Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Organometallics - Reactivity




Outline & simplify some simple general methods for the preparation of:
i)
ii)
iii)
iv)

metal carbonyl compounds
bis(cyclopentadienyl) metal compounds
metal-hydrogen bonds
metal-carbon single and multiple bonds



i) Metal carbonyl compounds
– They can be synthesized in 5 different ways:
1
...
The reduction of metal salts and oxides, as in the presence of
high-pressure CO(g), metal halides can be reduced with a reducing
agent such as copper, aluminium or hydrogen
...
Photolysis and thermolysis of mononuclear carbonyls generates bi- and multimetallic
Ligand substitution is another apparently straightforward process
...
On further heating this will eventually decompose into the metal and carbon
change in VE count, OS or d electron count
...

+
L
[M]
L
+
L'
[M]
L'

4
...


p
Valence electron count
p
5
...
Anionic
n
d-electron count
n
metal carbonylates can be obtained via reduction of dinuclear
complexes with sodium
...
The
be prepared from the carbonyl halides under pressurized CO(g)below uses mild photolytic methods to promote CO loss from 18 VE Fe(CO5)
...

reaction goes through a 16 VE Fe(CO)4 intermediate (dissociative mechanism) and not a

2
Fe(CO)5( -C8H14) 20 VE species (associative mechanism)
...
Apparently the rea
1
...

ligands
...
Substitution in
18 VE
16 VE
18 VE
17-electron complexes occurs via associative
mechanisms, with 19-electron intermediates
...
Reduction – the metal carbonyls react with reducing agents,
CO
such as Na(s) to give carbonylmetalate anions
...
Mercury can insert into the M-M bonds of some OC
CO
polynuclear metal carbonyls
...
Nucleophilic attack at CO – the CO ligands can be
Interestingly, associative substitution reactions starting from 4- or 5-coordinate coordinatively
Bonding in transition metal carbonyl complexes
converted to CO2 and CN ligands
saturated (18 VE) compounds areof   a   “ provided that a and,   as   mentioned,   avoids aspecial   pos
CO   is   an   example   possible acid”   ligand   mechanism exists that has   a   20 VE

intermediate
...
With electrophiles, as even though metal carbonyls are in low oxidation states, and transition metal chemistry in general
...

they are relatively unreactive towards many electrophiles
...
The basis orbitals for CO are taken as th
special bonding features
CO
L
alkylating agents, mild acids and mild oxidizing agents
...
highest occupied MO (HOMO) an
CO
likely to interact with the metal, namely are the

...
O MOs (LUMOs)
...

N
OC
OC
OC

year
...
So it is unsurprising that CO coordinates exclusively as a C-

18 VE, 3e donor
18 VE, 3e donor
18 VE, 1e donor
Perhaps unexpectedly, the HOMO NO CO is generally considered to be slightly a
linear NO
linear NO
bent of

nature
...
e
...
Using a metal carbonyl and cyclopentadiene, with a hydride complex appearing as intermediate
...










2
...
There is another, unusual method, where Hg(II) acts as oxidizing
agent
...
Using metal carbonyl halide and cyclopentadienyl reagent






4
...
Using metallocene and CO(g)














6
...
Reduction to cleave the metal bond and
form M-Na+ complexes






2
...
Ring substitution – CpV(CO)4, CpMn(CO)3 and CpRe(CO)3 readily undergo electrophilic substitution

4
...
Oxidative decarbonylation under vigorous conditions forms a non-bridged piano stool complex
...
Protonation, governed by the Lewis basicity of the central metal






2
...
Adduct formation, when moieties associate through hydride bridges
...



only as the dihydrogen compound (presumably because Re(+1) is more resistant to oxidative
M( 2-H2)(H) neutral the heaviest Group 8 element Os forms the Os(+4) classical tetrahydride
addition than 2(PR3)3, W(0))
...
  Kubas’s   compound   in   fact   exists   as   an   equilibrium   between   a  
Os(H)4(PR3)3
...
However, the cationic (but isoelectronic) rhenium3 analogue exists
Re
whereas: OC
OC
W
go through an unfavourable 20-electon intermediate
...
Ring slips are common in late transition metal complexes
...


iii) Metal-hydrogen bonds
W(0), dihydrogen
PR3

Re(+1),PiPr
dihydrogen
3

W(+2), dihydride
PR
3

Kubas’ compound shows that metal

(exclusively)
complexes can have a dihydride and a
CO H
(a) Alkylation of metal halides (transmetallation)
...

