organometallic chemistry between organic and...

Organometallic Chemistry

an overview of structures and reactions

Peter H.M. Budzelaar

Overview of Organometallic Chemistry2

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


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


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


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


Elements of interest

Organic elementsMain group metalsTransition metals


Organic vs organometallic reactivity

Organic chemistry:C-C / C-H : nearly covalentCδ+-Xδ- : 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


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

Cl

Sn

OMe

Cl Me

Me Zn Me

8-e

10-e

4-e



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

this is also the main application in organic synthesis

MeMgBr + Me2CO Me3COMgBr

note: this is a simplified pictureof the Grignard reaction



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"


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



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



Often ligands capable of donating 2-8 electronsPreference for π-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

O

C

2 e

C

carbene

2 e

olefin

2 e



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

donation

backdonation



“Forbidden” reactions ?

+?

Symmetry-forbiddenDoes not happen

Mo Mo+

No symmetryVery fast

a


Reactivity of the M-C bond

Polar ⇒ 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


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


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



Insertion and β-elimination

MeMgBr + MeC N Me2C=NMgBr

FeNNN Me FeN

NN Me FeNNN

Me

N

N NFeAr Ar

+

Ar =

FeNNN Me =



Insertion and β-eliminationHPdPPPdPP

H

Ph2P PPh2=PP



Oxidative addition / Reductive eliminationP

RhP

ClP

PRh

PH

P

H

Cl

H2"P" = PPh3



Oxidative addition / Reductive elimination

RhL

O

OL

RhL

O

OL

Me

I

L = P(OPh)3

RhMe

O

OI

L

L

L = PPh3

MeI



σ-bond metathesis

14 e

Lu CH3 Lu *CH3*CH4 Lu

C

*CH

H3

H3



Redox reactions

Homolysis

FeFe - e-

Et Hg Et Et Hg Et+



Reactivity of coordinated ligands


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


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


Working with organometallics

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

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

• IR• MS• EPR

Not:• GC• LC

organometallic chemistry between organic and...

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