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Theoretical investigations of a water oxidation catalyst for utility in artificial photosynthesis Nils Lenart Masters Thesis March 4, 2015 Supervisor: M˚ arten Ahlquist Examiner: Tore Brinck Division of Theoretical Chemistry and Biology Royal Institute of Technology Stockholm
Transcript
Page 1: Theoretical investigations of a water oxidation catalyst ...850144/FULLTEXT01.pdf · WOC = water oxidation catalyst PCET = proton coupled electron transfer WNA = water nucleophilic

Theoretical investigations of a water oxidation catalyst for

utility in artificial photosynthesis

Nils LenartMasters Thesis

March 4, 2015

Supervisor: Marten AhlquistExaminer: Tore Brinck

Division of Theoretical Chemistry and BiologyRoyal Institute of Technology

Stockholm

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Abstract

Water oxidation is the oxidative half-reaction of artificial photosynthesis. Realisa-tion of effective artificial photosynthesis could present a novel method for solar energyacquisition and storage. In this thesis the first reported mononuclear ruthenium basedwater oxidation catalyst following second order kinetics has been theoretically inves-tigated. Modifications to the catalyst had been reported to lead to reaction kineticsof first order. An explanation was sought by modelling and investigating multiple hy-potheses. Although no conclusive explanation was found, the results indicate that thereason may be in part due to an increased propensity of the complex to undergo acompeting first order rate mechanism.

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Sammanfattning

Vattenoxidationsreaktionen ar den oxidativa halvreaktionen i artificiell fotosyntes.Forverkligandet av artificiell fotosyntes kan leda till nya metoder for att alstra ochforvara solenergi. Den har avhandlingen behandlar en teoretisk studie av den forstamononukleara rutenium-baserade vattenoxidationskatalysatorn som foljer andra-ordningenskinetik. Forandringar av molekylen har setts leda till en kinetik av forsta ordningen ochen forklaring soktes genom att flertalet hypoteser testades med modellering. Nagonentydig forklaring aterfanns ej, men resultaten tyder pa att forklaringen delvis kanligga i en okad benagenhet hos katalysatorn att reagera via en konkurrerande forstaordningens reaktion.

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Contents

1 List of acronyms 4

2 Acknowledgement 5

3 Introduction 6

4 Background 64.1 The current state of global power consumption . . . . . . . . . . . . . . . . 64.2 Artificial photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.3 Water oxidation catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.4 Presentation of the problem . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5 Methodology 105.1 Molecular modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.2 Quantum chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115.3 Density functional theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.4 Computational details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6 Results and discussion 146.1 Spin density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146.2 Lowest unoccupied molecular orbital . . . . . . . . . . . . . . . . . . . . . . 156.3 Water nucleophilic attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166.4 RuV to RuVI oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186.5 Picoline/water substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . 206.6 Hydroperoxo stabilisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216.7 Six-coordinated species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

7 Conclusions 22

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1 List of acronyms

WOC = water oxidation catalystPCET = proton coupled electron transferWNA = water nucleophilic attackI2M = interaction of two metal complexesTON = turn-over numberTOF = turn-over frequencyPES = potential energy surfaceHF = Hartree-FockDFT = density functional theory

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2 Acknowledgement

I would like to acknowledge Marten Ahlquist for giving me the opportunity to realisethis project. I would like to thank him for his support and guidance, and interestingdiscussions, but also for the freedom I was given to explore my own ideas. Thanks also goout to Ying Wang who always had time to help me up to speed, both with the practicaland the theoretical, even during his busiest days of preparing his thesis defence. I alsowant to thank Rocio Marcos, Liqin Xue, Ivhan Tafner, Ting Fan, and Daniel Martenssonfor creating a wonderful environment to work in and the lively discussions we have hadabout all and everything.

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3 Introduction

The power consumption of the world is ever increasing. The continued increase makesresearch into alternative energy sources, and methods of utilising these, attractive. Amongthese are artificial photosynthesis which could present a novel method for solar energyutilisation. This thesis treats the investigation of a water oxidation catalyst, importantfor the potential utility in artificial photosynthesis. Firstly, some background is presentedfor perspective on the global energy situation and artificial photosynthesis, followed by apresentation of the specific problem investigated. Results are then presented, interspersedwith discussion, followed by some concluding thoughts.

4 Background

4.1 The current state of global power consumption

Recently the average global power consumption was estimated to an excess of 16.3 TWfor 2012 [1]. This figure is projected to increase rapidly, with current estimates predictinga doubling of the energy consumption by 2050 and a tripling by 2100 [2]. The issuewith rising energy demands lies not primarily in diminishing supply; fossil based energysources are expected to be available for a significant time ahead given a consumptionsimilar to the contemporary, but rather in the potential environmental effects of everincreasing carbon dioxide emissions [2]. Today an overwhelming majority of our energycomes from fossil sources, with only a small fraction, around 15 %, from other sources suchas biomass, nuclear, hydroelectric, and renewable wind and sunlight [3]. One way to offsetthe environmental impact of increasing energy consumption would be to replace existingenergy sources with carbon neutral energy.

