chem310 inorganic chemistry part 3. organometallic chemistry 1.introduction (types and rationale)...
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CHEM310 INORGANIC CHEMISTRY
Part 3
ORGANOMETALLIC CHEMISTRY
1. Introduction (types and rationale)
2. Molecular orbital (bonding) of CO, arrangement “in space”or ligand types (hapticity)
3. 16 and 18 electron rule (learning to count)
4. Synthesis, steric effects and reactivity - Wilkinsons catalyst (part 1)
5. Characterisation IR nmr etc.
6. Applications (oxidative addition b elimination)
What is organometallic chemistry?
Chemistry: structures, bonding and properties of molecules.
Organometallic compounds: containing direct metal-carbon bonds.
Either s or p bonds can occur
Main group:
(AlMe3)2
Structures s bonds and 3c-2e (or even 4c-2e) bonds Chem 210
Synthesis the first M-C bond
Reactivity nucleophilic and basic auxiliaries in organic synthesis source of organic groups for transition metals
Strong preference for s-donor groups but Cp is often p-bound (deceptively like with transition metals)
Electropositive metals: often 3c-2e or 4c-2e hydrides/alkyls
Cp2Mg Cp2Fe
(Me3Al)2(MeLi)4
As a Nucleophile
Addition to polar C=X bonds
(C=O, C=N, CºN)
Substitution at sp2 carbon
(often via addition)
R MO
+
O
R
M
R MO
OR'+
O
OR'R
M O
R- MOR'
Substitution at sp3 carbon does occur
but is far less easy and often has a multistep mechanism
Substitution at other elements:often easy for polar M-X bonds
(Si-Cl, B-OMe)
MeMgBr + B(OMe)3 BrMgMe
OMeB(OMe)2 MeOMgBr + MeB(OMe)2 Me3B
As a base More prominent in polar solvents think of free R- acting as base
Elimination
mechanism can be more complex than this
Metallation
chelate effect more important than inductive effect!
R M
H
X
+ RH + MX
R M
Me2N
H
Me2N
M
+ RH
b-hydrogen transfer
mainly for Al: for more electropositive elements, deprotonation
and nucleophilic attackare faster
for less electropositive elements, often no reaction
Al H
O O
HAl
Me W MeMe
MeMe
Me
OC FeCO
CO
CO
CO
Fe
Cl Ru CF2OC
ClPPh3
PPh3
C
WRO OR
OR
Ph
PhMgBr
MeLi
Chemistry: structures, bonding and properties of molecules.
Transition metal compounds
Some compounds do not contain metal-carbon bond, but they are usually
included in the field of organometallic chemistry. They include:
• Metal hydride complexes, e.g.
Et3P Pt PEt3
Cl
H
Ph3P Ru HH2
HPPh3
PPh3
• N2-complexes, e.g.
H3N Ru NH3N
NH3
NH3
NH3
N
2+
Ph2P Mo PPh2
Ph2P
PPh2N
N
N
N
• Phosphine complexes, e.g.
Ph3P Rh PPh3
Cl
PPh3
Ph3P RuCl
PPh3
PPh3Cl
In general, metals in organometallic compounds include: • main group metals• transition metals • f-block metals
In this course, transition metals are our main concern.
Exercise. Which of the following compounds is an organometallic compound?
