Organometallic Chemistry CHM 4317 · and detergentsare made via organometallic catalysis....

Organometallic ChemistryCHM 4317

between organic and inorganic

Organometallic Chemistry

Overview

• A real example: the Monsanto Acetic Acid process• Introduction: what is organometallic chemistry, and why should you care?• Electron counting: the basis for understanding structure and reactivity• An overview of Main-group and Transition metal chemistry• Main group metal chemistry and "Umpolung"• Intermezzo: characterization of organometallic compounds• Transition metal chemistry: overview of common ligands• Ligand substitution• Insertion and elimination• Oxidative insertion and reductive elimination• Applications in catalysis

Example: Acetic Acid synthesis

Acetic acid is an important industrial chemical.

The traditional synthesis uses bio-oxidation of ethanol obtained via fermentation:C6H12O6 ® 2 C2H5OH + 2 CO2

C2H5OH + O2 ® CH3COOH + H2OThis is not a clean and efficient process!

Industrial acetic acid synthesis:CH3OH + CO ® CH3COOH

Catalyzed by a rhodium complex.

Acetic Acid synthesis

Moderately complexcatalytic cycle:

H2O

HI CH3OH

CH3I

CH3COOH

CH3COI

Rh(CO)2I2-

MeRh(CO)2I3-

MeCORh(CO)I3-

MeCORh(CO)2I3-

CO

Acetic Acid synthesis

This cycle is knownin considerable detail:

To understand it,you need to be familiarwith electron countingand commonreaction types

CO

Rh(CO) I2 2-

MeRh(CO) I2 3-

MeCORh(CO) I2 3-

MeCORh(CO)I3-

CH I3CH COI3

CH COOH3

H O2

HI CH OH3

RhI16 e

RhIII18 e

RhIII16 e

RhIII18 e

oxidativeaddition

insertion

reductiveelimination

ligandbinding

What is organometallic chemistry ?

Strictly speaking, the chemistry of compounds containingat least one metal-carbon bond.

Metal hydrides are often included, H being considered as the "smallest organic group" (as in propyl, ethyl, methyl, hydride).

Metal-carbon bonds are often formed temporarily or potentially, so in practice many compounds are includedthat do not actually contain metal-carbon bonds.

(Ph3P)3RhClH2

O

OMe

LiO

OMeLi

Why should you care ?

Organometallic chemistry is the basis of homogeneous catalysis, which is the method of choice for clean and efficient synthesis of fine chemicals, pharmaceuticals and many larger-scale chemicals.

Many plastics (polythene, polypropene, butadiene rubber, ...)and detergents are made via organometallic catalysis.

Organometallic chemistry is also the basis for understanding important steps in heterogeneous catalysis reactions such as olefin hydrogenation and CO oxidation.

Organometallic compounds are used on a large scale as precursors for generation of semiconductors (AlN, GaAs, etc).

Silicone rubbers are one of the few classes of organometallic compounds used as "final products".

Course Objectives

By the end of this course, you should be able to:• Make an educated guess about stability and reactivity of a given

compound, based on (a.o.) electron counting rules• Propose reasonable mechanisms, based on "standard"

organometallic reaction steps, for many metal-catalyzed reactions

• Use steric and electronic arguments to predict how changes in reactants, metal or ligands affect the outcome of reactions

• Read a current research literature paper, understand and explain its content and significance

Useful Background Knowledge

Organometallic chemistry is a 4th-year course because it builds on:• Organic chemistry: reaction mechanisms, primarily nucleophilic

and electrophilic attack• Inorganic Chemistry: electronegativity; electron counting and

stability; properties of (transition) metals• Physical chemistry: orbitals and MO theory; free energy,

enthalpy and entropyYou will (now and then) need this background to understand the

material or make assignments etc

10

Between organic and inorganic...

Organic chemistry:• more or less covalent C-X bonds• rigid element environments• fixed oxidation states (better: valencies)

??Organometallic chemistry??

Inorganic chemistry:• primarily ionic M-X bonds, dative M-L bonds• variable and often fluxional environments• variable oxidation states

11

Organometallic reactivity

Since most organometallics are intermediates, the focus in organometallic chemistry is usually on understanding and tuning reactivity

This starts with analyzing reaction mechanisms in terms of elementary steps

The number of possible elementary steps is larger than in"pure organic" chemistry, but the ideas are similar

12

Organometallic structures

Knowledge of inorganic and coordination chemistry is useful to understand geometries, electron counts and oxidation states of organometallic compounds

Organometallics are more covalent and often less symmetricthan coordination compounds, so orbital symmetry argumentsare not as important

13

Trends in organometallic chemistry

Organometallic chemistry is concerned with all metals, in combination with all "organic" elements.

there are many metals !

Generalization is importantthe chemistry of e.g. Fe is not much more complicated

than that of C, but after that there are 80 more metals... we divide reactions in broader categories than organic chemists do

We concentrate on the M side of the M-C bond,and on how to tune its reactivity

14

Elements of interest

Organic elementsMain group metalsTransition metals

15

Organic vs organometallic reactivity

Organic chemistry:C-C / C-H : nearly covalentCd+-Xd- : polar (partly ionic)reactivity dominated by nucleophilic attack at CSN2 and SN1 like reactivity

Organometallic chemistry:C is the negative end of the M-C bond ("umpolung")reactivity dominated by electrophilic attack at C

or nucleophilic attack at Massociative and dissociative substitution at M

X

SN2 TS (10-e ?)

