Organometallic Chemistry

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Transcript Organometallic Chemistry

CHEM261 INORGANIC 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: (AlMe 3 ) 2

As a Nucleophile

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

Substitution at sp 2 carbon (often via addition) R M O

+ R O M R M O

+ OR' R O M OR' - MOR' R O

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.

Phosphine complexes, e.g.

N 2 -complexes, e.g.

Exercise. Which of the following compounds is an organometallic compound? 2+ OCH 3 NH 3 a) CH 3 O Ti OCH 3 OCH 3 b) H 3 N Cu NH 3 NH 3 In general, metals in organometallic compounds include: Cl O c) Cl Pt CH 2 CH 2 d) O Pt O

• •

main group metals transition metals

Cl O

f-block metals

e) Li Me Me Li Me Li Me Li f) Ph OC OC Co O C OC OC Co P Ph P CO CO Co C O Co CO CO In this course, transition metals are our main concern.

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(C 2 H 4 )Cl 3 ] • H 2 O

Confirmed to have H 2 C=CH 2 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 M CO CS M C C M M M CNR NO M M N 2 PR 3 M H H M M H X M M M M M 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

Types of bonds possible from Ligands Language: All bonds are coordination or coordinative Remember that all of these bonds are weaker than normal organic bonds (they are dative bonds) Simple ligands e.g. CH 3 , Cl , H 2 give s bonds

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 weak and 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 H 4 { ns, 3

´

np } for main group elements 9 { ns, 3

´

np, 5

´

(n-1)d } for transition metals d xy s d xz p x d yz p y p z d x2-y2 d z2

• • •

Therefore, every element wants to be surrounded by 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 called

electron-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 d 8 complexes) Addition of 2e- from 5th ligand converts complex to 5 CN 18e- , marginally more stable

Predicting reactivity 14 e

dissociative

(C

2

H

4

)PdCl

2

- C

2

H

4

(C

2

H

4

)

2

PdCl

2

16 e

CO CO

?

(C

2

H

4

)(CO)PdCl

2

(C

2

H

4

)

2

(CO)PdCl

2

associative

- C

2

H

4

16 e 18 e Most likely associative

Predicting reactivity

- CO

16 e

dissociative

Cr(CO)

5

MeCN

18 e

Cr(CO)

6

MeCN

?

Cr(CO)

6

(MeCN)

associative

20 e (Sterics!)

Cr(CO)

5

(MeCN)

18 e

- CO

Most likely dissociative

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 ("PPh 3 is similar to NH 3 ").

Rewrite the structure properly before you start counting.

covalent bond Cl Pd PPh 3 dative bond 1 e "bond" to the allyl fragment

Cl Pd PPh 3

2 e Pd = Cl¾ = P® = allyl = e-count 10 1 2 3 + ¾¾ 16 3 e

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

from periodic table 2. 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 treatment 7. 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 will employ 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 practical examples 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 ZrCl 2 (C 5 H 5 ) 2 Zr(4) + [2 x Cl(1)] + [2 x C 5 H 5 (5)] =16 TaCl 2 Me 3 Ta(5) + [2+ x Cl(1)] + [3 x M(1)] =10 WMe 6 W(6) + [6 x Me(1)] =12 Pt(PPh 3 ) 3 Pt(10) + [3 x PPh 3 (2)] =16 IrCl(CO)(PPh 3 )2 Ir(9) + Cl(1) + CO(2) + [2 x PPh 3 (2)] =16 What features do these complexes possess?

• Early transition metals (Zr, Ta, W) • Several bulky ligands (PPh • σ-donor ligands (Me) 3 ) • Square planar d8 e.g. Pt(II), Ir(I)

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 harsh conditions (high P and T) – Ni + 4CO

Ni(CO) 4 – Fe + 5CO

Fe(CO) 5 Reductive carbonylation – Useful when very aggressive conditions would be required for direct reaction of metal and CO » Wide variety of reducing agents can be used – CrCl 3 + Al + 6CO

AlCl 3 + Cr(CO) 6 – 3Ru(acac) 3 + H 2 + 12CO

Ru 3 (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 EPR

• •

Not used much: GC LC

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 Phosphine Ligand PH 3 PF 3 P(OMe) 3 PMe 3 PMe 2 Ph PEt 3 PPh 3 PCy 3 P(t-Bu) 3 P(mesityl) 3 Cone Angle 87 o 104 o 107 o 118 o 122 o 132 o 145 o 170 o 182 o 212 o

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 on activity 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 Addition c. Reductive Elimination d. 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

M Br M + Br -

b) Oxidative Addition Basic reaction:

L

n

M + X Y X L

n

M Y • • • • •

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

One reaction multiple mechanisms

Concerted addition, mostly with non-polar X-Y bonds H 2 , silanes, alkanes, O 2 , ...

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

L

n

M + X Y L

n

M X Y

A

Intermediate A is a s-complex Reaction may stop here if metal-centered lone pairs are not readily available Final product expected to have cis X,Y groups

X L

n

M Y

Stepwise addition, with polar X-Y bonds

HX, R 3 SnX, acyl and allyl halides, ...

low-valent, electron-rich metal fragment (Ir I , Pd (0) , ...)

L

n

M X Y L

n

M X Y

B

X L

n

M Y

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

Cis or trans products depends on the mechanism H 2 H OC Et 3 P Ir Cl PEt 3 H

cis

Ir(III) OC Et 3 P Ir Ir(I) PEt 3 Cl HI H OC Et 3 P Ir Cl PEt 3 I

cis

Ir(III) CH 3 Br OC Et 3 P CH 3 PEt 3 Ir Cl Br

trans

Ir(III)

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 SiR 3 + alkyl etc

• •

Often slow for: alkoxide + alkyl halide + alkyl

thermodynamic reasons?

We will do a number of examples of this reaction

Relative rates of reductive elimination L L Pd CH 3 CH 3 + solv -L L solv Pd CH 3 CH 3 RE LPd(solv) + CH 3 CH 3 Complex Ph 3 P Pd Ph 3 P CH 3 CH 3 MePh 2 P Pd MePh 2 P CH 3 CH 3 Rate Constant (s -1 ) 1.04 x 10 9.62 x 10 -3 -5 T( o C) 60 60 Ph P Ph Pd Ph P Ph CH 3 CH 3 4.78 x 10 -7 Most crowded is the fastest reaction 80

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 CO M R M R M

1,1

O

1,2

R

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

M Me CO M O Me M Me SO 2

CO, isonitriles (RNC) and SO 2 often undergo 1,1-insertion

M S Me O O

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

Standard Cossee mechanism

M R M R M R M

Why do olefins polymerise?

Driving force: conversion of a p-bond into a s-bond One C=C bond: 150 kcal/mol Two C-C bonds: 2

´

85 = 170 kcal/mol Energy release: about 20 kcal per mole of monomer (independent of mechanism) Many polymerization mechanisms Radical (ethene, dienes, styrene, acrylates) Cationic (styrene, isobutene) Anionic (styrene, dienes, acrylates) Transition-metal catalyzed (a-olefins, dienes, styrene)

R

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 hydrogen Bond Alkyl groups with β hydrogen tend to undergo β elimination M -CH 2 -CH 3

M - H + CH 2 =CH 2 Two examples

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.

To prevent beta-elimination from taking place, one can use alkyls that: Do not contain beta-hydrogens Are oriented so that the beta position can not access the metal center Would give an unstable alkene as the 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) 2 I 2 ] 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