ORGANOMETALLIC CHEMISTRY AND

CATALYSIS
 COURSE TOPICS

Reaction of organometallic complexes (oxidative addition, reductive

elimination, insertion, nucleophilic and electrophilic addition and abstraction,
coupling)

Catalytic application of organometallic complexes (hydrogenation, oxidation,

CO insertion, hydroformylation, Monsanto process, Wacker process,
polymerization, metathesis, C-H bond activation, coupling reactions)

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 NOBEL PRIZES IN
ORGANOMETALLIC CHEMISTRY

• 1912 Victor Grignard and Paul Sabatier for Grignard reagent and hydrogenation reactions

• 1963 Karl Ziegler and Giulio Natta on Ziegler-Natta catalyst, Olefin polymerization

• 1973 Geoffrey Wilkinson and Ernst Otto Fischer on sandwich compounds

• 2001 W. S. Knowles, Ryōji Noyori and Karl Barry Sharpless for asymmetric hydrogenation and oxidation
reactions

• 2005 Yves Chauvin, Robert Grubbs, and Richard Schrock on metal-catalyzed alkene metathesis

• 2010 Richard F. Heck, Ei-ichi Negishi, Akira Suzuki for palladium catalyzed cross coupling reactions

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Well-Known Organometallic Homogeneous Catalysts Used in Industry

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 Catalysis: A sustainable future

Catalyst
development
Circular Economy: Sustainable development
TOWARDS ZERO WASTE

Reduce (minimum use of raw materials)

Reuse (maximum reuse of products)
Recycle (high-quality reuse of raw materials)

Energy, Water, Climate change…………

Catalysis by Nature: Learn and inspire from Nature

• None of the biocatalysts evolved by nature

use any noble metal.
• No toxic chemicals

[Ni-Fe] hydrogenase
H2 production and activation
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 OUTLINE

1. Fundamental reactions in organometallic chemistry

2. Applications of organometallic chemistry to organic synthesis

References
• Organotransition metal chemistry: From bonding to catalysis by John F. Hartwig, University Science Books, 2010
• Organometallic chemistry and catalysis by Didier Astruc, Springer, 2007
• Applied Organometallic Chemistry and Catalysis by Robin Whyman, Oxford University Press, 2003
• Organometallic Chemistry by Gary O. Spessard and Gary L. Miessler, Prentice Hall, Inc., 1997
• The Organometallic Chemistry of the Transition Metals by Robert H. Crabtree, Wiley Interscience;
• Basic Organometallic Chemistry: Concepts, Syntheses and Applications by Gupta and Elias

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OXIDATION NUMBERS

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 THE THREE FAMILIES OF LIGANDS L, X AND Z

L type ligands

• They give two paired electrons to the metal centre

• The oxidation state of the metal is not modified

Examples: PR3, CO, Py, NR3, etc

X type ligands

• They give one single electron to the metal during the bond formation
• The valence number is modified of one unit

Examples: -Cl, -H, CH3, -CN, COR, -CH=CH2, -Ph…

Z-type ligand

• The metal gives two paired electrons to the ligand during the bond formation
• Examples: BX3, AlX3, SO2…
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11
 THE FUNDAMENTAL REACTIONS
IN ORGANOMETALLIC CHEMISTRY

Some fundamental reactions permit to explain in a rational way of the reactivity in organometallic chemistry.

1. Oxidative Addition (OA)

2. Reductive Elimination (RE)

3. Insertion – migratory insertion

4. b hydride elimination

5. Ligands Substitution

6. Nucleophilic and electrophilic addition and abstraction

7. Coupling
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 OXIDATIVE ADDITION

 The oxidative addition of an A-B molecule by a transition metal complex results in

- the cleavage of the A-B bond to form 2 new bonds M-A and M-B
- The increase of the number of oxidation state (OS) and the coordination number (CN).

