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 H H2 oxidative Ph3P PPh3 addition H PPh3 Coordination number increases by 2 units Rh Rh Ph3P Cl Ph3P Cl PPh3 2 new anionic ligands are added to the metal Rh+1 Rh+3
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 18E
Cl2 Ph3P CO Cl O
Cl PPh3 O2 O PPh3 Ir Ir
Ph3P CO Ph3P CO 16E Cl
Me Me MeI Cl PPh 3 I PPh3 Ir Ir Ph P CO 3 Ph3P CO I Cl
Homonuclear systems (H2, Cl2, O2, C2H2) Cis Heteronuclear systems (MeI) Cis or trans AJELIAS L7-S4 An important step in many homogeneous catalytic cycles
Hydrogenation of alkenes- Wilkinson catalyst H H2 oxidative Ph3P PPh3 addition H PPh3 Rh Rh Often thefirststepofmechanism Ph3P Cl Ph3P Cl PPh3 Rh+1 Rh+3
Methanol to acetic acid conversion- Cativa process CH3 I CO CH3I I CO Ir Ir I CO I CO Ir+1 Ir+3 I
Pd catalyzed Cross coupling of Ar-B(OH)2 and Ar-X – Suzuki Coupling
Br Br Ph3P Pd PPh3 Pd
Ph3P PPh3 Pd0 Pd+2 The more electron rich the metal, more easy is the 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
CH3 Ph2 Ph2 P CH P CH3 reductive elimination 3 Pt Pt + H3CCH3 165 °C, days P CH P CH3 3 Ph2 Ph2 CH3 2+ Pt4+ Pt
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 + L M Y [M-Y-X] M Y X d n d n
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
CH 3 Ph3P O OC OC CO C CH Mn + PPh3 Mn 3 OC CO OC 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 X
M A B M A 1, 1 - migratory insertion B X X A B 1, 2 - migratory insertion M M A B
CO O 1, 1-migratory CH2CH2R CCH2CH2R Ph3P insertion Ph3P Rh Rh OC PPh3 OC PPh3
H R 1, 2-migratory CH CH R Ph3P 2 2 Ph3P insertion Rh Rh OC PPh Ph3P 3 CO AJELIAS L7-S11
Stability of σ Bonded alkyl groups as ligands
Joseph Chatt 1962 - 68
Br PEt H PEt CH2 3 EtMgBr H3CH2C PEt3 Heat 3 Pt Pt Pt + Et P Br Et3P Br Et3P Br 3 Pressure CH2 poor yields unstable
Ph3P n - BuLi Pt cis PtCl2(PPh3)2 Ph3P
Ph P Ph3P + 3 Pt + Pt 60 °C, Ph P Ph3P 3 sealed tube
Ph3P CH3 Pt No decomposition 60 °C Ph3P CH3 sealed tube
Why does some σ bonded alkyl complexes decompose readily? 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 H H H H H C H C C M M C M C C H H H 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
H C Ph P Ph P Ph P 3 Et3P C 3 3 SiMe3 Pt Pt Pt Pt SiMe Ph P 3 Ph P Ph P 3 Et3P C 3 3 C H A B C D
No β-H β-H unable to MCCH unit will not be approach M coplanar 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
uncatalyzed gy r catalyzed Gibbs Ene
stable intermediate
Reactants Products
Heterogeneous 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 Easy Elaborate and sometimes from catalyst 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 + RhCl(PPh3)3 + CH3CH2OH + CH3CHO + 3 PPh3 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.
8 Square planar 16 electron d 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
PP The first step of this P catalytic cycle is the Cl Rh reductive H2 oxidative cleavage of a PPh3 to elimination 14e P addition generate the active form of the catalyst RCH2CH3 followed by oxidative addition of dihydrogen.
R H H CH 2 P P H Rh H2C Rh P P Cl Cl
R alkene 1, 2 -migratory P insertion coordination H H Rh P = PPh P Cl 3
R