Core Concepts of Organometallic Chemistry
Organometallic chemistry refers to the use of a metal bond to a carbon atom. This article covers the different metals and complexes involved in organometallic chemistry and their uses.
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Introduction to Organometallic Chemistry
Organometallic chemistry, as its name suggests, involves using a metal reagent or catalyst with an organic molecule. If the metal works as a catalyst, than it is returned to the same form that it started at by the end of the reaction.
Not all the metal reagents are catalysts however. For example, the Grignard reagent attaches alkyl groups but produces a magnesium salt. To create a Grignard reagent, magnesium inserts itself between a carbon and a halide.
An organometallic complex refers to a metal atom bonded to one or more organic groups. For example, palladium is a common metal in catalysis because it can create carbon-carbon bonds effectively. Triphenyl phosphorous, PPh3, is an organic compound that often forms a complex with palladium.
Metals Reagents in Organometallic Chemistry
Reagents, unlike catalysts, are consumed throughout a reaction and cannot be reused. There are many important metal reagents. Below some of the most important metalorganic reagents are discussed.
A Grignard reaction forms a secondary or tertiary alcohol from a ketone of aldehyde. The Grignard reagent is a halogen connected to magnesium(Mg) that also has an alkyl or aryl attached to it. The Grignard reagent is also an organometallic compound.
The C-Mg bond is polar, with a slight negative charge on carbon. As a result, the carbon can act as a nucleophile and form new bonds.
When a Grignard reagent attaches an R group to a carbonyl carbon, two electrons from the double bond to oxygen are pushed onto oxygen as lone pairs. After the R group adds, the lone pairs can reform the double bond and kick out a leaving group. The Grignard reagent then adds another R group, pushing a third lone pair back onto oxygen. In acid, this oxygen takes on a proton to form an -OH group.
If no leaving group is present, only one R group adds and an acidic workup step adds a proton to the oxygen.
Organolithium reagents work the same as Grignard reagents. The only difference is that two equivalents of Lithium are required. One stays with the alkyl halide’s R group, and the other forms a salt with the halide.
Tin is useful for reducing alkyl halides because of its relative stability as a radical. Tributylstannane is a tin compound with three butyl groups and a Hydrogen atom attached to a central tin atom. In the presence of light, a radical initiator, and an alkyl halide, tributylstannane replaces the halide and replaces it with a hydrogen atom.
Metals as Catalysts
Metal catalysts work in cycles rather than traditional mechanisms. Rather than being consumed at the end of a reaction, metal catalysts return to their condition at the beginning of the reaction. This is the definition of a catalyst.
Hydration is the process of adding H2 across a double or triple bond to make it a single or double bond. There are multiple ways to do this, all of which involve metal catalysts. The purpose of the catalyst is to break apart the hydrogen molecule and add a hydrogen atom to both sides of the bond.
Palladium, platinum, or nickel catalysts work for this reaction. To function, the metal is deposited onto carbon. These catalysts are so effective that they hydrogenate alkynes all the way to alkanes.
Fundamental Catalytic Reactions of Ligands
Ligands refer to groups bound to the metal. Together, the ligands and metal are called a complex. There are two types of ligands: L-type and X-type. L-type ligands do not have charges when they dissociate from the metal, whereas X-type ligands usually receive a negative charge after dissociation.
Rather than the metal and the ligand each providing an electron, electrons in ligand-metal bonds come entirely from the ligand.
In addition, all catalytic cycles involve the same series of fundamental steps.
Catalytic Steps in Organometallic Chemistry
- Ligand Dissociation: This is when a ligand takes the two electrons from its bond to the metal and leaves. It lowers the number of electrons in the catalytic complex by two but does not change the metal’s oxidation state.
- Ligand Association: This is when a ligan donates two electrons to a complex to form a bond. As with ligand dissociation, this does not change the oxidation state of the metal.
- Ligand substitution: This describes one ligand dissociating followed by another ligand associating. Often, L-type ligands will replace other L-type ligands. The same is true for X-type ligands. Transmetallation is a type of substitution that involves ligands moving to and from the metal catalyst and a second metal.
- Oxidative Addition: This is another method of adding ligands to a metal. It raises the electron count of the complex by 2.
- Reductive Elimination: This is the opposite of oxidative addition. The electron count of the metal decreases by 2.
- Migratory Insertion: This step involves a ligand inserting itself in between an existing metal-ligand bond. Additionally, this step must occur as a syn addition. This means that the ligand and the metal must join on the same face.
- ß-hydride elimination: This is the reverse of migratory insertion. The group that is beta to the metal moves to the metal.
Cross-Coupling Reactions Using Organometallic Chemistry
One of the major uses of metal catalysts is in cross-coupling reactions. Cross-Coupling is the process of forming a carbon-carbon single bond with the help of a catalyst. Transition metals work well as catalysts for these reactions. This type of reaction involves a metal catalyst attaching an electrophilic compound and nucleophilic compound together. Furthermore, there are multiple kinds of cross-coupling reactions. The type is determined by the nucleophile.
Additionally, many cross-coupling reactions are named after the scientist who developed them. For example, the Suzuki coupling reaction involves a boronic acid and an alkyl halide. The catalyst attaches the R groups from both compounds to each other.
Some cross-coupling reactions include the Heck reaction, Suzuki reaction, and the Negishi reaction. There are many more named cross-coupling reactions than we can list here.