chemistry of metals catalysis for sustainable development michael w.-y. yu department of applied...
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Chemistry of MetalsCatalysis for Sustainable Development
Michael W.-Y. YuDepartment of Applied Biology and Chemical Technology
The Hong Kong Polytechnic University
Chemistry is a unique science….
Drugs for better healthcare
Man-made fibers and materials for high performance apparel
Materials for device fabrication
Optical fiber for high speed communication
Chemical Synthesis improves quality of life
Most chemical products — from perfumes to plastics to pharmaceuticals — are
based on carbon, which currently is supplied by Earth's finite petroleum feedstocks
Industrial chemical processes generate large amounts of waste, the safe disposal of which imposes an increasing burden on the environment
We are facing challenges that our quality of life is becoming Unsustainable …..
Green Chemistry for achieving Sustainable Development
Innovation in catalysis… a route to green chemical synthesis
Catalysis speeds up a reaction, and can
also make new reactions possible that allow different starting materials to be used.
90% of industrial processes for production of fuels, plastics, drugs and other chemicals relies on catalysis
Development of new catalysts is critical for the development of more efficient, economic and greener technologies.
Catalysis – less energyEmploy non-toxic reagents and less wastes
Development of Metal Catalysis for sustainable development….. Hydrogen as fuel for tomorrowo Fuel cell technologyo Solar energy production of hydrogen
Chemical Synthesis via Activation of Inert Chemical bondso Activation of H-H bondo Activation of C-X (X = halide)o Activation of C-H bond
Hydrogen as fuel for tomorrow
Renewable Energy for the Future
• Highly exothermic reaction• Inexhaustible• Non-polluting• Zero carbon emission
Dihydrogen (H2)
2H2 + O2 2H2O
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What is a fuel cell?• Fuel is reacted with oxygen in an
electrochemical cell to produce energy.– Electricity is generated from oxidation of fuel supplied to
the anode and reduction of oxygen at the cathode.
• Controlled process!!!Oxidation of hydrogen:
cathode: O2(g) + 4e- + 4H+ 2H2O
anode: 2H2(g) 4H+ + 4e-
Each of the anode and cathode reactions are half-cell reactions.The overall cell reaction is: 2H2 + O2 2H2O
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Catalyst
Anode Cathode
O2H2
electrolyte
e-
H2O
Schematic diagram of fuel cell
A fuel cell works by catalysis, oxidizing the fuel on anode, and forcing the electrons pass through a circuit, hence converting them to electrical power
At the cathode, the oxidant (oxygen) is reduced and takes the electrons back in, combining them with protons to give water
• In acid medium, most metal cannot operate in such condition metal dissolution
• Noble metal (Pt) oxidation / reduction of surface within the potential range of interest
H2 oxidation
O2 reduction– 4 e- + 4 H protons
reduction (complex systems)
– O2 H2O2 or H2O
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Electron flow
H+
H+
H+ O2
WaterH2O
H2
Proton exchange membrane
Gas permeable electrode with platinum catalyst
Gas permeable electrode with platinum catalyst
2H2 4H+ + 4e-
O2 + 4H+ + 4e- 2H2O
The PEM fuel cell
A proton-exchange membrane (PEM) is used to separate the anode and cathode
Allows H+ to pass through while keeping the gases apart The protons reach cathode and react with oxygen to form water — the
only waste product is water, which is environmentally benign
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The H2/O2 fuel cell is ideal for driving environmentally friendly vehicles with
zero carbon emission
• Suitable for urban transportation.
Challenges:
1. Platinum is a rare metal$$$$ The operation cost of fuel cell becomes high CO poisoning with low tolerance
Development of new electrocatalysts for fuel cell becomes important!
- reducing platinum loading – metal alloy with Pt
- development of novel Pt-free materials like metal oxide-based catalystMo2C-ZrO2/C, transition metal marcocyclic compounds
2. Storage of hydrogen onboard
H2 gas oxidation
1. Mass transport of dissolved H2 to the surface:
H2(aq) H2(ads)
2. Chemisorption of hydrogen as atoms (breaking H-H bond) H2(aq) 2H(ads)
3. Ionization of hydrogen atoms H(ads) H+
(ads) + e-
4. Transport of the H+ ions away from the electrode surface H
+(ads) H+
(aq)
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H2(g) → 2H(g) DH = +436 kJ mol-1
Oxygen reduction
1. Large kinetic barrier for the oxygen reduction strong O=O bond: BDE = 463 kJ mol-1
2. For some electrocatalysts (Ag or Pt), two parallel reaction pathways are observed:
– Direct reduction of O2 to H2O (acid medium)
Eo = 1.23V
– An indirect reduction of O2 to H2O2 (acid medium)
Eo = 0.68V
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2H2OO2 + 4H+ + 4e-
H2O2O2 + 2H+ + 2e-
Catalyst is needed….
