Homogeneous Catalysis HMC-1- 2010 Dr. K.R.Krishnamurthy National Centre for Catalysis Research Indian Institute of Technology, Madras Chennai-600036
Homogeneous Catalysis- 1Basics Homogeneous Catalysis- General features Metal complex chemistry- Metals & Ligands –bonding & reactivity Reaction cycles Reaction types/ Elementary reaction steps Kinetics & Mechanism
Catalysis 1850 Berzelius C a ta ly z e d r x n p ro c e e d in g th r o u g h 1895 Ostwald: A catalyst is a a n in te r m e d ia te substance that changes the rate of a Ea chemical reaction without itself appearing into the products E a ∆G c a t a ly z e d Definition: a catalyst is a substance that increases the rate at which a R e a c ta n ts ∆G chemical reaction approaches equilibrium without becoming itself P ro d u c ts permanently involved. Catalysis is a kinetic phenomenon. R e a c tio n C o o r d in a te Catalysis –Types Heterogeneous Homogeneous Enzymatic/Bio Obeys laws of thermodynamics 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 characterizationHomogeneous 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 HomogeneousAspect Heterogeneous HomogeneousActivity Comparable ComparableReproducibility Difficulty in reproducibility Reproducible resultsSelectivity Heterogeneous sites. Difficult to control Relatively higher selectivity, easy to selectivity optimize, various types of selectivityReaction conditions Higher temp. & pressure, better thermal Lower temp. (<250ºC), Higher pressure, stability lower thermal stabilityCatalyst cost & High volume –low cost. Easy catalyst Low volume, high value. Recoveryrecovery recovery difficult. Major drawbackActive sites, nature Not well- defined, heterogeneous, Molecular active sites, very well defined,& accessibility but tunable, limited accessibility uniform, tunable & accessibleDiffusion limitations Susceptible, to be eliminated with proper Can be overcome easily by optimization reaction conditions of stirringCatalyst life Relatively longer, regeneration feasible Relatively shorter, regeneration may/may not be feasibleReaction kinetics Complex kinetics & mechanism, Difficult Reaction kinetics ,mechanism & catalyticmechanism & to establish & understand unequivocally activity could be established &catalytic activity at l, but days are not far-off understood with relative easemolecular levelSusceptibility to Highly susceptible Relatively less susceptible. Sensitive topoisons water & oxygenIndustrial Bulk/Commodity products manufacture Pharma, fine & specialty chemicalsApplication ~ 85% manufacture, ~15%
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 selectivityin the nickel catalyzed reaction of butadiene ( ) n ( )n ( ) n Scheme: 1,3-butadiene reactions on “Ni”
12 Principles of green chemistry1. Prevent waste2. Increase atom economy3. Use and generate no / less toxic chemicals4. Minimize product toxicity during function5. Use safe solvents and auxiliaries6. Carry out processes with energy economy (ambient temperature and pressure)7. Use renewable feedstocks8. Reduce derivatives and steps9. Use catalytic instead of stoichiometric processes10. Keep in mind product life time (degradation vs. biodegradation processes)11. Perform real-time analysis for pollution prevention12. Use safe chemistry for accident prevention Amenable for adoption in homogeneous catalysis
Basics - Reactivity of metal complexes A metal complex:The catalytic activity is influenced by the characteristics of the central metal ionsand attached ligands. MetalThe 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 Electron Cl PPh3 count Rh 1+ 16 Ph3P PPh3 H PPh3 Ph3P Rh 1+ 18 PPh3 CO + CH3 Zr 4+ 16 O - OC CO 1- 18 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 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 HeterogeneousFunctional similaritiesHomogeneous Functions HeterogeneousDissociation Metal-ligand bond breaking DesorptionAssociation Metal-ligand bond formation AdsorptionOxidative addition Fission of bond in substrate Dissoc. AdsorptionReductive 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
Metal complexes Metal complexes retain identity in solution Have characteristic properties- XRD,IR,UV,ESR Double salts exist as individual species
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 O P C C O Strong back donation-low C-O stretch Weak back donation-high C-O stretchThe σ-basicity and π-acidity can be studied by looking at the stretching frequencyof the coordinated CO ligands in complexes, such as Ni L(CO) 3 or Cr L(CO)5in 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 stretchThe IR frequencies represent a reliable yardstick for the electronic properties of aseries 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 thedifference 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 2056 N-Bu 4 2060 4-C6H4NMe3 5 2061 Ph 13 2069 4-C6H4F 16 2072 CH3O 20 2076 PhO 29 2085 CF3CH2O 39 2095 Cl 41 2097 (CF3)2CHO 54 2110 F 55 2111 CF3 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 complexes Cone angleCrystal structure determination, angles smaller than θ M Pvalues would suggest.Thermochemistry: heat of formation of metal-phosphine adducts.When electronic effects are small, the heats measured are a measure of thesteric hindrance in the complexes.Heats of formation decrease with increasing steric bulk of the ligand. Ligand, PR3; R = H θ value = 87 CH3O 107 n-Bu 132 PhO 128 Ph 145 i-Pr 160 C6H11 170 t-Bu 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 χ- and θ- values should be used with some reservationPredicting the properties of metal complexes and catalysts: Quantitative use of steric and electronic parameters (QALE) The use of χ- valaues in a quantitative manner in linear free energy relationships (LFER) Tolman’s equation: Property = a + b(χ) + cθ The property could be log of rate constant, equilibrium constant, etc. Refinements: Property = a + b (χ) + c(θ – θth)λ where, λ, the switching factor, reads 0 below the threshold and 1 above it. Refinement, the electronic parameter: Property = a(χd) + b(θ – θth)λ + c(Ear) + d(πp) + e where χd is used for σ-donicity and πp used for π-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 P P P X P X X Many examples show that the ligand bite angle is related to catalytic performancein 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 & 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: π electrons Whereas H2O and NH3 accept e- density from the metal, i.e., they act as Lewis Acids (π 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 π* (σ* for H2) acceptor orbitals match. The π ligands play important roles in a large number of homogeneous catalytic reactions.
