Organometallic Reactions and Catalysis Chapter 14 Gain or

Organometallic Reactions and Catalysis Chapter 14

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)4 L 2 + CO Mo(CO)5 L + L – Figure 14 -1 and Table 14 -1 (Article) – Rate dependence on cone angle and other factors. • Reaction follows the first-order rate law.

Oxidative Addition (OA) • Increases the coordination number and the oxidation state of the metal. • OA reactions of square-planar d 8 complexes. – trans-Ir(CO)Cl(PEt 3)2 (Figure 14 -3) • Changes in CN and oxidation state • Reactions may occur between ligands due to close proximity.

Reductive Elimination (RE, reverse of OA) • Decrease in coordination number and oxidation state of the metal. ( -C 5 H 5)2 Ta. H + H 2 ( 5 -C 5 H 5)Ta. H 3 • RE reaction rates are also affected by ligand bulk. How? (Table 14 -2)

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 negativelycharged complexes). [Co(CO)4]- + RX RCo(CO)4 + XRCo(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).

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

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

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.

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).

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.

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. CH 3 CH=CH 2 CH 3 CH 2 CHO

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

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.

Hydrogenation Catalyst • Selective hydrogenation can be observed if the ligand contains multiple double bonds. • Another hydrogenation catalyst, (PPh 3)2 Rh(CO)H, is very selective toward hydrogenation of only terminal olefins. • Asymmetric hydrogenation – If the catalyst, [L 2 RHS 2]+, bears an optically active diphosphane, prochiral unsaturated molecules can be hydrogenated to chiral products (enantiomeric selectivity). • L-Dopa (treatment of Parkinson’s disease).

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)

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

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

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.

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.

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.

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 + H 2 O • (on a metal surface)

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.

Examples of Heterogeneous Catalysts • Hydrogenation of alkenes on metal surfaces. – H 2 is dissociatively chemisorbed – Ethylene is associated – Hydrogen adds to produce an alkyl species – Another hydrogen atom coordinates and ethane leaves. – Actual species produced

Ziegler-Natta Polymerization • Ti. Cl 4 + Al(C 2 H 5)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

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.

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.

A Typical RAIRS Spectrum

The Labeling Study

Cyclic C 8 Systems on Pt(111)

1, 4 -Cyclohexadiene on Pt(111)