation of some IrOMCs into IrOx NPs, which were known to be efficient WOCs.[55] In the reality, the formation of IrOx NPs was demonstrated only in a few cases, mostly when CAN is exploited as sacrificial oxidant and under conditions often far from those used in catalysis (millimolar concentration of catalyst and up to molar concentration of sacrificial oxidant).[22],[43],[53] With NaIO4 as sacrificial oxidant, DLS evidence for the formation of IrOx NPs was provided by Fujita and co-workers when [Cp*Ir(6,6’-(OH)2-bpy)(H2O)]2+ (1 mM) was used as pre-catalyst.[56],[57] Particularly, they found that a transient species A, responsible for most of the catalytic activity, were likely IrOx NPs, having dimensions in the range 0.5-1.2 nm, thus containing 4 or 5 Ir atoms, as previously described by Harriman and co-workers.[58] Increasing the concentration of NaIO4, B species with higher dimensions (ca 120 nm), and much lower activity, formed. Crabtree and co-workers also reported evidence for IrOx NPs formation, using [Cp*2Ir2(µ-OH)3]OH (2.5 mM), with lag phases ranging from 5 min to 3 h after being injected into a 250 mM NaIO4 solution at room temperature.[38] They did not observe any particle formation for the same compound when a 125 mM NaIO4 solution was used and when various chelate ligands such as bpy, ppy, or pyalk were coordinated at Cp*Ir moiety.[38] The same group at Yale demonstrated that IrOx NPs formed when pyalk was used as ancillary ligand starting from [Ir(CO)2(pyalk)] (0.25 mM) precursor, with a 250 mM NaIO4 solution. However, when small aliquots of acetic acid were added (50 equivalents), IrOx NPs formation was delayed by 4 h.[59] This is rather interesting
because acetic acid is known to be one of the main products of Cp* oxidative transformation.[24] Finally, it was shown that complex 4 (2.5 mM solution) generates IrOx NPs when reacted with a 250 mM NaIO4 solution.[60] In many other cases where IrOMCs have been successfully applied in WO, their tendency to form IrOx NPs was studied without obtaining any evidence of that.[6],[61],[62],[63],[64],[41],[65],[66],[67] Although IrOx.nH2O nanoparticles 11 could not be properly tested, due to their transformation back to 10 under our experimental conditions, the results herein reported clearly indicate that their activity is smaller than that of molecular [Ir(OH)6]- (10), consistently with previously reported results indicating an improvement of performance when the dimensionality of the active species is decreased.[56],[68] This points to the conclusion that, whatever the real active species is, it must have a dimension smaller than that or IrOx.nH2O nanoparticles 11 (mean diameter of 2 nm) and larger or equal than that of [Ir(OH)6]- (10). So it has to be molecular in nature. On the other hand, because 10 is the fastest catalyst, the possibility that all precursors generate 10 as the real active species, due to the oxidative degradation of all organic ligands, cannot be excluded. Nevertheless, this hypothesis does not agree with the fast activation of all precursors (except 7, for wellknown reasons discussed above), which is comparable and, for 6, even faster than of 10 itself (Figure 4). NMR studies are in progress in our lab to clarify this important point on the nature of the active sites. Results will be reported in the due time.
Conclusions A systematic study on the catalytic activity of some archetypical iridium WOCs, exploiting NaIO4 as electron acceptor, has been herein described. Under exactly the same experimental conditions, the most active catalyst was found to be molecular [Ir(OH)6]2- (10), which exhibits a record TOF for NaIO4 driven WO of 554 min-1. Also when organometallic (1-7) and coordination complexes (8-9) were used as WOC precursors extremely high efficiencies were obtained with O2 yield always approaching 100% and TOF values, up to 476 min-1, following the order: 2 ≈ 4 > 6 ≈ 5 > 7 > 1 > 9 > 3 > 8. The observation of much lower activity of IrCl3.nH2O (9), thought to be responsible of IrOx.nH2O nanoparticle formation, with respect to many catalysts and especially the lower activity of IrOx.nH2O nanoparticles 11, having a mean diameter of 2 nm, than 10, suggest that the active site is molecular in nature.
