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Inorganic Chemistry
Year 3
Transition Metal Catalysis
Eighteen Electron Rule
Get the number of the group that the metal is in (this will be the number of d
electrons)
2.
Add to this the charge
3.
1.
Negative charge
More electrons
Add to the count
2.
Positive charge
Less electrons
Remove from the count
Add the number of donating ligands to the count
A stable complex has a total count of 18
Revision
1.
Eighteen Electron Rule - Test
16
HCo(CO)4
18
[RhI2(CO)2]-
16
Ni(η3-C3H5)2
16
Fe(CO)4(PPh3)
18
Revision
Rh(PPh3)3Cl
16 Electron Square Planar Complexes
Fe
Ru
Os
Have a strong preference for square planar 16 electron arrangements
Why?
-
Ligands provide 8 electrons, and the metal provides 8 electrons
-
dx2-y2 raised in energy above dz2
-
Back – bonding becomes less efficient
Revision
Complexes with d8 configuration
Oxidation State
Start at zero
2.
Ignore all formally neutral ligands (ie CO, PPh3)
3.
Add 1 for anionic ligand
4.
Positive change, electron removed, add 1
5.
Negative charge, electron added, subtract 1
6.
Give your result in roman numerals
Revision
1.
Oxidation State - Test
(I)
HCo(CO)4
(I)
[RhI2(CO)2]-
(I)
Ni(η3-C3H5)2
(II)
Fe(CO)4(PPh3)
(0)
Revision
Rh(PPh3)3Cl
LnL’M
- L’
LnM(S)
Ln M
A
+ L’
Product
B
Reaction Steps
Catalytic Cycles
LnM(S)
Substrate
LnL’M
- L’
Ln M
A
+ L’
Product
B
Reaction Steps
Catalytic Cycles

Reasonable 18 electron counts and coordination numbers

Types of reactions

Ligand association

Ligand dissociation

Migration

Elimination

Oxidative Addition

Reductive Elimination

Metallacycle formation

Transmetallation
Reaction Steps
Constructing Catalytic Cycles

18 electron
complexes
electronically
saturated

Most are coordination
saturated

Therefore exchange
of ligands require
initial dissociation

The rate of this
reaction can be
slow
Reaction Steps
Ligand Association / Dissociation

Complementary process

Requires two accessible oxidation states of 2 units apart

Change of co-ordination number is also 2
Reaction Steps
Oxidative Addition / Reductive
Elimination
Types of A – B
Favoured by:

Low oxidation number
H-H

Electron donating ligands
X-X

Anionic complexes
C-C

Vacant sites
H-X
C-X
Square planar complexes are ideal
for oxidative addition
M-X
Reaction Steps
Oxidative Addition

Low valent systems can attack the C-H bonds of an already bound ligand
Reaction Steps
Oxidative Addition
Ortho Metallation

The reverse of oxidative addition

Takes place in a cis sense

Favoured by the opposite factors:

High oxidation state

Crowded co-ordination sphere

Electron withdrawing ligands
Reaction Steps
Reductive Elimination
Insertion Reaction
Ligand Y migrates which enlarges ligand X
Can often be seen as a nucleophilic attack of Y on X
Migrating Ligand Y can be alkyl or halide
X Ligands can be a wide range, CO, CNR, CR2, C=C, O2, SO2
Reaction Steps
Migration
Alkyl / Hydride
Reaction Steps
Hydride Migration
Regiochemistry at Alkene
β-elimination
Reverse of hydride migration to an alkene
Coordination number increases during intermediate
Needs a vacant site
M-C-C-H group must be syn coplanar
Must have a beta-hydrogen for beta-elimination to take place
Reaction Steps
β - elimination
α-elimination
Alpha-elimination is the reverse of hydride migration to an
alkylidene (Carbene)
Usually not seen if beta-elimination is possible due to the less
favourable transition state
Reaction Steps
α - elimination
What reaction steps are required for the conversion…
Mn(CO)5Me + CO  Mn(CO)5(COMe)
Reaction Steps
Reaction Steps - Test
Draw a balanced scheme showing beta-hydride elimation from
[Cp2Ti(n-Pr)]+
Reaction Steps
Reaction Steps - Test
Reaction Steps - Test
Reaction Steps
Complex of the type Pt(PR3)4 can form PtCl2 (PR3)2 when reacted with HCl:
suggest an explanation for this result, including a reaction scheme naming the
elementary reactions involved and showing the byproducts.
Catalysed by: Wilkinson’s Catalysis RhCl(PPh3)3
Addition to Alkene
Hydrogenation
Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with
bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes
(a) Draw a catalytic cycle for the conversion of ethene and hydrogen to ethane with this
system
Addition to Alkene
Hydrogenation - Test
Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with
bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes
(a) Draw a catalytic cycle for the conversion of ethene and hydrogen to ethane with this
system
Addition to Alkene
Hydrogenation - Test
Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with
bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes
(b) Give electrons counts for each of the intermediates.
1
NA
2
12
3
14
4
16
5
14
Addition to Alkene
Hydrogenation - Test
Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with
bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes
(c) State the oxidation state of the metal centre in each of the intermediates
1
NA
NA
2
12
(I)
3
14
(III)
4
16
(III)
5
14
(III)
Addition to Alkene
Hydrogenation - Test
Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with
bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes
(d) Name each of the reaction steps in the catalytic cycle.
A Dis/Associaton
Ligand Exchange
B
Oxidative Addition
C Alkene Binding
D Hydride Migration
E
Reductive
Elimination
Addition to Alkene
Hydrogenation - Test

