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Homogeneous Catalysis

A part of 529-0502-00L Catalysis (J. A. van Bokhoven, A. Mezzetti)

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Prof. Dr. Antonio Mezzetti

Laboratory of Inorganic Chemistry ETH Hönggerberg HCI H 235 Tel.: 632 61 21 e-mail: mezzetti@inorg.chem.ethz.ch

Fr, 10.45 – 11.30 HCI J 4

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Topics:

1. Basic Concepts Oxidative addition, reductive elimination, migratory insertion, elimination An example: the hydrogenation of olefin catalyzed by [RhCl(PPh3)3] 2. Reactions with CO a. Hydroformylation (Rh, Co, Ir) (including asymmetric hydroformylation) b. Carbonylation of alcohols (Monsanto Process) c. Fischer – Tropsch process (CO reduction) 3. Reactions with Olefins a. Olefin oxidation b. Wacker process (olefin hydration) c. Carbene complexes d. Polymers by Metathesis e. The Shell Higher Olefins Process (SHOP) 4. Polymerization of Olefins a. Ziegler-Natta catalysts b. Metallocenes c. Stereoselective polymerization of olefins

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Learning Material and Books Skript

Password-protected, please paste into a browser and quit all error messages!):

https://www.mezzetti.ethz.ch/education/Kat Basics: R. H. Crabtree, The Organometallic Chemistry of the Transition Metals, Wiley, 2009 Industrial Processes: G. P. Chiusoli, P. M. Maitlis, Metal-catalysis in Industrial Organic Processes, RSC Publishing, 2008 Online: Catalysis - An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis Edited by: J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Basic Coordination Chemistry: J. Huheey, E. Keiter, R. Keiter, Anorganische Chemie – Prinzipien von Struktur und Reaktivität, de Gruyter

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Catalysis is a phenomenon related to kinetics! A Catalyst opens a new reaction pathway:

The equilibrium position is determined by by thermodynamic parameters of the reaction (!G0 < 0 " K > 1).

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Heterogeneous Catalysis

C2H4 + H2 Pt surface! "!!!! C2H6

How do soluble catalysts work?

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Learning Goals Understanding the basics of homogeneous catalysis with transition metal complexes. (Limitation: Transition metal complexes only!)

In a nutshell: „What happens at (and to) the metal during the catalytic reaction?“

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Catalysis: A Comparison heterogeneous homogeneous selectivity – + conditions – + (high T, P) (low T, P) separation + –

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New Concepts – Oxidative Addition / Reductive Elimination – Insertion / Elimination – Nucleophilic attack onto a Ligand – The best known example: [RhCl(PPh3)3]-catalyzed hydrogenation of olefins Source: Huheey

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Oxidative Addition

[Mn(L)m] + X–Y

low oxidation statecoordinatively unsaturated

(16-electron complex)

oxidation number increased by 2 units

coordinatively saturated(18-electron complex)

[M(n+2)XY(L)m]

Reductive Elimination

[M(n–2)(L)m] + X–Y

oxidation number= n – 2

coordinatively unsaturated(16-electron complex)

oxidation number = n

X and Y mutually cis

[MnXY(L)m]

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Examples Oxidative Addition:

LRhIL Cl

L

16-electron complex, free coord. site(s)(= unoccupied orbital(s)!) present

low oxidation state: Rh(I), d8

LRhIIIL H

L

H

Cl

H H+

coordinatively saturated(18-electron complex, coord. number = 6)

oxidation number increased by 2 units: Rh(III), d6

L = PPh3

Reductive Elimination:

SRh

L Cl

LSRh

L H

L

Cl

CH2CH2R

CH3CH2R+

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Which Factors determine the tendency of a complex to give oxidative additions? – Tendency to oxidation (electronic effects) " The electron richer the metal, the easier the oxidation: Good donors (e. g., trialkylphosphines) favor oxidative addition. " The tendency to oxidation increases upon descending in a triad, e. g. Co(I) < Rh(I) < Ir(I).

– Relative stability of the coordination numbers (steric effects) " The tendency to higher coordination numbers within a transition period decreases from left to right. Os(0) > Ir(I) > Pt(II) (all d8 systems): Os(0) has a greater tendency to oxidative addition than Pt(II). " Bulky ligands favor low coordination numbers, and therefore disfavor oxidative addition reactions.