Solar energy presents a practically renewable source of energy and solar power is envi-ronmentally benign during use. Photovoltaic devices, perhaps the most commonly recog-nised technology for solar energy, convert light into direct current. These are availableand receive increasingly more widespread use. A considerable issue with these devices isenergy storage; without practical and cheap technology for large-scale storage of electricenergy, solar energy cannot satisfy societal energy needs given the diurnal quality of suninsolation. A contrasting developing technology is that of artificial photosynthesis. Thisfield finds inspiration in natures strategy of solar energy conversion into chemical energy inthe process of photosynthesis, trying to create a method of doing this artificially. With anartificial photosynthetic device, ultimately, solar energy could be converted into reducedfuels, such as alcohols, from water and carbon dioxide. The storage and handling of suchfuels could also be practical in the existing energy infrastructure given its similarity tocurrently used fossil fuels.

6

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4.2 Artificial photosynthesis

The idea of artificial photosynthesis is to mimic the naturally occurring tactic of storingenergy in chemical bonds formed through the use of solar energy. In natural photosynthesislight is absorbed by chlorophylls and the energy is transferred to photosystem II wherewater oxidation occurs, producing protons and electrons:

2 H2Ohv

O2 + 4 H+ + 4 e–

The electrons are transported through the electron transport chain to photosystem I whereanother photon is absorbed and protons are reduced to produce nicotinamide adeninedinucleotide phosphate (NADPH):

NADP+ + H+ + 2 reduced ferredoxinferredoxin NADP+reductase

NADPH + 2 oxidised ferredoxin

NADPH has a reductive potential capable of reducing carbon dioxide [3]. Photosynthesiscan as such be seen as comprised of two half reactions, water oxidation and proton re-duction. Water oxidation is a key reaction in both natural and artificial photosynthesis.The proton reduction step in artificial photosynthesis can employ hydrogenase biomimeticcatalysts to produce hydrogen [4]

2 H+ + 2 e–cat

H2

or other catalysts to further reduce carbon dioxide with the hydrogen, e.g. to produceformic acid [5].

Many ideas exist as to the final construct of an artificial photosynthetic device. Oneis of a three-component unit of a photosensitiser, water oxidation catalyst and a protonreduction catalyst. Light hitting the photosensitiser creates charge separation in the unit,resulting in a potential able to drive the water oxidation at the anode and proton reduc-tion at the cathode. This project has been focused on the water oxidation half-reaction,specifically the investigation of one recently discovered water oxidation catalyst, thus thisarea will be further explored.

4.3 Water oxidation catalysts

The ruthenium based water oxidation catalysts are the most extensively studied. In 1982the Meyer group reported a dinuclear ruthenium based catalyst capable of water oxidation(figure 1 (a)) [13]. Gaseous dioxygen was found to be produced upon the addition of chem-ical oxidant CeIV. Their report of the first artificial catalyst capable of water oxidationpaved the way for the emergence of many ruthenium based complexes. The group of Thum-mel reported in 2005 the discovery of a mononuclear ruthenium complex capable of wateroxidation (figure 1 (b)) [14]. Recently the group of Sun reported that the introduction of

7

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N NO OO ORu

N

N

NN

RuN N

OH2 ON

NRu

NN

H2O N NO OO ORu

N

N

F

F

NN N

N NRuOH

H

N

N

(a) (b) (c) (d)

(13, 0.004 s-1) (260, 0.014 s-1) (2000, 18 s-1) (24 000, 1000 s-1)

Figure 1: Several ruthenium-based WOCs. The Meyer groups catalyst (a) was the firstartificial catalyst capable of water oxidation. The Thummel group then reported a mononu-clear ruthenium based WOC (b). The introduction of anionic ligands (c) improving thecatalytic performance was presented by Sun’s group and recent improvements have pro-duced a WOC with significantly enhanced activity (d). Turn-over number (TON) andturn-over frequency (TOF) are reported in parentheses respectively.

anionic ligands substantially increased the catalytic activity [6] . One such catalyst is theRuII(bda)(pic)2 (figure 1 (c)) which is the one specifically investigated in this project. Theeffect of the anionic ligands stems from the destabilisation of low oxidation states and thestabilisation of high oxidation states important for catalytic activity, promoting the oxida-tion of the complex and can be best understood by examining the catalytic mechanism ofthe catalyst (figure 2). Further modifications to the RuII(bda)(pic)2 catalyst has recentlyled to the synthesis of a highly active variant (figure 1 (d)) [7].

The current mechanistic understanding (figure 2) is that the catalyst in an acidic oxida-tive environment first undergoes a single electron transfer to form RuIII which associateswith water to form RuIII-H2O. The catalyst then goes through two consecutive proton-coupled electron-transfer (PCET) steps, forming RuIV-OH and RuV=O respectively. TheRuV=O species was found to have significant radicaloid character at the oxo oxygen pro-moting the bimolecular association of two complexes (figure 2 and 3). A mechanism wasdevised by which the O-O bond is formed in an oxyl radical coupling called I2M (interac-tion of two metal complexes) [19]. The rate of such a reaction follows second order kinetics.There is an alternative O-O bond formation mechanism where at the RuV=O state, solventwater reacts at the oxo oxygen in an nucleophilic attack denoted WNA (water nucleophilicattack). This is a first order rate mechanism with regards to the catalyst. Keep in mindthese two competing O-O bond forming mechanisms (figure 3) as they will be importantin the investigation of the Ru(bda)(pic)2 catalyst.

