Download Water Oxidation By Metal Complexes

Water Oxidation
By Metal Complexes:
Storing Solar Energy
Give It Up for the Sun King
MIT'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 photovoltaic
efficiency ~ 20%
• Pluses:
Sunshine is free.
• Minuses
Sun 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.
H
O
H
Bond Angle = 104.5o
not 109.5o
• O atom is very electronegative.
Electrons in bonds are not
shared equally. O is δ-, H are δ+.
Water has “structure”, high boiling point, acidity.
Oxidation of Water
• Reaction: 2 H2O Æ 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
Light Energy creates
powerful oxidizing agents.
Oxidation of Water provides the electrons used
in photosynthesis.
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Can We Use Transition Metals
to Oxidize Water?
21
22
23
44.956
47.88
50.942
51.996
50.942
39
40
41
42
43
88.91
91.224
92.906
95.94
(98)
101.07
102.91
106.42
107.87
57
72
73
74
75
76
77
78
79
180.95
183.85
190.2
192.22
195.08
Sc
Y
*La
138.91
Ti
Zr
Hf
178.5
V
Nb
Ta
24
Cr
Mo
W
25
Mn
Tc
Re
186.21
26
27
28
55.847
58.93
58.69
63.546
44
45
46
47
Fe
Ru
Os
Co
Rh
Ir
Ni
Pd
Pt
29
30
Cu
Zn
65.39
48
Ag
Cd
112.41
80
Au
Hg
196.97
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.
C
+
C
C
C
M
+
N
M
N
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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
L
M
L
L
L
Energy
L
dx2-y2, dz2
σ* orbitals
LUMO
dxy, dyz, dxz
πb or π*, varies with L
HOMO
Metals Bond Using d-Orbitals
In an isolated atom
all 5 d-orbitals are
degenerate.
In 6-coordinate
octahedral
complexes
the dz2 and dx2-y2
are σ*.
The dxy, dyz, dxz
can be πb, π* or
non-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.
Catalytic Oxidation of Water by an
Oxo-Bridged Ruthenium Dimer
•
•
•
•
[(bpy)2(OH2)RuORu(OH2)(bpy)2
4 e- oxidation equivalents as 2(Ru(V) Æ 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.
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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.
Reductive Wave
Ru(III) + e- Æ Ru(II)
Voltage
Scan
Irreversible
Current
Oxidative Reductive
Current
Oxidative Reductive
Reversible
No reductive wave.
Oxidized product reacts.
Voltage
Scan
Oxidative Wave
Ru(II) Æ Ru(III) + e-
Oxidative Wave
Ru(II) Æ Ru(III) + e-
Electrochemistry
• Ru(III)Ru(III) Æ Ru(III)Ru(IV)
couple observed ~ 0.77 V
• Second wave appears at 1.2 V.
2 e- process
Ru(III)Ru(IV) Æ Ru(IV)Ru(V)
• Followed by oxidation of
solvent. No solvent oxidation
w/o complex in background
scan.O2 observed at electrode.
J. Am. Chem. Soc. 104, 4029, 1982.
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Lots of Electrochemistry
• Pourbaix diagram: E vs. pH.
• Nernstian behavior:
function of [H+].
• Slope changes as the number
of protons lost with oxidation
changes.
Ru(III,III) ÆRu(III,IV)
2 H+ lost pH 4.3-6.5
1 H+ lost pH 6.5-8.5
• Ru(III,IV) ÆRu(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.
•J. Am. Chem. Soc. 107, 3855, 1985.
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 Æ Cheaper catalysts.
• Simpler structure Æ 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 complex
5000: 1 ratio
Run for 20 hrs
• O2 produced.
• 260 turnovers at a rate of
0.138 μmole/min
Inorg. 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+
• tpy = terpyridine – tridentate ligand.
N
N
N
• bpm = bipyrimidine – bidentate ligand.
N
Uncoordinated N atoms lead to
pH dependent chemistry.
N
• [Ru(II)(tpy)(bpm)(OH2)]2+ - 6 coordinate complex.
• Ruthenium will have several
oxidation states as it
participates in the oxidation of
water
J. Am. Chem. Soc.130, 16462, 2008.
N
N
N
N
N
Ru
N
N
OH2
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Cyclic Voltammetry
2 electron vs. 1 electron Processes
Charge Transferred = Area
Concentrations are known.
Proton Coupled
Electron Transfer
lowers oxidation energy.
J. Am. Chem. Soc.130, 16462, 2008.
More Cyclic Voltammetry
Ru(II) Æ Ru(IV)
Ru(IV) Æ Ru(V)
Presence of Ru(V) triggers H2O oxidation.
J. Am. Chem. Soc.130, 16462, 2008.
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Proposed Catalytic Cycle
• Water is oxidized catalytically by the complex using
reasonable potentials or chemical reagents.
Ce4+ + e- Æ Ce3+ Eo = 1.7 V
O2 production observed.
Several turnovers of the
Ruthenium complex
as the Ce4+is consumed.
J. Am. Chem. Soc.130, 16462, 2008.
New Dimer – Intramolecular Mechanism of
O2 Formation
• ([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.
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Two Mechanisms
• Intramolecular Bond Formation
• Nucleophilic Attack of Solvent
Labeling study supports intramolecular bond formation with
no fast exchange between ligated and free water
molecules.
J. Am. Chem. Soc.131, 2769, 2009
Even Newer Dimer
Ligand
Reductions
Metal Oxidations
• 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.
Inorg. Chem. 48, 2717, 2009.
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References
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“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 OxygenLabeling 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.
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