HOMOGENEOUS
CATALYSIS
CHOICE OF METAL LIGANDS
AND BASICS
A.V. Ramaswamy
Email: av_ram@hotmail.com
HETEROGENEOUS
1. All Catalytic processes
2. TON/TOF
3. Selectivity (Chemo-, Regio-,
and Enantio-)
4. Kinetics
5. Mechanistic
Understanding
4. Thermal Stability
5. Catalyst Life
6. Catalyst recovery/Separation
7. Heat recovery
8. Susceptibility to poisons
9. Macroscopic Diffusion
10. Applications
11. Catalysts/Costs
85%
Similar
High with zeolites
for Regio-selectivity
Could be complex
Surface reaction
Not simple, -Interface
-surface structure
- Non-homogeneity
High Temperatures
Longer
High
Design
More
Important
- Petroleum refining
- Petrochemicals, bulk
Chemicals and
intermediates
High volumes, lower
costs
HOMOGENEOUS
15%
Very High
for Enantio-selectivity
Somewhat simpler
Performance can be
easily explained and
understood at
molecular level
< 250oC
Shorter
Difficult/Serious problems
Simpler
Less susceptible
Not important
- Fine Chemicals
- Pharmaceutical
intermediates
Tailor-made plastics
Lower volumes and
more expensive
A Historical Perspective
Oldest
Metallo enzymes
Yeast whole cell
Fe porphyrin
Zn complexes
Ni complexes in
Hydrogenase enzyme
Cobalt corrin complexes
Man-made
1750
Lead chamber process
1920
Mercury sulfate
1950
Co complexes
Ni
(DuPont)
Co
(BASF)
(Shell)
Mo
1970
Rh
(Monsanto)
(Wilkinson)
whether they are truly homogeneous?
fermentation of sugars
Oxidation
decabrboxylation
H2 activation
C-C bond formation
SO2 to SO3 by N oxides (Homogeneous ?)
acetylene to acetaldehye (Wacker process)
oligomerization of ethene
hydrocyanation
Carbonylation
Hydroformhylation
Epoxidation of propene
Carbonylation
Hydrogenation
Lighter olefins
Asymmetric hydrogenation to L-Dopa
Ring opening polymerization
Homogeneous Catalysis – Recent, narrower definition
Involves (organo)-metallic complexes as the catalysts, should contain a bond
between a carbon atom and the metal
The reactions employing homogeneous catalysts that are not (organo) metallic
Complexes:
-General acid-base reactions (ester hydrolysis)
-Lewis acids as catalysts (Diels-Alder reaction)
-Organic catalysts (thiazolium ions in Cannizaro reaction)
-Porphyrin complexes (epoxidation, hydroxylation)
-Enzymatic processes
-Coordination complexes (polyester condensation)
Ligand Effects are extremely important in homogeneous catalysis by metal
complexes
Example: Various products from 1,3-butadiene with a “Ni” catalyst, where
by changing the type and the nature of the ligands
coordinated to “Ni” , dimerization, oligomerization and polymerization
reactions take place
Effect of ligands and valance states on the selectivity in the nickel
catalyzed reaction of butadiene
(
(
)
)
n
(
)n
n
Scheme: 1,3-butadiene reactions on “Ni”
Ligand Effects
A. Electronic Effects
P as donor element: Alkyl (aryl) phosphines and organo phosphites
Alkyl phosphines are strong bases, good σ-donor ligands
Organo phosphites are strong π-acceptors and form stable complexes with
Electron rich transition metals.
Metal to P bonding resembles, metal to ethylene and metal to CO
Which orbitals of P are responsible for π back donation?
Antibonding σ* orbitals of P to carbon (phosphine) or to oxygen (phosphites)
The σ-basicity and π-acidity can be studied by looking at the stretching frequency
of the coordinated CO ligands in complexes, such as Ni L(CO)3 or Cr L(CO)5
in which L is the P ligand.
1) Strong σ donor ligands → High electron density on the metal and hence a
substantial back donation to the CO ligands → Lower IR frequencies
Strong back donation and low C – O stretch
2) Strong π acceptor ligands will compete with CO for the electron back donation
and C-O stretch frequency will remain high
Weak back donation → High C – O stretch
The IR frequencies represent a reliable yardstick for the electronic properties of a
series of P ligands toward a particular metal, M.