a)
OCH3
TiCH3O OCH3
OCH3
b)
NH3
CuH3N NH3
NH3
2+
Cl Pt
Cl
Cl
c) CH2
CH2
-
d)O Pt O
O
O
Me
Li
Li
Li
MeMe
Me
Lie) CoCo
Co
P
Co
P COCO
COCO
OC
OC
OCOC
C O
CO
Ph
Ph
f)
A brief history of organometallic chemistry
1) Organometallic Chemistry has really been around for millions of years
Naturally occurring Cobalimins contain Co—C bonds
Vitamin B12
2) Zeise’s Salt synthesized in 1827 = K[Pt(C2H4)Cl3] • H2O
Confirmed to have H2C=CH2 as a ligand in 1868
Structure not fully known until 1975
3) Ni(CO)4 synthesized in 1890
4) Grignard Reagents (XMgR) synthesized about
1900 Accidentally produced while trying to make other
compounds Utility to Organic Synthesis recognized early on
5) Ferrocene synthesized in 1951 Modern Organometallic
Chemistry begins with this discovery (Paulson and Miller)
1952 Fischer and Wilkinson
Nobel -Prize Winners related to the area:
Victor Grignard and Paul Sabatier (1912)Grignard reagent
K. Ziegler, G. Natta (1963)
Zieglar-Natta catalyst
E. O. Fisher, G. Wilkinson (1973)
Sandwich compounds
K. B. Sharpless, R. Noyori (2001)
Hydrogenation and oxidation
Yves Chauvin, Robert H. Grubbs, Richard R. Schrock (2005) Metal-
catalyzed alkene metathesis
Common organometallic ligands
M H M C M CC M M
H
HM
H
XM
M
M PR3
M CO M CNR
M CS M NO
M N2
M M MM
M
M C
M C
Why organometallic chemistry ? a). From practical point of view:
* OMC are useful for chemical synthesis, especially catalytic processes,
e.g. In production of fine chemicals
In production of chemicals in large-scale
reactions could not be achieved traditionally
OBn
OBn
NMo
Ar
RORO CMe2Ph
H+
CN
I+ NEt3PhPh+ CN
+ HNEt3IPh
Ph
Pd(PPh3)3
* Organometallic chemistry is related to material sciences.
e.g. Organometallic Polymers
Pt C
PBu3
PBu3
C C Cn
Pt C
PBu3
PBu3
C C C
n
Small organometallic compounds: Precursors to films for coating (MOCVD)
(h3-C3H5)2Pd -----> Pd film
CH3CC-Au-CNMe -----> Au film
Luminescent materials
* Biological Science. Organometallic chemistry may help us to understand some enzyme-catalyzed reactions.
R
H
H
R
e.g. B12 catalyzed reactions.
b). From academic point of view:
* Organometallic compounds display many unexpected behaviors-
discover new chemistry- new structures e.g.
MM
M
H
HM
H
SiR3M
M C M CM
C C C C
H3N: M
New reactions, reagents, catalysts, e.g.
Ziegler-Natta catalyst, Wilkinson catalyst
Reppe reaction, Schwartz's reagent
Sharpless epoxidation, Tebbe's reagent
Types of bonds possible from Ligands
Language: All bonds are coordination or coordinative
Remember that all of these bonds are weaker than normal organicbonds (they are dative bonds)
Simple ligands e.g. CH3-, Cl-, H2 give s bonds
systems are different e.g. CO is a s donor and p acceptor
Bridging ligands can occur two metals
Metal-metal bonds occur and are called d bonds – they are weakand are a result of d-d orbital overlap
18 Electron Rule (Sidgwick, 1927)
• OM chemistry gives rise to many “stable” complexes - how can we tell by a simple method
• Every element has a certain number of valence orbitals:
1 { 1s } for H
4 { ns, 3´np } for main group elements
9 { ns, 3´np, 5´(n-1)d } for transition metals
pxs py pz
dxzdxy dx2-y2dyz dz2
• Therefore, every element wants to be surroundedby 2/8/18 electrons
– For main-group metals (8-e), this leads to the standard Lewis structure rules
– For transition metals, we get the 18-electron rule
• Structures which have this preferred count are calledelectron-precise
• Every orbital wants to be “used", i.e. contribute to binding an electron pair
The strength of the preference for electron-precise structures depends on the position of the element in the periodic table
• For early transition metals, 18-e is often unattainable for steric reasons - the required number of ligands would not fit
• For later transition metals, 16-e is often quite stable (square-planar d8 complexes)
• Addition of 2e- from 5th ligand converts complex to 5 CN 18e- , marginally more stable
Predicting reactivity
(C2H4)2PdCl2 (C2H4)(CO)PdCl2
(C2H4)PdCl2
(C2H4)2(CO)PdCl2
?
