transition metal complexes in organic synthesis · olefin polymerization and oligomerization...
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Transition metal complexes in organic synthesis
Hydrogenation
Methanol to acetic acid process
Olefin polymerization and oligomerization
Organometallic catalysts in industrial synthesis : Three Nobel Prizes 2000, 2005 and 2010
RHC CH2 + H2 RCH2CH3
CH3OH + CO CH3COOH
**
n **
n **
n
Isotactic polypropylene Syndiotactic polypropylene Atactic polypropylene
nC4-C8 40%C10- C18 40 %C20 & > 20 %
Organometallic complexes
Organometaliic complexes allow impossible reactions to occur.
Have direct metal to carbon bond.
The bond may be metal-carbon sigma bond to give metal alkyls such as R4Ti or by the interaction of metals with pi-system of unsaturated organic systems.
Metals whose energetically accessible d-orbitals allow efficient overlap with pi-orbitals of the carbon-carbon and carbon-heteroatom multiple bonds to give pi-complexes.
The moiety containing pi-bond may be neutral (R-CH=CH2), cation (R-CH=CH2+) or anion (R-CH=CH2-).
Organometallic ChemistryM-C bond can be a type or type bond
Organo transition metal compounds can be divided into two broad classes:1. Sigma bonded organometallics:In which a single carbon atom of the ligand is attached directly to the metal through a sigma
bond (two electron two centre bond). Ligands of this type increases electron density on the centralmetal atom. Examples are: R-Cu, R-TiCl3, R2TiCl2 etc.
2. Pi-bonded organometallicsIn which a metal atom (ion or complex species) is bonded to ligands such as CO, RNC, C=C andother unsaturated molecules. These ligands in addition to the presence of lone pair of electrons havein common the presence of low lying vacant orbitals of correct symmetry to form pi bonds byaccepting electrons from transition metal d-orbitals (back bonding). This decreases the electrondensity on metal atom. These can be further subdivided as; a) in which the ligand is bonded throughentire unsaturated hydrocarbon molecule and b) in which only one carbon of the ligand is within thebonding distance of the metal, examples CO, carbene, benzyne etc.
C
C
C
CM M
donation back donationfrom to *
*
M-C bond can be a type or type bond. The metal therefore acts as both Lewis acid (electron acceptor) and a Lewis base (electron donor). Back donation into pi-antibonding orbitals leads to weakening of
carbon-carbon double bond.
18 electron rule
The rule states that thermodynamically stable transition metal organometallic compounds are formedwhen the sum of the metal d electrons and the electrons conventionally considered as being supplied bythe surrounding ligands equals 18.
When this happens the metal acquires the electronic configuaration of next higher noble gas. In general, the conditions favouring adherence to the 18 electron rule are, an electron rich metal (one
that is in a low oxidation state) and ligands that are good -acceptors.
The requirement for 18 electrons comes from the need to fill one s, three p and five d orbitals
There are exceptions to 18 electron rule including complexes of Ni, Pt, Pd, Zr and Ti which all form stable 16-electron complexes.
The 16 electron complexes are very important in catalytic processes, which have one high energy vacant d-orbital, which act as a site for other ligands in chemical reactions.
The symbol indicates bridging normally we have 2 and rarely 3 bridgingExamples: 2-CO, 3-CO, 2-CH3, 2-H, 2-Cl, , 3-Cl, 2-OR, 2-PR2, 2-NR2
Examples:1-R, 1-Ar 2-C2R4 1-allyl, 3-allyl, 4- Cb, 5-Cp, 6-C6H6 8-C8H8 2-C60, 5-R5C60
Hapto ligands and Sandwich compounds
The hapto symbol, (eta) with a numerical superscript, provides a topological description by indicating the number ofcarbon atoms at a bonding distance to the metal. For example, if all the five carbon atoms of a cyclopentadienylmoiety are equidistant from a metal atom, we term it as 5-cyclopentadienyl.
Sandwich
Bent Sandwich Half Sandwich Triple decker & polycyclic
(5-C5H5)2Fe (6-C6H6)2Cr
Unique reactions in organometallic chemistry
• Ligand exchange
• Oxidative Addition
• Reductive Elimination
• Migratory Insertion
• - Hydrogen Elimination
1. Ligand exchange: A transition metal complex can lose a ligand by dissociation and combine with anotherligand by association process. For example, the rhodium complex can react with an alkene as follows:
It takes place in two steps; in the first step, one of the ligands dissociates. This leads to a complex in whichrhodium has only 16 valence electrons and is therefore coordinatively unsaturated.