• For dioxygen reduction reaction to take place, the
dioxygen bond must be weakened, • A strong interaction with the surface of the catalyst will
be necessary
– Electrocatalyst becomes important in the selectivity of product.
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Binding of dioxygen to metal
• End-on
• Side-on
• Bridging
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O O
M
M
O
O
O O
MM
Dioxygen is bonded to metal atom with π bond of O2 and metal surface.
Two bonds are formed with two metal centres in each end of the O2 molecule.
Dioxygen molecule is bonded between metal atom with bent structure.
Binding of O2 to metal leads to weakening of the O-O bond
M-O2 complexes are reactive…
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Overall result: Breaking of O=O bond to form 2H2O molecules
Mn+ O2
O
Mn+1
O Mn+ H2O2
Mn+ 2H2O
2e-/2H+
4e-/4H+superoxo
O O
Mn+2
peroxo
4e-/4H+
Mn+ 2H2O
O O
Mn+1 Mn+1
bridging peroxo
4e-/4H+
2Mn+ 2H2O
Where does the H2 come from…?
• Steam Methane Reforming High-temperature (800 – 900 oC) steam is
combined with methane in the presence of a Ni catalyst to produce hydrogen. This is the most common and least-expensive method of production in use today
• Water is the most abundant source of Hydrogen
2H2O(l) 2H2(g) + O2(g) DH = 285.9 kJ mol-1
• Turning water to H2 is a highly endothermic process
• Dependent on Fossil Fuel• CO2 emission!!!
Water electrolysis
• Zero carbon emission??• Great demand of high
quality water• Expensive
Learning from Nature….
• Higher green plants use solar energy to convert H2O into O2 and reducing equivalents in NADPH for reduction of CO2 to carbohydrates…
• This process is known as Photosynthesis
2H2O + 4hn O2 + 4H+ + 4e-
nCO2 + 2ne- + 2nH+ (CH2O)n
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Chlorophyll – pigments for Photosynthesis
• Macrocyclic structure • Conjugated C=C bond• Mg2+ cation (structure stabilization)
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hn
Ground state
p bonding
*p antibonding
chlorophyllExcited state
p bonding
*p antibonding
chlorophyll
A
electron transfer
“hole” – oxidizing!!
radical anion – reducing!!
chlorophyll cation
p bonding
*p antibondingA-
charge separation
Chlorophyll captures light
energy to form reactive chemical species….
Light Reactions
2H2O O2 + 4H+ + 4e-
• Active site (Oxygen Evolving Center) contains a Mn4 cluster• Four photons are required to effect 4e oxidation of 2H2O molecules
Oxygen Evolving Center (OEC) of PSIIFerreira, K. N., Iverson,T. M., Maghlaoui, K., Barber, J., Iwata, S. Science 2004, 303, 1831
• Light drives the oxidation of Mn to higher oxidation states• Highly oxidizing Mn would damage the associated proteins of the PSII complex;
protein being replaced every 30 minutes
Artificial Photosynthesis
Design a man-made catalytic system that mimics Nature for photo-driven water oxidation…
DyeA D OEC
e-e-e-e-
2H2O
O2 4H+
HEC
2H+
H2
e-
e-
anode cathodepermeablemembrane
hn
Dye-sensitized photovoltaic cells
• Photoexcitation of dye is followed by electron injection into the conduction band of the TiO2 film
