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The Ru-tpc Water Oxidation Catalyst and Beyond: Water Nucleophilic Attack Pathway versus Radical Coupling Pathway Ting Fan, Lele Duan, Ping Huang, Hong Chen, Quentin Daniel, Mårten S.G. Ahlquist, and Licheng Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03393 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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The Ru-tpc Water Oxidation Catalyst and Beyond: Water Nucleophilic Attack Pathway versus Radical Coupling Pathway Ting Fan,a Lele Duan,*b, § Ping Huang,c Hong Chen,b Quentin Daniel,b Mårten S. G. Ahlquist*a and Licheng Sun*b,d a

Division of Theoretical Chemistry & Biology, School of Biotechnology, KTH Royal

Institute of Technology, 10691 Stockholm, Sweden. b

Department of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm, Sweden.

c

Department of Chemistry – Ångström Laboratory, Box 523, Uppsala University, SE-751 20

Uppsala, Sweden d

State Key Laboratory of Fine Chemicals, DUT–KTH Joint Education and Research Center

on Molecular Devices, Dalian University of Technology (DUT), Dalian 116012, P. R. China. §

Current address: Department of Chemistry, Southern University of Science and Technology,

Shenzhen 518055, China. E-mail: [email protected]; [email protected]; [email protected]. Abstract Many Ru water oxidation catalysts have been documented in the literature. However, only a few can catalyse the O−O bond formation via the radical coupling pathway while most go through the water nucleophilic attack pathway. Understanding the electronic effect on the reaction pathway is of importance in design of active water oxidation catalysts. The Ru-bda (bda = 2,2′-bipyridine-6,6′-dicarboxylate) catalyst is one example that catalyses the O−O bond formation via the radical coupling pathway. Herein, we manipulate the equatorial backbone ligand, change the doubly charged bda2− ligand to a singly charged tpc− (2,2':6',2''terpyridine-6-carboxylate) ligand, and study the structure-activity relationship. Surprisingly, kinetics measurements revealed that the resulting Ru-tpc catalyst catalyses water oxidation via the water nucleophilic attack pathway, which is different from the Ru-bda catalyst. The O−O bond formation Gibbs free energy of activation (∆G‡) at T = 298.15 K was 20.2 ± 1.7 kcal mol−1. The electronic structures of a series of RuV=O species were studied by density function theory calculations, revealing that the spin density of ORu=O of RuV=O is largely

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dependent on the surrounding ligands. Seven coordination configuration significantly enhances the radical character of RuV=O. Key words. Water oxidation, ruthenium complex, artificial photosynthesis, DFT calculation, water splitting. Introduction The power consumption of the world is still heavily relied on fossil fuels, which are limited and environmentally unfriendly resources. Splitting water to oxygen and hydrogen, driven by solar energy, is one of the clean and sustainable ways to address energy crisis.1 Water splitting reaction consists of two half reactions: water oxidation (2 H2O → O2 + 4 H+ + 4 e−; E0 = 1.229 − 0.059×pH V vs NHE) and proton reduction (2 H+ + 2 e− → H2; E0 = −0.059×pH V vs NHE). The oxidation of water provides electrons and protons for hydrogen production. Water oxidation is considered to be the bottleneck of splitting water due to the large activation barrier caused by multiple electron/proton transfer steps and the O−O bond formation; in reality large overpotential is present resulting in even higher applied potential to reach satisfied water oxidation performance.2 Thereby, catalysts capable of catalysing water oxidation are highly desired. To achieve this goal, various approaches have been used to prepare water oxidation catalysts (WOCs), such as nanostructured metal oxide/hydroxide and multinuclear/mononuclear transition metal complexes.1,

3

Mononuclear transition metal

complexes have attracted a lot of attentions due to their high efficiency and advantages in mechanistic investigation. Currently, there are two well-accepted reaction mechanisms for the O−O bond formation by transition metal complexes, water nucleophilic attack (WNA) and interaction of two metal oxo species (I2M).4 These two pathways are representatively depicted in Figure 1.


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Figure 1. Representative illustrations of (A) WNA and 6-coordinate RuV=O versus (B) I2M and 7-coordinate RuV=O. Among the family of mononuclear ruthenium complexes,5 the Ru-bda catalysts [RuII(bda)(L)2] (bda = 2,2′-bipyridine-6,6′-dicarboxylate, L = typically nitrogen heterocycles such as pyridine) show very high efficiency at low overpotential using CeIV ((NH4)2[Ce(NO3)6]) as the oxidant under acid conditions.6 O2 evolution is a second order reaction with Ru-bda catalysts and the O−O bond formation goes through the I2M pathway.6b, 7

In comparison, many other mononuclear ruthenium metal complexes, such as

[Ru(tpy)(bpy)(OH2)]2+, catalysing the O−O bond formation via the WNA pathway.2b In addition, replacing the bda2− ligand with other tetra-dentate N2O2 ligands, for instance pda2− (H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid), tda2− ((2,2′:6′,2′′-terpyridine)-6,6′′dicarboxylate), bpaH22− (bpaH4 = 2,2′-bipyridine-6,6′-diphosphonic acid) and qpy-N,N′′′dioxide (qpy = 2,2′:6′,2′′:6′′,2′′′-quaterpyridine), changes the O−O bond formation mechanism from the I2M pathway to the WNA pathway.8 Such mechanistic difference is apparently caused by their coordination environments of the RuV=O species (see examples in Figure 2), such as 6-coordinate versus 7-coordinate and neutral versus negatively charged pyridyl ligands (leading to the charge difference of the RuV=O species).


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Figure 2. Structures of Ru water oxidation catalysts with different coordination spheres. To investigate the effects of coordination environments on the O−O bond formation mechanisms and understand why Ru-bda catalysts catalyse the O−O bond formation via the I2M pathway, we modified the H2bda ligand by replacing one of the carboxylic acid groups to a pyridyl unit and synthesized the Htpc (2,2':6',2''-terpyridine-6-carboxylic acid) ligand and its Ru complex [RuII(tpc)(pic)2]+ ([1]+; Figure 3). On the basis of collective observations from electrochemistry, stopped-flow UV-vis, mass spectrometry as well as the O2 evolution kinetics study, a detailed reaction mechanism was proposed for CeIV-driven water oxidation by [1]+ under acidic conditions. Together with DFT calculations, the intrinsic electronic properties of RuIV oxyl radical species (or RuV oxo species; RuIV-O• ↔ RuV=O) of a series Ru water oxidation catalysts (see their structures in Figure 2), were investigated and correlated to their O−O bond formation mechanisms.