Experimental Section All solvents and reagents were purchased from Sigma-Aldrich and used without any further purification. Purelab Option-R7 (Elga labwater) was used to purify water (Resistivity = 18 MΩ-cm; TOC < 20 ppb). Solutions of NaIO4 for kinetic experiments were prepared by dissolving NaIO4 (ACS reagent, ≥ 99.8%) in ultrapure water. Buffer solutions were prepared by mixing different volumes of previously prepared acid and base stock solutions (0.2 M). pH was checked using a Hanna Instrument pH-meter HI 2221. UV-Vis spectra were acquired on a Cary 8454 UV-Vis Diode
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Catalytic activity was evaluated using NaIO4 as sacrificial oxidant. The experiments were conducted at pH 7 by Na2HPO4/NaH2PO4 buffer (0.2 M). The formation of molecular oxygen, in the gas phase, was detected with a differential manometer Testo 521-1. Manometric measurements were performed using two homemade jacketed glass reactors coupled to the manometer. In a typical run, NaIO4, weighed with an analytical balance, was transferred directly into the measuring reactor and dissolved in 4.5 - 4.9 mL of buffered solution. The same amount of water was added to the reference reactor. Both reactors were equipped with a side arm to connect to the manometer and a septum to seal the system, which was kept at a constant temperature (T = 25°C). After ca. 20 min, equilibration was reached and the measurement was started. Afterward, 100 - 500 µL of water and catalyst solution were injected into the reference and measuring reactors. The concentration of the stock solution of catalyst was adjusted, depending on the desired final concentration of the catalyst, in order to have a final volume of 5 mL. The catalyst concentration ranged from 1 to 10.0 µM; that of NaIO4 was 5 – 40.0 mM.
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This work was financially supported by SABIC, PRIN 2015 (20154X9ATP_004) and University of Perugia (Fondo Ricerca di Base 2014 D.D. n. 170, 23/12/2014). AM thanks ETH Zürich for a visiting professorship and Christophe Copéret for the very kind hospitality in his laboratory, where this manuscript was written.
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Keywords: water oxidation • iridium complexes • iridium nanoparticles • kinetics • TOF and TON
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[36]
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The activity of archetypal iridium WOCs has been evaluated under exactly the same experimental conditions (pH 7, 25°C), exploiting NaIO4 as electron acceptor. [Ir(OH)6]2- was to found be the most active catalysts reaching a record value, for NaIO4 driven WO, of 554 min-1. Comparative kinetic analysis suggests that the active species has to be molecular in nature.
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Title
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FULL PAPER Benchmarking Water Oxidation Catalysts Based on Iridium Complexes: Clues and Doubts on the Nature of Active Species
Abstract: Water Oxidation (WO) is a central reaction in the photoand electro-synthesis of fuels. Iridium complexes have been successfully exploited as water oxidation catalysts (WOCs) with remarkable performances. Herein we report a systematic study aimed at benchmarking well-known Ir WOCs, when NaIO4 is used to drive the reaction. In particular, the following complexes were studied: cis-[Ir(ppy)2(H2O)2]OTf (ppy = 2-phenylpyridine) 1, [Cp*Ir(H2O)3]NO3 (Cp* = cyclopentadienyl anion) 2, [Cp*Ir(bzpy)Cl] (bzpy = 2-benzoylpyridine) 3, [Cp*IrCl2(Me2-NHC)] (NHC = Nheterocyclic carbene) 4, [Cp*Ir(pyalk)Cl] (pyalk = 2-pyridineisopropanoate) 5, [Cp*Ir(pic)NO3] (pic = 2-pyridine-carboxylate) 6, [Cp*Ir{(P(O)(OH)2}3]Na 7, and mer-[IrCl3(pic)(HOMe)]K 8. Their . 2reactivity was compared with that of IrCl3 nH2O 9 and [Ir(OH)6] 10. Most measurements were carried out in phosphate buffer (0.2 M), where 2, 4, 5, 6, 7, and 10 showed very high activity (yield close to -1 100%, TOF up to 554 min with 10, the highest ever observed for a WO driven by NaIO4). The found order of activity is: 10 > 2 ≈ 4 > 6 > 5 > 7 > 1 > 9 > 3 > 8. Clues concerning the molecular nature of the active species were obtained, whereas its exact nature remains doubtfully.