8

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2 RuII

2 RuIII-OH2

2 RuIV-OH

2 RuV=O

RuIV-O-O-RuIV

I2M

- 2 e-, - 2 H+

- 2 e-, - 2 H+

2 H2O

O2

- 2 e-

2 RuIII-OOH

2 RuIV-OO

WNA

2 H2O

2 H+

2 H2O2 O2

- 2 e-, - 2 H+

Figure 2: A general mechanism for the catalytic water oxidation with ruthenium complexes.Notice the divergence of the WNA and I2M pathway for different O-O bond formationmechanisms.

4.4 Presentation of the problem

The premise for this investigation is the experimental observation by the Sun group ofa change in measured reaction kinetics for the WOC Ru(bda)(pic)2 (bda = bipyridinedicarboxylic acid, pic = picoline) (figure 4). The kinetics were measured by the rate ofoxidant loss by absorption spectrometry at 360 nm (absorption peak of CeIV) versus thecatalyst concentration. Upon addition of catalyst to acidic (pH = 1) aqueous solution withCeIV (1.5 µM) as a chemical oxidant the unsubstituted form follows second order kineticswith regards to the catalyst, consistent with a bimolecular oxygen-oxygen bond formationstep as determining the rate (see previous subsection). Upon nitro substitution of the bda-ligand at the para-position, the measured kinetics are instead first order with respect to thecatalyst. Bromine and dibromine substitution results in kinetics between first and secondorder, with dibromine being closer to first order. Additionally, a mass compatible with anitro-substituted RuIV hydroperoxo species was detected by mass-spectrometry. The scopeof this project is to seek an explanation for the kinetic variation.

9

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WNA RuVOOH

H

RuO-O2 H+

I2M RuV ORuVO 2 Ru

O-O

Figure 3: WNA vs I2M mechanism.

N NO OO ORu

N

N

N NO OO ORu

N

N

N NO OO ORu

N

N

Br NO2

N NO OO ORu

N

N

BrBr

r ∝ [cat]2 r ∝ [cat]Decreasing rate order

H

Figure 4: The original unsubstituted catalyst and substitution with bromine, dibromine,and nitro at the para-position of the bda-ligand. The rate of water oxidation goes from2nd order from the left to 1st order to the right.

5 Methodology

5.1 Molecular modelling

To study a problem such as the one presented above a model of the physical behaviourof molecules is needed. A straight-forward way to model molecules would be to treatatoms as balls interconnected by springs, which can be modelled classically. Electrostaticinteractions are modelled by Coulombic interactions of point charges. This approach iscalled molecular mechanics. It is a very computationally effective method but cannotmodel the breaking or forming of bond, or any other movement of electrons since themodel does not take into account electronic behaviour. For this a model of electronicdistribution is needed.

Chemical reactions can be imagined as spatial rearrangement of electrons and atomicnuclei from one equilibrium geometry to another on a potential energy surface (PES) (figure5). Local minima on the surface corresponds to equilibrium geometries and saddle pointsconnecting two minima would correspond to transition states. Transition state theorystates that the rate of interconversion between two minima is given by the magnitude ofthe free energy barrier between the states assuming an equilibrium between reactant and

10

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Reaction coordinate

XNu-

XNu

Free energy

X-Nu

Figure 5: A potential energy surface (PES) depicting the interconnection of free energyand chemical reactivity.

transition state structure [Eyring, Polanyi, Evans, 1935]

k = κkBT

he

∆GRT

and thermodynamics assert that the relative free energies of states determines their equilib-rium populations, or in a non-equilibrium situation, the spontaneous reaction coordinate.Electrochemical reaction can also be related to free energy by the reduction potential

∆Gredox = −nF∆Eredox

As such, the problem presented in this report, involving the kinetic and electrochemicalbehaviour of one catalyst, can be studied by energy determination of the involved species.

5.2 Quantum chemistry

Quantum mechanics can accurately describe the behaviour of electrons and as such issuitable for modelling chemical behaviour. The energy of a time-independent system isfound by solving the time-independent Schrodinger equation [Schrodinger 1925]:

HΨ = EΨ

The energy is found as the eigenvalue to the quantum mechanical hamiltonian operatoron the wave-function ψ describing the system. The Hamiltonian contains the quantummechanical operators for potential and kinetic energy for the electrons and nuclei:

H =

electrons∑i

−~2

2me∇2

i +

nuclei∑A

−~2

2mA∇2

A +

electrons∑i

nuclei∑A

−e2ZA

riA+

electrons∑i>j

e2

rij+

nuclei∑A>B

e2ZAZB

rAB

11

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For chemical applications the Born-Oppenheimer approximation is invoked, stating thatthe movement of nuclei is negligible in comparison to electrons. This reduces the previousequation to:

H =electrons∑

i

−~2

2me∇2

i +electrons∑

i

nuclei∑A

−e2ZA

riA+

electrons∑i>j

e2

rij+

nuclei∑A>B

e2ZAZB

rAB

The electronic energy can now be solved followed by solving for the nuclear movement in theelectronic potential. This reduces the complexity of the problem but it is still considerablydemanding due to its many-body nature. One method for solving the energy of a systemwas presented by Hartree and Fock [Hartree, Fock, 1930], stating that the wave functioncan be described as one Slater determinant, that is a mathematical determinant consistingof spin orbitals in columns and electrons in the rows. Furthermore the spin orbitals aredescribed by a linear combination of basis functions formed with primitive Gaussian typefunctions defined by the basis set. The HF method utilises the variational principle statingthat the expectation value of the hamiltonian on any wave-function (the variational energy)is equal to or larger than the real energy:∫

