Chemistry of Metals

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. Chemistryis a unique science….

3. Chemical Synthesis improves quality of life Drugs for better healthcare Man-made fibers and materials for high performance apparel Materialsfor device fabrication Optical fiber for high speed communication

4. We are facing challenges that our quality of life is becoming Unsustainable ….. 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

5. Green Chemistry for achieving Sustainable Development

6. Innovation in catalysis… a route togreen chemical synthesis Catalysis speeds up a reaction, and can also make new reactions possible that allow different starting materials to be used. Catalysis – less energy Employ non-toxic reagents and less wastes 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.

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

8. Hydrogen as fuel for tomorrow

9. Renewable Energy for the Future Dihydrogen (H2) • Highly exothermic reaction • Inexhaustible • Non-polluting • Zero carbon emission 2H2 + O2 2H2O

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: 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

11. e- electrolyte Anode Cathode H2 O2 H2O Catalyst 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 H2 oxidation • O2 reduction • 4 e- + 4 H protons reduction (complex systems) • O2 H2O2 or H2O Schematic diagram of fuel cell • 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

12. Electron flow Gas permeable electrode with platinum catalyst Gas permeable electrode with platinum catalyst H2 H+ O2 H+ H+ Water H2O O2 + 4H+ + 4e- 2H2O 2H2  4H+ + 4e- Proton exchange membrane • 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 The PEM fuel cell

13. 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 electrocatalystsfor 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 • Storage of hydrogen onboard

14. H2 gas oxidation H2(g) → 2H(g) DH = +436 kJ mol-1 Mass transport of dissolved H2 to the surface: H2(aq) H2(ads) Chemisorption of hydrogen as atoms (breaking H-H bond) H2(aq) 2H(ads) Ionization of hydrogen atoms H(ads) H+(ads) + e- Transport of the H+ ions away from the electrode surface H+(ads) H+ (aq)

15. Oxygen reduction • Large kinetic barrier for the oxygen reduction strong O=O bond: BDE = 463 kJ mol-1 • 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

16. Catalystis 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.

17. Binding of dioxygen to metal Dioxygen molecule is bonded between metal atom with bent structure. 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. Binding of O2 to metal leads to weakening of the O-O bond End-on Side-on Bridging

18. M-O2 complexes are reactive… Overall result: Breaking of O=O bond to form 2H2O molecules

19. Where does the H2 come from…? • 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 • 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 • Dependent on Fossil Fuel • CO2 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 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

22. Chlorophyll – pigments for Photosynthesis • Macrocyclic structure • Conjugated C=C bond • Mg2+cation (structure stabilization)

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

24. 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

25. Oxygen Evolving Center (OEC) of PSII Ferreira, K. N., Iverson,T. M., Maghlaoui, K., Barber, J., Iwata, S. Science2004, 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 mimicsNature for photo-driven water oxidation… hn

27. 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)

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 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

29. Working principle p* ligand p* ligand formally one-electron reduced ligand hn • 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 dp(Ru) dp(Ru) formally Ru3+ center [Ru(bpy)3]2+ [Ru(bpy)3]2+* Excited state Ground state

30. 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

31. Metal-dihydrogen interaction Activation of Dihydrogen

32. Alkene Hydrogenation catalyst • 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) weak p-bond strong s-bond 2 X strong s-bonds Waste-free reaction!

33. H-H cleavage • 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

34. Rh-catalyzed homogeneous alkene hydrogenation Reversible changes of oxidation states: RhI RhIII

35. Chiral Technology for Drug Synthesis • 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%)

36. Asymmetric Hydrogenation DIPAMP (chiral at phosphorus) By Knowles in 60s (Nobel 2001) • Chiral ligands BINAP (Axially chiral backbone) By Noyori in 80s (Nobel 2001) DIOP (chiral at backbone) By Kagan in 70s DuPhos (chiral at backbone) By Burk in 90s

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. b -ketoesters: O H ( R )-BINAP Ru(II) 93-100 % yield 98-100 % ee O R ' O R ' H (70-103 atm) 2 C l O C H 3 R u H C 3 O C H 3 R u C l H C 3 1998 Murahashi, Chem. Rev. , 98 , 2599 Ketone Hydrogenation O O O P R R P P H O P O P O (R) (S) P O H higher energy lower energy

39. Ketone Hydrogenation: Examples

40. Ketone Hydrogenation: Selected practical examples Kawaguchi, T.; Saito, K.; Matsuki, K.; Iwakuma, T.; Takeda, M. Chem. Pharm. Bull. 1993, 41, 639. Thomassigny, C.; Greck, C. Tetrahedron: Asymmetry 2004, 15, 199.

41. Formation of Reactive Organopalladium Complexes Activation of Aryl Halides

42. Biaryls are important targets for organic synthesis Biaryls

43. 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

44. Oxidative Addition turned Aryl Halides to reactive Arylpalladium • Favored by strong s-donors (alkyl vs aryl phosphines) • Rate : tBu3P > Ph3P • 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. Developed in early 80s, by Akira Suzuki. Suzuki, Chem Rev. 1995, 95, 2457. Suzuki, J. Organomet. Chem. 1999, 576, 147 Suzuki coupling reaction

46. Catalytic cycle oxidative addition reductive elimination • 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 transmetallation

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 s-donors promote oxidative addition • Steric bulkiness of ligand increase activity of Pd(0) – more open for substrate interaction

49. The Next Challenge Activation of C-H Bond

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