Embed Size (px)
Citation preview
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