Ψ∗HΨdτ∫Ψ ∗Ψdτ

= Evar ≥ Eexact

A test wave-function is initiated and the energy is minimised with respect to the basis setcoefficients. Correlated electronic movement makes this into a multi-body problem. TheHartree-Fock method reduces the problem by producing a solution where every electronis optimised with respect to the mean field of all other electrons. This means that someelectronic correlation is omitted. Improvements to the Hartree-Fock method are possible,e.g. configuration interaction which describes the wave function as a linear combinationof several Slater determinants. This is possible due to the variational principle; expan-sion of the wave-function gives a more accurate energy. The problem is the increase incomputational cost. An alternative methodology is presented by density functional theory(DFT).

5.3 Density functional theory

Density functional theory is an alternative for solving the electronic Schrodinger equation.It is based on a theorem by Hohenberg and Kohn [1964] stating the existence of a functionalproducing the ground state energy of a system given the electronic density:

E = E(ρ(r))

The energy functional can be divided into terms of kinetic energy, nuclear electronic at-traction and electron electron repulsion. The attraction and repulsion are Coulombic in-teraction terms but the kinetic term is taken from HF theory. This means that also DFT

12

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takes the mean field approach. The exchange-correlation term in the density functionalcorrects for this, but there is no way of knowing the optimal form of the term. Thus, alarge amount of effort has been invested into finding accurate functionals.

The vast availability of functionals necessitates a rational choice based on the soughtimplementation. For chemical application the important aspects would be geometric andenergetic accuracy. Benchmarks are available for many functionals and the choice of theB3LYP and M06 functionals for this investigation is based on their performance. Compar-isons of several functionals in one study found B3LYP to have the overall best performancefor geometries (bond length, angle and dihedral) and atomisation energies [9]. Anotherstudy also found B3LYP to have favourable results [10]. A recently developed group offunctionals called the Minnesota density functionals, including the M06 functional, havebeen shown to treat medium range electron correlation well [11]. The M06 functional wasthus used for electronic energy calculations in this investigation. The treatment of mediumto long range electronic correlation is one of the caveats of DFT, their contribution is smallbut significant given the long range of interaction. Another group of functionals employempirical dispersion correction on sets of atoms such as the B3LYP-D3 functional. It hasbeen reported that the empirical dispersion correction reduces the mean energy error onone test set for the B3LYP functional [12]. One problematic consideration with DFT is thelack of a hierarchy in the method as no systematic method exists to improve the functional.This is in comparison to HF-based methods where the wave-function can be expanded toyield more accurate energies.

5.4 Computational details

All calculations were done with the quantum chemistry software package Jaguar 7.9 bySchrodinger. With the exception of single point energy corrections, all calculations em-ployed the B3LYP functional and LACVP** basis set at the DFT level of theory. Moleculargeometries were gas phase optimised until convergence (iterative energy difference of lessthan 0.010 kcal/mol). Vibrational frequency calculations were performed on optimisedgeometries to confirm local minima or first order saddle point (transition state) geom-etry and correct for zero point energy (ZPE) and vibrational enthalpy and entropy atT = 298 K. Solvation energy correction was performed with water as solvent (dielec-tric constant ε = 80.37) using Poisson-Boltzmann reactive field (PBF) method. Sin-gle point energy was recalculated at the optimised geometries with the M06 functionaland larger basis set LACV3P**++ with two added f-polarising functions on the metalcentre (as recommendation by Martin et al. [8]). The free energy was composed asG = E(M06/LACV 3P ∗ ∗ + +) + Gsolv + ZPE + H298 − 298 ∗ S298 + 1.9 (the last termis for correcting the difference between 1 M (g) and 1 M (aq)). Notice that the dispersioncorrected B3LYP-D3 functional has in this investigation for most structures also been usedfor single point energy correction for comparison to the M06 functional. In most cases thefunctionals were consistent in energy, in other cases the discrepancy has been reported and

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discussed.

6 Results and discussion

This section will be a presentation of the areas of inquiry and the hypotheses formulatedduring the project, followed by the obtained results and discussion.

Figure 6: The spin density localised at the oxygen of the RuV(bda)(pic)2O species.

6.1 Spin density

The I2M mechanism is one of the competing O-O bond forming mechanisms for theRuV(bda)(pic)2O species (figure 2 and 3). As it is a radical type coupling reaction be-tween two ruthenium oxo oxygens, the radicaloid character of this species can potentiallybe seen as a measure of the propensity for the coupling to occur. The radical character ofthis species can be visualised by the spin density on the oxygen (figure 6) We hypothesisedthat substitution on the bda-ligand could reduce the radical character on the oxygen of thereacting species, lowering the propensity for the complex to undergo I2M coupling. Thiscould force the complex to react primarily through the competing WNA pathway whichwould lead to the observed kinetic variation. The RuV species can be thought to be ofboth spin multiplicity two or four but the former was found to be lower in energy, thus

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multiplicity two was the state used for this species. As a measure of the radicaloid char-acter, the spin densities of the ruthenium and oxo oxygen were calculated using Mullikenpopulation analysis (table 1).