In the second step, rhodium associates with other ligand to become coordinatively saturated again.
Kr 4d8 5s1
When addition of ligands is accompanied by oxidation of the metal, it is called an oxidative addition reaction
L n M + X Y L n (X ) (Y )M
d n d n -2
• availability of nonbonded electron density on the metal,
• two vacant coordination sites on the reacting complex (LnM), that is, the complex must be coordinatively unsaturated,
• a metal with stable oxidation states separated by two units; the higher oxidation state must be energetically accessible and stable.
• ligands can be H-H, H-X, R-X, RCO-H, R-CO-X etc.
Requirements for oxidative addition
2. Oxidative addition
OX state of metal increases by 2 units
Coordination number increases by 2 units
2 new anionic ligands are added to themetal
RhPh3P
Ph3P PPh3
ClRh
Ph3P
H PPh3
Cl
H
PPh3
H2 oxidative addition
Rh+1 Rh+3
IrPh3P
Cl PPh3
CO
IrPh3P
Cl PPh3
CO
Cl
Cl
IrPh3P
O PPh3
CO
O
Cl
IrPh3P
I PPh3
CO
Me
Cl
IrPh3P
Cl PPh3
CO
Me
I
Cl2
O2
MeI
16E
18E
Examples of Oxidative addition : Cis or trans ?
Homonuclear systems (H2, Cl2, O2, C2H2) Cis
Heteronuclear systems (MeI) Cis or trans
PtPPh2
Ph2P CH3
CH3
CH3
CH3
reductive elimination PtPPh2
Ph2P CH3
CH3
+ H3C CH3165 °C, days
3. Reductive elimination
• a high formal positive charge on the metal,• the presence of bulky groups on the metal, and • an electronically stable organic product.
Cis orientation of the groups taking part in reductive elimination is a MUST
Almost the exact reverse of Oxidative Addition
Factors which facilitate reductive elimination
Oxidation state of metal decreases by 2 units
Coordination number decreases by 2 units
2 cis oriented anionic ligands form a stable bond and leave the metal
Pt4+Pt2+
Final step in many catalytic cycles
Hydroformylation ( conversion of an alkene to an aldehyde)
Sonogashira Coupling (coupling of a terminal alkyne to an aryl group
Cativa Process (Methanol to Acetic acid)
4. Migratory Insertion
YM
X
[M-Y-X] + LYM
L
X
d n d n
MnOC
OC CO
COOC
CH3
MnOC
OC C
COOC
Ph3P
+ PPh3
O
CH3
No change in the formal oxidation state of the metal
A vacant coordination site is generated during a migratory insertion (which gets occupied by the incoming ligand)
The groups undergoing migratory insertion must be cis to one another
These reactions are enthalpy driven and although the reaction is entropy prohibited the large enthalpy term dominates
1, 2-migratory insertion
Rh
CO
Ph3P
H
Ph3P
R
RhOC
Ph3P CH2CH2R
PPh3
RhOC
Ph3P
CO
PPh3
CH2CH2R
RhOC
Ph3P CCH2CH2R
PPh3
O1, 1-migratory insertion
M A
X
B M A
B
X
M
XA
BM A
B
X
1, 1 - migratory insertion
1, 2 - migratory insertion
Types of Migratory Insertion
5. -Hydride elimination
C
C
H
H
H
MH
HC
C
H
H
H
MH
H C
C
H
H
H
M
H
H
Beta-hydride elimination is a reaction in which an alkyl group having a hydrogen, bonded to a metal centre isconverted into the corresponding metal-bonded hydride and a bonded alkene. The alkyl must have hydrogens on thebeta carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex musthave an empty (or vacant) site cis to the alkyl group for this reaction to occur.
mechanism
Can either be a vital step in a reaction or an unwanted side reaction
No change in the formal oxidation state of the metal
-hydrogen elimination does not happen when
• the alkyl has no -hydrogen (as in PhCH2, Me3CCH2, Me3SiCH2)
• (ii) the -hydrogen on the alkyl is unable to approach the metal (as in C≡CH)
• the M–C–C–H unit cannot become coplanar
PtEt3P
Et3P C
C
C
C
H
HB
PtPh3P
Ph3P
C
PtPh3P
Ph3P
D
PtPh3P
Ph3P
A
SiMe3SiMe3
Select the most unstable platinum complex from the given list. Justify your answer
No -H -H unable to MCCH unit will not be approach M coplanar