CHEM261INORGANIC CHEMISTRY

Part 3

ORGANOMETALLIC CHEMISTRY

1. Introduction (types and rationale)

2. Molecular orbital (bonding) of CO, arrangement “in space” or ligand types (hapticity)

3. 16 and 18 electron rule (learning to count)

4. Synthesis, steric effects and reactivity - Wilkinsons catalyst (part 1)

5. Characterisation IR nmr etc.

6. Applications (oxidative addition b elimination)

What is organometallic chemistry?

Chemistry: structures, bonding and properties of molecules.

Organometallic compounds: containing direct metal-carbon bonds.

Either s or p bonds can occur

Main group:

(AlMe3)2

As a Nucleophile

Addition to polar C=X bonds(C=O, C=N, CºN)

Substitution at sp2 carbon(often via addition)

R MO+

O

R

M

R MO

OR'+

O

OR'R

M O

R- MOR'

Me W MeMe

MeMe

Me

OC FeCOCO

CO

CO

Fe

Cl Ru CF2OC

ClPPh3

PPh3

C

WRO OR

OR

Ph

PhMgBr

MeLi

Chemistry: structures, bonding and properties of molecules.

Transition metal compounds

Some compounds do not contain metal-carbon bond, but they are usually included in the field of organometallic chemistry. They include:

• Metal hydride complexes, e.g.

Et3P Pt PEt3

Cl

H

Ph3P Ru HH2

HPPh3

PPh3

• N2-complexes, e.g.

H3N Ru NH3N

NH3NH3

NH3

N

2+

Ph2P Mo PPh2Ph2P

PPh2N

N

N

N

• Phosphine complexes, e.g.

Ph3P Rh PPh3

Cl

PPh3

Ph3P RuCl

PPh3

PPh3Cl

In general, metals in organometallic compounds include: • main group metals• transition metals • f-block metals

In this course, transition metals are our main concern.

Exercise. Which of the following compounds is an organometallic compound?

a) OCH3

TiCH3O OCH3

OCH3

b)

NH3

CuH3N NH3

NH3

2+

Cl Pt

Cl

Clc) CH2

CH2

-

d)O Pt O

O

O

Me

Li

Li

Li

MeMe

Me

Lie) CoCo

Co

P

Co

P COCO

COCO

OC

OC

OCOC

C O

CO

Ph

Ph

f)

A brief history of organometallic chemistry

1) Organometallic Chemistry has really been around for millions of years

Naturally occurring Cobalimins contain Co—C bonds

Vitamin B12

2) Zeise’s Salt synthesized in 1827 = K[Pt(C2H4)Cl3] • H2O Confirmed to have H2C=CH2 as a ligand in 1868 Structure not fully known until 1975

3) Ni(CO)4 synthesized in 1890

4) Grignard Reagents (XMgR) synthesized about 1900

Accidentally produced while trying to make other compounds

Utility to Organic Synthesis recognized early on

5) Ferrocene synthesized in 1951 Modern Organometallic

Chemistry begins with this discovery (Paulson and Miller)

1952 Fischer and Wilkinson

Nobel -Prize Winners related to the area:

Victor Grignard and Paul Sabatier (1912)Grignard reagent

K. Ziegler, G. Natta (1963)

Zieglar-Natta catalyst

E. O. Fisher, G. Wilkinson (1973)

Sandwich compounds

K. B. Sharpless, R. Noyori (2001)

Hydrogenation and oxidation

Yves Chauvin, Robert H. Grubbs, Richard R. Schrock (2005) Metal-

catalyzed alkene metathesis

Common organometallic ligands

M H M C M CC M M

HH

MHX

M

M

M PR3

M CO M CNR

M CS M NO

M N2

M M MM

M

M C

M C

Why organometallic chemistry ? a). From practical point of view:

* OMC are useful for chemical synthesis, especially catalytic processes,

e.g. In production of fine chemicals

In production of chemicals in large-scale

reactions could not be achieved traditionally

OBn

OBn

NMo

Ar

RORO CMe2Ph

H+

CNI + NEt3PhPh+ CN+ HNEt3I

PhPh

Pd(PPh3)3

Types of bonds possible from Ligands

Language: All bonds are coordination or coordinative

Remember that all of these bonds are weaker than normal organicbonds (they are dative bonds)

Simple ligands e.g. CH3-, Cl-, H2 give s bonds

p systems are different e.g. CO is a s donor and p acceptor

Bridging ligands can occur two metals

Metal-metal bonds occur and are called d bonds – they are weakand are a result of d-d orbital overlap