Table 1: Spin densities calculated with Mulliken population analysis for RuV(bda)(pic)2Ospecies with substitutions.

Spin density at oxo oxygen Spin density at ruthenium

X = H 0.7246 0.3568

X = Br 0.7225 0.3585

X = NO2 0.7219 0.3583

Examination of the result indicates no correlation with substitution. All values are inaccordance with similar metal oxyl radical systems [16, 17]. Variation at the first decimalwould have made it more significant. By virtue of limited time and balancing priorities theI2M mechanism was disregarded to pursue an examination into the competing mechanism.It should be noted that these results by no means should be seen as conclusive evidence todisregard the I2M step as the source of kinetic variation among the substituted complexes.The reaction could still proceed through the I2M reaction but if a subsequent step hasbecome the rate determining step, the kinetics would be observed as first order. Theactual reaction barrier for the I2M mechanism should be modelled for a more conclusivecomparison.

6.2 Lowest unoccupied molecular orbital

Since the competing mechanism is nucleophilic attack by water at the ruthenium oxooxygen, we hypothesised that there could be a correlation of the energy of the lowest unoc-cupied molecular orbital (LUMO) for the reactant complex (figure 7). The LUMO energieswere calculated for the RuV(bda)(pic)2O species (table 2). The result indicates that thereseems to be a lowering of the LUMO energy correlated with substitution, predicting a lowerenergy product upon nucleophilic attack. This result gave merit to further investigationinto the WNA mechanism as described in the next section.

Table 2: LUMO energies for RuV(bda)(pic)2O species with substitutions.

LUMO energy (hartree)

X = H -0.2497

X = Br -0.2530

X = Br, X’ = Br -0.2560

X = NO2 -0.2618

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Figure 7: The LUMO of the RuV(bda)(pic)2O species is localised at the oxygen.

6.3 Water nucleophilic attack

The WNA mechanism is the other of the competing O-O bond forming pathways. It wasinitially modelled with three explicit water molecules (figure 8). The rationale for this wasthat the two hydrogen atoms of the attacking water would be stabilised in the transitionstate by hydrogen bonding to the other two water molecules. It was assumed that thisfeature would not be adequately captured by mere application of the solvation model.Examining the free energy profile, a lowering of the reaction free energy barrier correlatedwith substitution can be seen, but the magnitude of the barrier was still deemed too largeto account for the observed kinetic variation.

Building upon this hypothesis, the adequacy of three water molecules was questioned(figure 9). Intricate hydrogen bonding stabilisation could occur at the transition staterequiring more water to be accurately modelled. The reaction was thus modelled withfour, and subsequently, five water molecules (figure 11). The stabilising effect was foundto converge at four water molecules, thus a new free energy profile was computed (figure10).

Substitution on the bda-ligand seems to have a significant effect on the reaction barrier.Since the unmodified complex has previously been observed to obey second order kinetics[6], it is assumed that the barrier for WNA is significantly higher than that of the I2Mreaction, or else it would react readily with the ever present solvent water. Is the observedlowering of the barrier with substitution sufficient to explain the shift in reaction kinetics?The I2M electronic energy barrier has been reported to around 10 kcal/mol [19], but

16

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NN

O

O

O

ORuVO

N

N

HO HHOH

HOH

NN

O

O

O

ORuVO

N

N

HO H HOH

HOH

NN

OO

RuIIIO

N

N

HOH

HO

HOOH

OH

32.3

29.4

0

31.1

18.1

29.3

16.4

X

X

X

X = HX = Br

X = NO2

Gibbs free energy profile(kcal/mol)

Figure 8: The original free energy profile of WNA with three explicit water molecules.

Figure 9: The transition-state geometry of the WNA reaction with three explicit watermolecules. An empty space where another water could fit can be discerned.

difficulties in modelling this step should be highlighted. The observed second order kineticsindicate that the rate determining step is explicitly dependent on the proximal associationof two reactive complexes, this association must be accurately modelled for fair comparisonbetween the I2M and WNA barriers. Care should thus be taken when examining theenergy barrier directly without regard for the dynamic behaviour of the complexes insolution. Perhaps the WNA barrier, upon substitution, is low enough that the complex

17

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NN

O

O

O

ORuVO

N

N

HO HHOH

HOH

NN

O

O

O

ORuVO

N

N

HOH

HOH

HOH

NN

OO

RuIIIO

N

N

HOH

HO

HOOH

OH

31.3

27.8

0

28.8

15.7

24.618.0

X

X

X

X = HX = Br

X = Br, X' = Br

Gibbs' free energy(kcal/mol)

18.0

16.917.2

15.7

X = NO2

X'

(X' = H unless otherwise noted)

X'

X'

HO

H

HO

H

HO

H

Figure 10: Free energy profile of the WNA reaction at RuV(bda)(pic)2O with varyingsubstitution. Water attacks the oxo oxygen forming a highly intricate hydrogen bondingnetwork in the transition state followed by dissociation and proton transfer to one carboxylligand. Four explicit water molecules were used in the model. The lowering of the transitionstate barrier with substitution is more pronounced than with only three water and theproduct energies are more similar.

now primarily reacts with the solvent, with which it collides much more frequently thanwith another complex at high dilution.

