Organometallic Reactions and Catalysis_2
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Transcript of Organometallic Reactions and Catalysis_2
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Organometallic Reactions and CatalysisSARI EDI C
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Gain or Loss of LigandsCO dissociation In many cases to add another ligand.Dissociative and associative mechanismsMore complicated reactions.Dissociation of phosphine (steric effects)cis-Mo(CO)4L2 + CO Mo(CO)5L + LFigure 14-1 and Table 14-1 (Article)Rate dependence on cone angle and other factors.Reaction follows the first-order rate law.
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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 stateReactions may occur between ligands due to close proximity.
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Reductive Elimination (RE, reverse of OA)Decrease in coordination number and oxidation state of the metal.(-C5H5)2TaH + H2 (5-C5H5)TaH3RE reaction rates are also affected by ligand bulk. How? (Table 14-2)
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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 +ROH R(C=O)OR + HCo(CO)4(generates the ester from an alcohol).
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Modification of LigandsInsertion 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).illustrate1,2-insertion (bonds to the inserted molecule are made to adjacent atoms in that molecule).illustrate
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Insertion of LigandsHow is CO inserted in the complex shown previously (1,1-insertion)?Work through this and understand.1,2-insertions
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Hydride EliminationTransfer of a hydrogen atom from a ligand to a metal. elimination is the most common type. position on the alkyl ligand.StabilityAlkyl complexes that lack hydrogens are more stable.Coordinatively saturated complexes containing alkyl ligands are also more stable.
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AbstractionRemoval of a substituent from a ligand in which the coordination number of the metal does not change (can be removed by an acid).
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Organometallic Catalysts (hydroformylation)Converting terminal alkenes into other organic products.(oxo process) H and HCO are formally added across a double bond.Show reactionLargest-scale industrial process that is homogeneousMechanism 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.
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Comments on the Hydroformylation MechanismCO 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=CH2CH3CH2CH2CHO
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Union Carbide Hydroformylation ProcessContain Rh and bulky phosphine groups. How will this affect the mechanism?(Ph3P)3Rh(CO)HIn many cases, the linear/branched ration needs to high.The catalyst is also water-soluble.
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Hydrogenation of AlkenesWilkinsons catalystShow reaction (alkenes and alkynes)Show mechanism and discussStep 9 is slow, the sequence 123 is favored.The rate determining step is insertion, 4.The catalyst hydrogenizes terminal and internal olefins.Examine Table 14-3.
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Hydrogenation CatalystSelective 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 hydrogenationIf the catalyst, [L2RHS2]+, bears an optically active diphosphane, prochiral unsaturated molecules can be hydrogenated to chiral products (enantiomeric selectivity).L-Dopa (treatment of Parkinsons disease).
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Alkene MetathesisDemonstrate 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 complexThe 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)
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Heterogeneous CatalysisUsed much more extensively in industry than homogeneous catalysts. Robust at high temperatures.Easy to separate out the catalyst.
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Composition of Heterogeneous CatalystsUniform 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
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Surface LigandsIn 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.
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Surface-Sensitive TechniqusTemperature-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.
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Catalytic StepsMany parallels can be drawn in comparison to organometallic mechanisms studied previously.Chemisorption and physisorptionSimilar to interactions present in complexes with low oxidation states.Physisorption and chemisorption.
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Catalytic StepsSimilar 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)
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Diversity of SitesReal 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.
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Examples of Heterogeneous CatalystsHydrogenation of alkenes on metal surfaces.H2 is dissociatively chemisorbedEthylene is associatedHydrogen adds to produce an alkyl speciesAnother hydrogen atom coordinates and ethane leaves.Actual species produced
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Ziegler-Natta PolymerizationTiCl4 + Al(C2H5)3A 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
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Fundamental Studies of Hydrocarbons on Platinum SurfacesThe techniques used.Reflection-absorption infrared spectroscopy.Auger electron spectroscopyTemperature-programmed desorption/reaction spectroscopy.Others as needed.
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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.
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A Typical RAIRS Spectrum
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The Labeling Study
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Cyclic C8 Systems on Pt(111)
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1,4-Cyclohexadiene on Pt(111)