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Water Oxidation By Metal Complexes: Storing Solar Energy

Give It Up for the Sun KingMIT's Daniel Nocera has a recipe for taking solar power mainstream. It all starts with a tall glass of water.

"My idea is a simple one," Nocera has said. "If I take that sunlight and I take water, I can solve the entire energy problem."

They've come up with a system that can use the sun's oomph to split water molecules into their constituent elements, hydrogen and oxygen. The idea is that those elements could then be recombined in a fuel cell, releasing energy to run a household on cloudy days and at night, when ordinary solar systems stop producing.

By adding a chemical catalyst (cobalt and potassium phosphate) to plain water, his technique splits the molecules using just a volt or so of electricity.

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Other Media

Is it possible?

Solar Radiation• Typical house

in Florida averages~47 kWh/day.

• Typical photovoltaicefficiency ~ 20%

• Pluses:Sunshine is free.

• MinusesSun shines only during the day.

• Need to store collected energy.

http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/

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Water• Formula: H2O Mass: 18 grams/mole• Abundance: 1021 kilograms on earth.

Covers 71% of the earth’s surface.

• 4 electronic groups on central O atom. Tetrahedral electronic structure.Bent nuclear geometry.

• O atom is very electronegative.Electrons in bonds are notshared equally. O is δ-, H are δ+.Water has “structure”, high boiling point, acidity.

HO

HBond Angle = 104.5o

not 109.5o

Oxidation of Water• Reaction: 2 H2O O2 + 4 e- + 4 H+

A 4 electron process.• Standard Reduction Potential: Eo = +1.23 V• Standard Free Energy Change:

ΔGo = -nFEo = - 4*96,485 C/mole*+1.23 V = -474,706 J/mole = -475 kJ/mole

• Oxidation is the reverse of reduction.ΔGo = 475 kJ/mole

• Oxidation is an UPHILL process.• Single electron transfer requires more energy,

produces high energy radical intermediates.• Need multi-electron processes or ability to stabilize

intermediates.

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Reaction of Hydrogen & Oxygen

• Use a fuel cell to efficiently extract the energy.

Photosynthesis

Oxidation of Water provides the electrons used in photosynthesis.

Light Energy createspowerful oxidizing agents.

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Can We Use Transition Metals to Oxidize Water?

21Sc

44.956

39Y

88.91

57*La

138.91

22Ti

47.88

40Zr

91.224

72Hf

178.5

23V

50.942

41Nb

92.906

73Ta

180.95

24Cr

51.996

42Mo95.94

74W

183.85

25Mn

50.942

43Tc(98)

75Re

186.21

26Fe

55.847

44Ru

101.07

76Os190.2

27Co58.93

45Rh

102.91

77Ir

192.22

28Ni

58.69

46Pd

106.42

78Pt

195.08

29Cu

63.546

47Ag

107.87

79Au

196.97

30Zn65.39

48Cd

112.41

80Hg

200.59

Transition Metals are Lewis Acids• Lewis acids are electron acceptors.• Transition metals are σ-electron acceptors.

Can be π−electron acceptors or donors.• Ligands are Lewis bases, σ-electron donors.

Can be π−electron acceptors or donors.• Bonds formed are coordinate or dative bonds.

Weaker than covalent bonds.

NMM N+

C CC C+

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Coordination Number• Metal atoms commonly have 6, 5 or 4 ligands. • Common geometries are octahedral, square pyramid,

trigonal bipyramid, square planar and tetrahedral.• Ligation removes d-orbital degeneracy.• Observe Ligand Field (LF) & Charge Transfer

(MLCT or LMCT) transitions in visible spectrum.

L

M LL

LL

L

dxy, dyz, dxzπb or π*, varies with LHOMO

dx2-y2, dz2

σ* orbitalsLUMO

Ene

rgy

Metals Bond Using d-OrbitalsIn an isolated atomall 5 d-orbitals are degenerate.

In 6-coordinate octahedralcomplexesthe dz2 and dx2-y2

are σ*.

The dxy, dyz, dxzcan be πb, π* ornon-bonding.

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Transition Metals as Catalysts

• High coordination number –preassociation of reactants.