Y

- X Y

- XYX

Y

SN1 int (6-e)

(C2H4)2PdCl2 (C2H4)(CO)PdCl2

(C2H4)PdCl2

(C2H4)2(CO)PdCl2

?CO- C2H4

- C2H4CO

dissociative

associative

16

Main-group organometallics

s and p orbitals.8-e rule, usually.with a lot of exceptions

More electropositive and larger:higher coordination numbers,regardless of the number of electrons.

“Early" groups and not very electropositive:lower coordination numbers.

AlMe

AlMe

MeMeMe

Me

N

ClSnO

MeCl Me

Me Zn Me

8-e

10-e

4-e

17

Main-group organometallics

Metal is the "d+" side of the M-C bond.Chemistry dominated by nucleophilic attack of Cd- at electrophiles.

this is also the main application in organic synthesis

MeMgBr + Me2CO Me3COMgBr

note: this is a simplified pictureof the Grignard reaction

18

Main-group organometallics

M-M multiple bonds are fairly weak and rather reactivethey are a curiosity and relatively unimportant,

certainly compared to C-C multiple bonds

Bond strengths in kcal/mol:C-C 85 C=C 150N-N 40 N=N 100P-P 50 P=P 75

Multiple-bonded compounds often have unusual geometries

Me4C2"Me4Sn2"

19

Transition-metal organometallics

s, p and d orbitals18-e rule, sometimes 16-e

other counts relatively rareCO

Cr

CO

OC COCOOC

Fe

Ni

18-e

18-e

18-e

PtEt3P Me

ClEt3P

16-e

20

Transition-metal organometallics

Lower electron counts if metals are sterically saturated:

nor

Co

nor nor

nor

nor =

13-e

12-e

TiMeMe

16-e

Me

WMe

Me

MeMeMe

21

Transition-metal organometallics

Often ligands capable of donating 2-8 electronsPreference for p-system ligands (good overlap with d-orbitals)

Bonding to neutral ligands (olefin/diene/CO/phosphine)relatively weak

Important for catalysis!

allyl

3 e

cyclopentadienyl

5 e

OC

2 e

C

carbene

2 e

olefin

2 e

22

Transition-metal organometallics

An olefin complex: (Acac)Ir(NCMe)(C2H4)

donation

backdonation

23

Transition-metal organometallics

+?

Symmetry-forbiddenDoes not happen

Mo Mo+

No symmetryVery fast

a

“Forbidden” reactions ?

24

Reactivity of the M-C bond

Polar Þ reactive towards e.g.

• Water:Me3Al explodes with water; Me4Sn does not react.

• Oxygen:Me2Zn inflames in air; Me4Ge does not react.

• Carbonyl groups:MeLi adds at -80°C, Me3Sb not even at +50°C.

M-OH + H-C

M-O-O-C

M-O-C C

25

Reactivity of the M-C bond

Oxidation and hydrolysis: large driving force

Bond strengths in kcal/mol:Al-C 65 As-C 55 Si-C 74Al-O 119 As-O 72 Si-O 108Al-Cl 100 Si-Cl 91

26

Organometallic reaction steps

Ligand dissociation / coordination

Me3Al + NMe3 Me3Al-NMe3

6e 8e

Mo(CO)6 Mo(CO)5 + CO

18e 16e

note: free Me3Aldimerizes to Me6Al2

27

Organometallic reaction steps

Insertion and b-elimination

MeMgBr + MeC N Me2C=NMgBr

FeNNN Me FeN

NN Me FeNNN

Me

N

N NFeAr Ar

+

Ar =

FeNNN Me =

28

Organometallic reaction steps

Insertion and b-eliminationHPd

PPPdPP

H

Ph2P PPh2=PP

29

Organometallic reaction steps

Oxidative addition / Reductive eliminationP

RhP

ClP

PRh

PH

P

H

Cl

H2"P" = PPh3

30

Organometallic reaction steps

Oxidative addition / Reductive elimination

RhL

OO

L

RhL

OO

LMe

I

L = P(OPh)3

RhMe

OO

IL

L

L = PPh3

MeI

31

Organometallic reaction steps

s-bond metathesis

14 e

Lu CH3 Lu *CH3*CH4 Lu

C

*CH

H3

H3

32

Organometallic reaction steps

Redox reactions

Homolysis

FeFe - e-

Et Hg Et Et Hg Et+

Overview of Organometallic Chemistry

33

Organometallic reaction steps

Reactivity of coordinated ligands

34

Factors governing structure and reactivityof organometallic compounds

• M-C, M-X bond strengths

• Electronegativity of M (polarity of M-C etc bonds)

• Number of (d) electrons

• Coordination number

• Steric hindrance

35

Trends in the periodic table

Main group metals:• left and down: more electropositive• down: higher oxidation states less stable

Transition metals:• middle: strongest preference for 18-e• 2nd and 3rd row: strong preference for

paired electrons (low-spin states)• down: higher oxidation states more stable

36

Working with organometallics

Synthesis and reactivity studies (inert atmosphere!):• Glove box• Schlenk line, specialized glassware

Characterization:• Xray diffraction Þ structure Þ bonding• NMR Þ structure en dynamic behaviour• (calculations)

• IR• MS• EPR

Not:• GC• LC