CO- C2H4
- C2H4CO
dissociative
associative
Most likely associative
16 e
18 e
16 e
14 e
Cr(CO)6 Cr(CO)5(MeCN)
Cr(CO)5
Cr(CO)6(MeCN)
?
MeCN- CO
- COMeCN
dissociative
associative
Predicting reactivity
Most likely dissociative
16 e
18 e 18 e
20 e (Sterics!)
N.B. How do you know a fragment forms a covalent or a dative bond?
• Chemists are "sloppy" in writing structures. A "line" can mean a covalent bond, a dative bond, recognise/understand the bonding first
• Use analogies ("PPh3 is similar to NH3").
• Rewrite the structure properly before you start counting.
Pd = 10Cl¾ = 1P® = 2allyl = 3
+ ¾¾e-count 16
Cl
Pd
PPh3
covalentbond
dativebond
"bond" to theallyl fragment
Cl
Pd
PPh3
1 e 2 e
3 e
"Covalent" count: (ionic method also useful)
1. Number of valence electrons of central atom.
• from periodic table
2. Correct for charge, if any
• but only if the charge belongs to that atom!
3. Count 1 e for every covalent bond to another atom.
4. Count 2 e for every dative bond from another atom.
• no electrons for dative bonds to another atom!
5. Delocalized carbon fragments: usually 1 e per C (hapticity)
6. Three- and four-center bonds need special treatment
7. Add everything
N.B. Covalent Model:
18 = (# metal electrons + # ligand electrons) - complex charge
The number of metal electrons equals it's row number (i.e., Ti = 4e, Cr = 6 e, Ni = 10 e)
Hapto (h) Number (hapticity)
For some molecules the molecular formula provides insufficient information with which to classify the metal carbon interactions
The hapto number (h) gives the number of carbon (conjugated) atoms bound to the metal
It normally, but not necessarily, gives the number of electrons contributed by the ligand
We will describe to methods of counting electrons but we willemploy only one for the duration of this module
The two methods compared: some examples
N.B. like oxidation state assignments, electron counting is a formalism and does not necessarily reflect the distribution of electrons in the molecule – useful though
Some ligands donate the same number of electrons
Number of d-electrons and donation of the other ligands can differ
Now we will look at practicalexamples on the black board
Does it look reasonable ?
Remember when counting:
Odd electron counts are rare
In reactions you nearly always go from even to even (or odd to odd), and from n to n-2, n or n+2.
Electrons don’t just “appear” or “disappear”
The optimal count is 2/8/18 e. 16-e also occurs frequently, other counts are much more rare.
Exceptions to the 18 Electron Rule
ZrCl2(C5H5)2 Zr(4) + [2 x Cl(1)] + [2 x C5H5(5)] =16
TaCl2Me3 Ta(5) + [2+ x Cl(1)] + [3 x M(1)] =10
WMe6 W(6) + [6 x Me(1)] =12
Pt(PPh3)3 Pt(10) + [3 x PPh3(2)] =16
IrCl(CO)(PPh3)2 Ir(9) + Cl(1) + CO(2) + [2 x PPh3(2)] =16
What features do these complexes possess?