– Strength of the newly formed M–X- and M–Y-bonds relative to X–Y (see CO-insertion below)

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Mechanism X–Y apolar (H2, O2): Concerted reaction with a three-center transition state:

X–Y polar, electrophilic molecule (CH3I, HCl): SN2-Mechanism

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Insertion (Einschiebung) / Elimination Althought the oxidative addition of X–Y to M gives a complex that formally results from the insertion of a metal atom into the covalent X–Y-bond, the term “insertion” (Einschiebung) is reserved for reactions in which a molecule (which must possess a multiple bond) is inserted into a metal-ligand bond. The oxidation state of the metal remains unchanged:

+ X Y(L)n M Z (L)n M X

or

Y Z

(L)n M XZ

Y

Insertion(Einschiebung)

Elimination(Eliminierung)+ X Y(L)n M Z

Both reactions have fundamental significance in homogeneous catalysis!

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Olefin-Insertion into a M–H-Bond:

ClRh

L H

L

H

ClRh

LL

H

H

16 e–18 e–

Mechanism:

– Ligands involved in an insertion reaction are mutually cis. – The insertion reaction produces a free coordination site. – An elimination from an 18-electron complex can only occur upon dissociation of a ligand. Also elimination reactions from square planar complexes require a free coordination site:

ClPd

ClH2CCl C

H2

OH2–

ClPd

ClH

–– Cl

OH

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CO-Insertion into an M–C-Bond

OCMn

OC CO

CH3

CO

CO

+ CO

OCMn

OC CO

C

CO

CO O

CH3

Mechanism

OCMn

OC CO

CH3

CO

CO

+ 13CO

OCMn

OC 13CO

C

CO

CO O

CH3

is the only product – No 13C is incorporated into the acetyl group. – No 13CO trans to the acetyl group.

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Mechanism: insertion (Einschiebung) or migration (Wanderung)?

Methyl-Migration CO-Elimination Unambiguously confirmed by the inverse reaction:

25% 25% 50% 25% 75% Predicted for Me-migration / observed product distribution Predicted for elimination / not observed (Principle of microscopic reversibility: The direct and inverse reaction follow the same elementary steps)

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Thermodynamics of CO-Insertion Calculated Reaction Enthalpies:

OCMn

OC CO

CH3

CO

CO

+ CO

OCMn

OC CO

C

CO

CO O

CH3 !H0 = –54 ± 8 kJ / mol

OCMn

OC CO

H

CO

CO

+ CO

OCMn

OC CO

C

CO

CO O

H !H0 = +20 kJ / mol

the reaction does not occur!

J. A. Connor, M. T. Zafarani-Moattar, J. Bickerton, N. I. El Saied, S. Suradi, R. Carson, G. Al Takhin, H. A. Skinner, Organometallics 1982, 1, 1166.

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Nucleophilic Attack onto a Ligand A nucleophile (Nu–) can attack either the metal (" substitution) or a coordinated molecule. For instance:

MLn CO+ Nu–

CO

NuMLn

–

FeOC

OC

++ MeO–

solvent:MeOH

FeOC

OC OMe

The coordination to a metal activates olefins (and related compounds, such as allyls) toward nucleophilic attack (why?). The highest degree of activation occurs in metal complexes that either contain strong #-accepting ligands or are positively charged.

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Relative Reactivity of Coordinated Unsaturated Ligands Toward Nucleophiles In cationic 18-electron complexes, the reactivity decreases according to:

> > >>

Adapted form: S. G. Davies, M. L. H. Green, D. M. P. Mingos, Tetrahedron 1978, 34, 3047.

Note that nucleophilic attack onto an allyl complex is accompanied by 2 e– -reduction of the metal:

CH(CO2R)2Pd

Ph3P

Ph3P ++ –CH(CO2R)2 Pd

Ph3P

Ph3P

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H2-Activation Some of the catalytic reactions discussed in this course involve the activation of dihydrogen by transition metal complexes. This is a short overview of the possible activation pathways.

– Oxidative Addition (catalyst: dihydride complex)

(L)nM

H

H[M(L)n] + H2

– Hydrogenolysis (L)nM H[MX(L)n] + H2 + HX (catalyst: monohydride complex) – Heterolytic H2-Activation [M(L)n]n+ + H2 + B: [MH(L)n](n–1)+ + BH+ (catalyst: monohydride complex) – Homolytic H2-Activation (L)nM H+ H2(L)nM M(L)n 2 (catalyst: monohydride complex)