The phosphine ligand is one of the best ligands for hydrogenation

Good two electron donor with steric tuning of coordination sphere and metal
accessibility by changing the phosphine substituents (pi-acidity and tolman
cone angle)

Fast turnover needs

Bulky group favours dissociation

Good donors favour oxidative addition
Addition to Alkene
Phosphine Ligands

The hydrogenation of an
unsymmetrical alkene like this
will result in stereoisomers due
to a chiral compound

If you need a specific isomer you
need to use a chiral catalyst, and
to do this you need chiral ligands.

Chiral phosphines are the best
due to the above reasons
BINAP-(R)
Addition to Alkene
Chiral Hydrogenation
You do not need to use pure hydrogen and an alkene. A secondary alcohol and
an imine works just as good. Remember nitrogen has lone pair to coordionate
Arene Hydrogenation
Has cis stereospecificity. Only one face of an aromatic is hydrogenated
Addition to Alkene
Transfer Hydrogenation
Catalysed by: A range of well-defined complex: Fe, Mn, Co, Ru, Rh, Ir, Pd and Pt
examples.
Related to hydrogenation but with SiR3 as a ‘super-proton’
First developed by H2PtCl6
Strong tendency to form linear products
Addition to Alkene
Hydrosilation
Catalysed by: NiL4 where L = P[O(o-tolyl)]3
Addition of H-C≡N to double bonds catalysed by Ni, Cu and Pd complexes
Strong donor phosphines do not give turnover
Reaction tends to isomerise internal alkenes to give terminal products
Most important hydrocyanation is hydrocyanation of butadiene to give adiponitrile
Addition to Alkene
Hydrocyanation
oxo-reaction
Catalysed by: HCo(CO)4 / HRh(PPh3)2(CO)
Cobalt: Harsh Conditions. Moderate selectivity, competitive hydrogenation
Rhodium: Mild conditions. Excess phosphine needed for high selectivity
Addition of a hydrogen atom and a formyl (CHO) group to an alkene
Addition to Alkene
Hydroformylation
Suggest a plausible mechanism for the reaction and catalysed by Wilkinsons catalyst
Addition to Alkene
Addition to Alkene - Test
Suggest a plausible mechanism for the reaction and catalysed by Wilkinsons catalyst
Addition to Alkene
Addition to Alkene - Test
Hydroformylation
Carbonylation
Adding CO + H2 to a substrate
Adding just a CO to a substrate
The most important carbonylation reaction is the conversion of methanol to
acetic acid
R-OH + CO  R-COOH
Acetate Derivatives
Acetic Acid
Acetaldehyde
Methyl Acetate
Vinyl Acetate
H3CCOOH
H3CCHO
H3CCOOMe
H3CCOOCH=H2
Acetic acid is manufactured on large scale!
Carbonylation
Carbonylation
Cobalt-based
Reppe/BASF
Rhodium-based
Monsanto
[RhI2CO2]Iridium-based
BP Cativa
• Better than rhodium at forming alkyl bond
• Rate-limiting step is methyl migration
• Kinetics more complicated than Monsanto process
• Reaction ‘promoted’ by species such as [Ru(CO)3I2]2
• Mechanism likely to involve neutral and anionic species and competing processes
Carbonylation
Carbonylation Catalysts
Carbonylation
The Monsanto Process
Rhodium-based
Polyketone formation
Carbonylation
Catalyst: [PdL2(OR)]+

Grignard reagents couple with aryl halides
Catalyst: Ni(acac)2 / (dppe)NiCl2 / FeCl3 / CoCl2 / CrCl2
Coupling Reactions
Coupling
Grignard Reagents

Negishi coupling
Catalyst: (Ph3P)4Ni / (Ph3P)2PdCl2 | Coupling an aryl halide with an organozinc (RZnX)

Stille coupling
Catalyst: Pd(dba)3 | Coupling an aryl halide with an organotin reagent (RSnR’3)

Make sure you have no beta-hydrogens

Alkyl < benzyl ~ allyl < aryl < alkenyl < alkynyl (reactivity from transfer from tin)

Me3Sn best choice for control but tBu3Sn much safer

Bulky phosphine ligands and highly basic additives (CsF) facilitates cross coupling of aryl
chlorides
Coupling Reactions
Organometallic Coupling