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Figure 3. Chemical structures of a Ru-bda catalyst and [1]+. Experimental Section Materials and synthesis. The ligand [2,2':6',2''-terpyridine]-6-carboxylic acid (Htpc) was prepared according to the literature procedures.9 Perchloric acid (70%, 99.999% trace metals basis), [Ce(NO3)6](NH4)2 (99.99%), [Ce(NO3)6](NH4)2 (98.5%), HNO3 (70%, 99.999% trace metals basis) and [Ru(p-cymene)Cl2]2 dimer were purchased from Sigma-Aldrich. Water used in all measurements was purified by a Milli-Q Reference water purification system. All other reagents and solvents are commercially available and used directly without further purification. [Ru(tpc)(pic)2](PF6) ([1](PF6)). In a microwave vial were added Htpc (110 mg, 0.40 mmol), ruthenium precursor [Ru(p-cymene)Cl2]2 (120 mg, 0.20 mmol) and ethanol (5 mL). The mixture was purged with Ar for 5 min and the vial was placed in the microwave reactor (Biotage® Initiator+). The reaction mixture was heated under microwave irradiation at 140 °C for 35 min and then 0.2 mL of 4-picoline was added. After purged with Ar for 5 min, the mixture was further heated under microwave irradiation at 140 °C for 40 min. The volatile was removed under a rotary evaporator, the residues were dissolved in minimum amount of water, and aqueous NH4PF6 was added. The resulting precipitate was collected and purified by chromatograph on SiO2 (the acetone/KNO3/H2O system as eluent), and the major band was collected. The volume of solution was reduced under vacuum, and aqueous NH4PF6 was added. The precipitate was filtrated, washed with cold water and re-dissolved in minimum of methanol. Water was added again and the resulting precipitate was collected, washed with cold water and dried under vacuum, yielding [Ru(tpc)(pic)2](PF6) a dark red powder (105 mg, 37 % yield). 1H NMR (400 MHz, Methanol-d4) δ 9.59 (d, J = 4.8 Hz, 1H), 8.70−8.66 (m, 2H), 5 ACS Paragon Plus Environment

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8.52 (d, J = 8.1 Hz, 1H), 8.37 (d, J = 7.8 Hz, 1H), 8.11 – 8.03 (m, 4H), 7.84 (t, J = 5.9 Hz, 1H), 7.73 (d, J = 6.4 Hz, 4H), 6.98 (d, J = 6.0 Hz, 4H), 2.19 ppm (s, 6H). High-resolution mass: m/z+ = 564.0960 ([M−PF6]+), calcd: 564.0973. Elemental analysis: Calcd. for C28H24F6N5O2PRu•0.5H2O: C 46.87, H 3.51, N 9.76%; Found: C 46.83, H3.37, N 9.57%. [Ru(tpc)(pic)2]F ([1]F). The following metathesis procedure was carried out to exchange the PF6− anion to the F− anion. [Ru(tpc)(pic)2](PF6) (64 mg, 0.09 mmol) was fully dissolved in acetone (ca. 100 mL) with the assist of a sonicator, and then Bu4NF (1.0 M) in THF was added, leading to the formation of precipitate. To remove any Bu4NF, the precipitate was separated and re-dissolved in minimum MeOH, to which solution THF was added to induce precipitation. The precipitate was collected and dried under vacuum, giving the desired product [Ru(tpc)(pic)2]F as a dark red powder (43 mg, 82% yield). 1H NMR (500 MHz, D2O) δ 9.21 (d, J = 4.0 Hz, 1H), 8.49 (d, J = 8.1 Hz, 2H), 8.36 (d, J = 8.0 Hz, 1H), 8.29 (d, J = 8.0 Hz, 1H), 8.05 (dd, J = 16.6, 8.2 Hz, 2H), 7.99–7.87 (m, 2H), 7.76 (t, J = 6.0 Hz,1H), 7.66 (d, J = 6.5 Hz, 4H), 6.87 (d, J = 6.1 Hz, 4H), 2.13 ppm (s, 6H). High-resolution mass: m/z+ = 564.0959 ([M−F]+), calcd: 564.0973. Elemental analysis: Calcd. C28H24FN5O2Ru•H2O: C 55.99, H 4.36, N 11.66 %; Found: C 55.69, H 4.33, N 11.42 %. Physical Methods. The 1H NMR spectra were recorded with either 400 or 500 MHz of Bruker advance spectrometer. Elemental analyses was performed with a Thermoquest-Flash EA 1112 apparatus. High-resolution mass spectrometry was performed on a Q-Tof Micro mass spectrometer. Electrochemical measurements were performed with an Autolab potentiostat, using a glassy carbon disk (φ = 3 mm) as the working electrode, a platinum wire as the counter electrode and an aqueous Ag/AgCl (3 M) electrode as the reference electrode. All potentials reported herein were referenced to NHE (E(Ag/AgCl) = 0.21 V vs NHE). All the buffer solutions used in the electrochemistry study are phosphate buffers. The oxygen evolution was recorded by a pressure sensor and the final amount of oxygen was calibrated by GC (GC-2014 Shimadzu). See the detailed procedures in our earlier publications. 6b Kinetic studies on the stepwise oxidation of Ru complex were carried out by a stopped-flow module (Bio-Logic SFM300 coupled with a JM TIDAS UV-vis Diode Array spectrophotometer). The temperature was maintained by a thermostated bath (Polystate 36, 6 ACS Paragon Plus Environment

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Fisher Scientific). Rate constants were calculated by using global fitting within ReactLab KINETICS (version 1.1). Mass spectrometry measurements to capture the RuIII and RuIV intermediates were performed by using a Finnigan LCQ Advantage MAX mass spectrometer. After bulk electrolysis, the reaction solution was diluted with Milli-Q water and then directly injected to the mass spectrometer by syringe. X-band EPR measurements were performed on a Bruker ELEXYS E500 spectrometer equipped with a SuperX-EPR049 microwave bridge and a cylindrical TE011 ER 4122SHQE cavity. Temperature was controlled using an Oxford Instruments ESR 900 flow cryostat. Single crystals of [Ru(tpc)(pic)2](PF6) were obtained by slow evaporation of its methanol/water solution, and single crystals of [Ru(κ3N,N,N-tpc)(pic)2(MeCN)]2(PF6)3 were obtained from the acetonitrile/toluene solution of [Ru(κ3N,N,N-tpc)(pic)2(MeCN)](PF6). The data were collected with a Rigaku diffractometer. The crystal was kept at 100 K during data collection. The crystal structure was solved with the Shelxs implemented in Olex210 software package using Direct Methods and refined with Shelxl using Gauss-Newton minimisation.11 Due to the air sensitivity feature of the [Ru(κ3N,N,N-tpc)(pic)2(MeCN)]2(PF6)3 crystals, the diffraction intensity decrease very fast during the single crystal X-ray diffraction (SCXRD) data collection although we tried various method to protect the crystal, for the best data we achieved here, it is still just enough for resolving the crystal structure model of the complex but not possible to resolve all the solvent species in the lattice, so the diffraction intensity contributed by the disorder solvents molecules has been squeezed with Platon software,12 and the structure model was refined on a solvent free diffraction intensity data. From this structure model, an exact valence state of Ru cannot be deduced directly based on the refinement result. Density Functional Theory Calculations. All density functional theory (DFT) calculations were carried out with the Jaguar 8.6 program package by Schrödinger LLC. Molecular geometries were optimized at the Becke’s threeparameter hybrid functional and the LYP correlation functional (B3LYP) with the LACVP** basis set. And single-point energy corrections were performed with the B3LYP-D3 functional using LACV3P**++ basis set augmented with two f functions on the metal. Frequency calculations were performed on the optimized geometries to verify that the geometries correspond to minima on the potential energy surface (PES). On the basis of the gas phase 7 ACS Paragon Plus Environment