• Early transition metals (Zr, Ta, W)• Several bulky ligands (PPh3)• Square planar d8 e.g. Pt(II), Ir(I)• σ-donor ligands (Me)
Alkyl ligands:
Transition metal alkyl complexes important for catalysts e.g. olefin polymerization and hydroformylation thermodynamic
Problem is their weak kinetic stability(Thermally fine: M-C bond dissociation energies are typically 40-60 kcal/mol with 20-70 kcal/mol)
Simple alkyls are sigma donors, that can be considered to donate one or two electrons to the metal center depending on which electron counting formalism you use
Synthesis of Metal Alkyl Complexes1. Metathetical exchange using a carbon nucleophile (R-). Common reagents are RLi, RMgX (or R2Mg), ZnR2, AlR3, BR3, and PbR4. Much of this alkylation chemistry can be understood with Pearson's "hard-soft" principles
2. Metal-centered nucleophiles (i.e. using R+ as a reagent) Typical examples are a metal anion and alkyl halide (or pseudohalogen). for example:
NaFp + RX Fp-R + NaX [Fp = Cp(CO)2Fe]
3. Oxidative Addition. This requires a covalently unsaturated, low-valent complex (16 e- or less). A classic example:
4. Insertion- To form an alkyl, this usually involves an olefin insertion. The simplest generic example is the insertion of ethylene into an M-X bond, i.e.
M-X + CH2CH2 M-CH2CH2-X
Carbonyl Complexes
Bonding of CO
Electron donation of the lone pair on carbon s This electron donation makes the metal more electron rich - compensate for this increased electron density, a filled metal d-orbital may interact with the empty p* orbital on the carbonyl ligand
p-backbonding or p-backdonation or synergisticbonding
Similar for alkenes, acetylenes, phosphines, and dihydrogen.
What stabilizes CO complexes is M→C π–bonding
The lower the formal charge on the metal ion the more willing it is to donate electrons to the π–orbitals of the CO
Thus, metal ions with higher formal charges, e.g. Fe(II) form CO complexes with much greater difficulty than do zero-valent metal ions
For example Cr(O) and Ni(O), or negatively charged metal ions such as V(-I)
In general to get a feeling for stability examine the charges on the metals
Syntheses of metal carbonyls
Metal carbonyls can be made in a variety of ways.
For Ni and Fe, the homoleptic or binary metal carbonyls can be made by the direct interaction with the metal (Equation 1). In other cases, a reduction of a metal precursor in the presence of CO (or using CO as the reductant) is used (Equations 2-3).
Carbon monoxide also reacts with various metal complexes, most typically filling a vacant coordination site (Equation 4) or performing a ligand substitution reactions (Equation 5)
Occasionally, CO ligands are derived from the reaction of a coordinated ligand through a deinsertion reaction (Equation 6)
Synthesis of carbonyl complexes
Direct reaction of the metal
– Not practical for all metals due to need for harshconditions (high P and T)
– Ni + 4CO Ni(CO)4
– Fe + 5CO Fe(CO)5
Reductive carbonylation– Useful when very aggressive conditions would berequired for direct reaction of metal and CO
» Wide variety of reducing agents can be used– CrCl3+ Al + 6CO AlCl3 + Cr(CO)6
– 3Ru(acac)3 + H2 + 12CO Ru3(CO)12 +
N.B. From the carbonyl complex we can synthesize other derivatives
Main characterization methods:
• Xray diffraction Þ (static) structure Þ bonding
• NMR Þ structure en dynamic behaviour
• EA Þ assessment of purity
• (calculations)
Useful on occasion:
• IR
• MS
• EPR
Not used much:
• GC
• LC
The υCO stretching frequency of the coordinated CO is very informative
Recall that the stronger a bond gets, the higher its stretching frequency
M=C=O (C=O is a double bond) canonical structure
Lower the υCO stretching frequency as compared to the M-C≡O structure (triple bond)
Note: υCO for free CO is 2041 cm-1)
[Ti(CO)6]2- [V(CO)6]- [Cr(CO)6] [Mn(CO)6]+ [Fe(CO)6]2+
υCO 1748 1858 1984 2094 2204 cm-1
increasing M=C doublebonding
decreasing M=C doublebonding
