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Chemistry of Metals Catalysis for Sustainable Development Michael W.-Y. Yu Department of Applied Biology and Chemical Technology The Hong Kong Polytechnic.

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Presentation on theme: "Chemistry of Metals Catalysis for Sustainable Development Michael W.-Y. Yu Department of Applied Biology and Chemical Technology The Hong Kong Polytechnic."— Presentation transcript:

1 Chemistry of Metals Catalysis for Sustainable Development Michael W.-Y. Yu Department of Applied Biology and Chemical Technology The Hong Kong Polytechnic University

2 Chemistry is a unique science….

3 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

4 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 …..

5 Green Chemistry for achieving Sustainable Development

6 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 energy Employ non-toxic reagents and less wastes

7 Development of Metal Catalysis for sustainable development….. Hydrogen as fuel for tomorrow o Fuel cell technology o Solar energy production of hydrogen Chemical Synthesis via Activation of Inert Chemical bonds o Activation of H-H bond o Activation of C-X (X = halide) o Activation of C-H bond

8 Hydrogen as fuel for tomorrow

9 Renewable Energy for the Future Highly exothermic reaction Inexhaustible Non-polluting Zero carbon emission Dihydrogen (H 2 ) 2H 2 + O 2  2H 2 O

10 10 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: O 2(g) + 4e - + 4H +  2H 2 O anode: 2H 2(g)  4H + + 4e - Each of the anode and cathode reactions are half-cell reactions. The overall cell reaction is:  2H 2 + O 2  2H 2 O

11 11 Catalyst AnodeCathode O2O2 H2H2 electrolyte e-e- H2OH2O 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 H 2 oxidation O 2 reduction – 4 e- + 4 H protons reduction (complex systems) – O 2  H 2 O 2 or H 2 O

12 12 Electron flow H+H+ H+H+ H+H+ O2O2 Water H 2 O H2H2 Proton exchange membrane Gas permeable electrode with platinum catalyst 2H 2  4H + + 4e - O 2 + 4H + + 4e -  2H 2 O 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

13 13 The H 2 /O 2 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 catalystMo 2 C-ZrO 2 /C, transition metal marcocyclic compounds 2.Storage of hydrogen onboard

14 H 2 gas oxidation 1.Mass transport of dissolved H 2 to the surface: H 2(aq)  H 2(ads) 2.Chemisorption of hydrogen as atoms ( breaking H-H bond ) H 2(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) 14 H 2 (g) → 2H(g)  H = +436 kJ mol -1

15 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 O 2 to H 2 O (acid medium) E o = 1.23V – An indirect reduction of O 2 to H 2 O 2 (acid medium) E o = 0.68V 15

16 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. 16

17 Binding of dioxygen to metal End-on Side-on Bridging 17 Dioxygen is bonded to metal atom with π bond of O 2 and metal surface. Two bonds are formed with two metal centres in each end of the O 2 molecule. Dioxygen molecule is bonded between metal atom with bent structure. Binding of O 2 to metal leads to weakening of the O-O bond

18 M-O 2 complexes are reactive … 18 Overall result: Breaking of O=O bond to form 2H 2 O molecules

19 Where does the H 2 come from…? Steam Methane Reforming  High-temperature (800 – 900 o C) 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 2H 2 O(l)  2H 2 (g) + O 2 (g)  H = 285.9 kJ mol -1 Turning water to H 2 is a highly endothermic process Dependent on Fossil Fuel CO 2 emission!!!

20 Water electrolysis Zero carbon emission?? Great demand of high quality water Expensive

21 Learning from Nature …. Higher green plants use solar energy to convert H 2 O into O 2 and reducing equivalents in NADPH for reduction of CO 2 to carbohydrates… This process is known as Photosynthesis 2H 2 O + 4h  O 2 + 4H + + 4e - nCO 2 + 2ne - + 2nH +  (CH 2 O) n

22 22 Chlorophyll – pigments for Photosynthesis Macrocyclic structure Conjugated C=C bond Mg 2+ cation (structure stabilization)

23 23 h Ground state  bonding  antibonding chlorophyll Excited state  bonding  antibonding chlorophyll A electron transfer “hole” – oxidizing!! radical anion – reducing!! chlorophyll cation  bonding  antibonding A-A- charge separation Chlorophyll captures light energy to form reactive chemical species….