Re
whereas: OC
OC
method
...
Here Cy, rate ofis PR
the
protonation of the AlR3 ≥   ZnR2 as the 3
and zinc reagents
...

the dihydride
metal-carbonthe kinetic product which then undergoes oxidative addition forming dihydrogen
Re(+1),
W(0), dihydrogen
W(+2), dihydride



Finally we note that dihydrogen2compounds can act as intermediates on the pathway to classic
RLi
In addition, dihydrogen compounds
Example of a
R
dihydrides as shown below
...

H
P
P
H
P
terminal Fe–H (hydride) is fast so the
Ph 2
Ph2
Ph 2
2
CH2Ph
η -H2 is the kinetic product which then
The following example subsequently converts the first-formed Co-MeFe(+4), Co-acylby migratory
to Stabilised via
a dihydride
Fe(+2), dihydrogen
Fe(+2), hydride
undergoes oxidative addition forming
4 PhCH2MgCl
+
Kinetic product PhCH a M
Thermodynamic product
M--C
insertion Ph2 MCl 4(THF)2 Hof the iodide anion (avoiding 2 20 VE intermediate)
...

CH2 PhP 2 Fe
Fe
Fe
H
P
P
H
interactions

- 80 oC M = Zr, Hf
- 40 o
H
PhCH2C
H
P
P
H
P
iv) Metal-carbon single and multiple bonds
Ph 2
Ph2
Ph 2
WClMetal-alkane synthesis
too oxidising
used dihydride
6 is Fe(+2), hydride to use MeLi or Grignards so the milder AlMe 3 isFe(+4), (and gives a surprising
Fe(+2), dihydrogen
A wide range of methods are known for their synthesis – although this choice is dependent on the metal involve (eg
...
This is by far the most versatile synthetic
Kinetic product
Thermodynamic product
product)
...

can’t use oxidative addition starting from d materials)
...
The usual reagents are alkyl lithiums and Grignard reagents followed by alkyl aluminium
1
...
Uses alkyl lithiums & Grignard reagents,
Me
and zinc reagents
...
The alkylating power is RLi > RMgX > AlR3 ≥ ZnR2 as the covalency of the
3h
Me
6 AlMe
metal-carbon bond increases
...
W
and not Oh
...
] 2- and MoMe6 example is known as
[ZrMe6 This particular
- AlMe2Cl
Me
Me

are analogous
...
Although oxidative
2 generally cyclometallation) and is a Example of a
Me

R
Cp2 of
M
additionMCl2C–H bonds to metals is still a relatively rare "bent metallocene"
reaction, this intramolecular process is

M Ti, Zr, made
...

product is bimolecular processes of the type
entropically neutral and so not

R = alkyl, aryl

A+B→C
Li

CH3P
Li
R 2Ph
Example of
Cl
PPh3
Pt Stabilised by

PtCl2(PR3)24 PhCH MgCl
a metallacycle
2
MR 3P
MCl (THF)2
M--Cipso
Ph3 P
- HCl
PhCH2
Ph3P
H
Ph3P 4
Cl
Ir CH2 Ph
Ir
Ir
interactions

Zr, Hf
PPh 2
Ph3P
PPh 2
PhCH2
PPh 2M = oxidative
Ph3P
reductive

(b) Oxidative addition is a useful method for more electron-rich later metals with (normally) VE
addition
elimination
H
50
ortho
less oxidising to use MeLi or Grignards so the milder AlMe3 also carbolylate anions)
...
Oxidative addition, used for electron-rich later metals with less than 18 valence electrons
...
MO theory explains this as a 2nd order Jahn-Teller distortion
...
This method is I
CH2
Oh
...
 “Ate”  
[ZrMe6] 2-3 attack at 6
addition
- AlMe2Cl
Me
16 VE, Pt(0)
18 VE, Pt(+2)
16 VE, Pt(+2)
Me
are analogous
...
carbonate, sulfate)
...
From a carbonylate anion and an alkyl halide, and is an example of electrophilic attack at an electon-rich metal centre
...

n
The negative charge and high d electron count is stabilized by the π-acceptor CO ligands
Chelating dialkyls can also be made
...


