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Transcript
AJELIAS L7-S1 Unique reactions in organometallic chemistry • Oxidative Addition • Reductive Elimination • Migratory Insertion • β - Hydrogen Elimination AJELIAS L7-S2 Oxidative addition When addition of ligands is accompanied by oxidation of the metal, it is called an oxidative addition reaction LnM + XY Ln(X)(Y)M dn dn-2 OX state of metal increases by 2 units Ph3P Coordination number increases by 2 units Rh Ph3P 2 new anionic ligands are added to the metal PPh3 H2 oxidative addition Cl Rh+1 H H PPh3 Rh Ph3P Rh+3 Cl PPh3 Requirements for oxidative addition • availability of nonbonded electron density on the metal, • two vacant coordination sites on the reacting complex (LnM), that is, the complex must be coordinatively unsaturated, • a metal with stable oxidation states separated by two units; the higher oxidation state must be energetically accessible and stable. AJELIAS L7-S3 Examples of Oxidative addition : Cis or trans ? Cl Cl PPh3 Ir Ph3P Cl2 18E CO Cl O Cl PPh3 O2 O Ir Ph3P PPh3 Ir CO Ph3P CO Cl 16E Me MeI Cl Me PPh3 Ir Ph3P I PPh3 Ir CO I Homonuclear systems (H2, Cl2, O2, C2H2) Cis Heteronuclear systems (MeI) Cis or trans Ph3P CO Cl AJELIAS L7-S4 An important step in many homogeneous catalytic cycles Hydrogenation of alkenes- Wilkinson catalyst Ph3P PPh3 Rh H PPh3 Rh Cl Ph3P Rh+1 Cl Rh+3 PPh3 Methanol to acetic acid conversion- Cativa process CH3 I CO I CH3I Ir CO Ir CO I I CO I Ir+1 Ir+3 Pd catalyzed Cross coupling of Ar-B(OH)2 and Ar-X – Suzuki Coupling Br Ph3P Pd Br PPh3 Pd Ph3P Pd0 PPh3 Pd+2 The more electron rich the metal, more easy is the oxidative addition Often the first step of the mechanism Ph3P H H2 oxidative addition AJELIAS L7-S5 Oxidative addition involving C-H bonds and cyclo/ortho metallation Agostic interaction This type of reactions help to activate unreactive hydrocarbons such as methane – known as C-H activation AJELIAS L7-S6 Reductive elimination Almost the exact reverse of Oxidative Addition Ph2 P CH3 CH3 Pt P Ph2 CH3 CH3 reductive elimination 165 °C, days Ph2 P CH3 Pt P Ph2 Pt4+ + H3C CH3 CH3 Pt2+ Oxidation state of metal decreases by 2 units Coordination number decreases by 2 units 2 cis oriented anionic ligands form a stable σ bond and leave the metal Factors which facilitate reductive elimination • a high formal positive charge on the metal, • the presence of bulky groups on the metal, and • an electronically stable organic product. Cis orientation of the groups taking part in reductive elimination is a MUST AJELIAS L7-S7 Final step in many catalytic cycles Hydroformylation ( conversion of an alkene to an aldehyde) Sonogashira Coupling (coupling of a terminal alkyne to an aryl group Cativa Process (Methanol to Acetic acid) AJELIAS L7-S8 Migratory Insertion X L M [M-Y-X] Y +L M dn Y X dn No change in the formal oxidation state of the metal A vacant coordination site is generated during a migratory insertion (which gets occupied by the incoming ligand) The groups undergoing migratory insertion must be cis to one another CH3 OC CO + PPh3 Mn OC OC CO Ph3P O OC C CH3 Mn CO OC OC These reactions are enthalpy driven and although the reaction is entropy prohibited the large enthalpy term dominates AJELIAS L7-S9 Types of Migratory Insertion X M X A M B 1, 1 - migratory insertion A B X X A B M M B 1, 2 - migratory insertion A O CO Ph3P Rh OC Rh R 1, 2-migratory insertion OC PPh3 Ph3P CH2CH2R Rh OC Ph3P CO CCH2CH2R Rh PPh3 H Ph3P CH2CH2R 1, 1-migratory Ph P 3 insertion PPh3 AJELIAS L7-S11 Stability of σ Bonded alkyl groups as ligands Joseph Chatt 1962 - 68 Br Pt Et3P PEt3 EtMgBr Br H3CH2C Pt PEt3 Et3P poor yields unstable n - BuLi cis PtCl2(PPh3)2 Ph3P Et3P Br Ph3P CH2 CH2 Pt + 60 °C, sealed tube Pt + Pressure + Ph3P Ph3P Pt PEt3 Ph3P Pt Ph3P Br H Heat CH3 CH3 Ph3P Ph3P No decomposition 60 °C sealed tube Why does some σ bonded alkyl complexes decompose readily? Pt AJELIAS L7-S12 β-Hydride elimination Beta-hydride elimination is a reaction in which an alkyl group having a β