CrL(CO)5 or NiL(CO)3 as examples; L = P(t-Bu)3 as reference
The electronic parameter, χ (chi) for other ligands is simply defined as the
difference in the IR frequencies of the symmetric stretch of the two complexes
Ligand, PR3, R=
χ (chi)
IR Freq (A1) of NiL(CO)3 in cm-1
T-Bu
N-Bu
4-C6H4NMe3
Ph
4-C6H4F
0
4
5
13
16
2056
2060
2061
2069
2072
CH3O
PhO
CF3CH2O
Cl
(CF3)2CHO
F
CF3
20
29
39
41
54
55
59
2076
2085
2095
2097
2110
2111
2115
B. Steric Effects
1) Cone angle (Tolman’s parameter, θ) (Monodentate ligands)
From the metal center, located at a distance of
2.28 A from the phosphorus atom in the appropriate
direction, a cone is constructed with embraces all the
atoms of the substituents on the P atom, even though
ligands never form a perfect cone.
Sterically, more bulky ligands give less stable complexes
Cone angle
Crystal structure determination, angles smaller than θ
M
P
values would suggest.
Thermochemistry: heat of formation of metal-phosphine adducts.
When electronic effects are small, the heats measured are a measure of the
steric hindrance in the complexes.
Heats of formation decrease with increasing steric bulk of the ligand.
Ligand, PR3; R =
H
CH3O
n-Bu
PhO
Ph
i-Pr
C6H11
t-Bu
θ value =
87
107
132
128
145
160
170
182
An ideal separation between Steric and electronic parameters is not possible.
Changing the angle will also change the electronic properties of the phosphine
ligand.
Both the - and θ- values should be used with some reservation
Predicting the properties of metal complexes and catalysts:
Quantitative use of steric and electronic parameters (QALE)
The use of - valaues in a quantitative manner in linear free energy relationships
(LFER)
Tolman’s equation:
Property = a + b() + cθ
The property could be log of rate constant, equilibrium constant, etc.
Refinements:
Property = a + b () + c(θ – θth)
Where, , the switching factor, reads 0 below the threshold and 1 above it.
Refinement, the electronic parameter:
Property = a(d) + b(θ – θth) + c(Ear) + d(p) + e
Where d is used for -donicity and p used for -acceptor property;
Ear is for “aryl effect”.
For reactions having a simple rate equation, the evaluation of ligand effects with
the use of methods such as QALE will augment our insight in ligand effects,
a better comparison of related reactions, and a useful comparison between
different metals.
Bite angle effects (bidentate ligands)
Diphosphine ligands offer more control over regio- and stereoselectivity in many
catalytic reactions
The major dfiference between the mono- and bidentate ligands is the ligand
backbone, a scaffold which keeps the two P donor atoms at a specific distance.
This distance is ligand specific and it is an important characteristic, together with
the flexibility of the backbone
P
O
P
P
P
X
X
P
P
P
P
X
Many examples show that the ligand bite angle is related to catalytic performance
in a number of reactions.
Pt-diphosphine catalysed hydroformylation
Pd catalyzed cross coupling reactions of Grignard reagents with organic halides
Rh catalyzed hydroformylation
Nickel catalyzed hydrocyanation and
Diels-Alder reactions
Kinetic studies
Reaction rates: Dependence on the concentration of reactants
(and the products in some cases)
Useful in understanding the mechanism of
the reaction
Empirically derived rate expressions
Ligand dissociation:
leads to generation of catalytic active intermediate.
If the ligand is added to such a catalytic system,
the rate of the reaction decreases.
Examples: CO dissociation in Co-catalysed
hydroformylation
Phosphine dissociation in RhCl(PPh3) catalysed
hydrogenation
Cl- dissociation in the Wacker process
Michaelis-Menten Kinetics (Enzyme catalysed reactions)
Saturation kinetics
Rate = k.K[substrate][catalyst]/1 + K[substrate]
A complex is formed between the substrate and the catalyst by
a rapid equilibrium reaction. The equilibrium constant of this
reaction is K, and it is followed by the
rate-determining step with a rate constant, k.
Increasing the substrate concentration will increase the rate
initially, followed by more or less constant rate high substrate
concentration, when
K[substrate] ~ 1 + K[substrate]
at constant catalyst concentration,
a plot of (1/rate) vs. (1/(substrate)
will give a straight line.