• The dye is regenerated by a redox system (e.g. I- / I3- couple)
Ruthenium complexes as dye for photovoltaic cells
• Stable complexes, over 100 million turnovers (servicable for 20 years)• Carboxylic acid groups for metal anchoring to TiO2 (key to charge injection)• Tunable color by structure modification• Wide absorption range [400 (visible) – 900 nm (near IR)]
Figure extracted from Gratzel, M. Inorg. Chem. 2005, 44, 6841
Working principle
• Photoexcitation causes charge separation between Ru and the ligand Ru becomes one-electron oxidized; ligand becomes one-electron reduced
• Excited state is a stronger oxidant and reductant than its ground state
Ground state
d p (Ru)
*p ligand
[Ru(bpy)3]2+
Excited state
d p (Ru)
*p ligand
[Ru(bpy)3]2+*
formally Ru3+ center
formally one-electron reduced ligand
hn
Photoelectrochemical dehydrogentaion of alcohol and generation of hydrogen
• Electrochemical isopropanol oxidation by Ru-oxo• Platinum electrode for 2H+/H2 couple• Electrochemical water oxidation remains a challenge
[Ru(dpp)RuII(H2O)]4+TiO2
2H+ H2
photoanode
h
[Ru(dpp)RuIV=O]4+TiO2
[Ru*(dpp)RuII(H2O)]4+TiO2
e- e-
[Ru(dpp)RuII(H2O)]4+TiO2
cathode
2e- Pt
OH
O
ACTIVATION OF DIHYDROGENMetal-dihydrogen interaction
Alkene Hydrogenation
• Exothermic reaction (DH ~ 120 kJ mol-1)• Catalyst required: Pt, PtO2, Pd• Syn addition to C=C bond• BDE (H2) = 436 kJ mol-1 (critical reaction barrier)• C.f. BDEs (kJ mol-1) for: Cl-Cl (242); C-H (414)
catalystC C C C
H H
H H
weak p-bond strong s-bond
2 X strong s-bonds
Waste-free reaction!
• Metal-hydride formation from “M + H2”?
• Coordination of H2 to M
• Breaking of H-H bond• M-H covalent, polarized, reactive!!• c.f. 2Na + H2 2NaH
H-H cleavage
LnM H2 LnMH
HLnM
H
H
-+
metal-dihydrogen complex
metal-hydride (H-)
Rh-catalyzed homogeneous alkene hydrogenation
Ph3PRh
Ph3P
H
Cl
PPh3
H
Ph3PRh
Ph3P
H
Cl
H
18-electron, coordinatively saturated
16-electron, coordinatively unsaturated
PPh3Ligand DisscoiationIII III
Ph3PRh
Ph3P
H
Cl
H RhPh3P
HPPh3
HCl
Metal-Alkene -Complex
Ph3PRh
Ph3P
H
Cl
CC
H
Hydride Insertion
Metal-Alkyl Complex
Reductive Elimination
Ph3PRh
Ph3P
ClI III
IIIIIIAlkene Coordination
H2
H H
Reversible changes of oxidation states: RhI RhIII
• Preparation of stereochemically pure compounds• Enantiomers have different binding properties to
receptors thereby exhibiting different bioactivities• Hazard of serious side-effect (e.g. thalidomide)• Diastereomeric resolution (max. yield 50%)
Chiral Technology for Drug Synthesis
Asymmetric Hydrogenation
• Chiral ligands
Ph2PPPh2
P
P
Et
EtEt
Et
Me
Me O
OPPh2
PPh2
H
H
**
DIPAMP (chiral at phosphorus)By Knowles in 60s(Nobel 2001)
DIOP (chiral at backbone)By Kagan in 70s
BINAP(Axially chiral backbone)By Noyori in 80s(Nobel 2001)
DuPhos(chiral at backbone)By Burk in 90s
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Asymmetric Hydrogenation: application
Practical application of asymmetric hydrogenation in Eli Lilly Company (Making drugs).
FeP
Me
CF3
CF3
CF3
CF3
PPh2
L* =
Peroxime proliferator activated receptor (PPAR) agonist, for treatment of diabetes.
