Imperial CollegeLondon

Dr. Ed Marshall, M220, RCS 1e.marshall@imperial.ac.uk

Additional materials available on:www.ch.ic.ac.uk/marshall/3I3.html

Lecture notes also available on Blackboard

3I3 Slide 1

3I3 Advanced Organometallics

Lectures 1 - 4

Imperial CollegeLondonA question for you

What properties do you think are desirable for a catalyst?

• Cheap, robust and long-lived

• Low toxicity

• Lewis acidic metal centre – electronic unsaturation

• At least one vacant coordination site – coordinative unsaturation

• Variable oxidation states?

• Flexible metal-based frontier orbitals (energy, direction)

L MX

Large ligands (L) are often used to give coordinative (and electronic) unsaturation. If L bonds to M using a flexible mixture of orbitals, then M may also use a mixture of orbitals to bind to a substrate.

3I3-2

Imperial CollegeLondonThe next four lectures

Alkene and polyene ligands

Metal-carbon multiple bonds

Bonding, synthesis & reactivity

Alkene polymerisation

Olefin metathesis

3I3-3

Imperial CollegeLondonLearning objectives

1. Use simple MO theory to explain how a carbon-carbon p-cloud bonds to a metal.

2. To list methods used to synthesise metal complexes of alkenes and polyenes, and metal-carbon multiple bonds.

3. To describe typical reactions of these complexes.

4. To appreciate how polyene ligands may respond to the electronic needs of a metal, and how such a property is useful for catalysis.

5. To describe how cyclopentadienyl-based catalysts can be used to polymerise alkenes.

6. To outline the most important applications of olefin metathesis.

By the end of lecture 4, you should be able...

3I3-4

In order to get the most out of this course, it is worth making sure that you understand the following concepts…

• Crystal field theory versus molecular orbital theory

• LX ligand classifications

• How to count electrons and the 18 electron rule

• Metal-alkene bonding

Assumed Knowledge Imperial CollegeLondonAssumed knowledge

3I3-5

Section 1: Metal-alkene complexes

Imperial CollegeLondon3A Advanced Organometallics3I1 Advanced Organometallics

3I3-6

Note the similarity to CO ligands...

s-component: donation of C lone pair

p-component: backbonding into CO p*

s-component:C-C p → empty metal orbital

p-component:occupied metal d → empty C-C p*

The Dewar-Chatt-Duncanson Model of Metal-Alkene Bonding Imperial CollegeLondonDewar-Chatt-Duncanson model for metal-alkene bonding

3I3-7

C-C = 1.37 Å C-C = 1.43 Å

C-C = 1.49 Å C-C = 1.62 Å

C-C bond distance in ethene = 1.34 Å

Best Described as Metal-Alkenes or Metallacyclopropanes? Imperial CollegeLondon

H atoms nolonger planar

with the C-C bond

Metal-alkenes versus metallacyclopropanes

3I3-8

[Pt(C2H4)Cl3]2- versus [Pt(C2Cl4)(PPh3)2]

« Chem3D Embed » « Chem3D Embed »

No backbonding: “metal-alkene"

sp2 carbons

With backbonding: “metallacyclopropane"

sp3 carbons

The Concept Of Umpolung - Reversal Of Polarity Imperial CollegeLondon

2. Backbonding reduces d+ charge and reduces reactivity to nucleophiles

1. Free alkenes undergo electrophilic additions, but coordinated alkene ligands are susceptible

to nucleophilic attack

d+

The impact of metal coordination and backbonding on reactivity

Why sp3? Backbonding occurs to the p* antibonding orbital, therefore reducing the C-C bond order

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Appendix: Synthesis & Reactivity of Polyene LIgands Imperial CollegeLondon

Two common methods:

1. Addition to electron poor metal centres / displacement of other L-ligands

2. Reduction of a metal complex in the presence of the neutral -ene ligand

16e- 18e-

Oxidation state: N Oxidation state: N-2

Synthesis of metal-alkene complexes

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Synthesis Of Metal-Alkene Complexes Imperial CollegeLondon

1 (a) Addition to 16 electron species:

e.g. [Ir(CO)Cl(PPh3)2] + C60

[Ir(CO)Cl(PPh3)2C60]

16 e-

18 e-

1 (b) Displacement of other L-type ligands:

e.g. (h5-C5H5)2Zr(PMe3)2 + C2H4

(h5-C5H5)2Zr(C2H4)(PMe3)

18 e- 18 e-

Synthesis of metal-alkene complexes: examples

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2. Reduction of a metal in the presence of an alkene

nbd = norbornadiene

Rh(III)

Synthesis Of Metal-Alkene Complexes Imperial CollegeLondon

e.g. (h5-C5H5)2TiCl2 + 2NaC2H4

Ti(IV) Ti(II)

(h5-C5H5)2Ti(C2H4)

Synthesis of metal-alkene complexes

e.g. RhCl3 + CH3CH2OH + CH3CHO

Rh(I)[(nbd)Rh(m-Cl)]2

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Imperial CollegeLondonReactivity of metal-alkene complexes

Alkene ligands are often susceptible to nucleophilic attack

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Imperial CollegeLondonSummary of section 1

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1. Catalysis at a metal centre often requires a responsive metal (and therefore a responsive ligand set)

2. Binding an alkene to a metal often increases its susceptibility to nucleophilic attack

Most useful ligands are often those that can use different MOs to bind to a metal

Binding any organic fragment to a metal may activate it towards chemical modification