18 Electron Rule (Sidgwick, 1927)• OM chemistry gives rise to many “stable” complexes - how can we

tell by a simple method• Every element has a certain number of valence orbitals:

1 { 1s } for H4 { ns, 3´np } for main group elements9 { ns, 3´np, 5´(n-1)d } for transition metals

pxs py pz

dxzdxy dx2-y2dyz dz2

• Therefore, every element wants to be surroundedby 2/8/18 electrons– For main-group metals (8-e), this leads to the standard Lewis structure

rules– For transition metals, we get the 18-electron rule

• Structures which have this preferred count are calledelectron-precise

• Every orbital wants to be “used", i.e. contribute to binding an electron pair

The strength of the preference for electron-precise structures depends on the position of the element in the periodic table

• For early transition metals, 18-e is often unattainable for steric reasons - the required number of ligands would not fit

• For later transition metals, 16-e is often quite stable (square-planar d8 complexes)

• Addition of 2e- from 5th ligand converts complex to 5 CN 18e- , marginally more stable

Predicting reactivity

(C2H4)2PdCl2 (C2H4)(CO)PdCl2

(C2H4)PdCl2

(C2H4)2(CO)PdCl2

?CO- C2H4

- C2H4CO

dissociative

associative

Most likely associative

16 e

18 e

16 e

14 e

Cr(CO)6 Cr(CO)5(MeCN)

Cr(CO)5

Cr(CO)6(MeCN)

?MeCN- CO

- COMeCN

dissociative

associative

Predicting reactivity

Most likely dissociative

16 e

18 e 18 e

20 e (Sterics!)

N.B. How do you know a fragment forms a covalent or a dative bond?

• Chemists are "sloppy" in writing structures. A "line" can mean a covalent bond, a dative bond, recognise/understand the bonding first

• Use analogies ("PPh3 is similar to NH3").• Rewrite the structure properly before you start counting.

Pd = 10Cl¾ = 1P® = 2allyl = 3

+ ¾¾e-count 16

Cl

Pd

PPh3

covalentbond

dativebond

"bond" to theallyl fragment

Cl

Pd

PPh31 e 2 e

3 e

"Covalent" count: (ionic method also useful)1. Number of valence electrons of central atom.

• from periodic table2. Correct for charge, if any

• but only if the charge belongs to that atom!3. Count 1 e for every covalent bond to another atom.4. Count 2 e for every dative bond from another atom.

• no electrons for dative bonds to another atom!5. Delocalized carbon fragments: usually 1 e per C (hapticity)6. Three- and four-center bonds need special treatment7. Add everything

N.B. Covalent Model: 18 = (# metal electrons + # ligand electrons) - complex charge

The number of metal electrons equals it's row number (i.e., Ti = 4e, Cr = 6 e, Ni = 10 e)

Hapto (h) Number (hapticity)

For some molecules the molecular formula provides insufficient information with which to classify the metal carbon interactions

The hapto number (h) gives the number of carbon (conjugated) atoms bound to the metal

It normally, but not necessarily, gives the number of electrons contributed by the ligand

We will describe to methods of counting electrons but we willemploy only one for the duration of this module

The two methods compared: some examples

N.B. like oxidation state assignments, electron counting is a formalism and does not necessarily reflect the distribution of electrons in the molecule – useful though

Some ligands donate the same number of electrons

Number of d-electrons and donation of the other ligands can differ

Now we will look at practicalexamples on the black board

Does it look reasonable ?

Remember when counting:

Odd electron counts are rare

In reactions you nearly always go from even to even (or odd to odd), and from n to n-2, n or n+2.

Electrons don’t just “appear” or “disappear”

The optimal count is 2/8/18 e. 16-e also occurs frequently, other counts are much more rare.

Exceptions to the 18 Electron Rule

ZrCl2(C5H5)2 Zr(4) + [2 x Cl(1)] + [2 x C5H5(5)] =16

TaCl2Me3 Ta(5) + [2+ x Cl(1)] + [3 x M(1)] =10

WMe6 W(6) + [6 x Me(1)] =12

Pt(PPh3)3 Pt(10) + [3 x PPh3(2)] =16

IrCl(CO)(PPh3)2 Ir(9) + Cl(1) + CO(2) + [2 x PPh3(2)] =16

What features do these complexes possess?