 The complex undergoing the oxidative addition must

- have at most 16 valence electrons and a vacant site
- it occurs most readily with electron-rich metals and when the metal is in a relatively low oxidation state
- at least two non-bonding electrons on the metal in order to form the two bonds M-A and M-B.
 Influence factors
These factors are mainly related to i) the nature of the ligands, ii) the nature of the metallic centre which tend to
increase the electronic density on the metal

 Influence of the ligands

Electron donating ligands favor the reaction

P(alkyl)3 > P(Ar)3 > P(OMe)3 > P(OAr)3 > CO

 Influence of the metal centre

The oxidability of the metal favors the O.A. reaction

Q1. Fe(0), Co(I), Ni(II)

Q2. Ir(I), Rh(I), Co(I)

which one favor for OA and why?

Fe(0) > Co(I) > Ni(II)

Row 3rd > 2nd > 1st

Examples Ir(I) > Rh(I) > Co(I)
 Oxidative Addition
• Strongest donor ligands, less pi acceptor and high negative charge

• Complexes with 18 electrons do Not undergo OA without expulsion of

ligand

• OA will Not occur with higher oxidation state

• d0 metals can NOT do oxidative additions

• OA depends on the relative strength of A-B bond

Number of electrons in the starting and final metal complexes are always counted to
checkout the overall electron count, oxidation state and number of d electrons

OA of H2 works for RhCl(PPh3)3 but

slow for RhCl[CO(PPh3)2] why?

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 Oxidative Addition
Oxidative addition (OA) via C-H bond activation
OA may occur intramolecularly. Intramolecular additions of this type are called cyclometallations or, more
specifically, an orthometallation in the case shown

This type of reactions help to activate unreactive hydrocarbons such as methane – known as C-H activation

Oxidative addition at M-M bond

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Classes of A-B Polar Non- Ligands
polar with pi
system
HX H2, X2 O2
(X=halog
en)
R-X R-H Alkene

RCOX R3SiH Alkyne

Cis

Main pathways
Trans
1. Concerted
2. SN2
3. Radical

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 3-Center Concerted Addition
Concerted, or three-centre, OA is an associative reaction in which the incoming ligand first binds as a σ complex and
then undergoes bond breaking

Addition of H2

the product is the result of syn

addition of H2. Why?
J. Am. Chem. Soc. 1985, 107, 11, 3148–3160.

How do you confirm the formation of M-H?

D. R. Paterniti, P. J. Roman, Jr., and J. D. Atwood, Organometallics, 1997, 16, 3371.

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 3-Center Concerted Addition

π-acidic ligands love the equatorial sites of the TBP geometry, which are rich in electrons capable of π
bonding. As a consequence, π-acidic ligands get folded back preferentially and tend to end up cis to their trans
partners in the starting complex.

Dihydrogen may approach along two distinct trajectories. Placing π-acidic ligands in the equatorial plane of the
TBP transition state is favored.
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 3-Center Concerted Addition

These bonding principles have been applied to non-polar σ-bonds such

as H-H, C-H, Si-H, B-H and even C-C bonds.
The first stable dihydrogen metal
complex was isolated by Kubas. This is
thought to arise from metal
backbonding into the H-H σ* orbital

Kubas Acc. Chem. Res. 1988, 21, 120.

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 3-Center Concerted Addition
Addition of C–H • Oxidative addition of Si-H and Sn-H also concerted

σ-bond metathesis. Early d0 transition metal complexes.

Four-centered transition state

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Cyclometallations

Robert Howard Crabtree

Yale University

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 Agostic (to hold on to oneself) interactions
• In the mid1960s it was observed that certain transition metal complexes exhibited close
proximity of C-H bonds of their coordinating ligands to the metal atoms.

Maurice S. Brookhart
University of Houston

• In 1983 Brookhart and Green coined the word agostic to describe such weak
bonding interaction between hydrogen atoms of aliphatic C-H and aromatic C-H
bonds with metal centre

• Agostic interaction is defined as 3-centre 2-electron bonds.