• Lewis acids – ability to polarize ligands.- increase electrophilicity.- stabilize reactive intermediates.

• Weak bonds – ligands are labile.• Multiple oxidation states are accessible.

Electrons can be donated to substrates. Potentials can be “tuned” by ligand sphere.

• [(bpy)2(OH2)RuORu(OH2)(bpy)2• 4 e- oxidation equivalents as 2(Ru(V) Ru(III))• Ru-bridging O bond = 1.869 Å, Ru-Ru distance = 3.708 Å.• Hypothesize a distortion to bring the O atoms into close

contact as a bridging peroxo species.

J. Am. Chem. Soc. 104, 4029, 1982.

Catalytic Oxidation of Water by an Oxo-Bridged Ruthenium Dimer

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“It Looks Like A Duck”• Cyclic Voltammograms depend upon:

Thermodynamics – energy of oxidation or reduction.Kinetics of electron transfer.Kinetics of diffusion.Stability of oxidized or reduced product.

Scan

Cur

rent

Oxi

dativ

e R

educ

tive

Oxidative WaveRu(II) Ru(III) + e-

Reductive WaveRu(III) + e- Ru(II)

Voltage

Cur

rent

Oxi

dativ

e R

educ

tive

Oxidative WaveRu(II) Ru(III) + e-

Voltage

Reversible Irreversible

Scan

No reductive wave.Oxidized product reacts.

Electrochemistry

J. Am. Chem. Soc. 104, 4029, 1982.

• Ru(III)Ru(III) Ru(III)Ru(IV) couple observed ~ 0.77 V

• Second wave appears at 1.2 V.2 e- process

Ru(III)Ru(IV) Ru(IV)Ru(V) • Followed by oxidation of

solvent. No solvent oxidation w/o complex in background scan.O2 observed at electrode.

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Lots of Electrochemistry

•J. Am. Chem. Soc. 107, 3855, 1985.

• Pourbaix diagram: E vs. pH.• Nernstian behavior:

function of [H+].• Slope changes as the number

of protons lost with oxidation changes.Ru(III,III) Ru(III,IV)2 H+ lost pH 4.3-6.51 H+ lost pH 6.5-8.5

• Ru(III,IV) Ru(IV,V)1 H+ lost pH 6.5-8.5 in 2 e- oxidation.

• Thermodynamics of H2O oxidation to O2 by Ru(V,V) or Ru(IV,V) is favorable. Only Ru(V,V) has 4 equivalents.

Oxygen Labeling• Synthesize dimer with labeled aquo groups 18OH2.• Mass spectrometry of O2 produced.

Monitor peaks at 32, 34 & 36 amu (m/z+).Monitor at 28 to correct for air leakage.

• Corrected yields:16O16O 23%, 16O18O 64%, 18O18O 13%

• Conclusions:Oxidation of bound water to O2 occurs.Rule out direct attack of H2O on bound O atom as

this would give primarily singly labeled product.Many possible pathways: direct intra-molecular coupling of aquo groups & inter-molecular coupling.

Inorg. Chem. 29, 3894, 1990.

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Water Oxidation by Mono-Metallic Complexes

• Simpler mechanisms of mononuclear catalysts may be useful for optimizing systems.

• Simpler synthesis Cheaper catalysts.• Simpler structure More robust catalyst.

Oxo bridge of Ru dimer prone to cleavage.

Bulky Substituents Prevent Dimerization• Tert-butyl substituted

bis(1’,8’-naphthpyrid-2’-yl)pyridine• Sealed reaction vessel charged with:

1 mmole Ce4+

2x10-4 mmole complex5000: 1 ratioRun for 20 hrs

• O2 produced. • 260 turnovers at a rate of

0.138 μmole/minInorg. Chem. 47, 11763, 2008.

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Rate Law is First Order in Catalyst

• No dimerization• Unimolecular

catalyst.

• Catalyst recovery indicates no involvement of RuO2 as active species.

Inorg. Chem. 47, 11763, 2008.

[Ru(II)(tpy)(bpm)(OH2)]2+

• Ruthenium will have severaloxidation states as it participates in the oxidation of water

• tpy = terpyridine – tridentate ligand.