IR spectra and metal-carbon bonds
Bridging CO groups can be regarded as having a double bond C=O group, as compared to a terminal C≡O, which is more like a triple bond:
MM-C≡O C=O
M
~ triple bond~ double bond
terminal carbonyl bridging carbonyl(~ 1850-2125 cm-1) (~1700-1860 cm-1)
the C=O groupin a bridgingcarbonyl is morelike the C=O ina ketone, whichtypically hasυC=O = 1750 cm-1
Bridging versus terminal carbonyls
Bridging CO between 1700 and 2200 cm-1
bridgingcarbonyls
terminalcarbonyls
OC
FeOC
OC
C
CFe
CO
CO
COC
O
O
O
Bridging versus terminal carbonyls in [Fe2(CO)9]
Summary
1. As the CO bridges more metal centers its stretchingfrequency drops – same for all p ligands– More back donation
2. As the metal center becomes increasingly electron rich the stretching frequency drops
Alkene ligands
Dewar-Chatt-Duncanson model
The greater the electron density back-donated into the p* orbital on the alkene, the greater the reduction in the C=C bond order
Stability of alkene complexes also depends on steric factors as well
An empirical ordering of relative stability would be: tetrasubstituted < trisubstituted < trans-disubstituted < cis-disubstituted < monosubstituted < ethylene
Alkyne ligands:
Similar to alkenesAlkynes tend to be more electropositive-bind more tightly to a transition metal than alkenes -alkynes will often displace alkenes
Difference is 2 or 4 electron donorsigma-type fashion (A) as we did for alkenes, including a pi-backbond (B)
The orthogonal set can also bind in a pi-type fashion using an orthogonal metal d-orbital (C)
The back-donation to the antibonding orbital (D) is a delta-bond-the degree of overlap is quite small - contribution of D to the bonding of alkynes is minimal The net effect p-donation - alkynes are usually non-linear in TM complexes
Resonance depict the bonding of an alkyne. I is the metallacyclopropene resonance form
Support for this versus a simple two electron donor, II, can be inferred from the C-C bond distance as well the R-C-C-R angles
III generally does not contribute to the bonding of alkyne complexes.
Ally ligands:
Allyl ligands are ambidentate ligands that can bind in both a monohapto and trihapto form The trihapto form can be expressed as a number of difference resonance forms as shown here for an unsubstituted allyl ligand: Important applications
Dihydrogen Ligands:
Metal is more electropositive than hydrogen
Hydrogen acts as a two electron sigma donor to the metal center.
The complex is an arrested intermediate in the oxidative addition of dihydrogen
How does this affect the oxidation state of the meta?
Dihydrogen complexes Bonding is “simple” a 3C-2electron bond.
H2 - neutral two electron sigma donor
One could also describe a back-donation of electrons from a filled metal orbital to the sigma-* orbital on the dihydrogen
Electronic Attributes of Phosphines
Like that of carbonyls
As electron-withdrawing sigma-donating capacity decreases
At the same time, the energy of the p-acceptor (sigma-*) on phosphorous is lowered in energy, providing an increase in backbonding ability.
Therefore, range of each capabilities –tuning rough ordering -CO stretching frequency indicator- low CO stretching frequency- greater backbonding to M
Experiments such as this permit us to come up with the following empirical ordering:
Phosphine Ligand
Cone Angle
PH3 87o
PF3 104o
P(OMe)3 107o
PMe3 118o
PMe2Ph 122o
PEt3 132o
PPh3 145o
PCy3 170o
P(t-Bu)3 182o
P(mesityl)3 212o
Cone Angle (Tolman)
Steric hindrance:
A cone angle of 180 degrees -effectively protects (or covers) one half of the coordination sphere of the metal complex
You would expect a dissociation event to occur first before any other reaction -steric bulk (rate is first order -increasing size)
This will also have an effect onactivity for catalysts
N.B. “flat” can slide past each other
For example Wilkinson's catalyst(more later)
Has a profound effect on the reactivity!