hydrogen, σ bonded to a metal centre is converted into the corresponding metal-bonded hydride and a π bonded alkene. The alkyl must have hydrogens on the beta carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex must have an empty (or vacant) site cis to the alkyl group for this reaction to occur. No change in the formal oxidation state of the metal mechanism H H H C M C H H H H H H C H H C M C M H C H H H Can either be a vital step in a reaction or an unwanted side reaction AJELIAS L7-S13 β-hydrogen elimination does not happen when • the alkyl has no β-hydrogen (as in PhCH2, Me3CCH2, Me3SiCH2) • (ii) the β-hydrogen on the alkyl is unable to approach the metal (as in C≡CH) • the M–C–C–H unit cannot become coplanar Select the most unstable platinum σ complex from the given list. Justify your answer Ph3P SiMe3 SiMe3 Pt Ph3P A No β-H Et3P C C H Ph3P Pt Et3P C B β-H unable to approach M C Pt Ph3P H Ph3P Pt Ph3P C MCCH unit will not be coplanar D Problem solving Classify the following reactions as oxidative addition, reductive elimination, (1,1 / 1,2)migratory insertion, β- H elimination, ligand coordination change or simple addition (a) [RhI3(CO)2CH3]− → {RhI3(CO)( solvent)[C(O)CH3]}− (b) Ir(PPh2Me)2(CO)Cl + CF3I → Ir(I)(CF3)(PPh2Me)2(CO)Cl (c) TiCl4 +2 Et3N → TiCl4 (NEt3)2 (d) HCo(CO)3(CH2=CHCH3) + CO → CH3CH2CH2Co(CO)4 Step 1. determine the oxidation state of the metal in reactant and product Step 2. count the electrons for reactant and product Step 3. see if any ligand in the reactant has undergone change AJELIAS L7-S14 Homogeneous catalysis using organometallic Catalysts A catalyst typically increases the reaction rates by lowering the activation energy by opening up pathways with lower Gibbs free energies of activation (G). Gibbs energy of activation Gibbs Energy uncatalyzed catalyzed stable intermediate Reactants Heterogeneous Products Homogeneous Homogeneous versus Heterogeneous Catalysis Parameter Heterogeneous Homogeneous Phase Gas/solid Usually liquid/ or solid soluble in the reactants Required temperature High Low ( less than 250°C) Catalyst Activity Low High Product selectivity Less (often mixtures) More Catalyst recycling Simple and cost effective Expensive and complex Reaction mechanism Poorly understood Reasonably well understood Product separation from catalyst Easy Elaborate and sometimes problematic Fine tuning of catalyst Difficult Easy AJELIAS L7-S15 Heterogeneous Catalyst- Catalytic Converter of a Car Platinum and Palladium Platinum and Rhodium Chemistry at the molecular level – Poorly understood Home assignment : See Youtube video ‘Catalysis’ AJELIAS L7-S16 Comparing different catalysts; Catalyst life and Catalyst efficiency Turnover Number (TON) TON is defined as the amount of reactant (in moles) divided by the amount of catalyst (in moles) times the percentage yield of product. A large TON indicates a stable catalyst with a long life. Turnover Frequency (TOF) It is the number of passes through the catalytic cycle per unit time (often per hour). Effectively this is dividing the TON by the time taken for the reaction. The units are just time–1 . A higher TOF indicates better efficiency for the catalyst AJELIAS L7-S17 Wilkinson’s Catalyst for alkene hydrogenation RhCl3 (H2O)3 + CH3CH2OH + 3 PPh3 RhCl(PPh3)3 + CH3CHO + 2HCl + 3H2O Wilkinson’s catalyst: The first example of an effective and rapid homogeneous catalyst for hydrogenation of alkenes, active at room temperature and atmospheric pressure. Square planar 16 electron d8 complex (Ph3P)3RhCl Discovered by G Wilkinson as well as by R Coffey almost at the same time (1964–65) Conventional Catalytic cycle for hydrogenation with Wilkinson’s catalyst P P Rh P Cl P Cl reductive elimination P Rh 14e The first step of this catalytic cycle is the cleavage of a PPh3 to generate the active form of the catalyst followed by oxidative addition of dihydrogen. P H2 oxidative addition P RCH2CH3 R CH2 H2C H H Rh P H Rh P P P Cl Cl R 1, 2 -migratory insertion alkene coordination P H H Rh P Cl R P = PPh3