CHEM529 - Copy.pptx

Organometallic Chemistry

Mohammad S IqbalDepartment of ChemistryForman Christian College

Lahore

Course content

General properties of organometallic complexes, synthesis of organotransitiion metal complexes, -complexes, η2-bonded olefin complexes, acetylene complexes, η4- and η6- η7- and η8-bonded polyene complexes, η3-allylic complexes, η3-cyclopentadienyl and related complexes, η5-arene metal complexes, bonding in transition organometallic complexes, some common properties and reactions of transition metal organometallic complexes, application of transition metal complexes to catalysis, polymerization and oligomerization of olefins and dienes, synthesis with carbon monoxide, olefin metathesis, hydrogenation and related reactions, transition metal complexes with metal carbon multiple bonds, transition metal hydrides, supported transition metal catalysts, complexes of sigma and pi bonded ligands .

Organometallic Compounds

• Interface of inorganic chemistry and organic chemistry

• Chemical compounds containing direct metal-carbon bonds

• Compounds containing metal-element bonds having predominantly a covalent character

• Widely used in homogeneous catalysis

General Properties

• The metal-carbon bond in organometallic compounds has a character intermediate between ionic and covalent.

• Primarily ionic metal-carbon bonds are encountered either when the metal is very electropositive (as in the case of Group 1 or Group 2 metals) or when the carbon-containing ligand exists as a stable carbanion.

• Carbanions can be stabilized by resonance (as in the case of the aromatic cyclopentadienyl anion) or by the presence of electron-withdrawing substituents (as in the case of the triphenylmethyl anion).

General Properties

• Bonding in compounds like sodium acetylide and triphenylmethylpotassium is primarily ionic.

• The character of metal-carbon bonds in the organometallic compounds of transition meta ls less ionic

• Organometallic compounds with bonds that have characters in between ionic and covalent are very important in industry, as they are both relatively stable in solutions and relatively ionic to undergo reactions.

18 Electron Rule

Many stable organometallic

compounds obey this rule

Synthesis of carbonylate anions

• Direct reduction of a carbonylFe(CO)5 + Na/THF [Fe(CO)4]2-

+ CO

• Reductive cleavage of a metal-metal bond2Na + Mn2(CO)10 2NaMn(CO)5

• Disproportionation by use of a good donor3Co2(CO)8 + 12py 2[Co(py)6][Co(CO)4]2 + 8CO

• Reaction of bound CO with base3Fe(CO)5 + 4OH- [Fe3(CO)11]2- + CO32- + 2H2O +

3CO

Basicity of carbonylate anions

• Many carbonylate anions are capable of acting as Bronsted bases

[Mn(CO)5]- + H+ HMn(CO)5

• HCo(CO)4 has a similar acid strength to HCl

Nucleophilicity of carbonyl anions

Carbonylate anions act as good nucleophiles making them useful reagents for synthesis of a variety of organometallic species

[Mn(CO)5] + MeI MeMn(CO)5 + I

[Co(CO)4] + MeCOI Co(CO)4(MeCO) + I

[Mn(CO)5] + ReBr(CO)5 (CO)5Mn

Re(CO)5 + Br

Sandwich Compounds

Synthesis of FerrocenePauson and his student prepared it by the reaction:

2C5H5MgBr + FeCl2 C5H5FeC5H5 + MgBr2 + MgCl2

• The dicyclopentadiene dimer is thermally cracked into cyclopentadiene

• Place KOH and 1, 2- Dimethoxyethane in a flask• Stirred and purge with nitrogen• Add cyclopentadiene to the mixture under the

same conditions• The solution turns to reddish-brown color

• Add DMSO to FeCl2.H2O in a glove box and stirr for approximately five minutes

• Add FeCl2 solution drop-wise so that the entire solution in about thirty minutes

• A dark green solution thus obtained is maintained for thirty more minutes

• Add ice cold HCl and stop the nitrogen flow • Stirr the mixture for about fifteen minutes and

collect the orange precipitate Buchner funnel and then spread on a watch glass to dry.

Metallocenes

• Compounds with organic ring ligands sandwiching the metal

Synthesis of Cp complexes• NaCp is a good source of nucleophilic Cp

2NaCp + MnCl2 MnCp2 + 2NaCl• This approach works for Fe, Co and Ni

Ferrocene is very stable and the rings can bemodified by electrophilic substitution or by metallation to produce a nucleophile that can then be reacted witha wide range of electrophiles

• Rings in metallocenes as quite mobile. • There are very low barriers between different

conformers

Fluxionalproperties

Metal-Alkene Compounds

• As in metal-carbonyls, electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of the alkenes and alkynes.

• This electron-transfer strengthens the metal-ligand bond and weakens the C-C bonds within the ligand.

• Electronegative substituents exhibit greater backbonding. For example, tetrafluoroethylene, tetracyanoethylene, and hexafluoro-2-butyne are strong backbonding ligands.

