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