Introduction Water oxidation (WO) to molecular oxygen is considered the ideal reaction for producing electrons and protons to be exploited in the reductive generation of solar fuels.[1],[2] This has caused an explosion of interest for developing efficient water oxidation catalysts (WOCs).[3],[4] Tremendous progresses have been obtained over the last few years with material-based,[5],[6],[7] molecular[8],[9],[10],[11],[12],[13],[14] and heterogenized WOCs.[15],[16],[17],[18] Because WO is an endergonic redox process, the evaluation of WOCs’ performance are carried out by coupling it with a markedly exergonic redox semi-reaction, utilizing chemical and photochemical oxidants,[19] or applying a high electrochemical redox potential. The scales of activity that are deduced from those experiments strongly depend on the nature of the selected oxidant.[20] Even when a single oxidant is considered, a quantitative comparison of WOCs performances
[a]
[b]
are extremely difficult because different research groups carry out the catalytic tests under different conditions, thus simply contrasting TOF (turnover frequency) and TON (turnover number) values may have a little sense. The above considerations convinced us to carry out a comparative evaluation of the performances of molecular WOCs based on iridium, which attracted much attention over the last ten years,[21] due to their remarkable TOF and TON values, under exactly the same experimental conditions. It was decided to test them using NaIO4 as sacrificial oxidant because, being less aggressive than cerium ammonium nitrate (CAN) [22],[23],[24] and not showing by itself any tendency to form nanoparticles, as CAN does, it avoids complications in the kinetic studies. Furthermore, Reek and co-workers have already reported an elegant and comparative study for some Ir WOCs, using CAN as sacrificial oxidant.[25] The following catalysts were selected for this study (Scheme 1): cis-[Ir(ppy)2(H2O)2]OTf (ppy = 2-phenylpyridine) 1, the first Ir molecular WOC, which is also the first organometallic WOC in general, reported by Bernhard in 2008;[26] [Cp*Ir(H2O)3]NO3 (Cp* = cyclopentadienyl anion) 2, the tris-acquo Cp* complex independently reported by Crabtree[27] and our group in 2010;[28] [Cp*Ir(bzpy)Cl] (bzpy = 2-benzoylpyridine) 3,[28],[29],[30] analogous to the first IrCp* WOC reported by Crabtree [Cp*Ir(ppy)Cl] in 2009,[31] but slightly more active and more soluble in water; [Cp*IrCl2(Me2-NHC)] (Me2-NHC = N-dimethylimidazolin-2ylidene) 4,[32],[33],[34] as a representative of the Ir carbene WOCs;[35],[36] [Cp*Ir(pyalk)Cl] (pyalk = 2-pyridine-isopropanoate) 5,[37],[38] perhaps, the most investigated Ir WOC;[39] [Cp*Ir(pic)NO3] (pic = 2-pyridine-carboxylate) 6, apparently the fastest Ir WOC with both CAN[40] and NaIO4;[41] [Cp*Ir{(P(O)(OH)2}3]Na 7,[17] which easily liberates up to three coordination sites by the oxidative expulsion of phosphate; mer[IrCl3(pic)(HOMe)]K 8,[42] bearing the pic ligand as 6, thus potentially able to generate the same active species under the assumption that Cp* completely degrades;[24] IrCl3.nH2O 9[43] and [Ir(OH)6]2- 10, prepared according to the procedure reported by Mallouk.[44],[45],[46],[47] The results of this study, beside to reliably determine the relative activity of the various catalysts, allow also to obtain some clues concerning the active species, which appears to have a molecular nature.
