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Homogeneous CatalysisHMC-1- 2010
Dr. K.R.KrishnamurthyNational Centre for Catalysis ResearchIndian Institute of Technology, Madras
Chennai-600036
Homogeneous Catalysis- 1
BasicsHomogeneous Catalysis- General features
Metal complex chemistry- Metals & Ligands –bonding & reactivity
Reaction cycles
Reaction types/ Elementary reaction steps
Kinetics & Mechanism
Catalysis1850 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.
Obeys laws of thermodynamics
A + B + [CAT] Ck1
k-1
K =k1
k-1
Reaction Coordinate
G
GReactants
Products
Ea
E acatalyzed
Catalyzed rxn proceeding through
an interm ediate
Catalysis –Types 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 phaseMostly soluble organometallic complexes are used as catalystsCharacterized by high TON & TOFOperate under milder process conditionsAmenable 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
Terephthalic acid -PTA
Acetic acid & acetyl chemicals
Aldehydes and alcohols- Hydroformylation
Adiponitrile- Hydrocyanation
Detergent-range alkenes- SHOP- Oligomerization
Alpha Olefins (C4- C20)- Dimerization &
Oligomerization
Total fine chemicals manufacture
Olefins Polymerization (60% uses Ziegler-Natta)
Production (Milln.MTA)
50
7
6
1
1
4
< 1
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 possibleTailor made catalyst systems are possible
( )n
( )n
( )n
Effect of ligands and valance states on the selectivity in the nickel catalyzed reaction of butadiene
Scheme: 1,3-butadiene reactions on “Ni”
Types of selectivity
O
OH
O
OH
Chemoselectivity
O
OHydrogenation Hydrofomylation
Regioselectivity
OHR
OHR
OHR
Diastereoselectivity
Hydrogenation
COOR'
R NHCOR"
COOR'
R NHCOR"
COOR'
R NHCOR"Hydrogenation
Enantioselectivity
Types of selectivity
12 Principles of green chemistry
1. Prevent waste
2. Increase atom economy
3. Use and generate no / less toxic chemicals
4. Minimize product toxicity during function
5. Use safe solvents and auxiliaries
6. 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 catalysisAmenable 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 byRule 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 dissociationNiL4 ↔ NiL3 + L
Many complexes have electron counts less that 16
Metal complexes-Electron counts for activity
Co
COOC
OC CO
-
Zr
CH3
O
+
Rh
H
COPPh3
PPh3Ph3P
RhCl PPh3
PPh3Ph3P
Oxidation state Electron count
1+ 16
1+ 18
4+ 16
1- 18
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.
start here
precatalyst
A
B
C
D
catalyst
substrate
substrate
products
Homogeneous CatalysisRole 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 FunctionsHeterogeneous
Dissociation Metal-ligand bond breaking Desorption
Association Metal-ligand bond formation Adsorption
Oxidative addition Fission of bond in substrate Dissoc. Adsorption
Reductive elimination Bond formation towards product 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
RhAr3P
Cl PAr3
PAr3
RhCl
Ar3P S
PAr3
RhCl
Ar3P H
PAr3
H
S
RhCl
Ar3P H
PAr3
H
RhCl
Ar3P CH2CH2H
PAr3
H
S
PAr3
S ligand dissociation
H-H
oxidativeaddition
S
substratecoordination
insertion/migration
CH3CH3
reductiveelimination
S
rearrangement
irreversible
Metal complexes
Metal complexes retain identity in solutionHave characteristic properties- XRD,IR,UV,ESRDouble salts exist as individual species
Co-ordination complex
Ligands-Types
Ligands-Types
Alkene additions
R + E-Nu R
E
Nu
R
Nu
E
Anti-MarkovnikovMarkovnikov
E = H, BR2, Si, Hg, SnNu = halogen, CN, CHO, OH, CO, COOR, NR2
Wacker Oxidation- Catalyst & Chemical cycles
CatalystChemical
Hydrogenation cycles
Ligand 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)
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 frequenciesStrong back donation and low C – O stretch
A. Electronic Effects
PC O P
