Download MS PowerPoint - Catalysis Eprints database

Homogeneous Catalysis
HMC-1- 2010
Dr. K.R.Krishnamurthy
National Centre for Catalysis Research
Indian Institute of Technology, Madras
Chennai-600036
Homogeneous Catalysis- 1
Basics
Homogeneous Catalysis- General features
Metal complex chemistry- Metals & Ligands –bonding & reactivity
Reaction cycles
Reaction types/ Elementary reaction steps
Kinetics & Mechanism
Catalysis
1850 Berzelius
1895 Ostwald: A catalyst is a
substance that changes the rate of a
chemical reaction without itself
appearing into the products
Definition: a catalyst is a substance
that increases the rate at which a
chemical reaction approaches
equilibrium without becoming itself
permanently involved.
Catalysis is a kinetic phenomenon.
Catalyzed rxn
proceeding through
an intermediate
Ea
Ea
catalyzed
G
Reactants
G
Products
Reaction Coordinate
Catalysis –Types
Obeys laws of thermodynamics
Heterogeneous
Homogeneous
Enzymatic/Bio
Photo/Electro/Photo-electro
Phase transfer
Homogeneous Catalysis
Reactions wherein the Catalyst components and substrates of the reaction
are in the same phase, most often the liquid phase
Mostly soluble organometallic complexes are used as catalysts
Characterized by high TON & TOF
Operate under milder process conditions
Amenable to complete spectroscopic characterization
Homogeneous processes without a heterogeneous counterpart:
Pd-catalyzed oxidation of ethylene to acetaldehyde (Wacker process)
Ni-catalyzed hydrocyanation of 1,3-butadiene to adiponitrile (DuPont)
Rh- and Ru-catalyzed reductive coupling of CO to ethylene glycol
Enantioselective hydrogenation, isomerization, and oxidation reactions.
Catalysis- Heterogeneous Vs Homogeneous
Aspect
Heterogeneous
Homogeneous
Activity
Reproducibility
Comparable
Difficulty in reproducibility
Comparable
Reproducible results
Selectivity
Heterogeneous sites. Difficult to control
selectivity
Relatively higher selectivity, easy to
optimize, various types of selectivity
Reaction conditions
Higher temp. & pressure, better thermal
stability
Lower temp. (<250ºC), Higher pressure,
lower thermal stability
Catalyst cost &
recovery
High volume –low cost. Easy catalyst
recovery
Low volume, high value. Recovery
difficult. Major drawback
Active sites, nature
& accessibility
Not well- defined, heterogeneous,
but tunable, limited accessibility
Molecular active sites, very well defined,
uniform, tunable & accessible
Diffusion limitations
Susceptible, to be eliminated with proper
reaction conditions
Can be overcome easily by optimization
of stirring
Catalyst life
Relatively longer, regeneration feasible
Relatively shorter, regeneration may/may
not be feasible
Reaction kinetics
mechanism &
catalytic activity at
molecular level
Complex kinetics & mechanism, Difficult
to establish & understand unequivocally l,
but days are not far-off
Reaction kinetics ,mechanism & catalytic
activity could be established &
understood with relative ease
Susceptibility to
poisons
Highly susceptible
Relatively less susceptible. Sensitive to
water & oxygen
Industrial
Application
Bulk/Commodity products manufacture
~ 85%
Pharma, fine & specialty chemicals
manufacture, ~15%
Homogeneous catalysis-Major industrial
processes
Processes/Products
Production (Milln.MTA)
Terephthalic acid -PTA
50
Acetic acid & acetyl chemicals
7
Aldehydes and alcohols- Hydroformylation
6
Adiponitrile- Hydrocyanation
1
Detergent-range alkenes- SHOP- Oligomerization
1
Alpha Olefins (C4- C20)- Dimerization &
4
Oligomerization
Total fine chemicals manufacture
<1
Olefins Polymerization (60% uses Ziegler-Natta)
60
Homogeneous catalysis-Features
Cone Angle
Transition-metal catalysts- Features / Potential
Activity & Selectivity can be controlled in several ways:
Strength of metal-ligand bond can be varied
Variety of ligands can be incorporated into the coordination sphere
Specific ligand effects can be tuned- constituents
Variable oxidations states are feasible
Variation in coordination number can be possible
Tailor made catalyst systems are possible
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”
Types of selectivity
Types of selectivity
12 Principles of green chemistry
1.