Reactivity

• Early TM organometallics are oxophilic– tend to form compounds with O or some other

hard donor bound to the metal• Early TM organometallics tend to undergo C-H

bond cleavage reactions• The C-H bond activation is a potentially useful

property. Methane is a very abundant feedstock for such reactions

M – Alkenes• Alkenes can bind to a metal center and act as

both a σ donor and π acceptorOne of the earliest such complexes was Zeise’s salt

K2[PtCl4] + H2C=CH2 + SnCl2 K[PtCl3(η2-C2H4)] + KCl

Bond ing in M – Alkenes

σ-Donation from the π bonding orbital of alkene to the metal and π acceptor involving the π* orbital on the alkene

M – Allyl

• Virtually all transition metals can form η3-allyl complexes, but few are synthetically useful. Pd is most widely studied and has broad utility. Allyl complexes of Rh, Ir, Ru and Mo are becoming more important.

• These are usually quite stable but still reactive.

Interconversion of syn and anti is observed– mechanism involvesη3 η1 η3

Applications

• As catalysts• Synthesis of alkene-polymers. All the world's

polyethylene and polypropylene are produced via organometallic catalysts, usually heterogeneously via Ziegler-Natta catalysis.

• Acetic acid is produced via metal carbonyl catalysts in the Monsanto process and Cativa process.

• Most synthetic aldehydes are produced via hydroformylation.

Applications

• Most of larger alcohols are produced by hydrogenation of hydroformylation derived aldehydes.

• Wacker process is used in the oxidation of ethylene to acetaldehyde.

• Organolithium, organomagnesium, and organoaluminium compounds are highly basic and highly reducing. They catalyze many polymerization reactions.

Applications• Some semiconductors are produced from

trimethylgallium, trimethylindium, trimethylaluminium and related nitrogen / phosphorus / arsenic / antimony compounds. These volatile compounds are decomposed along on a heated substrate via metalorganic vapor phase epitaxy process for applications such as light emitting diodes (LEDs) fabrication.

• Organometallic compounds may be found in environment, such as organolead and organomercury compounds. They are highly toxic.

Polymerization of olefins – Polyolefins

• Polymer properties depend on –Molar ration of monomers–Equilibrium properties: temperature,

pressure–The linkage pattern is dictated by the

functional groups–The catalyst used

Ptterns of polymerization

Isotactic polymersIn isotactic macromolecules all the substituents are located on the same side of the macromolecular backbone. An isotactic macromolecule consists of 100% meso diads. Polypropylene formed by Ziegler-Natta catalysis is an isotactic polymer.

Syndiotactic polymersIn syndiotactic or syntactic macromolecules the substituents have alternate positions along the chain. The macromolecule consists 100% of racemo diads. Syndiotactic polystyrene, made by metallocene catalysis is an example for Syndiotactic polymer.

Atactic polymersAtactic macromolecules have substituents placed randomly along the chain.

Copolymers

Classes of olefin polymerization

There are four main classes of olefin polymerization catalysts, with widely varying selectivities

• Free radical• Anionic• Cationic• Insertion

Insertion polymerization

• Propene polymerization by successive olefin insertions into a metalalkyl bond

• A positively charged metal catalyst, which is connected to a chain end bearing a partial negative charge, binds and polarizes a monomer molecule.

• The coordination sphere of the metal controls the stereochemistry of olefin coordination and insertion, which enhances the stereoregularity of the chain growth.

Insertion Polymerization Catalysts• Heterogeneous

– Titanium-based catalysts. Discovered Karl Ziegler and applied by Giulio Natta to the stereospecific polymerization of propene (isotactic polypropylene)

– Chromium-containing catalysts, discovered by Paul Hogan and Robert Banks, usually called Phillips catalysts. The catalyst is prepared by adsorption of a chromium compound, mostly chromium trioxide, onto an amorphous silica support and subsequent reduction by exposure to ethene. They are used to produce high-density polyethylene (HDPE) with particularly high molar mass

• Homogeneous– Zirconium(IV) and titanium(IV) complexes,

mostly cyclopentadienyl complexes (either sandwich- or half-sandwich).

– Nickel(II) and palladium(II) complexes with nitrogen (imine) ligands.

– These catalysts give access to polyolefins with a wider choice of properties not easily accessible otherwise.

Stereoregularity in Ziegler-Natta Catalysts

Polymerization of dienes

• Synthetic RubberThe most important synthetic rubber is Neoprene which is produced by the polymerization of 2-chloro-1,3-butadiene

Olefin Metathesis

R2 R2

R1 R1Catalyst

R2

R1

R2

R1

RCM

ROMn n

n

ADMET ROMP

R1 R2 R2R1

CM

Ring Closing Metathesis(RCM)

Ring Opening Metathesis(ROM)

Acyclic Diene Metathesis(ADMET)

Ring Opening Metathesis Polymerization

(ROMP)

Cross Metathesis(CM)