G. Menendez Rodriguez, G. Gatto, Prof. Dr. Cristiano Zuccaccia and Prof. Dr. Alceo Macchioni Department of Chemistry, Biology and Biotechnology University of Perugia Via Elce di Sotto 8, 06123 – Perugia (Italy) E-mail: [email protected] Prof. Dr. Alceo Macchioni Department of Chemistry and Applied Biosciences ETH Zürich Vladimir-Prelog-Weg 2, CH-8093 – Zürich (Switzerland) Supporting information for this article is given via a link at the end of the document.
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Gabriel Menendez Rodriguez,[a] Giordano Gatto,[a] Cristiano Zuccaccia[a] and Alceo Macchioni*[a][b]
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FULL PAPER the complex activation and speciation of WOCs. Two fitting examples are illustrated in Figure 1. The first refers to catalyst 1 and shows that, even when an almost ideal and monoexponential TON trend is observed, the derivative (TOF) exhibits some noise that is perfectly averaged by the PB fitting (Figure 1, left). When a rather slow catalyst as 8 is considered (Figure 1, right), the derivative of the experimental TON versus t trend exhibits a lot of oscillations and an extended regime of linearity, where the TOF remains constant.
Results and Discussion Catalytic tests were prevalently performed in water at pH 7 by 0.200 M phosphate buffer at 25 °C. Kinetics was carried out using differential manometry, which allows obtaining reliable TON and TOF parameters. For each catalyst, the orders in catalyst and NaIO4 were determined by varying the catalyst concentration (1 µM, 2.5 µM, 5 µM and 10 µM) at [NaIO4] = 20 mM and the NaIO4 concentration (5 mM, 10 mM, 20 mM, 40 mM) at [cat] = 5 µM, respectively. Under those conditions of micromolar catalyst concentration and millimolar sacrificial oxidant concentration, all previous DLS measurements did not show any evidence of nanoparticles formation, for some catalysts considered in this paper[41] and even for catalysts that, in principle should be me more prone to form them.[48] Some kinetic measurements were duplicate and triplicate to evaluate the error in TOF and TON values that was found to be approximately equal to 10 %. Data are reported in Table 1S.
Fitting methodology and analysis of kinetic data Rate of O2 production as a function of time (t) [and TOF versus reaction conversion (X)] trends were obtained by the derivative of experimental O2 versus t trends. Due to extreme sensitivity of manometry to very small ambient alterations, the O2 versus t experimental profiles exhibited some apparently irrelevant oscillations that, nevertheless, makes their numerical derivative noisy (Figure 1). It was thought that an obvious solution to this problem was to fit O2 versus t trends with whatever mathematical function, capable of perfectly following the trend but averaging oscillations, and perform the derivative of the resulting analytical fitting equation. After many unsuccessful attempts, using the mathematical functions implemented in standard software, excellent fittings of the measured O2 versus t trends by manometry were obtained with a composite mathematical function developed by Peters and Baskin (PB) for distinguishing sigmoidal and bi-linear growth profiles of plant roots (Supporting Information).[49] It has the big advantage to perfectly follow the trends of O2 productions even when asymmetric sigmoidal profiles, with extended linear regions, are present. These trends are often observed in WO catalysis due to
Figure 1. Experimental (grey) and PB fitting (red) of TON (top) and TOF (bottpm) versus t trends for catalyst 1 (left, [cat] = 5 µM, [NaIO4] = 40 mM) and 8 (right, [cat] = 2.5 µM, [NaIO4] = 20 mM).
As illustrated in Figure 1 (right), the PB mathematical equation nicely averages the oscillations, even in this extreme case, and perfectly follows the experimental trend. All O2 versus t data were fitted with the PB function. The derivative of the PB fits provided reliable reaction rate trends as function of time. Reaction rate over catalyst concentration led to TOF, which was plotted versus X [= 2nO2/n(NaIO4)0]. A summary of the most important kinetic and catalytic data is reported in Table 1. Particularly, Table 1 shows the ranges of TOFmax and yield, and the order in NaIO4 (n) and catalyst (m) for each catalyst. Yield was evaluated by the ratio between the observed TON over that expected, based on the concentrations of NaIO4 and catalyst.