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optimized geometries, the solvation energies were estimated by the single-point calculations using the Poisson-Boltzmann reactive field implemented in Jaguar (PBF) in water. The Gibbs free energies were calculated based on the equation G = E(B3LYP-D3/LACV3P**++ 2f on Ru) + Gsol + ZPE + H298 – TS298 + 1.9 kcal/mol (1.9 kcal/mol is a concentration correction to the free energy of solvation which by default is calculated at 1M (g) to 1M (aq) in Jaguar). The standard redox potential of hydrogen electrode is 4.281 V and the corresponding reference free energy of solvation of a proton is −1101 kJ/mol.13 In the calculation of redox potential and pKa values, one or two additional water molecules were included. Coordinates of all optimized structures are available upon request. Results and discussion Synthesis and characterization. Complex [1](PF6) was prepared by a two-step one-pot reaction (Figure 4). First, the ruthenium precursor [Ru(p-cymene)Cl2]2 was complexed with the ligand Htpc under microwave irradiation. Then, the resulting Ru intermediate was reacted with excess 4-picoline, yielding the designed complex [1](PF6) after addition of aqueous NH4PF6. [1](PF6) was fully characterized by 1H NMR, UV-vis, mass spectrometry, elemental analysis as well as X-ray crystallography. The solubility of [1](PF6) in water is very poor. Thereby, water soluble [1]F was prepared from [1](PF6) by metathesis (interchanging PF6− to F−; Figure 4). For mechanistic studies on water oxidation, for instance stoichiometric oxidation of water oxidation catalysts, the water soluble catalyst is more suitable since it allows to avoid using any organic co-solvents and therefore reduce the side reactions, such as the oxidation of organic co-solvents.

Figure 4. The synthetic routes of [1](PF6) and [1]F. The crystal structure of [1](PF6) is resolved and its cationic part is shown in Figure 5 (a and c). The Ru atom is in a strongly distorted octahedral configuration with an O1−Ru1−N3 angle 8 ACS Paragon Plus Environment

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of 122.45ο, which is similar to the previously reported configuration of Ru-bda catalysts (O−Ru−O angle: 122.12ο−122.99ο).14 This angle is much larger than 90ο in an ideal octahedron, which is believed to play a vital role to form seven-coordinate Ru intermediates during water oxidation process.

Figure 5. X-ray crystal structures of [1](PF6) (a: side view; c: top view) and [Ru(κ3N,N,Ntpc)(pic)2(MeCN)]2(PF6)3 (b: side view; d: top view. One Ru unit is displayed herein) with thermal ellipsoids at 30% probability. Hydrogen atoms and counter ions are omitted for clarity. The carboxylate ligation in [1]+ is labile. Complex [Ru(κ3N,N,N-tpc)(pic)2(MeCN)]+, named as [1− −NCCH3]+, is formed upon addition of acetonitrile to [1]+ (Figure 6), as evidenced by means of 1H NMR. Figure 7 shows the aromatic region of 1H NMR spectra of [1]+ and [1− −NCCH3]+. In D2O, the 4-picoline ligands of [1]+ display two doublets at δ=6.87 and δ=7.66 ppm in the aromatic region. Upon addition of acetonitrile-d3 into the solution, these two doublets are downfield shifted to δ=6.97 ppm and δ=7.81 ppm, respectively. The Ru center of [1− −NCCH3]+ is more electron deficient in comparison with that of [1]+ due to the de-coordination of the carboxylate from the Ru center. The electron deficient Ru center thus withdraws electron density from axial 4-picoline ligands, leading to the downfield shift of 4picoline protons (the resonance peak of methyl group, which is not shown in Figure 7, shifted from δ=2.13 ppm to δ=2.24 ppm). In contrast, the de-coordinated carboxylate is more electron rich than the coordinated carboxylate, thereby its neighbouring proton is upfield 9 ACS Paragon Plus Environment

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shifted from δ=9.21 ppm to δ=9.10 ppm. To our delight, the crystal structure of the acetonitrile coordinating complex was obtained and is displayed in Figure 5 (b and d). During the long-time crystal growth, part of [1− −NCCH3]+ molecules were oxidized to RuIII states, and the crystals should be obtained as [Ru(κ3N,N,N-tpc)(pic)2(MeCN)]2(PF6)3. Unfortunately, we were not able to assign the valence of these two Ru atoms based on the resolved molecular species in the unit cell due to the poor quality of the diffraction data. But the crystal structure obtained from the SCXRD data clearly showed that the Ru−O(carboxylate) bond was cleaved due to the coordination of acetonitrile. Such a phenomenon has also been observed for the Ru-bda and Ru-pda (H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid) catalysts.8a, 15

Figure 6. Reaction scheme of [1]+ to [1− −NCCH3]+ in the presence of NCCH3.

Figure 7. The aromatic region of 1H NMR (500 MHz) spectra of [1]+ in (A) D2O and (B) D2O/CD3CN (v/v = 9:1). The spectra of A and B are corresponding to [1]+ and [1− −NCCH3]+, respectively. CeIV-driven water oxidation. The catalytic performance of [1]+ was evaluated by using CeIV as a terminal oxidant at acidic conditions (equation 1). Figure 8 depicts the typical oxygen evolution time plot. A total of 10 ACS Paragon Plus Environment

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360 turnovers were achieved within 5 hours. Kinetics measurements revealed that the oxygen evolution was first order in catalyst (Figure 9) and zero order in CeIV (Figure S1), with a rate constant kO2 = 0.17 ± 0.005 s−1 (rate law: rate = kO2[cat.]). The TOF of [1]+, which value was equal to the first order rate constant, is 0.17 ± 0.005 s−1. In comparison with the Ru-bda catalysts which catalyse water oxidation through a bimolecular pathway, [1]+ catalyses water oxidation via a single molecular pathway. The rate determining step very likely involved the energy demanding O−O bond formation step. The catalytic performance of [1]+ was much worse than that of the Ru-bda catalysts which catalyse water oxidation via the I2M pathway but similar to other Ru catalysts that catalyse water oxidation via the WNA pathway.1 Thereby, the structural similarity/difference as well as the mechanistic difference between the Ru-tpc catalyst and the Ru-bda catalyst provide a good chance to study the structure-activity relationship (see the DFT calculations on the spin density of RuV=O species vide infra). .