• bpm = bipyrimidine – bidentate ligand.Uncoordinated N atoms lead to

pH dependent chemistry.• [Ru(II)(tpy)(bpm)(OH2)]2+ - 6 coordinate complex.

N

N

N

NN

NN

N

Ru

OH2

N N

N NJ. Am. Chem. Soc.130, 16462, 2008.

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Cyclic Voltammetry2 electron vs. 1 electron Processes

Charge Transferred = AreaConcentrations are known.

Proton Coupled Electron Transferlowers oxidation energy.

J. Am. Chem. Soc.130, 16462, 2008.

More Cyclic VoltammetryRu(II) Ru(IV)

Ru(IV) Ru(V)

J. Am. Chem. Soc.130, 16462, 2008.

Presence of Ru(V) triggers H2O oxidation.

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Proposed Catalytic Cycle• Water is oxidized catalytically by the complex using

reasonable potentials or chemical reagents.

J. Am. Chem. Soc.130, 16462, 2008.

O2 production observed.

Several turnovers of theRuthenium complex

as the Ce4+is consumed.

Ce4+ + e- Ce3+ Eo = 1.7 V

• ([Ru(II)trpy(OH2)]2(μ-bpp))3+

bpp = 2,6 bis(pyridyl)pyrazolate• Greater rigidity than μ-oxo dimer.• Oxygen labeling experiment.

J. Am. Chem. Soc.131, 2769, 2009.

New Dimer – Intramolecular Mechanism of O2 Formation

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• Intramolecular Bond Formation

• Nucleophilic Attack of Solvent

Labeling study supports intramolecular bond formation with no fast exchange between ligated and free water molecules.

Two Mechanisms

J. Am. Chem. Soc.131, 2769, 2009

Even Newer Dimer

• Low potential metal oxidations due to anionic carboxylate ligands.

• Structural features:Metal centers are anti.Metal complexation by C atom in the ring.

• Water converted to oxygen confirmed by isotopic ratio.

Metal Oxidations

LigandReductions

Inorg. Chem. 48, 2717, 2009.

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References• “Catalytic Oxidation of Water by an Oxo-Bridged Ruthenium Dimer”. S.W.

Gersten, G.J. Samuels, T.J. Meyer. J. Am. Chem. Soc. 104, 4029, 1982.• “Structure and Redox Properties of the Water Oxidation Catalyst

[(bpy)2(OH2)RuORu(OH2)(bpy)2]4+”. J.A. Gilbert, D.S. Eggleston, W.R. Murphy, D.A. Geselowitz, S.W. Gersten, D.J. Hodgson, T.J. Meyer. J. Am. Chem. Soc. 107, 3855, 1985.

• “Water Oxidation by [(bpy)2(OH2)RuORu(OH2)(bpy)2]4+. An Oxygen-Labeling Study ”. D.A. Geselowitz, T.J. Meyer. Inorg. Chem. 29, 3894, 1990.

• “A New Family of Ru Complexes for Water Oxidation”.R. Zong, R.P. Thummel. J. Am. Chem. Soc. 127, 12802, 2005.

• “One Site is Enough. Catalytic Water Oxidation by [Ru(II)(tpy)(bpm)(OH2)]2+

and [Ru(II)(tpy)(bpz)(OH2)]2+”. J.J. Concepcion, J.W. Jurss, J.L. Templeton, T.J. Meyer. J. Am. Chem. Soc. 130, 16462, 2008.

• “Mononuclear Ruthenium(II) Complexes That Catalyze Water Oxidation”. H.W. Tseng, R. Zong, J.T. Muckerman, R. Thummel. Inorg. Chem. 47, 11763, 2008.

• “Oxygen-Oxygen Bond Formation by the Ru-Hbpp Water Oxidation Catalyst Occurs Solely via an Intramolecular Reaction Pathway”. S. Romain, F. Bozoglian, X. Sala, A. Llobet. J. Am. Chem. Soc. 131, 2768, 2009.

• “ A New Dinuclear Ruthenium Complex as an Efficient Water Oxidation Catalyst”.Y. Xu, T. Akermark, V.Gyollai, D. Zou, L. Eriksson, L. Duan, R Zhang, B. Akermark, L. Sun. Inorg. Chem. 47, 2717, 2009.