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Scheme 1. Ir WOCs considered in this study.
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FULL PAPER
Entry
Cat
TOFmax -1 (min )
Yield (%)
Xmax
1
1
78 ± 6 215 ± 19
86/100
2
2
180 ± 16 444 ± 37
3
3
4
[a]
m
n
0.05/0.25
0.89
0.49
96/100
0.07/0.25
0.91
0.45
25 ± 2 68 ± 6
70/100
0.10/0.34
0.78
0.49
4
145 ± 14 394 ± 35
97/100
0.05/0.27
0.87
0.46
5
5
179 ± 16 369 ± 33
85/100
0.04/0.31
0.89
0.35
6
6
165 ± 15 465 ± 41
92/100
0.06/0.16
0.98
0.48
7
7
129 ± 11 408 ± 36
89/100
0.07/0.28
0.91
0.49
8
8
6 ± 0.8 10 ± 0.9
17 /92
[b]
0.04/0.12
0.85
-0.15
9
9
60 ± 6 116 ± 10
80/100
0.07/0.25
0.98
0.47
10
10
204 ± 20 554 ± 55
92/100
0.04/0.26
0.87
0.26
[a]
Xmax is defined as the reaction conversion at which TOFmax is observed. Due to the low TOF, kinetic experiments with a large excess of NaIO4 were stopped before the end (it is presumable that they go to completion as observed for all other catalysts). [b]
Six graphs are reported in SI for each catalyst, showing: 1) O2 versus t at different catalyst concentration; 2) rate of O2 production at different catalyst concentration; 3) TOF versus X at different catalyst concentration; 4) O2 versus t at different NaIO4 concentration; 5) rate of O2 production at different NaIO4 concentration; 6) TOF versus X at different NaIO4 concentration. An example of such graphs is reported in Figure 2 for catalyst 6. Looking at the plots of Figure 2 and analogous ones reported in SI, it is immediately clear that catalysts 1-10 are extremely efficient. From O2 versus t and rate versus t trends, it is evident that all moles of O2, expected based on the amount of NaIO4 used, are formed at all catalyst and NaIO4 concentrations. TOF versus X plots are instead useful to graphically evaluate the order in catalyst according to reaction progress kinetic analysis (RPKA) proposed by Blackmond.[50],[51] For instance, because the TOF versus X plots at different catalyst concentration values show a substantial overlapping (Figure 2 and SI) it can be immediately concluded that reaction order in catalyst is 1. TOF versus X plots can also provide precious information about the tendency of catalysts to activate (vide infra).
Figure 2. Kinetic trends for catalyst 6 (water solution at pH 7 by phosphate buffer, 25 °C) at different catalyst (top: black, 1 mM; red, 2.5 mM; blue, 5 mM; green, 10 mM) and NaIO4 (bottom: grey, 5 mM; orange, 10 mM; blue, 20 mM; brown, 40 mM) concentration.
The order in catalyst, m, approaches 1 for all catalysts, as mention above (Table 1). It is interesting to notice that m = 1.6/1.7 was found for catalysts 2, 4 and 5, when WO reaction is driven by CAN.[25] This was attributed to the possibility of WOCs to form multimetallic species exhibiting a higher activity than the single site precursors.[25] Apparently, the formation of such multimetallic species in unlikely when NaIO4 is used under our experimental conditions. The order in NaIO4, evaluated at the time of TOFmax, is close to 0.5 for all catalysts except 8. For the latter an apparent zero order is observed. Because the maximum activity is reached at different X and t values on varying both catalyst and NaIO4 concentration, such an order cannot be used to deduce reliable mechanistic information. As a matter of fact, logkobs/log[NaIO4] plots carried out at low X values, for which the initial rate approximation can be considered valid, provide an order ranging from 0.7 to 0.9 for all catalysts (SI).