2 H2O + 4 CeIV O2 + 4 H+ + 4 CeIII

(1)

35 400

30

350 25

300

20

250

15

200

TON

O2 (µmol)

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150

10

100 5

50

0

0 0

1

2

3

4

5

t (h)

Figure 8. Oxygen evolution curve. [CeIV] = 0.083 M, [Cat] = 26.4 µM and V = 3 mL pH 1.0 HNO3 with 3.3% CF3CH2OH.


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(a)

40

52.8 µM 79.3 µM 105.7 µM 132.1 µM 211.3 µM

O2 (µmol)

30

20

10

0 0

(b)

100

200

300

t (s) 40 O2 formation rate linear fitting 30

O2 rate (µM s−1)

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20

10

0 0

50

100 [cat] (µM)

150

200

250

Figure 9. (a) Oxygen evolution curve at various concentrations of catalyst. [CeIV] = 0.083 M, [Cat] = 52.8-211.3 µM and V = 3 mL pH 1.0 HNO3 with 3.3% CF3CH2OH. (b) O2 rate versus [cat.]. Electrochemistry. The electrochemistry of [1]+ was studied to understand the redox events of the Ru-tpc catalyst. Cyclic voltammograms and square wave voltammograms of [1]+ at pH 1.0 and pH 11 were displayed in Figure 10. At pH 1.0, the complex exhibits a reversible redox couple at 0.91 V vs NHE (all the redox potentials reported herein are versus NHE) and a weak return wave at Epred = 1.31 V. At pH 11.0, two obvious oxidation waves (E1/2 = 0.51 V and E1/2 = 1.27 V) were observed from the CV and SWV curves. To understand these redox processes, the potential versus pH diagram (Pourbaix diagram) of [1]+ was constructed and shown in Figure 11. Below pH 4.0, the redox event only involves an electron transfer and is therefore assigned to the RuII/III process. Above pH 4.0, the first redox couple becomes pH


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dependent with a slop of 58.7 mV/pH, indicating a ne−/nH+ (n = 1 or 2, in consideration of the electrochemical properties of Ru complexes) process.

The second oxidation wave is not pH dependent above pH 4.0, and three possibilities were considered: (i) [RuIII]2+ → [RuIV]3+ + e− (direct oxidation of [1]2+ to [1]3+). However, this is ruled out by carrying out the electrochemistry of [1] + in less coordinating organic solvent dichloromethane (DCM). Only one oxidation peak RuIII/II was observed in the scan range 0.44 −1.84 V (Figure S2). Meanwhile, the Ru-bda catalyst which contains a more electron rich bda2− ligand than the tpc− ligand also displayed a single oxidation wave RuIII/II in DCM up to 1.8 V (not shown). Neither Ru-tpc nor Ru-bda displayed the RuIV/III redox couple in DCM. (ii) [RuIII−OH]+ → [RuIV−OH]2+ + e−. This process will generate a stable [RuIV−OH]2+ species, which seems not likely to occur because the [RuIV−OH]2+ species usually have very low pKa values and undergo deprotonation simultaneously upon its formation under basic conditions. IV

For

instance,

the

pKa

values

of

[RuIV(tpy)(bpy)(OH)]3+

+

IV

[Ru (bda)(pic)2(OH)] are < 0 and 5.5, respectively. Because [Ru (tpc)(pic)2(OH)] IV

and 2+

is

+

more electron deficient than [Ru (bda)(pic)2(OH)] , a lower pKa value than 5.5 is expected for [RuIV(tpc)(pic)2(OH)]2+. Accordingly, the formation of stable [RuIV−OH]2+ under basic conditions is excluded. (iii) [RuIV=O]+ → [RuV=O]2+ + e−. This is reasonable by comparing the RuV=O/RuIV=O oxidation potentials of [1]+, [Ru(bda)(pic)2] and [Ru(tpy)(bpm)(OH2)]2+ (bpm = 2,2′-bipyrimidine) complexes. The electron donating ability of tpc− is stronger than that of tpy but weaker than that of bda2−. The resulting oxidation potentials of RuIV=O → RuV=O are in the right trend [Ru(bda)(pic)2] (1.10 V) < [1]+ (1.27 V) < [Ru(tpy)(bpm)(OH2)]2+ (1.65 V).6b, 16


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Figure 10. CVs and SWVs of complex of [1]+ in pH 1.0 HNO3 aqueous solution (upper) and pH 11 phosphate buffer (lower). Working electrode = glassy carbon, scan rate for CV = 100 mV s−1. [RuV=O]2+ + e-

[RuIV=O]+

[RuII]+

[RuIII]2+ + e-

[RuII]+ + H2O

[RuIV=O]+ + 2H+ + 2e-

Figure 11. Potential versus pH diagram of [1]+. The buffer system used in this study is phosphate buffer. At pH > 4, there are two kinetic processes for the two electron oxidation of [RuII]+ to [RuIV=O]+: (1) 2[RuII]+ + 2H2O → 2[RuIII−OH]+ + 2e− + 2H+ and (2) 2[RuIII−OH]+ → [RuII]+ + [RuIV=O]+ + H2O. Combination of equations 1 and 2 leads to the final 2e−/2H+ oxidation reaction: [RuII]+ + H2O → [RuIV=O]+ + 2e− + 2H+.


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ACS Catalysis

Regarding the first redox event above pH 4.0 conditions, it could be either (i) [RuII]+ + H2O → [RuIII−OH]+ + e− + H+ while the oxidation wave of RuIII/IV is not detectable by CV/SWV techniques due to the slow kinetics or (ii) [RuII]+ + H2O → [RuIV=O]+ + 2e− + 2H+. Controlled potential electrolysis of [1]F in pH 7.0 phosphate buffer at the first wave yielded the pass of 1.4 equivalent electrons, pointing to a mixed oxidation processes involving the oxidation of [RuII]+ → [RuIII−OH]+ and the disproportionation of [RuIII−OH]+ to [RuII]+ and [RuIV=O]+. However, the disproportionation of [RuIII−OH]+ is relatively slow. This is evidenced by the concentration dependent CV measurements (Figure S3). Since the disproportionation of [RuIII−OH]+ (2 [RuIII−OH]+ → [RuII]+ + [RuIV=O]+ + H2O) is second order in the concentration of [RuIII−OH]+, the reversibility of the first redox couple becomes worse and worse upon increasing the concentration of [1]+, and a new emerging peak at Epred = 0.54 V grows gradually, which is proposed to the corresponding [RuIV=O]+ reduction ([RuIV=O]+ + H+ + e− → [RuIII−OH]+). The slow disproportionation of [RuIII−OH]+ enabled us to detect both [RuIII−OH]+ and [RuIV=O]+ by mass spectrometry upon the injection of the resulting solution from bulk electrolysis. Unfortunately, due to the one mass unit difference of these two species, we were not able to totally separate these two species, and their mass spectra overlap with each other. By overlapping the isotope distribution patterns of [RuIII−OH]+ and [RuIV=O]+ species in an 11:9 ratio, we were able to closely assemble the asobserved mass spectra (Figure 12). Moreover, the presence of RuIII species after bulk electrolysis was further proved by the EPR experiment. The recorded EPR spectrum of this sample showed a characteristic signal for a low spin S =1/2 centre with rhombic anisotropy (Figure 13). The g-tensors were calculated as gx = 2.32, gy = 2.18, gz = 1.90, which are typical for low spin RuIII species.


