Stoichiometric Reactions of Organometallic Complexes

1. Stoichiometric Reactions of Organometallic Complexes • Catalytic reactions of organometallic complexes are described in terms of sequences of stoichiometric reactions. These often include: • Ligand Coordination Ligand Dissociation • Oxidative Addition Reductive Elimination • Insertion b-elimination • The terms used to describe stoichiometric reactions represent only the result of the process. • no mechanistic significance • several elementary steps may be involved, but we will assign elementary kinetic rate expressions to them. • Under industrial conditions, reactive intermediates are too short-lived to be detected by spectroscopic methods. • Many steps proposed in catalytic mechanisms are based on known reactions of more stable systems at milder conditions.

2. Stoichiometric Reactions and Catalytic Cycles • Catalytic cycle proposed for the hydroformylation of an a-olefin by HCo(CO)4. 1 2 3 7 4 6 5

3. Coordination and Dissociation • Coordination of an additional ligand without a change of the oxidation state of the metal is an addition reaction • the coordination number and EAN of the complex increases without a change of the formal charge of the metal (ox. state) • The reverse process of addition is dissociation • In many cases, addition-dissociation reactions are reversible, leading to a dynamic equilibrium between reaction products. • Ligand substitution reactions of relatively stable complexes have been studied intensively, leading to mechanistic terms such as associative, dissociative, and interchange (CHEM 321).

4. Restricting-Assisting Coordination:Catalyst Design • A. Selective hydrogenation • to produce Ivemectin. • Can different coordination • affinities be used to improve • selectivity? • B. Improved NBR hydrogenation activity for catalysts requiring ligand dissociation: • k’=3.57*10-3 s-1 k’=1.41*10-3 s-1

5. Oxidative Addition - Reductive Elimination • A low-valent transition metal complex reacting with XY to yield a product in which both the oxidation number and coordination number of the metal are increased undergoes oxidative addition. • The reverse process is reductive elimination. In catalytic processes, the eliminated molecule most often differs from the one added oxidatively. • In the following example, the complex is a coordinatively unsaturated, 16 electron complex. • IrI, d8, 16 electron IrIII, d6, 18 electron • 4 coordinate 6 coordinate

6. Factors Influencing Ox-Addition/Red-Elimination • The position of the oxidation/reduction equilibrium is dependent on: • nature of the metal and ligands • nature of XY and the M-X, M-Y bonds formed • reaction medium • Note: • Increased electron density about the metal centre favours oxidative addition • Higher oxidation states are usually more stable for the heavier metals (IrIII vs RhIII).

7. Substances Undergoing Oxidative Addition

8. Insertion Reactions • Reactions wherein a group is inserted into a metal-ligand bond are called insertions: • Note once again that insertion refers only to the result of the reaction, and the term carries no mechanistic implications. • Olefin insertion into a metal-hydride bond (hydrogenation):

9. Insertion Reactions: Transition States • Depending on whether the reaction is a 1,1-insertion or a 1,2-insertion, either a three-centre or a four-centre transition state can describe the process. • A three-centre transition state accounts for the insertion of CO into a metal-alkyl bond: • This is a key step in catalytic hydroformylation reactions. • A four-centre transition state accounts for olefin insertion into a metal-alkyl: • This is a propagation reaction in Ziegler-Natta polymerization.

10. Insertion or Migration? • Kinetic studies have generated a debate about whether the apparent insertion step is actually ligand insertion or migration of an alkyl group. • CO insertion into a metal-alkyl to generate a metal-acyl bond. • Complexes such as CH3Mn(CO)5, CpFe(CO)2CH3, and CpMo(CO)3CH3 • Although the mechanism is not important to our treatment of catalytic reactions, terminology is confusing. Reactions of this type are often called insertions, migrations, even migratory insertions.

11. Restricting-Assisting Insertion: Catalyst Design • Enantioselective Hydrogenation: • Stereoselective Olefin Polymerization:

12. b-Hydride Transfer - Alkene Elimination • The reverse reaction of insertion of an alkene into a metal-hydride bond is b-hydride elimination. • b-elimination reactions of metal alkyl complexes can occur if: • a vacant coordination position is available on the metal centre, and b-hydrogen is present • b-hydride elimination tends to make higher alkyl compounds unstable. The alkyls of most isolable complexes do not carry a labile b-hydrogen: (CH3)6W and Ti(CH2C6H5)4 . • This reaction is important in olefin polymerization processes, as the abstraction of a hydride from the alkyl to generate an olefin complex limits the length of the polyolefin chain.

13. Other Reactions: s-bond and p-bond Metathesis • s-bond metathesis involves exchange of M-X and Y-Z sigma bonds to give M-Z and X-Y (or M-Y and X-Z) through a concerted mechanism proceeding through a four-centre transition state. • Oxidation states are unchanged, making this reaction prevalent amongst early do metals, as opposed to the later transition metals that may prefer ox. addition-red. elimination. • p-bond metathesis involves metals containing M=C, M=N, or M=O functionality. It can be viewed as a [2+2] cycloaddition – retrocycloaddition, leading ultimately to substituent exchange.

14. Other Reactions: Transmetallation • Transmetallations are a class of s-bond metathesis reactions in which M-X and M’-Y swap substituents to give M-Y and M’-X. • Reactions of electronegative metals such as tin are not well understood, but underlie important catalytic substitution chemistry, such as in this Stille coupling example. • Transmetallations from electropositive Li and Mg to transition metal halides are consistent with SN2 displacements.