Creation of ‘vacant site’ and coordination of the substrate
The function of a catalyst is to bring the reactants together
and lower the activation barrier for the reaction.
To bring the reactants together, a metal center must have
a vacant site.
Compare a homogeneous catalyst and a solid heterogeneous
catalyst. The latter has a surface.
Homogeneous catalyst in a condensed phase:
How do we create a vacant site?
The solvent molecules will always be coordinated to the
metal ion.
There is a competition between the desired substrate and the
other potential ligands present in the solution.
Classical way to look: Substitution reactions
Associative and Dissociative mechanisms
Basic Chemical Concepts
A metal complex: The catalytic activity is influenced by the characteristics of the
central metal ions and attached ligands.
The metal:
The oxidation state and the electron count of the valence shell
of the metal ion.
A fully ionic model is implicit.
Oxidation state
PPh3
Cl
Rh
Ph3 P
1+
Electron
count
16
1+
18
4+
16
1-
18
PPh3
H
Ph3 P
PPh3
Rh
PPh3
CO
+
CH3
Zr
O
CO
OC
Co
OC
CO
Rule of effective atomic number (EAN) or the 18 e- rule
Coordinative unsaturation
High reactivity
Reasons
When the electron count is less than 18, the metal complex is
often classified as
Coordinatively unsaturated.
Easy displacement of weakly bound ligands;
e.g., Zr Complex, THF can be easily replaced by the substrate
and solvent molecules.
Influenced by bulkier ligands; Steric constraints
NiL4
↔
NiL3 + L
Many complexes have electron counts less that 16
Important properties of ligands
1. Ligands: CO,
R2C=CR1, PR3 and
H- (N2, NO, etc.)
All ligands behave as Lewis bases and the M acts as a Lewis acid
Alkenes:  electrons
Whereas H2O and NH3 accept e- density from the metal, i.e., they act as
Lewis Acids ( acid ligands)
The donation of e- density by the metal atom to the ligand is referred to
as back donation.
H2 acts as a Lewis acid.
Also, Lewis acid-like behaviour of CO, C2H4 and H2 in terms of overlaps
between empty orbitals of the ligand and the filled metal orbitals of
compatible symmetry.
Back donation is a bonding interaction between the metal atom and
the ligands, because the signs of the donating metal ‘d’ orbitals and
the ligand * (* for H2) acceptor orbitals match.
The  ligands play important roles in a large number of homogeneous
catalytic reactions.
2. Another set of ligands: Alkyl, Allyl and alkylidene ligands
Alkyl ligands: Two reactions
a) Addition of RX to unsaturated metal center
M
+
R
R
M
X
Oxidation state: +n
valence electrons: p
X
+n+2
p-2
b) Insertion of alkene into a metal-H or an existing metal-C bond
M
R
R
H
H
M
H
H
Reactivity of metal-alkyls: kinetic instability towards conversion by -hydride
elimination.
Others:
H
H
H
-hydride elimination
R
M
Agostic interaction
Metallocycle formation
M
R
H
M
+
R
X
H
R
M
M
R
R
M
H
H
H
H
X
H
H
H
M
R
M
R
H
H
OR
M
M
R
R
H
M
R
M
R
M
H
M
Important Reaction Steps Homogeneous Catalysis
There are 5 types of reactions (and their reverse)
which, in combination, account for most homogeneous
catalytic cycles involving hydrocarbons
1. Ligand Coordination and Dissociation
2. Insertion and Elimination
3. Nucleophilic attack on coordinated ligands
4. Oxidative addition and Reductive elimination
5. Oxidation and Reduction
Important Reaction types
1. Oxidative addition and Reductive elimination
Electronic ligand effects are highly predictable in oxidative addition reactions;
-donors (alkylphosphines) strongly promote the formation of high-valence states
and thus oxidative additions.
Compexation of halides to Pd(0) increases e- density and facilitates
oxidative addition.
Phosphites and CO, on the other hand, reduce e- density on the
metal and thus the oxidative addition is slower or may not occur at all.
Reductive elimination is simply the reverse reaction of oxidative addition.
The formal valence state of the metal is reduced by two.
And the total electron count of the complex is reduced by two.
In a catalytic cycle, the two reactions always occur pair-wise.
Reductive eliminations can be promoted by stabilization of the low-valent state of the
product.
This means ligands that are good -acceptors, bulky ligands, and ligands
preferring bite angles more suited for tetrahedral than for square planar complexes,
when we deal with group 10 metals.