ACS Catalysis

Experimental 579.99

1.0 0.8 0.6

Normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 0.2 0.0 572

574

576

578

Calculated

1.0

580

582

584

586

588

582

584

586

588

580.09

0.8 0.6 0.4 0.2 0.0 572

574

576

578

580

m/z

Figure 12. (Upper) Observed mass spectra of Ru species after bulk electrolysis at 1.0 V and (lower)

the

sum

of

calculated

mass

spectra

of

[RuIII(tpc)(pic)2(OH)]+

and

[RuIV(tpc)(pic)2(O)]+ with a ratio of 11:9.

gx

gy gz

Figure 13. EPR spectrum of the RuIII state of [1]+ together with the background of its RuII state. Conditions: solvent = pH 7.0 phosphate buffer, microwave frequency = 9.27 GHz, microwave power = 0.200 mW, modulation frequency = 100 kHz, modulation amplitude = 10 G and T = 9.0 K. DFT calculations on the redox events. To get a better understanding of the electrochemical properties of [1]+, DFT calculations were performed. First, neither a six coordinate [RuII-OH2]+ nor a six coordinate [RuIII-OH2]2+, where a water molecule coordinates to the Ru centre while one of the Ru−Ocarboxylate bonds cleaves, is stable. Second, several possible RuIII species of [1]+ at pH = 0 were calculated and 16 ACS Paragon Plus Environment

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the results were depicted in Figure 14. Apparently, the six coordinate [RuIII(tpc)(pic)2]2+ is the most stable form among these structures under highly acidic conditions. Under more basic conditions, a six coordinate [RuIII(κ3N,N,N-tpc)(pic)2(OH)]+ gets stabilized and the pKa value of [RuIII(κ4N,N,N,O-tpc)(pic)2]2+•H2O is calculated to be 7.85. For example, at pH = 8.0, [RuIII(κ3N,N,N-tpc)(pic)2(OH)]+ is more stable than the six coordinate [RuIII(tpc)(pic)2]2+ by 0.2 kcal/mol. Third, the [RuIV−OH]2+ and [RuIV=O]+ species are seven coordinate and in the pentagonal bipyramidal configuration, which is similar to the calculated structure of [RuIV(bda)(pic)2(OH)]+. The calculated pKa value of [RuIV(tpc)(pic)2(OH)]2+ is 6.74.

Figure 14 Free energy profiles for possible RuIII species of [1]+ at pH = 0. The relative solvation corrected Gibbs free energies are given in kcal/mol. Table 1 shows the calculated potentials of possible redox processes. The calculated oxidation potential of [RuII]+ to [RuIII]2+ is 0.70 V (Table 1, reaction A), close to the experimentally observed oxidation potential of 0.91 V. Under basic conditions, the first redox potential becomes pH dependent and a ne−/nH+ process. Thus it could be either [RuIII−OH]+/[RuII]+ or [RuIV=O]+/[RuII]+. At pH = 8, E([RuIII−OH]+/[RuII]+)calcd = 0.69 V (Table 1, reaction B) is slightly higher than E([RuIV=O]+/[RuIII−OH]+)calcd = 0.68 V (Table 1, reaction D). This is in agreement with the proposed 2e−/2H+ oxidation process from [RuII]+ to [RuIV=O]+ because the redox potential of [RuIV=O]+/[RuIII−OH]+ is similar to that of [RuIII−OH]+/[RuII]+ and disproportionation of [RuIII−OH]+ could happen and form [RuIV=O]+. In addition, the second redox potential [RuV=O]2+/[RuIV=O]+ at basic conditions is independent of pH and is 1.27 V 17 ACS Paragon Plus Environment

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in experiment while the calculated E([RuV=O]2+/[RuIV=O]+)calcd = 1.17 V (Table 1, reaction F). Since little information of the electrochemistry of [1]+ in acidic conditions (pH < 4.0) was obtained in experiments, DFT calculations were used to construct the Pourbaix diagram of [1]+ and to shed light on the oxidation sequence of [1]+ under acidic conditions, especially the CeIV-driven water oxidation conditions. The calculated Pourbaix diagram is depicted in Figure 15. We would like to point out that the pKa values of [RuIII(tpc)(pic)2(OH2)]+ and [RuIV(tpc)(pic)2(OH)]+ were predicted a little far away from the experimental values while this phenomenon was often seen in other documented literatures.17 At pH < 6.74, the oxidation sequence of [1]+ is as follows: (i) [RuII]+ → [RuIII]2+ + e−, (ii) [RuIII]2+ + H2O → [RuIV−OH]2+ + H+ + e−, and (iii) [RuIV−OH]2+ → [RuV=O]2+ + H+ + e− (Table 1, reaction G). [RuV=O]2+/[RuIV−OH]2+ is pH dependent while E([RuV=O]2+/[RuIV=O]+)calcd = 1.57 V at pH = 0. At 6.74 < pH < 7.85, the second oxidation involves a 1e−/2H+ process: [RuIII]2+ + H2O → [RuIV=O]+ + 2H+ + e− (Table 1, reaction E), followed by a 1e− oxidation process giving [RuV=O]2+. Above pH=7.85, the oxidation sequence is the same as that we observed in the experiments: [RuII]+ → [RuIV=O]+ → [RuV=O]2+. Table 1 The calculated redox potential. All calculated potential values are referred to NHE. Reaction

E (V vs NHE)

(A) [Ru (tpc)(pic)2] + e → [Ru (tpc)(pic)2]

0.70[a]

(B) [RuIII(κ3N,N,N-tpc)(pic)2(OH)]+ + H+ + e− → [RuII(tpc)(pic)2]+ + H2O

0.69 (pH = 8)[b]

(C) [RuIV(tpc)(pic)2(OH)]2+ + H+ + e− → [RuIII(tpc)(pic)2]+ + H2O

1.34 (pH = 0)

(D) [RuIV(tpc)(pic)2(O)]+ + H+ + e− → [RuIII(κ3N,N,N-tpc)(pic)2(OH)]+

0.68 (pH = 8)

(E) [RuIV(tpc)(pic)2(O)]+ + 2H+ + e− → [RuIII(tpc)(pic)2]+ + H2O

0.91 (pH = 7)

(F) [RuV(tpc)(pic)2(O)]2+ + e− → [RuIV(tpc)(pic)2(O)]+

1.17

(G) [RuV(tpc)(pic)2(O)]2+ + H+ + e− → [RuIV(tpc)(pic)2(OH)]2+

1.57 (pH = 0)

III

2+

−

II

+

[a] Two explicit water molecules were included for [RuIII(tpc)(pic)2]2+ and [RuII(tpc)(pic)2]+ in the calculation. [b] Two explicit water molecules were included for [RuII(tpc)(pic)2]+ and one explicit water molecule was included for [RuIII(κ3N,N,N-tpc)(pic)2(OH)]+ in the calculation.