• Early transition metals (Zr, Ta, W)• Several bulky ligands (PPh3)• Square planar d8 e.g. Pt(II), Ir(I)• σ-donor ligands (Me)

Syntheses of metal carbonyls

Metal carbonyls can be made in a variety of ways.

For Ni and Fe, the homoleptic or binary metal carbonyls can be made by the direct interaction with the metal (Equation 1). In other cases, a reduction of a metal precursor in the presence of CO (or using CO as the reductant) is used (Equations 2-3).

Carbon monoxide also reacts with various metal complexes, most typically filling a vacant coordination site (Equation 4) or performing a ligand substitution reactions (Equation 5)

Occasionally, CO ligands are derived from the reaction of a coordinated ligand through a deinsertion reaction (Equation 6)

Synthesis of carbonyl complexes

Direct reaction of the metal

– Not practical for all metals due to need for harshconditions (high P and T)

– Ni + 4CO Ni(CO)4

– Fe + 5CO Fe(CO)5

Reductive carbonylation– Useful when very aggressive conditions would berequired for direct reaction of metal and CO

» Wide variety of reducing agents can be used– CrCl3+ Al + 6CO AlCl3 + Cr(CO)6

– 3Ru(acac)3 + H2 + 12CO Ru3(CO)12 +

N.B. From the carbonyl complex we can synthesize other derivatives

Main characterization methods:

• X-ray diffraction (static) structure bonding• NMR structure en dynamic behaviour• EA assessment of purity• (calculations)Useful on occasion:• IR• MS• EPRNot used much:• GC• LC

Phosphine Ligand

Cone Angle

PH3 87o

PF3 104o

P(OMe)3 107o

PMe3 118o

PMe2Ph 122o

PEt3 132o

PPh3 145o

PCy3 170o

P(t-Bu)3 182o

P(mesityl)3 212o

Phosphine ligands are important Cone Angle (Tolman)

Steric hindrance:

A cone angle of 180 degrees -effectively protects (or covers) one half of the coordination sphere of the metal complex

                                                         

               

You would expect a dissociation event to occur first before any other reaction - steric bulk (rate is first order - increasing size)

This will also have an effect onactivity for catalysts

N.B. “flat” can slide past each other

For example Wilkinson's catalyst(more later)

Has a profound effect on the reactivity!

Reaction chemistry of complexes

Three general forms:

1. Reactions involving the gain and loss of ligands a. Ligand Dissoc. and Assoc. (Bala)b. Oxidative Additionc. Reductive Eliminationd. Nucleophillic displacement

2. Reactions involving modifications of the ligand a. Insertion

b. Carbonyl insertion (alkyl migration) c. Hydride elimination (equilibrium)

3. Catalytic processes by the complexes Wilkinson, Monsanto

Carbon-carbon bond formation (Heck etc.)

a) Ligand dissociation/association (Bala)

• Electron count changes by -/+ 2

• No change in oxidation state

• Dissociation easiest if ligand stable on its own(CO, olefin, phosphine, Cl-, ...)

• Steric factors important

MBr

+ Br-M

b) Oxidative Addition

Basic reaction:

• Electron count changes by +/- 2(assuming the reactant was not yet coordinated)

• Oxidation state changes by +/- 2• Mechanism may be complicated The new M-X and M-Y bonds are

formed using:• the electron pair of the X-Y bond• one metal-centered lone pair

LnM +X

YLnM

X

Y

One reaction multiple mechanisms

Concerted addition, mostly with non-polar X-Y bondsH2, silanes, alkanes, O2, ...

Arene C-H bonds more reactive than alkane C-H bonds (!)

Intermediate A is a s-complex

Reaction may stop here if metal-centered lone pairsare not readily available

Final product expected to have cis X,Y groups

X

YLnM

X

YLnM + LnM

X

YA

Stepwise addition, with polar X-Y bonds– HX, R3SnX, acyl and allyl halides, ...

– low-valent, electron-rich metal fragment (IrI, Pd(0), ...)

Metal initially acts as nucleophile

– Coordinative unsaturation less important

Ionic intermediate (B)

Final geometry (cis or trans) not easy to predict

Radical mechanism is also possible

X YLnM

B

LnM X Y LnMX

Y

OC Ir ClPEt3

Et3P

OC Ir H

PEt3

Et3P

H

Cl

OC Ir I

PEt3

Et3P

H

Cl

OC Ir Cl

PEt3

Et3P

CH3

Br

Ir(I)

Ir(III)

Ir(III)

Ir(III)

H2

cis

cis

trans

HI

CH3Br

Cis or trans products depends on the mechanism

c) Reductive elimination

This is the reverse of oxidative addition - Expect cis elimination

Rate depends strongly on types of groups to be eliminated.