6.4 RuV to RuVI oxidation

Upon oxidation of the RuV(bda)(pic)2O species to a highly oxidised RuVI(bda)(pic)2Ospecies (singlet state, multiplicity one), optimisation of the geometry underwent a barri-erless WNA mechanism (figure 12) leading to the hydroperoxo species RuIV-OOH. Thistransformation was also modelled with geometry optimisation in solvent model to makesure it was not an artefact of gas-phase geometry optimisation. The highly oxidised RuVI

species seems to be so electrophilic as to react spontaneously with water, but the feasi-bility for this reaction to occur is not trivial. Merely regarding the change in free energyof the reaction (figure 12) would indicate that it could be a favourable reaction under thereductive potential of CeIV reported to be around 1.5 to 1.7 V, although it reveals nothingas to the dynamics of the reaction.

It has been proposed for another mononuclear ruthenium WOC following first orderkinetics, Ru(bpy)(tpy), that the actual O-O bond formation mechanism could be a WNAconcerted electron transfer [15]. The reaction would as such proceed through the concomi-

18

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NN

O

O

O

ORuVO

N

N

HO HHOH

HOH

NN

O

O

O

ORuVO

N

N

HOH HOH

HOH

31.3

0

24.6X

X

X = H

X = NO2

HOH

H O H

(a) Four explicit water molecules.

NN

O

O

O

ORuVO

N

N

HO HHOH

HOH

NN

O

O

O

ORuVO

N

N

HOH HOH

HOH

26.7

0

26.2X

X

X = HX = NO2

HOH

H O H

HOH

HOH

(b) Five explicit water molecules.

Figure 11: The effect of four (a) and five (b) explicit water molecules respectively formodelling the WNA barrier for unsubstituted and nitro-substituted catalyst. There was adiscrepancy in the unsubstituted barrier height for the unsubstituted structure with fivewater molecules, but was disregarded since geometry convergence was difficult to attainwith the additional water molecule for these structure.

NN

O

O

O

ORuVO

N

N

HO HHOH

HOH

H O H- e- -H+

NN

O

O

O

ORuIV O

N

N

HO HOH

HOH

HO H

NN

O

O

O

ORuIV O

N

N

HO HOH

HOH

H O H

X X X

- e-, -H+

X = H, 1.69 VX = NO2, 1.65 V

H

Figure 12: The catalyst undergoes spontaneous WNA upon oxidation from RuV to RuVI.

tant oxidation of the catalyst during the nucleophilic attack by solvent water. This couldbe imagined to occur here as well. Perhaps the effective WNA barrier in a strongly oxi-dising environment is actually lower than predicted from only nucleophilic attack by waterdue to a concomitant electron transfer during the reaction. For better understanding ofthis process the kinetics of the reaction should be modelled, or an experimental strategycould be devised to test this hypothesis. There was no correlation of the potential withsubstitution, as such it is not seen as an explanation to the variation of reaction kineticswith the substitutions.

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6.5 Picoline/water substitution

The substitution of the axial picoline ligands with water was also studied as a mode ofdeactivation of the catalyst. Both the associative reaction pathway (figure 13a) where theleaving of the picoline ligand is concerted with the entering of water, and the dissocia-tive pathway (figure 13b) where picoline leaves first and water enters subsequently wascomputed. The barriers were found to be too large (around 30 kcal/mol) for these reac-tions to be significant contributors to the major reaction pathway and no correlation wasfound for the substitutions. No significant difference was found between the associativeand dissociative pathway.

26.3

0

NN

O

O

O

ORuV O

N

N

HO HHOH

HOH

NN

O

O

O

ORu O

N

N

HO

H

HO HHO

H

NN

O

O

O

ORu O

N

H OH HOHH

OH

26.5

27.0

0.1

0.4

13.6X

XX

X = HX = Br

X = NO2

+ Pic

(a) Associative pathway.

NN

O

O

O

ORuVO

N

N

HO HHOH

HOH

NN

O

O

O

ORuVO

N

H OH HOHH

OHN

N

O

O

O

ORuVO

N

HO HHOH

HOH

26.6

0

27.2

0.1

0.4

13.6

X = HX = Br

X = NO2

27.5

X

X

X

(b) Dissociative pathway.

Figure 13: Free energy profile of associative (a) and dissociative (b) pathways of picol-ine/water exchange for RuV(bda)(pic)2O.

The computed profiles are of substitution at the RuV state. It was in retrospect realisedthat substitution may be more prevalent for the low oxidation state species, such as RuII-OH2 or RuIII-OH2. The larger electronic density at ruthenium of the less oxidised speciesmay weaken the Ru-N bond by electronic repulsion. If further investigation would beconducted in this area, these species should be examined, but since no correlation was found

20

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for substitution at the higher oxidation state, attention was shifted to other hypothesesgiven the available time.

6.6 Hydroperoxo stabilisation

The experimental mass-spectrometer finding of a mass consistent with a nitro-substitutedRuIV-OOH hydroperoxo species inspired the idea of a stable species upon substitution,hindering the release of dioxygen. It can be thought that even with an I2M reaction, ifa later step is stable and becomes rate-determining the reaction kinetics will still be firstorder. Specifically, the species where the hydroperoxo proton had been transferred to onecarboxyl group was investigated (figure 14).