Reaction chemistry of complexes
Three general forms:
1. Reactions involving the gain and loss of ligands a. Ligand Dissoc. and Assoc. (Bala)b. Oxidative Additionc. Reductive Eliminationd. Nucleophillic displacement
2. Reactions involving modifications of the ligand a. Insertion
b. Carbonyl insertion (alkyl migration) c. Hydride elimination (equilibrium)
3. Catalytic processes by the complexes Wilkinson, Monsanto
Carbon-carbon bond formation (Heck etc.)
a) Ligand dissociation/association (Bala)
• Electron count changes by -/+ 2
• No change in oxidation state
• Dissociation easiest if ligand stable on its own
(CO, olefin, phosphine, Cl-, ...)
• Steric factors important
MBr
+ Br-M
b) Oxidative Addition
Basic reaction:
• Electron count changes by +/- 2(assuming the reactant was not yet coordinated)
• Oxidation state changes by +/- 2
• Mechanism may be complicated The new M-X and M-Y bonds are formed using:
• the electron pair of the X-Y bond
• one metal-centered lone pair
LnM +X
YLnM
X
Y
One reaction multiple mechanisms
Concerted addition, mostly with non-polar X-Y bondsH2, silanes, alkanes, O2, ...
Arene C-H bonds more reactive than alkane C-H bonds (!)
Intermediate A is a s-complex
Reaction may stop here if metal-centered lone pairsare not readily available
Final product expected to have cis X,Y groups
X
YLnM
X
YLnM + LnM
X
Y
A
Stepwise addition, with polar X-Y bonds
– HX, R3SnX, acyl and allyl halides, ...
– low-valent, electron-rich metal fragment (IrI, Pd(0), ...)
Metal initially acts as nucleophile
– Coordinative unsaturation less important
Ionic intermediate (B)
Final geometry (cis or trans) not easy to predict
Radical mechanism is also possible
X YLnM
B
LnM X Y LnMX
Y
OC Ir Cl
PEt3
Et3P
OC Ir H
PEt3
Et3P
H
Cl
OC Ir I
PEt3
Et3P
H
Cl
OC Ir Cl
PEt3
Et3P
CH3
Br
Ir(I)
Ir(III)
Ir(III)
Ir(III)
H2
cis
cis
trans
HI
CH3Br
Cis or trans products depends on the mechanism
c) Reductive elimination
This is the reverse of oxidative addition - Expect cis elimination
Rate depends strongly on types of groups to be eliminated.
Usually easy for:
• H + alkyl / aryl / acyl
– H 1s orbital shape, c.f. insertion
• alkyl + acyl
– participation of acyl p-system
• SiR3 + alkyl etc
Often slow for:
• alkoxide + alkyl
• halide + alkyl
– thermodynamic reasons?
We will do a number of examples of this reaction
Complex Rate Constant (s-1) T(oC)
PdCH3Ph3P
Ph3P CH3
PdCH3MePh2P
MePh2P CH3
PdCH3P
P CH3
PhPh
PhPh
1.04 x 10-3 60
60
80
9.62 x 10-5
4.78 x 10-7
Relative rates of reductive elimination
Most crowded is the fastest reaction
PdCH3L
L CH3
+ solv
-L
PdCH3L
solv CH3
RELPd(solv) + CH3 CH3
Special case:Nucleophilic Attack on a Coordinated CO acyl anion
Fisher carbene
This is Fischer carbene It has a metal carbon double bond
Such species can be made for relatively electronegativemetal centers N.B. mid to late TMs
Fischer carbenes are susceptible to nucleophilic attack atthe carbon
Fischer carbenes act effectively as σ donors and p acceptors
The empty antibonding M=C orbital is primarily on the carbon making it susceptible to attack by nucleophiles
Other type is called a Shrock carbene (alkylidene)
Characteristic Fischer-type Schrock-type
Typical metal (Ox. State)
Middle to late T.M.Fe(0), Mo(0) Cr(0)
Early T.M.Ti(IV), Ta(V)
Substituents attached to carbene carbon
At least one highly electronegative heteroatom
H or alkyl
Typical other ligands
Good p acceptors Good s and p donors
Electron count 18 10-18
Nucleophilic displacement
Ligand displacement can be described as nucleophilic substitutions
O.M. complexes with negative charges can behave as nucleophilesin displacement reactions Iron tetracarbonyl (anion) is very useful
RXR[Fe(CO)4]2- [ Fe(CO)4]-
CO
H+
OX
R
[ Fe(CO)4]-RO
H+ OH
R
R H
O
XR
X2
O2
R'X O
OHR
O
R'R
Modifications of the ligand
a) Insertion reactions
Migratory insertion!