Comparative analysis of catalysts performance 2, 4, 5, 6, 7 and 10 catalysts exhibit extremely high TOFmax values (> 300 min-1, Figure 3), whereas the order of reactivity of the other catalysts is the following: 1>9>3>8 Interestingly, compound 8, bearing the pic-ligand, but missing the Cp*-one, is the slowest catalyst (TOFmax ≤ 10 min-1), whereas 6 having both pic- and Cp*-ligands is (one of) the fastest (TOFmax ≤ 465 min-1). 8 is also slower than IrCl3.nH2O (9) that, in principle, could be generated by 8 after pic and MeOH degradation/detachment from the metal center. Complex 1 is more than three times faster than 3, both bearing a pyridinecyclometallated ligand. In strict contrast, when CAN is used as sacrificial oxidant 3 shows a TOF 8.4-12.7 min-1,[29] which is 4771 times higher than that of 1 (0.18 min-1),[26] confirming that the scale of activity can be strongly dependent on the choice of the oxidant. Coming to the fastest catalysts, the best performance is observed for [Ir(OH)6]2- (10), which reaches the highest ever
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Table 1. Summary of kinetic and catalytic performances of catalysts 1-10 (water solution at pH 7 by phosphate buffer, 25 °C).
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FULL PAPER As mentioned above, all catalysts, except 8 at very high [NaIO4]/[cat] ratio, are able to accomplish all expected cycles based on the concentration of NaIO4.
Figure 4. TOF versus X (left) and t (right) for catalytic experiments performed with [cat] = 5 µM and [NaIO4] = 5 mM (water solution at pH 7 by phosphate buffer, 25 °C).
Figure 3. TOF values for the studied catalysts as a function of catalyst (left, [NaIO4] = 20 mM) and NaIO4 (right, [cat] = 5 µM) concentration (water solution at pH 7 by phosphate buffer, 25 °C).
Complexes 2, 4, 5, 6 and 7 also exhibit remarkable TOF values not that far from that of 10 (Figure 3). For instance, at [NaIO4] = 20 mM; [cat] = 5 µM, which are the typical conditions in our lab, the order of activity is the following: 10 (402 min-1) > 2 (320 min-1) ≈ 4 (316 min-1) > 6 (288 min-1) ≈ 5 (286 min-1) > 7 (255 min-1) Attempts to evaluate the performance of IrOx.nH2O nanoparticles 11 (mean diameter of 2 nm), prepared according to the procedure reported by Mallouk,[52] were conducted (SI), despite we were aware that 11 are stable only at acidic pH, whereas at neutral/basic pH they are in equilibrium with 10. The 10/11 solutions thus obtained show TOF values that are significantly lower than those of solutions containing only 10, at the same iridium and NaIO4 concentrations.
It is worth noticing that catalyst 6, bearing the pic-ligand is easier to activate than the other fast catalysts. 0.16 is the highest Xvalue at which TOFmax is reached for 6, whereas it is >0.25 for 2, 4, 5, 7 and 10 (Table 1). This phenomenon is even more evident when experiments with the lowest [NaIO4]/[cat] ratio are considered (Figure 4, top); 6 shows a TOFmax when X = 0.08, whereas TOFmax occurs at X > 0.20 for all other catalysts, except 8 (X = 0.08), which is extremely slow but it also bears the picligand (Figure 4). Also in terms of time of activation, 6 is the fastest, followed by 4, 2, 5, and 7, focusing on the most active catalysts (Figure 4, bottom). It is not surprising that 7 needs more time than other catalysts to reach TOFmax because it must undergo at least an oxidative expulsion of H3PO4 before becoming active.[17] Some catalytic experiments were carried out using 20 mM of NaIO4 and 5 µM of catalysts at pH 7, regulated by the addition of KOH, for 2, 4, 5, 6 and 10. Results are reported in SI and graphically illustrated in Figure 5. A decrease of activity is observed for all catalysts but the same scale than in buffered solution is found. It is presumable that the presence of phosphate bases facilitates the proton removal from the water molecule that is attacking the Ir=O moiety in the rate determining
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reported TOF = 554 min-1 ([NaIO4] = 40 mM; [cat] = 5 µM), when NaIO4 is used as electron acceptor. Because 10 is known to be an intermediate of the formation of IrOx.nH2O nanoparticles (11),[52] its monomeric nature was checked before each single catalytic experiment by recording the UV-Vis spectrum. Differently from 11, which has an absorption band at 580 nm, 10 has no visible absorbance but a strong UV-band at 313 nm.[44]
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step, as proposed by Meyer for ruthenium complexes.[15] Interestingly, at pH 7, but in the absence of the phosphate buffer, IrOx.nH2O nanoparticles 11 exhibit a much smaller TOF (ca 200 min-1, [cat] = 5 mM, [NaIO4] = 20 mM), which might be due to their higher difficulty to disaggregate into 10.