Me
Me-Cl

Li
CO
L
CO
Li L
R3P
Ir
Example of
Ir

Pt
PtCl2(PR3)2
Br metallacycle L
L
Br
a

trans oxidative
R 3P
Cl

addition
18 VE, Ir(+3)
16 VE, Ir(+1)

(b) Oxidative addition is a useful method for more electron-rich later metals with (normally) VE
[Cp*Fe(dppe)(H)2]+
...


29

PR 3
R 3P
R 3P

Pt

CH2
CH2

Me-I

Me

cis oxidative

I

Pt

CH2
CH2

- C2 H4

Me
I

Pt

PR 3
PR 3


...
3RP yb deppart si seiceps e

ot tcepser htiw( elbatsnu na elpmaxe gniwollof eht nI
...
The following is

Here is an example of ethyl to hydride conversion
...


(c) Reductive elimination reactions
...
The

-H elimination reactions
...
process
...
The rate of reaction is reductive PR3 which
(c) Reductive elimination reactions
...
The
2
...
6

ation reactions
...
Because these types ofis compoundsofcannot addition
...

-H elimination reactions
...


of ethyl to hydride conversion
...
The rate of reaction is inhibited by PR3 which

reactions
...


era esehT
...
Several examples of thisare known for almost all instances
...

compounds
Organometallics - Reactivity not an exhaustive list and numerous examples have already been mentioned

have already seen, metal-alkyl compounds have a wide rangeThe reactions
...
of rate of reaction inhibited is
As you
(a) -Elimination
Where
cannot
Reactions of metal alkyl of the reactions
...
Hydridescompounds 6 to before 10-eliminationmetals in occur (see above) -elimination
group

the intermediate CpFe(CO)Bu
-Htransition
elimination
(b) -H eliminationnumeroussometimes are known for almostisall been mentioned above
...
Several examples of The product
not an exhaustive list andcan trap backexamplestake over
...
occurs
...
The first example is formation of of reactions
...
The of the typeknownis inhibitedall instances
...
These are
of ethyl to hydride and numerous examples are (L)M=CRalmost bothPR3M–C
rate of reaction for 2 with by a which

(formally CR ) species
(a) insertion of an alkene into a M–H bond
...
Where
can trap back the intermediate CpFe(CO)Bu before detailelimination occurs
...

as hydrometallation (i
...
The -Elimination across C=C carbene (neutral CR2) or Schrock alkylidene
addition of M–H
4
...
Alkyl forms via
sometimes take over
...
Where -elimination cannotaddition (see above) -elimination
Fischer – cf hydrogenation: H–H occur across
Mo-Me) cationic methylene-hydride species is trapped by PR3
...
The product is a Fischer carbene (neutral CR2) or Schrock alkylidene
2C=C)
...
type2-(L)M=CRthewiththis haveMo which been mentioned above
...

(b) -H 2 ) species of the CR-CH2PR3 in 2 end and not already remains Mo(+4)
...
These are
discussed is an example of ethyl to hydride conversion
...
have in following In the is inhibited following unstable (with to
later a the course
...
The by PR is
As you have already seen, metal-alkyl compounds

Mo-Me) cationic methylene-hydride by PR Note
Mo-Me) exhaustivethe intermediate CpFe(CO)Buareis trappedspeciesall3
...
PR3
...
by

+
+
-CH2PR in the end and not
-CH2PR-Elimination reactions
...
occur Mo(+4)
...
The product is a Fischer carbene (neutral CR2) or Schrock alkylidene

(formally CR22-) species of the type (L)M=CR2 with both a reactions
...

and component
...
Diazomethane, CH2N2 is a
Here is an following ethyl to an unstable (with respect to
discussed in more detail later in the course
...
The rate of reaction is inhibited by PR3 which
Metal-alkane reactions
source of CH2 but should be handled with extreme care in non-ground glass jointed equipment
...
α-elimination – where β-elimination cannot occur, α-elimination can takeover, giving a Fischer carbine (neutral CR2) or a
Mo-Me) cationic methylene-hydride species is trapped by PR3
...