2.
3.
4.
5.
6.
Prevent waste
Increase atom economy
Use and generate no / less toxic chemicals
Minimize product toxicity during function
Use safe solvents and auxiliaries
Carry out processes with energy economy (ambient temperature and
pressure)
7. Use renewable feedstocks
8. Reduce derivatives and steps
9. Use catalytic instead of stoichiometric processes
10. Keep in mind product life time (degradation vs. biodegradation processes)
11. Perform real-time analysis for pollution prevention
12. Use safe chemistry for accident prevention
Amenable for adoption in homogeneous catalysis
Catalysts affect both rate & selectivity
Chemo selectivity
Regio selectivity
Diastereo & Enantio Selectivity
Basics - Reactivity of metal complexes
A metal complex:
The catalytic activity is influenced by the characteristics of the central metal ions
and attached ligands.
Metal
The oxidation state and the electron count (EC) of the valence shell of the metal ion
are the critical parameters for activity. A fully ionic model is implicit.
Activity of a metal complex is governed by
Rule of effective atomic number (EAN) or the 18 e- rule
EC=18- Co-ordinative saturation
Inactive
EC < 18- Co-ordinative unsaturation
Activity
Easy displacement of weakly bound ligands;
e.g., Zr Complex, THF can be easily replaced by the substrate and solvent
molecules.
Influenced of bulkier ligands; Steric constraints- Easy ligand dissociation
NiL4
↔
NiL3 + L
Many complexes have electron counts less that 16
Metal complexes-Electron counts for activity
Oxidation state
1+
Electron
count
16
1+
18
4+
16
1-
18
PPh3
Cl
Rh
Ph3P
PPh3
H
Ph3P
PPh3
Rh
PPh3
CO
+
CH3
Zr
O
CO
OC
Co
OC
CO
Homogeneous Catalysis- Reaction cycle
The catalytically active species must
have a vacant coordination site (total
valence electrons = 16 or 14) to allow
the substrate to coordinate.
Noble metals (2nd and 3rd period of
groups 8-10) are privileged catalysts
(form 16 e species easily).
In general, the total electron count
alternates between 16 and 18.
Ancillary ligands insure stability and a
good stereoelectronic balance.
One of the catalytic steps in the
catalytic cycle is rate-determining.
Homogeneous Catalysis
Role of ‘vacant site’ and Co-ordination of the substrate
Catalyst provides sites for activation of reactant (s)
Through surface/site activation the activation barrier for reaction is reduced.
In homogeneous as well as heterogeneous catalysts such active sites are
normally referred to as vacant site/ co-ordinatively unsaturated site (cus).
Substrates on adsorption at cus get activated
In a typical homogeneous catalyst the active site is a cus in a metal
complex
In heterogeneous catalysis, similar cus exist
In homogeneous phase, metal complexes are fully saturated with ligand &
solvent molecules
There is a competition between the desired substrate and the other potential
ligands present in the solution for co-ordination with metal ion.
Nature of interaction/binding between Metal- ligand-substrate-solvent
governs overall activity & selectivity
These interactions/exchange takes place via different routes:
Substitution
Associative
Dissociative
Homogeneous Vs Heterogeneous
Functional similarities
Homogeneous
Dissociation
Association
Oxidative addition
Reductive elimination
Functions
Metal-ligand bond breaking
Metal-ligand bond formation
Fission of bond in substrate
Bond formation towards product
Heterogeneous
Desorption
Adsorption
Dissoc. Adsorption
Association
Wilkinson’s catalyst: Oxidative addition of H2
H2 adds to the catalyst before the olefin.
