Slide Kuliah Ttm-13-1.pptx

Catalytic Reaction Cycle Haryo Pandu Winoto , Ph.D

Muqodimah/Opening

Organometallic • Counting e• Application

Summary • Summary

Advanced Inorganic Chemistry • Let’s see what can we do….

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Let’s start it slowly….. • Elementary Steps in Catalytic Processes When a coordination compound functions as a catalyst, there are usually several steps in the process. The entire collection of steps constitutes the mechanism of the reaction. Before describing several of the important catalytic processes, we will describe the types of reactions that often constitute the elementary steps.

Ones should understand the elementary step in order to master the art of catalyst design.

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Elementary step - Ligand Substitution • The very essence of the catalytic schemes that involve coordination compounds is that reactants, other ligands, and solvent molecules must enter and leave the coordination sphere of the metal. • As a complex functions as a catalyst, it is often necessary for one ligand to enter the coordination sphere of the metal and another to leave (before or as the other ligand enters).

LnM-X + Y → LnM-Y + X

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Elementary step – Oxidative Addition • Oxidative addition is a process by which an atom is simultaneously oxidized and the number of bonds to it is increased as groups are added • The term oxad is used to denote this type of reaction • The nature of the oxad reaction requires that the following conditions generally apply : • It must be possible to change the oxidation state of a metal • In most case the metal must be able to increase its coordination number by 2 • Diatomic molecules such as H2, Cl2, or HCl add in cis positions when reacting as gases or as solutions in nonpolar solvents that do not assist in separating the molecules. In polar solvents, molecules that dissociate are not restricted to entering in cis positions.

PtF4 + F2 → PtF6 5

Elementary step – Oxidative Addition-2 • Mechanisms of oxidative addition : • Concerted pathway

• SN2-type

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Elementary step – Oxidative Addition-3 • Mechanisms of oxidative addition : • Ionic The ionic mechanism of oxidative addition is similar to the SN2 type in that it involves the stepwise addition of two distinct ligand fragments. The key difference being that ionic mechanisms involve substrates which are dissociated in solution prior to any interactions with the metal center. • Radical

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Elementary step – Reductive Elimination • A reductive elimination reaction is the opposite of oxidative addition. The coordination number of the metal decreases (usually by two units) as the oxidation state decreases. The following reaction illustrates the reductive elimination of C2H6, which is one step in hydrogenation using Wilkinson’s catalyst. • In catalytic processes, reductive elimination is essential in order to remove the product from the coordination sphere of the metal.

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Elementary step – Insertion Reaction • In an insertion reaction, an entering group becomes bonded to a metal and a ligand while being positioned between them • Formally, such a reaction can be shown as : LnM-X + Y → LnM-Y-X in which the entering ligand is inserted between M and X • Molecules such as O2, CS2, CO, C2H4, SO2, and SnCl2 are among those that undergo insertion reaction

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Elementary step – Migratory Insertion Reaction • A migratory insertion is a type of reaction in organometallic chemistry wherein two ligands on a metal complex combine • It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products • However, often the two are used interchangeably because the mechanism is sometimes unknown • Therefore, migratory insertion reactions or insertion reactions, for short, are defined not by the mechanism but by the overall regiochemistry wherein one chemical entity interposes itself into an existing bond of typically a second chemical entity 10

Elementary step – Migratory Insertion Reaction-2 • In the migratory insertion, a ligand that is viewed as an anion (X) ligand in and a ligand that is viewed as neutral couple, generating a new anionic ligand. The anion and neutral ligands that react are adjacent. • If the precursor complex is coordinatively saturated, migratory insertion often result in a coordinatively unsaturated product. • A new (neutral) ligand can then react with the metal leading to a further insertion. The process can occur many times on a single metal, as in olefin polymerization. • The anionic ligand can be: H− (hydride), R− (alkyl), acyl, Ar− (aryl), or OR− (alkoxide). The ability of these groups to migrate is called their migratory aptitude. The neutral ligand can be CO, alkene, alkyne, or in some cases, even carbene. 11

Elementary step – Migratory Insertion Reaction-3 (CO-Insertion) • CO inserts into a metal-alkyl bond via migratory insertion • The key concept is that both the CO and the alkyl groups are ligands on the same metal

