Organometallic Reactions and Catalysis

1. Organometallic Reactions and Catalysis Chapter 14

2. Gain or Loss of Ligands • CO dissociation • In many cases to add another ligand. • Dissociative and associative mechanisms • More complicated reactions. • Dissociation of phosphine (steric effects) • cis-Mo(CO)4L2 + CO  Mo(CO)5L + L • Figure 14-1 and Table 14-1 (Article) • Rate dependence on cone angle and other factors. • Reaction follows the first-order rate law.

3. Oxidative Addition (OA) • Increases the coordination number and the oxidation state of the metal. • OA reactions of square-planar d8 complexes. • trans-Ir(CO)Cl(PEt3)2 (Figure 14-3) • Changes in CN and oxidation state • Reactions may occur between ligands due to close proximity.

4. Reductive Elimination (RE, reverse of OA) • Decrease in coordination number and oxidation state of the metal. (-C5H5)2TaH + H2 (5-C5H5)TaH3 • RE reaction rates are also affected by ligand bulk. How? (Table 14-2)

5. Nucleophlic Displacement (attack by a Lewis base) • A strong nucleophile would be a ligand with strong electron-donating character. • Organometallic complexes can behave as nucleophiles in displacement reactions (especially negatively-charged complexes). [Co(CO)4]- + RX  RCo(CO)4 + X- RCo(CO)4 + CO  R(C=O)Co(CO)4 (acyl complex) R(C=O)Co(CO)4 +R’OH  R(C=O)OR’ + HCo(CO)4 (generates the ester from an alcohol).

6. Modification of Ligands • Insertion – a molecular fragment appears to insert itself into a metal-ligand bond. • Many reaction mechanism can be complicated. • 1,1-insertion (both bonds are made to the same atom). • illustrate • 1,2-insertion (bonds to the inserted molecule are made to adjacent atoms in that molecule). • illustrate

7. Insertion of Ligands • How is CO inserted in the complex shown previously (1,1-insertion)? • Work through this and understand. • 1,2-insertions

8. Hydride Elimination • Transfer of a hydrogen atom from a ligand to a metal. •  elimination is the most common type. •  position on the alkyl ligand. • Stability • Alkyl complexes that lack  hydrogens are more stable. • Coordinatively saturated complexes containing alkyl ligands are also more stable.

9. Abstraction • Removal of a substituent from a ligand in which the coordination number of the metal does not change (can be removed by an acid).

10. Organometallic Catalysts (hydroformylation) • Converting terminal alkenes into other organic products. • (oxo process) H and HCO are formally added across a double bond. • Show reaction • Largest-scale industrial process that is homogeneous • Mechanism was suggested by Heck and Breslow in 1961. • Examine each step in the cycle and characterize the reaction according to type. • 18-,16-electron cycling is common.

12. Comments on the Hydroformylation Mechanism • CO pressure has to be controlled carefully. Why? • Rate-determining step is the insertion of the olefin (alkene). • Main purpose of the reaction is to produce butanal from propene. CH3CH=CH2CH3CH2CH2CHO

13. Union Carbide Hydroformylation Process • Contain Rh and bulky phosphine groups. How will this affect the mechanism? • (Ph3P)3Rh(CO)H • In many cases, the linear/branched ration needs to high. • The catalyst is also water-soluble.

15. Hydrogenation of Alkenes • Wilkinson’s catalyst • Show reaction (alkenes and alkynes) • Show mechanism and discuss • Step 9 is slow, the sequence 123 is favored. • The rate determining step is insertion, 4. • The catalyst hydrogenizes terminal and internal olefins. • Examine Table 14-3.

17. Hydrogenation Catalyst • Selective hydrogenation can be observed if the ligand contains multiple double bonds. • Another hydrogenation catalyst, (PPh3)2Rh(CO)H, is very selective toward hydrogenation of only terminal olefins. • Asymmetric hydrogenation • If the catalyst, [L2RHS2]+, bears an optically active diphosphane, prochiral unsaturated molecules can be hydrogenated to chiral products (enantiomeric selectivity). • L-Dopa (treatment of Parkinson’s disease).

18. Alkene Metathesis • Demonstrate • Propene and 1-butene (what are the 4 new products that may form from methathesis?) • Ring-opening metathesis (ROM) • Chauvin mechanism is most widely accepted. • Involved a carbene complex • The carbene reacts with an alkene to form a metallocyclobutane intermediate. The intermediate can either revert to reactants or form new products. • Schrock metathesis catalysts are most effective and the most studied (available commercially). • Ring-closing methathesis (page 545)

20. Heterogeneous Catalysis • Used much more extensively in industry than homogeneous catalysts. • Robust at high temperatures. • Easy to separate out the catalyst.

21. Composition of Heterogeneous Catalysts • Uniform – bulk of the high-surface area serves as the catalyst. • ZSM-5 (zeolite) • Multiphase – high-surface-area material serves as a support for the active catalyst. • Pt/Re on alumina

22. Surface Ligands • In many cases, the nature of the surface ligand is inferred by comparison of IR spectra with those of organometallic or inorganic complex. • Terminal and bridging CO.

23. Surface-Sensitive Techniqus • Temperature-programmed desorption (mass spectroscopy). • Photoelectron spectroscopy (XPS and Auger) • Low-Energy Electron Diffraction (LEED) • Scanning Tunneling and Atomic Force Microscopies. • Vibrational Techniques (RAIRS and HREELS). • Many othes.

24. Catalytic Steps • Many parallels can be drawn in comparison to organometallic mechanisms studied previously. • Chemisorption and physisorption • Similar to interactions present in complexes with low oxidation states. • Physisorption and chemisorption.

25. Catalytic Steps • Similar to homogeneous catalysis, there is also a balance between strong enough adsorption for the reaction to occur and weak enough desorption that the species can be removed for further reactions. • HCOOH  CO + H2O • (on a metal surface)

26. Diversity of Sites • Real surfaces possess a large diversity of surface types. Each surface type may have a different reactivity and/or produce different products. • Lower selectivity. • Most reactive sites.

27. Examples of Heterogeneous Catalysts • Hydrogenation of alkenes on metal surfaces. • H2 is dissociatively chemisorbed • Ethylene is associated • Hydrogen adds to produce an alkyl species • Another hydrogen atom coordinates and ethane leaves. • Actual species produced

28. Ziegler-Natta Polymerization • TiCl4 + Al(C2H5)3 • A titanium alkyl complex is produce. • Ethylene or propylene associates and inserts into the titanium-carbon bond. • The 1,2-insertion continues. • Mechanism has proved difficult to understand. In Miessler and Tarr

29. Fundamental Studies of Hydrocarbons on Platinum Surfaces • The techniques used. • Reflection-absorption infrared spectroscopy. • Auger electron spectroscopy • Temperature-programmed desorption/reaction spectroscopy. • Others as needed.

30. Reflection-Absorption Infrared Spectroscopy (RAIRS) • The dynamic dipole moment must have a component normal to the surface to be visible. • The intensity of the vibration signature reveals orientation information. • Position of the signature indicates identity of species on the surface.

31. A Typical RAIRS Spectrum

32. The Labeling Study

33. Cyclic C8 Systems on Pt(111)

34. 1,4-Cyclohexadiene on Pt(111)