The last step of the catalytic cycle is irreversible. This is very useful
because a kinetic product ratio can be obtained. S-Solvent
Metal complexes
Metal complexes retain identity in solution
Have characteristic properties- XRD,IR,UV,ESR
Double salts exist as individual species
Co-ordination complex
Ligands-Types
Ligands-Types
Alkene additions
Wacker Oxidation- Catalyst & Chemical cycles
Catalyst
Chemical
Hydrogenation cycles
Ligand Effects
A. Electronic Effects
P as donor element: Alkyl (aryl) phosphines (PR3) 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)
P
C
Strong back donation-low C-O stretch
O
P
C
Weak back donation-high C-O stretch
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
O
Triethyl phosphite
Trimethyl phosphite
Triphenyl phosphite
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
Ligands - Types & properties
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.
Acids & Bases
Lewis acids
A Lewis acid accepts a pair of
electrons from other species
Bronsted acids transfer protons
while Lewis acids accept electrons
A Lewis base transfers a pair of
electrons to other species
BF3- Lewis acid; Ammonia- Lewis base
2. 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
M-Alkyl-Single bond- M-C
M-Alkylidene-Double bond M=C
M-Allyl group
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:
-hydride elimination
H
H
Agostic interaction
Metallocycle formation
H
R
M
M
R
H
Interaction between metal & α- H of alkyl group that
weakens C-H bond but does not break
Homogeneous Catalysis –Key reaction steps
1. Ligand Coordination and Dissociation
2. Oxidative addition and Reductive elimination
3. Insertion and Elimination
4. Nucleophilic attack on coordinated ligands
5. Oxidation and Reduction
1. Ligand Coordination and Dissociation
Basis
Easy coordination of substrate to the metal center-activation
Facile elimination of product from the metal coordination sphere- Desorption ?
Requirement
Co-ordinative unsaturation- active centre
Highly labile metal complex- activity
Substitution- addition-dissociation-migration
Examples
Many square-planar complexes
EC are highly active.
with 16e
ML4 complexes of Pd(II), Pt(II) and Rh(I)
are commonly used as catalysts.
E.g., Wilkinson’s catalyst
Ph3P
Ph3P
Cl
Rh
PPh3
2. Oxidative Addition & Reductive Elimination
Oxidative Addition
Addition of a molecule AX to a complex
Steps
Dissociation of the A—X bond
Coordination of the two fragments to the metal center
A
L
L
M
L
L
+ AX
L
L
M
L
X
L
Reductive Elimination
Reverse of oxidative addition:
Steps
Formation of a A—X bond
Dissociation of the AX molecule from the coordination sphere
Examples of Oxidative addition
Examples of reductive elimination
3. Insertion and Elimination
Insertion : Migration of alkyl (R) or hydride (H) ligands from the metal center
to an unsaturated ligand
L
R
L
+
M
C O
M
H
O
C R
M
CH 2
CH 2
M
CH 2CH 3
Elimination:
Migration of alkyl (R) or hydride (H) ligands from a ligand to the metal center
e.g., β-hydride elimination
H
M
CH2
CH3
M
CH2
CH2
H
M
H
CH2
CH2
-C2H4
M
+Sol
Sol
3. Insertion reactions : Migratory insertion - Examples
H
H
M
M
R
M
R
M
CO
CO
R
M
Insertion of CO into M-R bond
Migratory insertion of R in M-CO
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
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 Coordinated Ligands
A (+)ve charge on a metal-ligand complex tends to activate the coordinated C
atom toward attack by a nucleophile.
Pd
L
L
H H 2+
OH2
L C
H
C
R
L
L
H
L H
Pd
C C OH
H R
+
+ H+
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
+
]+
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
H+
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 –Elementary steps
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
Kinetic studies
Reaction rates
Dependent 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.
Addition of ligand in such a catalytic system, the rate of the reaction
decreases.
Examples
CO dissociation in Co-catalyzed hydroformylation
Phosphine dissociation in RhCl(PPh3) catalyzed 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.
K -The equilibrium constant of this reaction
k- rate constant for rate-determining step
Increasing the substrate concentration will increase the rate
initially, followed by more or less constant rate
At high substrate concentration, when
K[substrate] ~ 1 + K[substrate]
At constant catalyst concentration, plot of (1/rate) vs. (1/(substrate)
will give a straight line.
Homogeneous Catalysis- Kinetics & Mechanism
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.
5. Catalysis-An integrated approach- R.A.van Santen, Piet W.N.M. van Leeuwen,
J.A.Moulijn &B.A.Averill