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Elementary step – Migratory Insertion Reaction-4 (CO-Insertion) Effect on Reaction rates : • Steric effects strain – Increasing the steric strain of the chelate backbone in square planar complexes pushes the carbonyl and methyl groups closer together, increasing the reactivity of insertion reactions • Oxidation state – Oxidation of the metal tends to increase insertion reaction rates. The main rate-limiting step in the mechanism is the migration of the methyl group onto a carbonyl ligand, oxidizing the metal by imparting a greater partial positive charge on the acetyl carbon, and thus increasing the rate of reaction • Lewis acids – Lewis acids also increase the reaction rates, for reasons similar to metal oxidation increasing the positive charge on the carbon. Lewis acids bind to the CO oxygen and remove charge, increasing the electrophilicity of the carbon. This can increase the reaction rate by a factor of up to 108, and the complex formed is stable enough that the reaction proceeds even without additional CO to bind to the metal • Electronegativity of the leaving group - Increasing the electronegativity of the leaving alkyl group stabilizes the metalcarbon bond interaction and thus increases the activation energy required for migration, decreasing the reaction rate • Trans-effect – Ligands in an octahedral or square planar complex are known to influence the reactivity of the group to which they are trans. This ligand influence is often referred to as the trans-influence, and it varies in intensity between ligands. A partial list of trans-influencing ligands is as follows, from highest trans-effect to lowest:aryl, alkyl > NR3 > PR3 > AsR3 > CO > Cl. 13

Elementary step – Migratory Insertion Reaction-5 (Trans Effect) • In inorganic chemistry, the trans effect is the labilization (making more reactive) of ligands that are trans to certain other ligands, which can thus be regarded as trans-directing ligands • The intensity of the trans effect (as measured by the increase in rate of substitution of the trans ligand) follows this sequence: F−, H2O, OH− < NH3 < py < Cl− < Br− < I−, SCN−, NO2−, SC(NH2)2, Ph− < SO32− < PR3, AsR3, SR2, CH3− < H−, NO, CO, CN−, C2H4

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Elementary step – Migratory Insertion Reaction-6 (Trans Effect 2) In the following reaction instead of adding NH3 into PtCl4 we have to add Cl- ion into Pt(NH3)4 due to greater Cl- trans-effect.

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Elementary step – Migratory Insertion Reaction-6 (Migratory aptitude) • Migratory aptitude is the relative ability of a migrating group to migrate in a rearrangement reaction. • This can be affected by the leaving group (whichever gives a more stable carbocation) depends upon the electron density of the migrating group i.e. Hydride > Phenyl = Tert-Alkyl > Sec-Alkyl > Primary Alkyl > Methyl

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Catalytic cycle (Summary of Ligand type) Ligand

Abbreviation Charge

Hydride (H)

monoanionic

Carbonyl (CO)

Neutral

Cyano (CN)

Monoanionic

Methyl

Me

Monoanionic

Cyclopentadienyl

Cp

Monoanionic

Carbonate

Dianionic

Pyridine

Py

Neutral

BiPyridine

BiPy

Neutral

Triphenylphosphine

PPh3

Neutral

Water

Aq

Neutral

Acetylacenate

Acac

Neutral

Thiocyano

Neutral

Chloro/Iodo/….

Monoanionic

Ethylenediaminetetraaceto

EDTA

Monoanionic

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Revisiting the 18 rules

z=26 x (Oxidation number of Fe can be found through following steps) Total complex charge = Fe Oxi. Number + 6. CN Charge -4 = x + 6(-1) x=2 L (Monodentate) = 1 EAN = Z – x + 2nL EAN = 26 – 2 + 2(6.1) = 36

z=27 x (Oxidation number of Co can be found through following steps) Total complex charge = Co Oxi. Number + 3. en Charge (en = ethylene diamine) 3 = x + 3(0) x=3 L (Bidentate) = 2 EAN = Z – x + 2nL EAN = 27 – 3 + 2(3.2) = 36 18

Example-1 Wilkinson Catalyst

Let us name each of the step and discover the oxidation number of Rh in every step….

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Example-2 Monsanto Process

Let us name each of the step and discover the oxidation number of Rh in every step….

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Example-3 Ziegler Natta Process

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Thank you

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