2+
Schrock alkylidene (CR(b)) of the type (L)M=CR2, with both a M-C σ and π component
...

2
-CH2PR3 in the end and not Mo-H elimination reactions
...


)evoba ees( rucco tonnac noitanimileto
to hydride conversion
...
The overall reaction is shown generally below for H2 and  alkanes  R΄–H

are best established for d0 transition metal compounds

trapped to give the final ML4 d8 square planar products
...
By way of example,

6

eactions
...
The

ed from M–R + CO2)
...


(e)

rmediate CpFe(CO)Bu before -H elimination occurs
...
must be cis reductive elimination from d ML6 compounds proceeds via
groups to be eliminated Interestingly to each other for this concerted process
...


the rhodium complex shown below undergoes reductive elimination of acetone or benzophenone at

6
RT
...
The
(c) Reductive elimination reactions
...


(d) Migratory Insertion reactions
...
You have already seen an a
(d) Migratory reaction with CO
groups to be eliminated must be cis to each other for this concerted process
...
Many substrates insert into M–R bonds
...
Many
the rhodium complex shown below undergoes carbodiimideelimination of acetone or forming an 2-amidinate+ligand (the N substrates insert into M–R bonds
reductive inserting into a Ti–Me bond benzophenone 5at
below involves a seen an alkyl insertion reaction with CO
(d) Migratory Insertion reactions
...
As, mentioned, this is the reverse of oxidative addition
...
Interestingly Many substrates insert into d bonds
...

equivalent of carboxylate compounds proceeds via CO
3
...
The groups to be eliminated must be cis to each other,
(p22, Mn(CO)5Me + CO)
...
By way of example,
groups to be eliminated must be cis to M–R other equivalent of carboxylate which can be formed from M–R + CO )
...
trappedTi–Me bond final ML4 shown below undergoes(the N elimination of acetone or benzophenone at
intermediates which are then into a to give the forming an 8 square planar products
...
Interestingly
equivalent of carboxylate which can be formed from M–R + CO2)
...
As, mentioned, this is the reverse 8of oxidative addition
...


groups to be eliminated must be cis to each other for this concerted process
...
or
(c) Reductive elimination reactions
...
already seen an alkyl elimination from d CO
groups to be have Interestingly cis to each other reaction with ML6 compounds proceeds via ML
(d) Migratory Insertion reactions
...
By way of example, 5 and then ML3

8
13
intermediates which are then trapped to elimination of
(p22, Mn(CO)5Me + CO)
...

Many substrates insert into M–R bonds
...
Interestingly Ti–Me bond forming from2 d6 ML6 compounds N
below involves a carbodiimide inserting into areductive elimination an -amidinate ligand (theproceeds via ML5 and then ML3

8
intermediates which are then As, 2)
...
trapped to give this is ML4 d square planar products
...
The
4
...
By way of example,

the rhodium complex shown below undergoes reductive elimination of acetone or benzophenone at
6

RT
...


33


31

33

0
(e) Bond metathesis reactions
...
Several examples of this have already been mentioned above
...
Because these types ofmetathesis reactions
...


such as WMe6 and Cp2Zr(R)Cl (R = alkyl), or lanthanide alkyls suc

33
Bond metathesis reactions
...
The overall reaction is shown generally

such as WMe6 and Cp2Zr(R)Cl (R = alkyl), or lanthanide alkyls such as Cp*2LuMe where oxidative

Organometallics - Reactivity

Bond metathesis reactions
...
The overall reaction istransformations
...
Alkane bond σ-metathesis is rare
...
Alkenes can be other
undergo oxidative addition, the bottom route is not permitted
...

processes
...
They immediately enter olefin polymerisation processes

catalysis)
...
Again you will be given much more on this in the 3 year (organometallic reactions and
Alkenes can be introduced directly or prepared by modification of another ligand (e
...
alkyl)
...