• Depending on the connectivity the name will be varied like a, b, etc
Proc. Natl. Acad. Sci. 2007, 104 (17): 6908–14.
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 Agostic Vs Anagostic interactions

The first structurally characterized -agostic-metal-alkyl compound

Proc. Natl. Acad. Sci. 2007, 104 (17): 6908–14.

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 SN2 pathway
• These reactions proceed via a polar transition state that is accompanied by an inversion of the stereochemistry
at the atom of attack by the metal center.
• The SN2 mechanism is often found in the addition of methyl, allyl, acyl, and benzyl halides, as is the case for
Vaska’s complex.

• The more nucleophilic the metal, the greater its

reactivity in SN2
• Steric hindrance at carbon slows the reaction
MeI > EtI > tBuI
• A better leaving group, X at carbon, accelerates
the reaction for this mechanism RI > RBr > RCl

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Ligands with pi system

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 Radical Pathways
• Less desirable are oxidative additions involving radicals, because these reactive intermediates tend to give
undesired side-reactions
a) Non-chain radical mechanism

The nonchain variant applies to OA of alkyl halides, RX, to Pt(PPh3)3 (RX = MeI, EtI, or PhCH2Br).

• A one-electron transfer from M to the RX σ* forms ·PtXL2 and

R·. This radical pair rapidly recombines to give the product
• The reaction goes faster as R· becomes more stable and easier to
form, giving rise to increasing reactivity in the order: Me < 1° < 2° < 3°.

b) Radical chain mechanism

• A radical initiator, Q· (e.g., a trace of air or peroxide in the

solvent), may be required to substitute for R· in the first cycle to
set the process going.
• Chain termination steps, such as recombination of two R· to give
R2, limits the number of possible cycles.

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Oxidative Addition

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 Oxidative coupling
• Two L type ligands, coordinated to the same metal center are coupled to lead to a metallacycle via the formation
of a  bond between the two L type ligands.

• The coordination number (CN) is not modified

• The oxidation state (OS) increase of two units, which explain the “oxidative” term for this coupling.

• The oxidative coupling is the key step for the formation of carbon cycles and in olefin metathesis reactions.

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 REDUCTIVE ELIMINATION
• The reductive elimination is the reverse fundamental reaction of the oxidative addition. In general, the two
X type ligands must be in a cis position in order to undergo reductive elimination and lead to a X-X
molecule.

• The reductive elimination results in the

decrease of the oxidation state (OS)
and the coordination number (CN)

Example:
It is often the last step in catalytic cycles
(hydrogenation, hydroformylation, cross-
coupling reactions…).

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31
 Factors Favouring Reductive Elimination (RE)

 High oxidation state on the metal

 Presence of bulky ligands-Increase steric strain
 Cis orientation of leaving groups
 Formed low-valent complex after RE must be stable
 Less e- donating or pi acceptor ligands accelerated RE

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 Reductive Elimination (RE)

• It’s been observed in a number of cases that a ligand dissociates from octahedral complexes to form a
distorted TBP geometry brings the eliminating groups closer to one another to facilitate elimination.

• Square planar complexes may either take on an additional fifth ligand or lose a ligand to form an odd-coordinate
complex before reductive elimination.

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 Reductive Elimination (RE)

RE can be promoted by:

• Increasing the bite angle
of the ligand
• Increasing electrophilicity
of metal center (e.g. π-
acids)
• Ligand dissociation

Large bite angles of diphosphines have been shown to enhance the rates of reductive elimination by bringing the
two departing ligands closer together. 34
 LIGAND SUBSTITUTION

Associative ligand substitution: is often

called square planar substitution because16
e-, d8 square planar complexes generally
undergo ligand substitution via an
associative mechanism (the M-Nu bond is
formed before the M-X bond breaks).