C OStrong back donation-low C-O stretch Weak back donation-high C-O stretch
Trimethyl phosphiteTriethyl 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 stretchThe 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 referenceThe electronic parameter, χ (chi) for other ligands is simply defined as the difference in the IR frequencies of the symmetric stretch of the two complexesLigand, PR3, R= χ (chi) IR Freq (A1) of NiL(CO)3 in cm-1
T-Bu 0 2056N-Bu 4 20604-C6H4NMe3 5 2061Ph 13 20694-C6H4F 16 2072
CH3O 20 2076PhO 29 2085CF3CH2O 39 2095Cl 41 2097(CF3)2CHO 54 2110F 55 2111CF3 59 2115
B. Steric Effects1) 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 complexesCrystal structure determination, angles smaller than θ 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 θ value = 87CH3O 107n-Bu 132PhO 128Ph 145i-Pr 160C6H11 170t-Bu 182
P MCone angle
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
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
O
P P
X
PX
PP
P
P P
X
Ligands - Types & properties1. 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 acidsA Lewis acid accepts a pair of electrons from other species
Bronsted acids transfer protonswhile 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
Oxidation state: +n +n+2valence electrons: p p-2
b) Insertion of alkene into a metal-H or an existing metal-C bond
Reactivity of metal-alkyls: kinetic instability towards conversion by -hydrideelimination.Others:-hydride elimination
Agostic interaction
Metallocycle formation
M +R
X
M
R
X
MH H
R R
HM
H
M
M
R
HH H
R
H
M-Alkyl-Single bond- M-CM-Alkylidene-Double bond M=CM-Allyl group
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
ExamplesMany square-planar complexes with 16e EC are highly active.
ML4 complexes of Pd(II), Pt(II) and Rh(I) are commonly used as catalysts.
E.g., Wilkinson’s catalyst
RhPh3P
Cl
PPh3
Ph3P
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
M
L
LLL + AX M
L
LX
A
L
L
Reductive EliminationReverse 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
Elimination: Migration of alkyl (R) or hydride (H) ligands from a ligand to the metal centere.g., β-hydride elimination
M C
R
OL + M C
L
R
O
M
H
CH2
CH2M CH2CH3
M CH2 CH3 M CH2
H CH2
M
H
CH2
CH2M Sol
H-C2H4
+Sol
3. Insertion reactions : Migratory insertion - Examples
M
HM
H
M
R
MR
CO
MH
CO
MR
M R
O
M H
O
Insertion of olefin into M-H bond
Insertion of CO into M-R bond
Insertion of olefin into M-R bond
Insertion of CO into M-H bond
Migratory insertion of R in M-CO
M HHM
M
R COM
O
R
Insertion reactions are ‘cis’ in character
Rh
L
L H
Rh
L
L
MH n
MH
+ n
Insertion
Polymer chain termination by ß-elimination
ß-elimination
L = PPr3i
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
LCL CPd
L
LL
C
C
HH
H R
2+OH2 H
H
H
R
OH
+
+ 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.
The extent of the reactivity of the ligand is reflected in the rate constants
Pd2+
+ H2O[ Pd
OH]+ + H+
Ti4+
OO
R
H+ Ti
4+O
R
H+ O
Fe CO + HO- Fe
OH
O -
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 + H + Co(II)
R + O2 R O O R HR O O H + R
+ Co(II)R O O H + Co(III)OHR O + Co(III)R O O H R O O + H + Co(II)AND
The Catalytic Cycle –Elementary steps
MLn+1 ⇋ MLn + L
MLn+ + H2 ⇋ H2MLn
H2MLn + alkene ⇋ H2MLn(alkene)
H2MLn(alkene) ⇋ HMLn(alkyl)
HMLn(alkyl) → MLn + alkane
Example: A metal complex catalyzed hydrogenation of an alkene
Alkene + H2 → 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
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.
Rate = k.K[substrate][catalyst]/1 + K[substrate]
Homogeneous Catalysis- Kinetics & Mechanism
a. Kinetic studies and mechanistic insight
i) Macroscopic rate lawii) Isotope labelling and its effect on the rate
or stoichiometryiii) Rate determining stepiv) 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