COOH
OEtBnO
COOH
OEtBnOH2
[Rh(NBD)2]BF4
MeOH
L*
COOH
OEtO
O
HN
Me
Me
Houpis, Org. Lett. 2005, 7,
38
Ketone Hydrogenation
R OR'
O O
R OR'
OH O(R)-BINAP Ru(II)
H2 (70-103 atm)
93-100 % yield98-100 % ee
b-ketoesters:
Ru
RuP
P O
O
Cl
H
H3C
OCH3
(R)
P
P
Cl
H
O
O
OCH3(S)
higher energy
Murahashi, Chem. Rev. 1998, 98, 2599
H3C
lower energy
P
P PPh2
PPh2=
39
Ketone Hydrogenation: Examples
R Y
OO
R' R'
Y = OR, SR, NR2
R Y
OOH
R' R'
Asymmetric hydrogenation ofketo esters
PR (OR'')2
OO
R' R'
PR (OR'')2
OOH
R' R'
Asymmetric hydrogenation ofketo phosphonates
R
O
S
ONa
OO R
OH
S
ONa
OO
Asymmetric hydrogenation ofketo sulfonates
HYDROGENATION OF FUNCTIONALIZED KETONES
R
O
OHR
OH
OHAsymmetric hydrogenation ofhydroxy ketones
R
O
NR'2 R
OH
NR'2Asymmetric hydrogenation ofamino ketones
40
Ketone Hydrogenation: Selected practical examples
O
O
N
MeO OMe
COPh
trans-RuCl2[(R)-Xyl-BINAP][(R)-DAIPEN]
t-BuOK, i-PrOH, 25 ¢XC, 8 atm H2
O
OH
N
MeO OMe
COPh
97% ee>99% yield
HO
OH
NH
MeO OMe
HCl
(R)-denopamine
O
OO
Me
Me
OTBS
MeO
O O
glycoside derived -keto ester
O
OO
Me
Me
OTBS
MeO
O OH
(S)-config. (R)-config.
O
OO
Me
Me
OTBS
MeO
O OH
%dr (S)/(R) = >99/1
%de = >98%
RuBr2[(R)-BINAP]
45 oC, 1 atm. H2
MeOH, 24 h
Thomassigny, C.; Greck, C. Tetrahedron: Asymmetry 2004, 15, 199.
Kawaguchi, T.; Saito, K.; Matsuki, K.; Iwakuma, T.; Takeda, M. Chem. Pharm. Bull. 1993, 41, 639.
ACTIVATION OF ARYL HALIDESFormation of Reactive Organopalladium Complexes
Biaryls are important targets for organic synthesis
Biaryls
Cross Coupling Reactions – little by-products
• Aryl halides are poor electrophiles for SN1 / SN2• Due to sp2 hybridized C-X bond:
– Lower polarity – Stronger C-X bond
• Catalyst is required for Biaryl Coupling Reactions
X
M = main group element(e.g. Li, MgX, B, Sn)
nucleophileelectrophile C-C bond formation
A AB B
X = halide
M
PR3
PdR3P
R3PPR3
0
four-coordinate18e species
2 PAr3
PR3
Pd
PR3
two-coordinate14e species
X R3P
Pd
R3P X
four-coordinate16e species
R3P
Pd
PR3X
isolated trans-complex
0 II II
d10 d8
R3P
Pd
R3P X
Oxidative Addition
concerted three-center T.S.
Oxidative Addition turned Aryl Halides to reactive Arylpalladium
Favored by strong s-donors (alkyl vs aryl phosphines)
Rate : tBu3P > Ph3PReactivity trend: C-I > C-Br >>> C-Cl >>> C-Fc.f. Mg + ArX ArMgX (Grignard reagent)
Coordinatively unsaturatedElectron rich
Stable square planar complex
45
Suzuki coupling reaction
X
X = I, OTf, Br, Cl(recent application)
R
palladium complex
base(R'O)2B
R R''R''
Developed in early 80s, by Akira Suzuki.
Suzuki, Chem Rev. 1995, 95, 2457.Suzuki, J. Organomet. Chem. 1999, 576, 147
Catalytic cycle
• Oxidative addition: Pd(0) Pd(II) + C-X bond breaking • Ar’ group transfer from B Pd “transmetallation”• Reductive elimination: C-C bond formation step + Pd(0) regeneration
Pd(0) Ar-X
Pd(II)X
ArPd(II)Ar'
Ar
Ar Ar'
Ar'-MMX
General catalytic cycle for cross coupling
M = borate
oxidative addition
transmetallation
reductive elimination
Examples
Muller, D.; Fleury, J-P. Tetrahedron Lett. 1991, 32, 2229
(OH)2B O
NEt2
CONEt2
Br
O
NEt2
CONEt2
Pd(PPh3)4
Na2CO3
DME, reflux
Fu, J-M.; Sharp, M. J.;Snieckus, V. Tetrahedron Lett. 1988, 29, 5459
CN
Cl (OH)2B
CN NN
NNH
N
O
HO2C
Valsartan (antihypertensive drug)
Na2CO3H2O/toluene/dmso 120 oC
PdCl2 / L
89% yieldca. 2000 TON
P
HO3S
SO3H
HO3S
L =
Haber, S; Egger, N. US Patent 2000, 6-140-265
O
B(OH)2
RO
OR O
OR
OMeO
OMe O
OR
IPdL4
K2CO3
L = phosphine ligand
O
OR
OR
O
RO
OMeO
OMe O
OR
95% yield
Ligands for Suzuki Coupling Reactions
• Strong s-donors promote oxidative addition• Steric bulkiness of ligand increase activity of Pd(0) – more
open for substrate interaction
P
3
P
3P R
2
R = BuR = Me
CH3
P
3
PR2
R = PhR = CyR = tBu
PCy2
Fe
Ph
PhPh
PhPh
PtBu2
N
N
RR
R'
RR
R'
PCy3 PtBu3P(oTolyl)3
Q-Phos
N-heterocyclic carbene
R, R' = MeR = iPr, R' = H
ACTIVATION OF C-H BONDThe Next Challenge
• Using pre-functionalized substrates (e.g. aryl halides)• Expensive• Derived from simple aromatics by halogenation
Coupling reactions with C-H bond???