Acids & BasesLewis acidsA Lewis acid accepts a pair ofelectrons from other speciesBronsted acids transfer protonswhile Lewis acids accept electronsA Lewis base transfers a pair ofelectrons to other species BF3- Lewis acid; Ammonia- Lewis base
2. Alkyl, Allyl and alkylidene ligandsAlkyl ligands: Two reactions M-Alkyl-Single bond- M-C M-Alkylidene-Double bond M=Ca) Addition of RX to unsaturated metal center M-Allyl group R R M + M X X Oxidation state: +n +n+2 valence electrons: p p-2b) Insertion of alkene into a metal-H or an existing metal-C bond R R M M H H H HReactivity of metal-alkyls: kinetic instability towards conversion by β-hydrideelimination.Others:α-hydride elimination H H H M RAgostic interaction M R HMetallocycle formation
Interaction between metal & α- H of alkyl group thatweakens 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 DissociationBasis 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-migrationExamples E.g., Wilkinson’s catalystMany square-planar complexes with 16eEC are highly active. Ph3P Cl ML4 complexes of Pd(II), Pt(II) and Rh(I) Rhare commonly used as catalysts. Ph3P 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 L M L + AX L M X L L LReductive EliminationReverse of oxidative addition:Steps Formation of a A—X bond Dissociation of the AX molecule from the coordination sphere
3. Insertion and Elimination Insertion : Migration of alkyl (R) or hydride (H) ligands from the metal center to an unsaturated ligand R L H O CH2L + M C O M C R M M CH2CH3 CH2 Elimination: Migration of alkyl (R) or hydride (H) ligands from a ligand to the metal center e.g., β-hydride elimination H CH2 H H -C2H4 CH2 M CH2 CH3 M CH2 M M Sol CH2 +Sol
3. Insertion reactions : Migratory insertion - Examples H HM M Insertion of olefin into M-H bond R RM M Insertion of olefin into M-R bond O RM Insertion of CO into M-R bond CO M R Migratory insertion of R in M-CO O HM Insertion of CO into M-H bond M H CO
Insertion reactions are ‘cis’ in character M H M H O R CO M R M
L H L Insertion Rh Rh ß-elimination L L L = PPr3i M +M H n H n Polymer chain termination by ß-elimination
4. Nucleophilic Attack on Coordinated LigandsA (+)ve charge on a metal-ligand complex tends to activate the coordinated Catom toward attack by a nucleophile. H H 2+ + OH2 H H L C L L Pd L Pd C C OH + H+ L C L H R H R
Nucleophilic attack on a coordinated ligandUpon coordination to a metal center, the electronic environment of the ligandundergoes a change. The ligand may become susceptible to electrophilic ornucleophilic attack. OH Pd 2+ + H2O [ Pd ]+ + H+ R O R 4+ + Ti 4+ O + O Ti O H H O - Fe CO + 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 + H + Co(II) R + O2 R H R O O R O O H + RR O O H + Co(II) R O + Co(III)OH R O O H + Co(III) R O O + H + Co(II) AND
The Catalytic Cycle –Elementary stepsExample: 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 studiesReaction rates Dependent on the concentration of reactants and the products in some cases Useful in understanding the mechanism of the reaction Empirically derived rate expressionsLigand 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 bya rapid equilibrium reaction. K -The equilibrium constant of this reaction k- rate constant for rate-determining step Increasing the substrate concentration will increase the rateinitially, 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 & Mechanisma. 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, ESRc. Studies on model compoundsd. 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 methodscan resolve such uncertainties to a large extent.
Reference Books1. 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