(a) Ligand substitution into an existing complex is often effective (e
...
Alkenes can Fe(CO) 5
lkene compounds play a major role in many industrial transformations
...
The first alkene compound was
years before it processes
rectly cyclooctene on pagefeedstocks
...
g
...

of olefin

recognised   as   such
...
g
...
Again you will be given much more on 4this in the 3rd year (organometallic reactions and 5
h cyclooctene on page 20)
...


Cl
Cl CH2
C2H4
ognised   as   such
...
g
...

Cl
lkenes can be introduced directly or prepared by modification of

Zeise's salt (1827)
2
a) Ligand substitution into an existing complex is often effective (e
...
Cl CH2
the reaction of Fe(CO) 5
Cl
C2H4
Although alkenes are Pt
usually poor ligands with respect was Cl
Pt
Cl
Cl
ith cyclooctene on page 20)
...
  Zeise’s   salt   was   first   obtained  
34
acceptor ligand (see below):
There are 3 ways in which metal-alkene complexes can be synthesized
...
σ-metathesis reactions, for d transition metal compounds like WMe6 or lanthanide alkyls (as oxidative addition cannot
directly from bond metathesis being very rare still)
...
They immediately Because polymerisation compounds cannot
(alkane

PMe3

eptor ligand (see below):
Cl

- PMe3

Cl

CH2 2
CH

CH2



1
...
In this example RhCl3·H23 is reduced from Rh(+3) to Rh(+1) Ti EtOH, which complex
by
is in turn
Ti
orks because the Ti(+2) compound Cp2Ti(PMe3)2 is so electron-rich and C2H4 can act as a Zeise's salt (1827)



PMe3
- PMe3
oxidised to EtCHO
...
Reduction of a
PMe3
(c) Modification of an alkyl PMe
ligand reduced from this example by hydride EtOH, which
be
Ti

e
...
 The  hypothetical  monomer  “RhCl( 2-C2Rh )2”  would  be  a  14  VE  compound  and
H4
EtOH
compound in the
CH2

reflux
dimerises via two 2-Cl bridges:
presence of C2H4

b) Reduction of a higher oxidation state compound in the presence of C2H4 is also an effective

- Rh(+3) to Rh(+1) by EtOH, which is in turn
oute
...
Modification of an
OC
OC
2
C2H4
Cl
Rh

xidised to EtCHO
...
page 33)
...
The alkene-hydride compounds formed like this are
Reactions of metal alkene compounds
C2H
Cl
Rh
hydride compounds formed this way are 4
often intermediates
often intermediates and so may not be directly observed
...
other products and so may
2 RhCl 3 H2O
Cl
Rh
not be directly observed
...
g
...
A further example of alkene insertion into a M–R
There are 3 ways in which these compounds react
...
Insertion reactions – into metal-alkyne bonds or into metal-hydride bonds
...

below, proceeding through a 16 VE CpNiCH2CH2R intermediate (not shown):
59
bonding MO to a vacant acceptor AO on the metal
...
This particular
via CpNiCH2 CH2R

bonding scheme is known as the Dewar-Chatt-Duncanson model
...
The titanocene
compound below is an example with protonation taking place at the somewhat reduced C2H4



R = Ph, CH2C3H5
Ni

CH2

CH2CH2

Ni

H2C

CH2

R
50 atm
R
(b) Reactions with electrophiles can take place with CH2
more reduced alkenes
...
Overall it is a double-deprotonation of H2O leading to a
(b)

2-oxo

ligand:

Reactions with electrophiles can take place with more reduced alkenes
...

2
...
Overall it is a double-deprotonation of H2O leading to a 2-oxo ligand:
H2O

CH2
CH

Ti

2

Ti

CH2

CH2

CH2

Ti

Ti

CH2

H

Ti

H2O

CH2

O




Ti



Ti

CH2

Ti

O

H


H


(c) Reactions with nucleophiles is one of the most important reaction types of coordinated
3
...
(c) Reactions with nucleophiles – i
...
migratory insertion into M–H or M–alkyl (see above) – or
This can be internal attack is one of the most important reaction types of coordinated

attack (as shown)
alkenes
...
e
...