H MA Bcatalyst
A B
Strong bond energyNon-polar
Formation of Arylpalladium from arenes
• Electrophilic attack of Pd2+ on arenes leads to C-H bond cleavage• pKa: 44 (butane); 37 (C6H6); • Limitation: homocoupling; isomeric products
PdCl2NaOAc
HOAc, heat
Pd
H
X
HX
Pd X
X
X = Cl- or OAc-
HX
Pd
II
II
Pd(0)
oxidants
electrophilic attack by Pd2+
deprotonation
reductive elimination
Cyclopalladated Complexes
• Cyclopalladation of arenes [Ryabov, A. D. Chem. Rev. 1990, 90, 403]
• Five-membered metallacycle
• Example:
C HPd2+
H+
cyclopalladated complex
D
CPd
D
D = donor group (e.g. O, N)
N MeOH NPd OAc
2
+ Pd(OAc)2r.t.
Results from W.-Y. Yu (PolyU)
Suzuki Coupling via C-H Activation
N
O CH3
HN
O CH3
Ar
Ar B(OH)2Pd(OAc)2 (5 mol%), Cu(OTf2) (1 equiv)
Ag2O (1 equiv), toulene, 120oC, 24h(2 equiv)
Shi, Z. and co-workers, Angew. Chem. Intl. Ed. 2007, 46, 5554
Yu, J.-Q. and co-workers, J. Am. Chem. Soc. 2007, 129, 3570
• Using Pd(II) salt as catalyst• Oxidizing agents: Ag(I), Cu(II), benzoquinone
CO2Na
HAr B
O
O
Pd(OAc)2 (10 mol%), benzoquinone (0.5 equiv)
Ag2CO3 (1 equiv), tBuOH, 120oC, 3h
Me
CO2Na
Ar
Me
(1 equiv)
Direct C-H Arylation Methods
Pd(OAc)2 (5 mol%)
AgOAc (1 equiv),
HN
O
Et
I tBu
Br CF3CO2H, 110-120oC
NH
O
Et
Br
tBu
Daugulis, O. and co-workers, J. Org. Chem. 2007, 72, 7720
NH
I+ Ar
BF4-
Pd(OAc)2 (10 mol%),
AcOH,100oC, 12h NAr
Sanford, M. and co-workers, J. Am. Chem. Soc. 2005, 127, 7330
NH
ArO
O O
OAr
Pd(OAc)2 (10 mol%),
CH3CN, AcOH,100oC, 2h(2 equiv)
NAr
Yu, W.-Y. and co-workers @ PolyU (Org. Lett. 2009, 11, 3174 )
Activation of inert chemical bond by metal
• Coordination of stable H-H, C-X and C-H to transition metal
• Weakening of the bonding in reagent molecules
• Generation of reactive “M-reagent” species
MM
Stable Reactive
Activation stepsubstrate
product
Catalysis and Sustainable Development
By catalysis, we can.. Design processes for developing renewable energy technologies Synthesize value compounds from raw materials
o Cost effective (fewer steps)o Environmental friendly (atom economy, no toxic wastes)
Role of metal ion in catalysis Non-redox metal ion (e.g. Mg2+) provide structure support for the
active site Redox active metal ions mediate multiple electron transfer reactions
for oxidation and reduction of substrates Variable oxidation states and coordination mediate bond breaking
and bond formation