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1.8

[RuIV-OH]2+ + H+ + e-

[RuIII]2+ + H2O

[RuV=O]2+ + H+ +e-

[RuIV-OH]2+

1.6 1.4

[RuIV=O]+

[Ru IV -OH] 2+

1.2

E (V vs NHE)

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ACS Catalysis

1.0

[RuV=O]2+ [RuV=O]2+ + e-

[RuIII]2+ + H2O

[RuIII]2+

[RuIV=O]+ + 2H+ + e-

0.8 0.6

[RuII]+

[RuIV=O]+

[RuIII]2+ + e[RuII]+

0.4

[RuIV=O]+ + 2H+ + 2e-

[RuII]+ + H2O

0.2

1

2

3

4

5

6

7

8

9

10 11 12 13 14

pH

Figure 15. Calculated Pourbaix diagram for [1]+ in aqueous solution. Kinetics study. The consecutive electron transfer steps of the catalytic cycle was thoroughly investigated by the means of stopped-flow UV-vis technique. The spectral change of [1]+ versus time was monitored upon mixing various stoichiometric amount of CeIV with [1]+ under pH 1.0 conditions. All the rate constants were obtained by global fitting of the time dependent spectra using a software called ReactLab Kinetics. All kinetics studies were performed at room temperature (21 oC) unless otherwise stated. Upon addition of one equivalent of CeIV, [1]+ was rapidly oxidized to [1]2+ (equation 2) with a rate constant k1 = 7.53 × 105 M−1s−1 (Figure S4). As a consequence, the typical metal-to-ligand charge-transfer absorption (MLCT) bands of [1]+ at 516 and 470 nm was bleached (not shown). When two equivalents of CeIV was mixed with [1]+, the Ru-tpc complex was firstly oxidized to [RuIII]2+ state within 0.02 s, followed by a relatively slow process that is the oxidation of [RuIII]2+ to [RuIV−OH]2+ (equation 3) with k2 = 8.09 × 104 M−1s−1 (Figure S5).

[RuII]+ + CeIV → [RuIII]2+ + CeIII

(2)

[RuIII]2+ + H2O + CeIV → [RuIV−OH]2+ + CeIII + H+ (3) The formation of [RuV=O]2+ and [RuIII-OOH]+ was achieved by addition of three equivalents of CeIV to [1]+. Under these conditions, the RuII/III process was too fast to follow and was ignored. The observed oxidation processes are [RuIII]2+ to [RuIV−OH]2+ (equation 3, k2 is fixed to be 8.09 × 104 M−1s−1 during the global fitting), [RuIV−OH]2+ → [RuV=O]2+ (equation 4) as well as [RuV=O]2+ → [RuIII-OOH]+ (equation 5). From the global fitting analysis, the 19 ACS Paragon Plus Environment

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rate constants k3 and kO−O were calculated to be 1.99 × 103 M−1s−1 and 8.83 × 10−3 s−1, respectively. The spectra of [RuIII]2+, [RuIV−OH]2+ and [RuV=O]2+ were calculated using singular value decomposition (SVD) analysis with ReactLab Kinetics and shown in Figure 16(b). Meanwhile, we obtained the time-dependent species distribution profile as depicted in Figure 16(c). The intermediates [RuIV−OH]2+ and [RuV=O]2+ are respectively dominant at 0.6 s and 29 s in the reaction solution. The formation of [RuV=O]2+ is rather slow, which may not be the case under excess CeIV conditions as we explain above.

[RuIV−OH]2+ + CeIV → [RuV=O]2+ + CeIII + H+

[RuV=O]2+ + H2O [RuIII-OOH]+ + H+

(4) (5)


(a)

0.05 0.00 -0.05

Absorbance at 335.6 nm

0.55

0.5

Experimental data Fit

0.45

0.4

0.35 0

20

40

80

60

100

120

140

t (s)

(b) [RuIII ]2+ [RuIV-OH]2+

8000

[RuV=O] 2+

ε (M-1 cm-1)

[RuIII-OOH] +

6000 4000 2000 0 300

400

500

600

700

wavelength (nm)

(c) 60

[RuIII ]2+ [RuIV-OH]2+

50

Concentration (µM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Residual

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[RuV=O] 2+ 40

[RuIII-OOH] +

30 20 10 0 0

20

40

60

80

100

120

140

t (s)

Figure 16. Kinetics for the generation of RuIII-OOH upon mixing three equivalents of CeIV and [1]+ (6.25 × 10−5 M; pH 1.0 HClO4). (a) Absorbance changes at 335.6 nm (blue dots) and


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the fitting curve (green). (b) Calculated absorption spectra of Ru-based intermediates. (c) Concentration profiles of Ru species. Table 2 summarizes all the above discussed rate constants. Under stoichiometric CeIV conditions, k1 and k2 are much larger than k3, indicating that the oxidation [RuIV−OH]2+ to [RuV=O]2+ is considerably difficult in comparison with the oxidation of [RuII]+ and [RuIII]2+. Nevertheless, those electron transfer steps are much faster than the O−O bond formation step (small kO–O), in agreement with that the O−O bond formation is the rate determining step. The reaction at various temperatures (5−21oC) were also conducted and the kO–O values obtained. On the basis of Eyring plot, we calculated the activation parameters of the WNA step: ∆H‡ = 5.5 ± 0.8 kJ mol−1 and ∆S‡ = −265.2 ± 3.0 J mol−1. The Gibbs free energy of activation (∆G‡) at T = 298.15 K was then calculated to be 20.2 ± 1.7 kcal mol−1: the entropy contributes significantly more than the enthalpy to the Gibbs free energy of activation, which is in line with the decrease of the disorder for the WNA step. Table 2. Rate constants of CeIV-driven water oxidation by [1]+ at room temperature (21oC) at pH 1.0. rate constant

reaction equation

kO2 = 0.17 s−1

equation 1[a]

k1 = 7.53 × 105 M−1s−1

equation 2

k2 = 8.09 × 104 M−1s−1

equation 3

k3 = 1.99 × 103 M−1s−1

equation 4

kO–O = 8.83 × 10−3 s−1

equation 5

[a ]

CeIV is in large excess.