Organotransition metal catalytic chemistry performs several reactions: alkene hydrogenation, hydrocyanation,
hydroformylation, acetic acid synthesis, alkene oxidation, alkene isomerisation, alkene metathesis, alkene and alkyne
oligomerisation, metathesis, and alkene polymerisation
...



is useful if
is favourable
...

63


The more common complexes used in catalysis are π-systems,
carbonyl, alkl and hydride ligand complexes
...


Some key processes include the Wacker Process, the BP-Monsanto Process and the Oxo Reaction
...
Heterogeneous catalysts are also used, such as those used in the hydrogenation of alkenes by H2(g)
and Pd on carbon and hybrid catalysts are also used
...
These advantages
include the phase differences between the reactants/products and the catalyst itself
...





Organometallics - Reactivity




Cycle 1

-

Under the conditions used, the I ions present react with methanol to set up an appreciable concentration of iodomethane in
the first step of the reaction
...

This step is followed by methyl migration, yielding a 16-electron acyl complex
...

Water then hydrolyses the acetyl iodide to acetic acid and regenerates HI
...
Acetic Acid Synthesis (Monsanto Process, 1970’s)
iodomethane
...
> 99%
30 bar

This is an asymmetric hydrogenation reaction, using a homogeneous, metal carbonyl catalyst
...

Organometallics - Reactivity




Cycle 2

Palladium(0) is most widely used in homogeneous catalysis
1321
Palladium(0) is most widely used in homogeneous cataly
which is then ready for another cycle
...
The steps leading to the other regioisomeric aldehyde and
ligands on the metal are omitted for clarity
...

O

π-complex formation

reductive elimination

START HERE

π-complex formation
reductive elimination
O
START HERE
H
which is then ready for another cycle
...
H
This is the catalytic cycle of the Oxo
H H
π-complex formation
reductive elimination
O
START
Reaction
H H HERE
O
M H
H
O
The OXO process is the
H
H
M
M H
M H
H
M
hydroformylation of alkenes that uses

H

H

O
M

H

carbonylation

H

catalytic
cycle
of the
OXO
process

catalytic
cycle
carbonylation
of the
OXO
process

two migratory insertions to make

M

H

catalytic
higher value aldehydes
...
Hydrometallation occurs product aldehyde and the metalby initial πThe mechanisms of of the alkene and are worth discussion
...

complex formation followed by addition of the metal to one end
complex formation followed by addition of the metal to

other
...
discussion
...
reaction is another migrat
from theformation the carbon atom of aof the metalBoth ofend of regioisomers are formed
...
to one these the alkene and hydrogen to the
ligand
...

Hydrometallation occurs by initial π- complex formation followed by addition of the metal to one end of the
O
other
...
The carbonyl insertion reaction is another migration
C
O
alkene and hydrogen to the other
...

metal
M
M
C
M H
these regioisomers are formed
...

carbonylation
π-complex formation
H
H
carbonylation
π-complex formation
hydrometallation

Insertion reactions are reversible
carbonylation
π-complex formation
hydrometallation
Insertion reactions are reversible
The reverse process, decarbonylation, is also fast but can be arrested by maintaining a pressure of
reversible
Insertion reactions arethe reaction mixture
...
The reverse of hydrometallation involves the elimi
carbon can be to form an alkene complex
...
The reverse of hydrometallation involves the elimina- alkyl to form an alkene complex
...
It requires a vacant site on the a metal the
tion of a hydride from the adjacent carbon of a known as β-hydride elimination or simply β elimination
...
This process is
number of ligands increases in the process and so is favoured by a shortage of ligands as in 16-elecknown as β-hydride elimination or simply β elimination
...
The metal and the hydride must be syn to each other on the carbon chain for the
number of ligands increases in the process and so iscomplexes
...
be possible
...
So β elimination is an important to be possible

---