NN

OO

RuIV O

N

N

OH

OO

X

X = H, 8.8 kcal/molX = NO2, 6.3 kcal/mol

+ 3 H2O

NN

OO

RuIV O

N

N

O

OHO

X

+ 3 H2O

Figure 14: Structures from carboxyl ligand dissociation of the RuIII-OH2 species.

The results did not show any significant lowering of energy upon proton transfer to thecarboxyl ligand. Neither is there a large difference between the unsubstituted and nitro-substituted structure. Still, given the mass-spectrometry finding, this hypothesis could befurther explored. Perhaps explicit water molecules in the geometry could be tried and theredox potential to the further oxidised RuV species should be calculated.

6.7 Six-coordinated species

A structure, where a carboxyl ligand had dissociated from the ruthenium centre, was foundearly in the investigation. This inspired a hypothesis that substitution promotes the for-mation of a six-coordinated species early in the catalytic cycle. All known mononuclearsix-coordinated ruthenium WOCs have been found to follow 1st order kinetics. The forma-tion of a six-coordinated could bring the complex into an alternative pathway, hindering

21

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the interaction of two catalytic complexes, inhibiting the I2M mechanism. Possibly byprohibiting hydrogen bonding interactions between complexes. The carboxyl dissociationof the RuIV-OH species was first modelled (figure 15).

NN

O

O

O

ORuIV OH

N

N

X

NN

O

O

O

O-

RuIV O

N

N

X

X = H, 10.0 kcal/molX = NO2, 7.3 kcal/mol

- H+

Figure 15: The conversion of seven- to six-coordinated species at the RuIV state.

The dissociation was a thermodynamically unfavourable uphill reaction, although therewas considerable discrepancy between the M06 and B3LYP-D3 functional. This entailedthe investigation of the RuIII species (figure 16).

Six-coordinated ruthenium structures have previously been found to follow first orderkinetics. The removal of one carboxyl group from the centre would also destabilise thehighly oxidised states of ruthenium. In accordance with the improved performance of thecatalysts by the introduction of anionic ligand, if the bda-substitution promotes the sixcoordinated state it could lead to worsened catalytic activity. The barrier for conversionbetween the seven- and six-coordinated RuIII species should be investigate. If this is lowupon substitution, which could be explained by electron withdrawal by the substitutedgroup through the conjugated system, the complex could gain access to the six-coordinatedstates. The six-coordinated geometry could also be unfavourable for the bimolecular radicalcoupling, perhaps hindering stabilising hydrogen bonding. The barrier for the formationof the six-coordinated species should also be investigated at the RuIV and RuV state, aswell as the possibility of O-O bond formation.

7 Conclusions

The change in reaction kinetics upon bda-substitution is not explained by a change in spindensity for the reacting species. Neither does it seem to be a problem with degradation,

22

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N NO OO ORu

Pic

Pic

N NOO

O

OHRu

Pic

PicOH

OH2

N NOO

O

O-

Ru

Pic

PicOH2

N NOO

O

O-

Ru

Pic

PicOH

N NOO

O

O-

Ru

Pic

PicO

+ H+O+HHHH

(RuIII)

X = H0.87 kcal/mol(-0.18 kcal/mol)

X = NO20.33 kcal/mol(-0.75 kcal/mol)

X = H-1.10 kcal/mol(6.28 kcal/mol)

X = NO2-1.47 kcal/mol(6.76 kcal/mol)

X = H2.86 kcal/mol(7.93 kcal/mol)

X = NO23.19 kcal/mol(8.08 kcal/mol)

X = H7.10 kcal/mol(13.15 kcal/mol)

X = NO25.33 kcal/mol(11.17 kcal/mol)

X X

XX

Figure 16: The conversion of seven- to six-coordinated species at the RuIII state. Severalstructures were modelled with single-point energy with M06 functional compared to theB3LYP-D3 functional in parentheses. There are large discrepancies between the functionalsand structures except for the lower right structure.

at least not at the RuV state, as seen by the large uncorrelated barrier for picoline/waterexchange. The hypothesis that a later step after bimolecular I2M coupling has been affectedhas not yielded any explanation but has been largely unexplored in this investigation.Further study of this idea may be warranted.

The most promising finding was regarding the WNA pathway. There is some loweringof the WNA reaction barrier, possibly by electron withdrawal by the substituted groups,promoting the nucleophilic attack by water. Can this lowered barrier explain the change inreaction kinetics? Does the catalyst predominantly perform water oxidation through O-Obond formation by water nucleophilic attack upon substitution? It has been noted that thelack of second order mechanism not necessarily means that the reaction does not proceedthrough the I2M coupling mechanism. The I2M mechanism should be investigated to gainmore insight into this complicated mechanism. The behaviour of the complex in solutioncould be modelled with molecular dynamics simulations which could lead to qualitativeinsights to the interaction of two complexes, as contrasted by the hitherto studied reactionbarrier which only depicts the kinetics of two reacting species when they are already inproximity.

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Another interesting finding is the spontaneous procedure of WNA upon oxidation ofRuV. As mentioned by the group of Pushkar [15], the idea of a WNA concerted electrontransfer is interesting. Especially in a highly oxidising environment,a thousand-fold molarexcess of oxidant to catalyst in the experimental findings preceding this investigation, andthe ever present water, perhaps often the solvent water is not at an equilibrium distancebut much closer to the oxo oxygen promoting the oxidation of the complex. An experimentshould be devised to investigate this as a possible mechanism.