Usually easy for:• H + alkyl / aryl / acyl

– H 1s orbital shape, c.f. insertion

• alkyl + acyl

– participation of acyl p-system• SiR3 + alkyl etc

Often slow for:• alkoxide + alkyl• halide + alkyl

– thermodynamic reasons?

We will do a number of examples of this reaction

Complex Rate Constant (s-1) T(oC)

PdCH3Ph3P

Ph3P CH3

PdCH3MePh2P

MePh2P CH3

PdCH3P

P CH3

PhPh

PhPh

1.04 x 10-3 60

60

80

9.62 x 10-5

4.78 x 10-7

Relative rates of reductive elimination

Most crowded is the fastest reaction

PdCH3L

L CH3

+ solv

-L

PdCH3L

solv CH3

RELPd(solv) + CH3 CH3

Modifications of the ligand

a) Insertion reactions

Migratory insertion!

The ligands involved must be cis - Electron count changes by -/+ 2

No change in oxidation state

If at a metal centre you have a s-bound group (hydride, alkyl, aryl)

a ligand containing a p-system (olefin, alkyne, CO) the s-bound

group can migrate to the p-system

1. CO, RNC (isonitriles): 1,1-insertion

2. Olefins: 1,2-insertion, b-elimination

M

R

MR

MR

COM

O

R

1,1 1,2

1,1 Insertion

The s-bound group migrates to the p-system

if you only see the result, it looks like the p-system has inserted into the M-X bond, hence the name insertion

To emphasize that it is actually (mostly) the X group that moves, we use the term migratory insertion (Both possible tutorial)

The reverse of insertion is called elimination

Insertion reduces the electron count, elimination increases it

Neither insertion nor elimination causes a change in oxidation state

a- elimination can release the “new” substrate or compound

In a 1,1-insertion, metal and X group "move" to the same atom of the inserting substrate.

The metal-bound substrate atom increases its valence

CO, isonitriles (RNC) and SO2 often undergo 1,1-insertion

1,2 insertion (olefins)

Insertion of an olefin in a metal-alkyl bond produces a new alkyl

Thus, the reaction leads to oligomers or polymers of the olefin

• polyethene (polythene)• polypropene

MMe

SO2

MS Me

O OM

Me

CO

MMe

O

MR

MR

M

R

MR

Standard Cossee mechanism

Why do olefins polymerise?

Driving force: conversion of a p-bond into a s-bondOne C=C bond: 150 kcal/molTwo C-C bonds: 2´85 = 170 kcal/molEnergy release: about 20 kcal per mole of monomer(independent of mechanism)

Many polymerization mechanismsRadical (ethene, dienes, styrene, acrylates)Cationic (styrene, isobutene)Anionic (styrene, dienes, acrylates)Transition-metal catalyzed (a-olefins, dienes, styrene)

Two examples

b Hydride elimination (usually by b hydrogens)

Many transition metal alkyls are unstable (the reverse of insertion)the metal carbon bond is weak compared to a metal hydrogenBond Alkyl groups with β hydrogen tend to undergo β elimination

M -CH2-CH3 M - H + CH2=CH2

To prevent beta-elimination from taking place, one can use alkyls that:

Do not contain beta-hydrogensAre oriented so that the beta position can not access the metal centerWould give an unstable alkene as the product

A four-center transition state in which the hydride is transferred to the metal An important prerequisite for beta-hydride elimination is the presence of an open coordination site on the metal complex - no open site is available - displace a ligand metal complex will usually have less than 18 electrons, otherwise a 20 electron olefin-hydride would be the immediate product.

Catalysis (homogeneous)Reduction of alkenes etc.

The size of the substrate has an effect on the rate of reaction

Same reaction different catalyst

Alternative starting material

The Monsanto acetic acid process

Methanol - reacted with carbon monoxide in the presence of a catalyst to afford acetic acid

Insertion of carbon monoxide into the C-O bond of methanol

The catalyst system - iodide and rhodium

Iodide promotes the conversion of methanol to methyl iodide, Methyl iodide - the catalytic cycle begins:

1. Oxidative addition of methyl iodide to [Rh(CO)2I2]-

2. Coordination and insertion of CO - intermediate 18-electron acyl complex 3. Can then undergo reductive elimination to yield acetyl iodide and regenerate

our catalyst

Catvia Process

Wacker process (identify the steps)

Identify the steps