Dissociative ligand substitution is

most favoured in coordinatively saturated 18e- complexes.
• M-X bond is fully broken before the M-Nu bond
forms, thereby avoiding an energetically unfavourable
20e- intermediate.
• Analogous in many ways to SN1reactions.

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Associative Substitution: Nature of the nucleophile

• Basicity of the incoming ligand

(nucleophile) plays only a minor role in its
reactivity for soft metal centers.
• In general, the softest (i.e. most
polarizable) nucleophiles react fastest with
soft metals like Pt(II) via associative
substitution.

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 Associative Substitution: Sterics

Sterically shielding the positions

above and below the plane of the
square planar complex can lead to
significant decreases in the rates
of associative substitution.

Pearson J. Chem. Soc. 1961 2207.

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Reference:
M.C. White/Q. Chen Chem
153

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Ring-slippage chemistry of transition metal cyclopentadienyl and indenyl
complexes; Chem. Rev. 1987, 87, 2, 307–318.

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Ligand dissociation: Heat or Light

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Ligand dissociation: Solvent effect

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Organometallic Reactions

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43
Insertion Reactions

CO ligands give 1,1 insertion, and alkene

ligands give 1,2 insertion.

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 MIGRATORY INSERTION

• If two alkyl groups are present, more electron-rich

group migrate (n-alkyl> PhCH2)

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 Migration Vs Insertion

We generally do NOT worry about the exact pathway, that is why we use the term “migratory insertion” to indicate
that either directional pathway is fine.
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47
 ALKENE INSERTION
• The insertion of coordinated alkenes into M–H or M-R bonds leads to metal alkyls and constitutes a key
step in a variety of catalytic reactions including alkene polymerization, the most commercially important
reaction.
• Like 1,1-insertions, 1,2-insertions generate a vacant site on the metal, which is usually filled by an external
ligand.

The transition state for insertion has an essentially coplanar M−C−C−H arrangement, and this implies that both insertion
and elimination also require the M−C−C−H system to be capable of becoming coplanar.

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ALKENE INSERTION
Hydrozirconation

Source: Professor M. Christina White Group

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50
ELIMINATION

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 b-Hydride Elimination
Beta-hydride elimination is a reaction in which an alkyl group having a b hydrogen,  bonded to a metal centre is
converted into the corresponding metal-bonded hydride and a  bonded alkene. The alkyl must have hydrogens on the
beta carbon. The metal complex must have an empty (or vacant) site cis to the alkyl group for this reaction to occur.

To prevent b-Hydride elimination

No change in the formal oxidation state of the metal

H H H H H
H H H
C H C C
M
M C M C C Can either be a vital step in a reaction or an unwanted
H H H H H H side reaction

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 Transmetalation
The transfer of an organic group from one metal center to another. The process involves no formal change in
oxidation state for either metal.
Transmetalation reagents

In a simple picture, the metal accepting the R

group is the electrophile and the M-R bond being
transferred is the nucleophile. M-R bond formation
may or may not be simultaneous with M’-X bond
formation, depending on the nature of X and the 53
actual complexes involved
 ADDITION AND ABSTRACTION
In reductive elimination or migratory insertion, ligand transformations occur within the coordination sphere of the
metal. In contrast, we now look at outer sphere processes in which direct attack of an external reagent can take
place on a ligand without prior binding of the reagent to the metal.

• Favored when the metal fragment is a poor π base but a good σ acid.
• Complex bears a net positive charge or has electron-withdrawing
ligands
• reduce the hapticity hapticity of the ligands

• A nucleophile abstracts a cationic fragment, such as H+

or Me+

• Favored when the metal is a weak σ acid but a strong π base

• Complex has a net anionic charge, a low oxidation state, and
good donor ligands
• Increase increase the hapticity hapticity of the ligand

• Electrophile abstracts an anionic fragment, such as H−

or Cl-
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