The ligands involved must be cis - Electron count changes by -/+ 2
No change in oxidation state
If at a metal centre you have a s-bound group (hydride, alkyl, aryl)
a ligand containing a p-system (olefin, alkyne, CO) the s-bound
group can migrate to the p-system
1. CO, RNC (isonitriles): 1,1-insertion
2. Olefins: 1,2-insertion, b-elimination
M
R
MR
MR
COM
O
R
1,1 1,2
1,1 Insertion
The s-bound group migrates to the p-system
if you only see the result, it looks like the p-system has inserted into the M-X bond, hence the name insertion
To emphasize that it is actually (mostly) the X group that moves, we use the term migratory insertion (Both possible tutorial)
The reverse of insertion is called elimination
Insertion reduces the electron count, elimination increases it
Neither insertion nor elimination causes a change in oxidation state
a- elimination can release the “new” substrate or compound
In a 1,1-insertion, metal and X group "move" to the same atom of the inserting substrate.
The metal-bound substrate atom increases its valence
CO, isonitriles (RNC) and SO2 often undergo 1,1-insertion
1,2 insertion (olefins)
Insertion of an olefin in a metal-alkyl bond produces a new alkyl
Thus, the reaction leads to oligomers or polymers of the olefin
• polyethene (polythene)
• polypropene
MMe
SO2
MS Me
O O
MMe
CO
MMe
O
MR
MR
M
R
MR
Standard Cossee mechanism
Why do olefins polymerise?
Driving force: conversion of a p-bond into a s-bondOne C=C bond: 150 kcal/molTwo C-C bonds: 2´85 = 170 kcal/molEnergy release: about 20 kcal per mole of monomer(independent of mechanism)
Many polymerization mechanismsRadical (ethene, dienes, styrene, acrylates)Cationic (styrene, isobutene)Anionic (styrene, dienes, acrylates)Transition-metal catalyzed (a-olefins, dienes, styrene)
Two examples
b Hydride elimination (usually by b hydrogens)
Many transition metal alkyls are unstable (the reverse of insertion)the metal carbon bond is weak compared to a metal hydrogenBond Alkyl groups with β hydrogen tend to undergo β elimination
M -CH2-CH3 M - H + CH2=CH2
To prevent beta-elimination from taking place, one can use alkyls that:
Do not contain beta-hydrogensAre oriented so that the beta position can not access the metal centerWould give an unstable alkene as the product
A four-center transition state in which the hydride is transferred to the metal An important prerequisite for beta-hydride elimination is the presence of an open coordination site on the metal complex - no open site is available - displace a ligand metal complex will usually have less than 18 electrons, otherwise a 20 electron olefin-hydride would be the immediate product.
Catalysis (homogeneous)Reduction of alkenes etc.
The size of the substrate has an effect on the rate of reaction
Same reaction different catalyst
Alternative starting material
The Monsanto acetic acid process
Methanol - reacted with carbon monoxide in the presence of a catalyst to afford acetic acid
Insertion of carbon monoxide into the C-O bond of methanol
The catalyst system - iodide and rhodium
Iodide promotes the conversion of methanol to methyl iodide,
Methyl iodide - the catalytic cycle begins:
1. Oxidative addition of methyl iodide to [Rh(CO)2I2]-
2. Coordination and insertion of CO - intermediate 18-electron acyl complex 3. Can then undergo reductive elimination to yield acetyl iodide and regenerate
our catalyst
Catvia Process
Wacker process (identify the steps)
Identify the steps