Figure 5. TOF of some fastest catalysts for catalytic experiments performed at pH 7 at 25 °C in buffered (red) and unbuffered (black) solutions with [cat] = 5 µM and [NaIO4] = 20 mM.
On the nature of active species Soon after the first reports on the activity of Ir organometalliccomplexes (IrOMCs) in WO catalysis, a hot controversy about the nature of active species raised in the literature, stimulated by some papers[22],[43],[53],[54] reporting evidence on the transformation of some IrOMCs into IrOx NPs, which were known to be efficient WOCs.[55] In the reality, the formation of IrOx NPs was demonstrated only in a few cases, mostly when CAN is exploited as sacrificial oxidant and under conditions often far from those used in catalysis (millimolar concentration of catalyst and up to molar concentration of sacrificial oxidant).[22],[43],[53] With NaIO4 as sacrificial oxidant, DLS evidence for the formation of IrOx NPs was provided by Fujita and co-workers when [Cp*Ir(6,6’-(OH)2-bpy)(H2O)]2+ (1 mM) was used as pre-catalyst.[56],[57] Particularly, they found that a transient species A, responsible for most of the catalytic activity, were likely IrOx NPs, having dimensions in the range 0.5-1.2 nm, thus containing 4 or 5 Ir atoms, as previously described by Harriman and co-workers.[58] Increasing the concentration of NaIO4, B species with higher dimensions (ca 120 nm), and much lower activity, formed. Crabtree and co-workers also reported evidence for IrOx NPs formation, using [Cp*2Ir2(µ-OH)3]OH (2.5 mM), with lag phases ranging from 5 min to 3 h after being injected into a 250 mM NaIO4 solution at room temperature.[38] They did not observe any particle formation for the same compound when a 125 mM NaIO4 solution was used and when various chelate ligands such as bpy, ppy, or pyalk were coordinated at Cp*Ir moiety.[38] The same group at Yale demonstrated that IrOx NPs formed when pyalk was used as ancillary ligand starting from [Ir(CO)2(pyalk)] (0.25 mM) precursor, with a 250 mM NaIO4 solution. However, when small aliquots of acetic acid were added (50 equivalents), IrOx NPs formation was delayed by 4 h.[59] This is rather interesting
because acetic acid is known to be one of the main products of Cp* oxidative transformation.[24] Finally, it was shown that complex 4 (2.5 mM solution) generates IrOx NPs when reacted with a 250 mM NaIO4 solution.[60] In many other cases where IrOMCs have been successfully applied in WO, their tendency to form IrOx NPs was studied without obtaining any evidence of that.[6],[61],[62],[63],[64],[41],[65],[66],[67] Although IrOx.nH2O nanoparticles 11 could not be properly tested, due to their transformation back to 10 under our experimental conditions, the results herein reported clearly indicate that their activity is smaller than that of molecular [Ir(OH)6]- (10), consistently with previously reported results indicating an improvement of performance when the dimensionality of the active species is decreased.[56],[68] This points to the conclusion that, whatever the real active species is, it must have a dimension smaller than that or IrOx.nH2O nanoparticles 11 (mean diameter of 2 nm) and larger or equal than that of [Ir(OH)6]- (10). So it has to be molecular in nature. On the other hand, because 10 is the fastest catalyst, the possibility that all precursors generate 10 as the real active species, due to the oxidative degradation of all organic ligands, cannot be excluded. Nevertheless, this hypothesis does not agree with the fast activation of all precursors (except 7, for wellknown reasons discussed above), which is comparable and, for 6, even faster than of 10 itself (Figure 4). NMR studies are in progress in our lab to clarify this important point on the nature of the active sites. Results will be reported in the due time.