Lastly, the briefly investigated formation of a six-coordinated complex could poten-tially also be a part of an explanation for the kinetic variation. The finding of one six-coordinated structure in equal population at thermodynamic equilibrium to the seven-coordinated structure gives merit to this hypothesis. Much more investigation is needed,especially the barrier of interconversion and also the possibility of O-O bond formationthereafter.

In essence, the investigated catalyst as been found to follow complex mechanisms, andmuch more thought and study is necessary to comprehend its full behaviour. At the veryleast, this investigation has highlighted some interesting directions for further studies, andsome which may not be as promising. The road towards the catalysts for tomorrows solarenergy is long, but every step is necessary.

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References

[1] International Energy Agency.2012 Key World Energy Statistics 2012. Paris, France: International Energy Agency.(http://www.iea.org)

[2] Nathan S. Lewis, Daniel G. NoceraPowering the planet: chemical challenges in solar energy utilization.PNAS 2006, 103, 15729?15735

[3] James Barber, Phong D. TranFrom natural to artificial photosynthesisJ. R. Soc. Interface 2013 10, 20120984

[4] Thomas B. Rauchfuss, Maria E. Carroll, Bryan E. Barton, Patrick J. CarrollSynthetic Models for the Active Site of the [FeFe]-Hydrogenase: Catalytic ProtonReduction and the Structure of the Doubly Protonated IntermediateJ. Am. Chem. Soc. 2012, 134, 18843?18852

[5] Etsuko Fujita, James T. Muckerman, Yuichiro HimedaInterconversion of CO2 and formic acid by bio-inspired Ir complexes with pendentbasesBiochimica et Biophysica Acta 2013, 1827, 1031-1038

[6] Licheng Sun, Lele Duan, Andreas Fischer, Yunhua Xu*Isolated Seven-Coordinate Ru(IV) Dimer Complex with [HOHOH]- Bridging Ligandas an Intermediate for Catalytic Water OxidationJ. Am. Chem. Soc. 2009, 131, 10397-10399

[7] Licheng Sun, Lele Duan, Andreas Fischer, Yunhua XuHighly efficient and robust molecular water oxidation catalysts based on rutheniumcomplexesChem.Commun., 2014, 50, 12947-12950

[8] Jan M. L. Martin, Andreas SundermannCorrelation consistent valence basis sets for use with the Stuttgart?Dresden?Bonnrelativistic effective core potentials: The atoms Ga?Kr and In?XeJ. Chem. Phys., 2001, 114, 3408 - 3420

[9] Charles W. Bauschlicher Jr.A comparison of the accuracy of different functionalsChemical Physics Letters 1995, 246, 40-44

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[10] Larry A. Curtiss, Paul C. Redfern, Krishnan RaghavachariAssessment of Gaussian-3 and density-functional theories on the G3/05 test set ofexperimental energiesJ. Chem. Phys., 2005, 123, 124107

[11] Donald G. Truhlar, Yan ZhaoBenchmark Data for Interactions in Zeolite Model Complexes and Their Use for As-sessment and Validation of Electronic Structure MethodsJ. Phys. Chem. C, 2008, 112, 6860-6868

[12] Stefan Grimme, Tobias SchwabeDouble-hybrid density functionals with long-range dispersion corrections: higher ac-curacy and extended applicabilityPhys. Chem. Chem. Phys., 2007, 9, 3397?3406

[13] Thomas J. Meyer, Susan W. Gersten, George J. SamuelsCatalytic Oxidation of Water by an Oxo-Bridged Ruthenium DimerJ. Am. Chem. Soc., 1982, 104, 4029-4030

[14] Randolph P. Thummel, Ruifa ZongA New Family of Ru Complexes for Water OxidationJ. Am. Chem. Soc., 2005, 127, 12802-12803

[15] Yulia Pushkar, Dooshaye Moonshiram, Vatsal Purohit, Lifen Yan, Igor AlperovichSpectroscopic Analysis of Catalytic Water Oxidation by [RuII(bpy)(tpy)H2O]2+ Sug-gests That RuV=O Is Not a Rate-Limiting IntermediateJ. Am. Chem. Soc., 2014, 136, 11938-11945

[16] Marcus Lundberg, Margareta R. A. Blomberg, Per E. M. SiegbahnOxyl Radical Required for O?O Bond Formation in Synthetic Mn-CatalystInorg. Chem., 2004, 43, 264-274

[17] Mu-Hyun Baik, Xiaofan Yangcis,cis-[(bpy)2RuVO]2O4+ Catalyzes Water Oxidation Formally via in Situ Genera-tion of Radicaloid RuIV-O?J. Am. Chem. Soc., 2006, 128, 7476-7485

[18] Mu-Hyun Baik, Xiaofan YangThe Mechanism of Water Oxidation Catalysis Promoted by [tpyRu(IV)dO]2L3+: AComputational StudyJ. Am. Chem. Soc., 2008, 130, 16231?16240

[19] Timofei Privalov, Jonas Nyhln, Lele Duan, Bjrn kermark, Licheng SunEvolution of O2 in a Seven-Coordinate RuIV Dimer Complex with a [HOHOH]-

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Bridge: A Computational StudyAngew. Chem. Int. Ed., 2010, 49, 1773-1777

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