DFT calculations on the RuV=O species. The electronic properties of the RuV=O species are important since it is the active species and control the O−O bond formation pathways via either I2M or WNA. We systematically examined a series of RuV=O species by changing their formal charge from +1 to +3 and their coordination sphere from six coordination to seven coordination (Figure 2). Table 3 shows the spin density located on the oxygen atom of RuV=O and the energy of LUMO orbital which is π* orbital of RuV=O. The spin density and the energy of LUMO are calculated in 22 ACS Paragon Plus Environment

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water using the PBF (Poisson-Boltzmann reactive filed) model. Generally, 7-coordinate complexes have higher spin density on the ORu=O atom than 6-coordinated complexes while 6-coordinated complexes have lower LUMO energy than 7-coordinated ones. Table 3. Spin density located on oxygen and LUMO energy of different RuV=O species Complex

Spin density of ORu=O

E(LUMO) / eV

V

+

0.66973

-4.37

V

2+

0.62994

-4.52

[Ru (qpy)(pic)2(O)]

0.60064

-4.74

[RuV(pdc)(pic)2(O)]+

0.57206

-4.93

[RuV(bpc)(bpy)(O)]2+

0.57942

-5.41

[RuV(tpy)(bpy)(O)]3+

0.53055

-5.98

[Ru (bda)(pic)2(O)] [Ru (tpc)(pic)2(O)] V

3+

Discussion Complex [1]+ reacts with acetonitrile readily to yield complex [1− −NCCH3]+, indicating the labile ligation of carboxylate. This ligand exchange phenomena was also observed for several other Ru water oxidation catalysts that contain negatively charged carboxylate ligands, such as [Ru(bda)(pic)2], [Ru(pda)(pic)2] (H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid) as well as [Ru(pdc)(pic)3] (H2pdc = 2,6-pyridinedicarboxylic acid).15, 18 The RuII center of [1]+ has accepted six ligands and its coordination sphere is saturated. A labile carboxylate ligation then becomes essential to create an open site for H2O/OH− coordinating to the Ru center. For instance, the OH− ligand can coordinate to the RuIII state of [1]+ above pH 4. At pH > 4, the oxidation of [RuII]+ affords six-coordinate [RuIII(κ3N,N,N-tpc)(pic)2(OH)]+ which then slowly disproportionates to [RuII]+ and seven-coordinate [RuIV(tpc)(pic)2(O)]+. Thus the redox potential wave of [RuIV=O]+/[RuIII-OH]+ was not observed in the CV/SWV measurements. The disproportionation of [RuIII-OH]+ to [RuII]+ and [RuIV=O]+ species has also been observed for [Ru(NPM)(pic)2(OH2)]2+ (NPM = 4-t-butyl-2,6-di-(1',8'-naphthyrid-2'-yl)pyridine).17b For the RuIII state of [1]+, DFT calculations did not reveal any stable form of [RuIII−OH2]2+, indicating that neither six coordinate nor seven coordinate [RuIII−OH2]2+ species is present under the catalytic conditions. The catalytic performance of [1]+ is much worse than that of the Ru-bda catalyst. Kinetics measurements reveal that [1]+ catalyzes the O−O bond formation in a different mechanism 23 ACS Paragon Plus Environment

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from the Ru-bda catalyst: WNA for [1]+ versus I2M for the Ru-bda catalyst. On the basis of experimental observations and DFT calculations (calculated Pourbaix diagram), water oxidation mechanism catalyzed by [1]+ under acidic conditions was proposed as shown in Figure 17. At pH 1.0, the oxidation of [RuII]+ gives [RuIII]2+ which is the most stable form at the RuIII state under acidic conditions. According to the DFT calculations, [RuIII]2+ is lower in energy by 13.9 and 10.7 kcal/mol compared with seven-coordinate [RuIII(tpc)(pic)2(OH)]+ and six-coordinate [RuIII(κ3N,N,N-tpc)(pic)2(OH)]+, respectively. This oxidation process occurs in a faster rate (k1 = 7.53 × 105 M−1s−1) than that of [Ru(tpy)(bpm)(OH2)]2+ (2.4 × 103 M−1s−1).16 The next oxidation step is [RuIII]2+ to [RuIV−OH]2+ with a rate constant k2 = 8.09 × 104 M−1s−1, which is relatively slow in comparison with the first oxidation step. This step most likely involves first the equilibrium between [RuIII]2+ and [RuIII−OH2]2+ and the subsequent oxidation of [RuIII−OH2]2+ to [RuIV−OH]2+. Due to the low concentration of [RuIII−OH2]2+, the oxidation wave of [RuIII]2+ to [RuIV−OH]2+ was barely observable by CV and SWV at pH 1.0. Oxidation of [RuIV−OH]2+ yields the active and oxidized [RuV=O]2+. Water nucleophilic attack on the [RuV=O]2+ species affords the O−O bond formation with a rate constant kO–O = 8.83 × 10−3 s−1, which number is close to those of [RuV(tpy)(bpm)(O)]3+ (kO–O = 9.6 × 10−3 s−1) and [RuV(bpc)(bpy)(O)]2+ (kO–O = 1.1 × 10−2 s−1; Hbpc = 2,2′bipyridine-6-carboxylic acid).16, 19 kO−O under stoichiometric CeIV conditions is much smaller than the global rate constant kO2 (0.17 s−1) observed under large excess CeIV conditions. This is most likely due to that under excess CeIV conditions the intermediate [RuV=O]2+ concentration is higher, leading to a faster O2 production. The oxidation power of CeIV is dependent on the ratio of [CeIV]/[CeIII] according to the Nernst equation. Under stoichiometric CeIV conditions, the ratio of [CeIV]/[CeIII] decreases dramatically along with the reaction time so as to the oxidation power of CeIV. However, under large excess CeIV conditions, the ratio of [CeIV]/[CeIII] does not change too much in the beginning. Consequently, the oxidation of [RuIV−OH]2+ to [RuV=O]2+ is more favourable under large excess CeIV conditions, resulting in a higher concentration of [RuV=O]2+ as well as faster O2 production.


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Figure 17. Proposed water oxidation mechanism catalysed by [1]+ at pH 1.0. We have been frustrated by why the Ru-bda catalyst catalyses water oxidation via the I2M pathway while most of other Ru complexes via the WNA pathway. Which pathway one catalyst will proceed with, is controlled by the electronic structure of its active species, usually RuV=O. If a catalyst performs water oxidation via the I2M mechanism, its RuV=O species should display a large radical character on the O atom of RuV=O and meanwhile its LUMO energy should not be too close to the HOMO of the substrate water (HOMOwater = −8.16 eV). To favour the WNA pathway, a catalyst should have the opposite electronic structure: low spin density on the O atom and a low LUMO level of the RuV=O. We herein calculated the electronic structures of six RuV=O species and analysed their spin density of ORu=O and LUMO orbitals of RuV=O. First, no matter what their formal charge is +1, +2 or +3, 7-coordinate complexes give substantially higher spin density on the ORu=O atom than 6coordinated complexes while 6-coordinated complexes have lower LUMO energy than 7coordinated ones. [RuV(bda)(pic)2(O)]+ displays the highest spin density on ORu=O (0.66973) and it catalyses water oxidation via the I2M pathway while [RuV(tpc)(pic)2(O)]2+ has slightly lower spin density (0.62994) and it catalyses water oxidation via the WNA pathway. The LUMO levels of the two species are similar to each other. It seems that the spin density on ORu=O has to be close to 0.67 for a catalyst proceeding via the I2M pathway. Then, another question comes: to what extend the surrounding ligands could influence these two factors. Variation of the formal charge from +3 to +1 results in the increase of the spin density by 0.07 and 0.04 for 7-coordinate and 6-coordinate complexes, respectively, while this also leads to the increase of the LUMO energy by 0.37 and 1.05 eV, respectively. The formal charge has dramatic effect on the LUMO energy of the 6-coordinate complexes. Changing the 25 ACS Paragon Plus Environment