Conclusions A systematic study on the catalytic activity of some archetypical iridium WOCs, exploiting NaIO4 as electron acceptor, has been herein described. Under exactly the same experimental conditions, the most active catalyst was found to be molecular [Ir(OH)6]2- (10), which exhibits a record TOF for NaIO4 driven WO of 554 min-1. Also when organometallic (1-7) and coordination complexes (8-9) were used as WOC precursors extremely high efficiencies were obtained with O2 yield always approaching 100% and TOF values, up to 476 min-1, following the order: 2 ≈ 4 > 6 ≈ 5 > 7 > 1 > 9 > 3 > 8. The observation of much lower activity of IrCl3.nH2O (9), thought to be responsible of IrOx.nH2O nanoparticle formation, with respect to many catalysts and especially the lower activity of IrOx.nH2O nanoparticles 11, having a mean diameter of 2 nm, than 10, suggest that the active site is molecular in nature.
Experimental Section All solvents and reagents were purchased from Sigma-Aldrich and used without any further purification. Purelab Option-R7 (Elga labwater) was used to purify water (Resistivity = 18 MΩ-cm; TOC < 20 ppb). Solutions of NaIO4 for kinetic experiments were prepared by dissolving NaIO4 (ACS reagent, ≥ 99.8%) in ultrapure water. Buffer solutions were prepared by mixing different volumes of previously prepared acid and base stock solutions (0.2 M). pH was checked using a Hanna Instrument pH-meter HI 2221. UV-Vis spectra were acquired on a Cary 8454 UV-Vis Diode
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Catalytic activity was evaluated using NaIO4 as sacrificial oxidant. The experiments were conducted at pH 7 by Na2HPO4/NaH2PO4 buffer (0.2 M). The formation of molecular oxygen, in the gas phase, was detected with a differential manometer Testo 521-1. Manometric measurements were performed using two homemade jacketed glass reactors coupled to the manometer. In a typical run, NaIO4, weighed with an analytical balance, was transferred directly into the measuring reactor and dissolved in 4.5 - 4.9 mL of buffered solution. The same amount of water was added to the reference reactor. Both reactors were equipped with a side arm to connect to the manometer and a septum to seal the system, which was kept at a constant temperature (T = 25°C). After ca. 20 min, equilibration was reached and the measurement was started. Afterward, 100 - 500 µL of water and catalyst solution were injected into the reference and measuring reactors. The concentration of the stock solution of catalyst was adjusted, depending on the desired final concentration of the catalyst, in order to have a final volume of 5 mL. The catalyst concentration ranged from 1 to 10.0 µM; that of NaIO4 was 5 – 40.0 mM.
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This work was financially supported by SABIC, PRIN 2015 (20154X9ATP_004) and University of Perugia (Fondo Ricerca di Base 2014 D.D. n. 170, 23/12/2014). AM thanks ETH Zürich for a visiting professorship and Christophe Copéret for the very kind hospitality in his laboratory, where this manuscript was written.
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The activity of archetypal iridium WOCs has been evaluated under exactly the same experimental conditions (pH 7, 25°C), exploiting NaIO4 as electron acceptor. [Ir(OH)6]2- was to found be the most active catalysts reaching a record value, for NaIO4 driven WO, of 554 min-1. Comparative kinetic analysis suggests that the active species has to be molecular in nature.
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Title