24 Light Reactions 2H 2 O  O 2 + 4H + + 4e - Active site (Oxygen Evolving Center) contains a Mn 4 cluster Four photons are required to effect 4e oxidation of 2H 2 O molecules

25 Oxygen Evolving Center (OEC) of PSII Ferreira, 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

26 Artificial Photosynthesis Design a man-made catalytic system that mimics Nature for photo-driven water oxidation… h

27 Dye-sensitized photovoltaic cells Photoexcitation of dye is followed by electron injection into the conduction band of the TiO 2 film The dye is regenerated by a redox system (e.g. I - / I 3 - couple)

28 Ruthenium complexes as dye for photovoltaic cells Stable complexes, over 100 million turnovers (servicable for 20 years) Carboxylic acid groups for metal anchoring to TiO 2 (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

29 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  (Ru)  ligand [Ru(bpy) 3 ] 2+ Excited state d  (Ru)  ligand [Ru(bpy) 3 ] 2+ * formally Ru 3+ center formally one-electron reduced ligand h

30 Photoelectrochemical dehydrogentaion of alcohol and generation of hydrogen Electrochemical isopropanol oxidation by Ru-oxo Platinum electrode for 2H + /H 2 couple Electrochemical water oxidation remains a challenge

31 ACTIVATION OF DIHYDROGEN Metal-dihydrogen interaction

32 Alkene Hydrogenation Exothermic reaction (  H ~ 120 kJ mol -1 ) Catalyst required: Pt, PtO 2, Pd Syn addition to C=C bond BDE (H 2 ) = 436 kJ mol -1 (critical reaction barrier) C.f. BDEs (kJ mol -1 ) for: Cl-Cl (242); C-H (414) catalyst weak  -bondstrong  -bond 2 X strong  -bonds Waste-free reaction!

33 Metal-hydride formation from “M + H 2 ”? Coordination of H 2 to M Breaking of H-H bond M-H covalent, polarized, reactive!! c.f. 2Na + H 2  2NaH H-H cleavage

34 Rh-catalyzed homogeneous alkene hydrogenation Reversible changes of oxidation states: Rh I  Rh III

35 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

36 Asymmetric Hydrogenation Chiral ligands 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

37 37 Asymmetric Hydrogenation: application Practical application of asymmetric hydrogenation in Eli Lilly Company (Making drugs). L* = Peroxime proliferator activated receptor (PPAR) agonist, for treatment of diabetes. Houpis, Org. Lett. 2005, 7,

38 38 Ketone Hydrogenation ROR' OO ROR' OHO (R)-BINAP Ru(II) H 2 (70-103 atm) 93-100 % yield 98-100 % ee  -ketoesters: Ru Ru P P O O Cl H H 3 C OCH 3 (R) P P Cl H O O OCH 3 (S) higher energy Murahashi,Chem. Rev. 1998,98, 2599 H 3 C lower energy P P

39 39 Ketone Hydrogenation: Examples

40 40 Ketone Hydrogenation: Selected practical examples 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.

41 ACTIVATION OF ARYL HALIDES Formation of Reactive Organopalladium Complexes

42 Biaryls are important targets for organic synthesis Biaryls

43 Cross Coupling Reactions – little by-products Aryl halides are poor electrophiles for S N 1 / S N 2 Due to sp 2 hybridized C-X bond: – Lower polarity – Stronger C-X bond Catalyst is required for Biaryl Coupling Reactions

44 Oxidative Addition turned Aryl Halides to reactive Arylpalladium  Favored by strong  -donors (alkyl vs aryl phosphines)  Rate : tBu 3 P > Ph 3 P  Reactivity trend: C-I > C-Br >>> C-Cl >>> C-F  c.f. Mg + ArX  ArMgX (Grignard reagent) Coordinatively unsaturated Electron rich Stable square planar complex

45 45 Suzuki coupling reaction Developed in early 80s, by Akira Suzuki. Suzuki, Chem Rev. 1995, 95, 2457. Suzuki, J. Organomet. Chem. 1999, 576, 147

46 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 oxidative addition transmetallation reductive elimination

47 Examples Muller, D.; Fleury, J-P. Tetrahedron Lett. 1991, 32, 2229 Fu, J-M.; Sharp, M. J.;Snieckus, V. Tetrahedron Lett. 1988, 29, 5459 Haber, S; Egger, N. US Patent 2000, 6-140-265

48 Ligands for Suzuki Coupling Reactions Strong  -donors promote oxidative addition Steric bulkiness of ligand increase activity of Pd(0) – more open for substrate interaction

49 ACTIVATION OF C-H BOND The Next Challenge

50 Using pre-functionalized substrates (e.g. aryl halides) Expensive Derived from simple aromatics by halogenation Coupling reactions with C-H bond??? Strong bond energy Non-polar

51 Formation of Arylpalladium from arenes Electrophilic attack of Pd 2+ on arenes leads to C-H bond cleavage pK a : 44 (butane); 37 (C 6 H 6 ); Limitation: homocoupling; isomeric products electrophilic attack by Pd 2+ deprotonation reductive elimination

52 Cyclopalladated Complexes Cyclopalladation of arenes [Ryabov, A. D. Chem. Rev. 1990, 90, 403] Five-membered metallacycle Example: Results from W.-Y. Yu (PolyU)

53 Suzuki Coupling via C-H Activation 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

54 Direct C-H Arylation Methods Daugulis, O. and co-workers, J. Org. Chem. 2007, 72, 7720 Sanford, M. and co-workers, J. Am. Chem. Soc. 2005, 127, 7330 Yu, W.-Y. and co-workers @ PolyU (Org. Lett. 2009, 11, 3174 )

55 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 M M StableReactive Activation step substrate product

56 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. Mg 2+ ) 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


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