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coordination sphere from 6-coordinate to 7-coordinate, the spin density increases by 0.098, 0.051 and 0.070 for charge +1, +2 and +3 species, respectively, while the LUMO energy increases by 0.56, 0.89 and 1.24 eV, respectively. The coordination sphere has substantial influence on the spin density and also on the LUMO of charge +3 species. Nevertheless, negatively charged ligands increase the spin density and LUMO energy, and the seven coordination configuration is essential for the high spin density on ORu(V)=O. On the basis of spin density and LUMO energy, [RuV(bda)(pic)2(O)]+ is prone to undergo the I2M pathway and [RuV(tpy)(bpy)(O)]3+ undergoes the WNA pathway, which is in line with the experimental observations. Another plausible effect of the formal charge is the propensity to form the proposed prereactive dimer from two free monomers, that precedes the O−O bond forming step of the I2M reaction. We recently showed that the reaction I2M of two Ru(bda) units has minimal barriers and that the formation of the prereactive dimer likely determines the reactivity.20 Combined with the results from this study we conclude that low formal charge and high spin density are prerequisites for the I2M pathway. Conclusions We have investigated the properties and water oxidation mechanism of [RuII(tpc)(pic)2]+ ([1]+). The Pourbaix diagram of [1]+ is rather simple compared to [RuII(bda)(pic)2] at high pH. This is because of disproportionation of [RuIII(tpc)(pic)2(OH)]+ to [RuII(tpc)(pic)2]+ and [RuIV(tpc)(pic)2(O)]+ according to electrochemistry and corresponding DFT calculations. On the basis of collective observations from electrochemistry, stopped-flow UV-vis, mass spectrometry, O2 evolution kinetics study as well as DFT calculations, a detailed reaction mechanism was proposed for CeIV-driven water oxidation by [1]+ under acidic conditions. The O−O bond formation is the rate determining step. Complex [1]+ that catalysed the O−O bond formation via the WNA pathway is similar to many other mononuclear ruthenium metal complexes bearing neutral backbone ligands but different from Ru-bda catalysts. In order to investigate the effects of coordination environments on the O−O bond formation mechanisms, the intrinsic electronic properties of RuIV oxyl radical species (or RuV oxo species; RuIV−O• ↔ RuV=O) of a series Ru water oxidation catalysts (Figure 2) with 6- versus 7-coordinated and neutral versus negatively charged pyridyl ligands, were compared. Negatively charged ligands increase the spin density and LUMO energy, and the seven coordination configuration is essential for the high spin density on ORu(V)=O. Our results are important in


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designing new generation of Ru-based water oxidation catalysts and may inspire the design of earth abundant transition metal based water oxidation catalysts. Supporting Information Crystallographic data for [Ru(tpc)(pic)2](PF6) and [Ru(κ3N,N,N-tpc)(pic)2(MeCN)]2(PF6)3 (CIF). Electrochemistry and kinetic measurements data (PDF). Acknowledgement We thank the Swedish Research Council, K & A Wallenberg Foundation, Wenner-Gren Foundation, the Swedish Energy Agency, the Department of Chemistry at KTH, the National Natural Science Foundation of China (21120102036, 91233201), and the National Basic Research Program of China (973 program, 2014CB239402) for financial support of this work. References (1) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B., Chem. Rev. 2014, 114, 11863-12001. (2) a) Romain, S.; Vigara, L.; Llobet, A., Acc. Chem. Res. 2009, 42, 1944-53; b) Concepcion, J. J.; Jurss, J. W.; Brennaman, M. K.; Hoertz, P. G.; Patrocinio, A. O. T.; Murakami, I. N. Y.; Templeton, J. L.; Meyer, T. J., Acc. Chem. Res. 2009, 42, 1954-65. (3) a) Dionigi, F.; Strasser, P., Adv. Energy Mater. 2016, 1600621; b) Han, L.; Dong, S.; Wang, E., Adv. Mater. 2016, 28, 9266-9291. (4) a) Sala, X.; Maji, S.; Bofill, R.; García-Antón, J.; Escriche, L.; Llobet, A., Acc. Chem. Res. 2014, 47, 504-516; b) Berardi, S.; Drouet, S.; Francas, L.; Gimbert-Surinach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A., Chem. Soc. Rev. 2014, 43, 7501-7519. (5) a) Zeng, Q.; Lewis, F. W.; Harwood, L. M.; Hartl, F., Coord. Chem. Rev. 2015, 304– 305, 88-101; b) Duan, L.; Wang, L.; Li, F.; Li, F.; Sun, L., Acc. Chem. Res. 2015, 48, 20842096; c) Tong, L.; Thummel, R. P., Chem. Sci. 2016, 7, 6591-6603. (6) a) Wang, L.; Duan, L.; Wang, Y.; Ahlquist, M. S. G.; Sun, L., Chem. Commun. 2014, 50, 12947-12950; b) Duan, L.; Bozoglian, F.; Mandal, S.; Stewart, B.; Privalov, T.; Llobet, A.; Sun, L., Nat. Chem. 2012, 4, 418-423; c) Duan, L.; Araujo, C. M.; Ahlquist, M. S. G.; Sun, L., Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15584-15588. (7) Nyhlen, J.; Duan, L.; Akermark, B.; Sun, L.; Privalov, T., Angew. Chem. Int. Ed. 2010, 49, 1773-1777. (8) a) Tong, L.; Duan, L.; Xu, Y.; Privalov, T.; Sun, L., Angew. Chem. Int. Ed. 2011, 50, 445-449; b) Matheu, R.; Ertem, M. Z.; Benet-Buchholz, J.; Coronado, E.; Batista, V. S.; Sala, X.; Llobet, A., J. Am. Chem. Soc. 2015, 137, 10786-10795; c) Xie, Y.; Shaffer, D. W.; Lewandowska-Andralojc, A.; Szalda, D. J.; Concepcion, J. J., Angew. Chem. Int. Ed. 2016, 55, 8067-8071; d) Liu, Y.; Ng, S.-M.; Yiu, S.-M.; Lam, W. W. Y.; Wei, X.-G.; Lau, K.-C.; Lau, T.-C., Angew. Chem. Int. Ed. 2014, 53, 14468-14471. (9) Chen, X.-Y.; Bretonnière, Y.; Pécaut, J.; Imbert, D.; Bünzli, J.-C.; Mazzanti, M., Inorg. Chem. 2007, 46, 625-637.


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TOC

Seven coordination and negatively charged ligands enhance the radical character of the O atom of RuV=O species.


The Ru-tpc Water Oxidation Catalyst and Beyond ... - ACS Publications

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