Sonogashira coupling
Catalyst: (MeCN)2PdCl2
Copper alkynyls (Cu-C≡C-R) couple to organic electrophiles under harsh conditions
Palladium salts allow same reaction under mild conditions
A base is required to remove the HX liberated
Coupling Reactions
Organometallic Coupling
Catalyst: Can be Pd(0) or Pd(II) pre-catalyst
Coupling Reactions
Organometallic Coupling
Mechanism
Catalyst: Can be Pd(0) or Pd(II) pre-catalyst | Note number of ligands can change PdLn
Coupling Reactions
Mizoroki-Heck reaction
Mechanism
A palladium-catalysed coupling may be carried out using either a Suzuki or a
Stille protocol. What are the advantages of each approach?
Stille
• Organotin complexes
• Reaction is facile (low temp, fast), but starting materials are still stable in air so
practically east
Suzuki
• Organoboron complexes
• Low toxicity of starting material and byproducts
• Some boronic acids commercially availiable
Coupling Reactions
Coupling Reactions - Test
Carbenes

Carbene is defined as a neutral divalent carbon. Free carbenes can be formed. Looking
at metal-carbine complexes

Metal in low oxidation state

Heteroatom substituents on carbene cation
Carbenes
Fischer Carbenes
`
Schrock Carbenes

Metal in high oxidation state

Substituents on carbene carbon usually carbon or hydrogen

Carbene may be called an alkylidene
Nucleophilic Carbenes

Ideally carbine ligands introduced like phosphines using free carbine

Wanzlick proposed stabilisation using heteroatom substiuents
`
`
`
Metal-Carbene Bonding
• As with phosphines, this allows bonding from
carbene to metal (lone pair in sp2 orbital) and
back-bonding (to empty p orbital)
• Back bonding is significant in fisher and
schrock carbenes but not with nucleophilic
carbenes
• For electron counting all carbenes donate two
electrons
• For oxidation state:
• Fisher / Shrock carbenes (alylidenes) are
dianionic (-2)
• Nucleophillic carbenes are neutral
(phosphines)
Carbenes
• Free carbenes may be either in triplet (sp) or
singlet (sp2) state
• Bonding to metals is best described by invoking
singlet carbenes
Carbenes / Coupling - Test
[Ar = 2,6-(i-Pr)2C6H3]?
N-Heterocyclic carbene ligand will bind
tightly to metal preventing formation of Pd
metal, pyridine ligand is labile so rapidly
forms free site
Carbenes
Why might the complex below be a better catalyst for both of these coupling
routes than the more ‘traditional’ palladium-phosphine systems
Metathesis
Alkene Methesis – Very small free energy differences
Schrock’s Catalysts
Highly active
React with hindered alkenes
Control reactivity by altering alkoxide groups
Poor functional group tolerance
Grubb’s Catalysts
3 different types
Undergo the following mechanism
Metathesis
Equilibria must be driven (for example loss of gas)
Metathesis
Metathesis
Mechanism
The Chauvin
Mechanism
Metathesis in synthesis
Metathesis
Metathesis / Carbene - Test
Metathesis
Treating EtOCH=CH2 with RuCl2(PCy3)2(CHPh) (Grubbs’ I) does not lead to catalytic turnover but gives a stable metal complex and one equivalent of a styrene (PhCH=CH2) as a
byproduct. Predict the structure of this product and explain why the reaction is
stoichiometric.
Ziegler Catalyst
TiCl4
Activated by AlEt3
Mild Conditions
Controlled process: Yields HIGH DENSITY linear polymer
TACTICITY
ISOTATIC
SYNDIOTATIC
ATATIC
Alkene Polymerisation
Alkene Polymerisation
Alkene Polymerisatioin
Cossee-Arlman Mechanism
Alkene Polymerisatioin
Metallocene Mechanism
Hydrolysis of AlMe3 gives MAO [-Al(Me)-O-]n
Alkene Polymerisation
MAO
Methylaluminoxane
Triphenylcarbenum (trityl) salts
[Ph3C]+ powerful alkyl
abstractor
Alkene Polymerisation
Co-Catalysts and Activators
Alkene Polymerisation
Ligand Symmetry and Tacticity
Essential Features:
• Electron Deficiency
• Vacant coordination site adjacent to the growing polymer chain
• Steric protection at the metal centre to prevent beta-hydride elimination
Cations and Anions
Alkene Polymerisation
Alkene Polymerisation Catalysts
Show how MAO activates metal halide complexes containing bidentate ligands to act as
polymerisation catalysts: use the general formula (L-L)MX2 to represent the pre-catalyst
What other role does MAO play in the reaction?
What is the mechanism for polymerisation of alkenes by the activated catalyst?
Alkene Polymerisation
Alkene Polymerisation - Test
Alkene Polymerisation
PAST EXAM PAPER
MODEL ANSWERS
Hydroformylation
Alkene Metathesis
Alkene Polymerisation
Alkene Polymerisation
Transfer Hydrogenation
Alkene Polymerisation
Alkene Polymerisation