2. Migratory insertion Reactions
In homogeneous catalytic reactions, old bonds are usually
broken by Oxidative addition reactions and
New bonds are formed by
Reductive elimination and insertion reactions.
How does insertion take place ?
A more appropriate description of a number of insertion
reactions is
“migratory insertion”
2. Insertion reactions : Migratory insertion - Examples
H
H
M
M
R
M
R
M
CO
CO
R
M
Insertion of CO into M-R bond
O
H
M
Insertion of olefin into M-R bond
O
R
M
Insertion of olefin into M-H bond
M
H
Insertion of CO into M-H bond
Insertion reactions are ‘cis’ in character
M H
M
H
O
R CO
M
M
R
3. -Hydride elimination
This is also called -elimination reactions:
Hydride abstractions can occur from carbon atoms in
other positions also,
But elimination from the -carbon is more common.
Insertion of an alkene into a metal-hydrogen bond and
-elimination reaction have a reversible relationship.
It is possible to study this reversible equilibrium by
NMR spectroscopy.
e.g., a hydrido-ethylene complex of Rh
L
H
L
Insertion
Rh
Rh
ß-elimination
L
L
L = PPr3i
M
M
H
n
H
+
Polymer chain termination by ß-elimination
n
4. Nucleophilic attack on a coordinated ligand
Upon coordination to a metal center, the electronic environment of the ligand
undergoes a change. The ligand may become susceptible to electrophilic or
nucleophilic attack.
Pd
2+
+
H2O
OH
[ Pd
+
]+
H+
R
Ti
4+
O
+
O
Ti
4+
R
H
H
O
Fe
CO
O
+
O
+
HO-
-
Fe
OH
The extent of the reactivity of the ligand is reflected in the rate constants
5. Oxidation and Reduction
During a catalytic cycle, metal atoms frequently alternate
between two oxidation states:
Cu2+/Cu+
Co3+/Co2+
Mn3+/Mn2+
Pd2+/Pd
Catalytic Oxidation: generating alcohols and carboxylic acids
The metal atom 1) initiates the formation of the radical R•
2) contributes to the formation of R-O-O• radical
R H + Co(III)
R
R O O H + Co(II)
+ O2
R + H + Co(II)
R O O
R O + Co(III)OH
R H
AND
R O O H + R
R O O H + Co(III)
R O O + H + Co(II)
The Catalytic Cycle and the Intermediates
Example: A metal complex catalyzed hydrogenation of an alkene
→
Alkene + H2
Alkane
MLn+1
⇋
MLn + L
MLn+ + H2
⇋
H2MLn
H2MLn + alkene
⇋
H2MLn(alkene)
H2MLn(alkene)
⇋
HMLn(alkyl)
HMLn(alkyl)
→
MLn + alkane
Catalytic Cycle and Catalytic Intermediates
a. Kinetic studies and mechanistic insight
i) Macroscopic rate law
ii) Isotope labelling and its effect on the rate
or stoichiometry
iii) Rate determining step
iv) Variation of ligand structure and its
influence on ‘k’
b. Spectroscopic investigations
‘in-situ’ IR, NMR, ESR
c. Studies on model compounds
d. Theoretical calculations
Limitations:
- Kinetic studies are informative about the slowest step only,
not other steps.
- Spectroscopic investigations of a complex requires a
minimum concentration.
- It is possible that the catalytically active intermediates
never attain such concentrations and therefore,
not observed.
-The species that are seen by spectroscopy may not be
involved in the catalytic cycle!
However, a combination of kinetic and spectroscopic methods
can resolve such uncertainties to a large extent.
Reference Books
1. Homogeneous Catalysis: The Applications and Chemistry
of Catalysis by soluble Transition Metal Complexes,
G.W. Parshall and S.D. Ittel,
Wiley, New York, 1992.
2. Applied Homogeneous Catalysis with Organometallic
Compounds,
Vols 1 & 2, edited by B. Cornils and W.A. Herrmann, VCH,
Weinheim, New York, 1996.
3. Homogeneous Catalysis: Mechanisms and Industrial
Applications,
S. Bhaduri and D. Mukesh, Wiley, New York, 2000.
4. Homogeneous catalysis: Understanding the Art,
Piet W.N.M. van Leeuwen,
Kluwer Academic Publishers, 2003.