CHEM 352 -Note inorganic chemistry notes

Chem352

1. Organometallic Chemistry
Lecture Note2
1

2. Group 2 Organometallic Compounds
•Organometallic compounds of beryllium are
pyrophoric in air and unstable in water.
•They can be prepared by transmetallation from
methylmercury in a hydrocarbon solvent:
•Another synthetic route is by halogen exchange or
metathesis reactions in which a beryllium halide
reacts with an alkyllithium compound.
•LiCH
3 (sol) + BeCl
2 (s) Be(CH
3)
2 (s) + LiCl (s)
Hg(CH
3
)
2
(sol) + Be (s) Be (CH
3
)
2
(sol) + Hg (s)
2

3. Group 2 Organometallic Compounds
•The products are the lithium halide and an
alkylberyllium compound.
•In this way, the halogen and organic groups are
transferred between the two metal atoms.
2BuLi (sol) + BeCl
2 (sol) (Bu)
2Be (sol) + 2LiCl(s)
❖The driving force for this and similar reactions is the
formation of the halide of the more electropositive
metal.
3

4. Group 2 Organometallic Compounds
•Grignard reagents (organomagnesium compounds)
in ether can also be used in the synthesis of
organoberyllium compounds:
2RMgCl (sol) + BeCl
2 (sol) R
2Be (sol) + 2 MgCl
2 (s)
NB: R is an alkyl or aryl group.
•Methylberyllium, Be(CH
3)
2, is predominantly a
monomer in the vapour phase and in hydrocarbon
solvents, it adopts a linear structure, as expected
from the VSEPR model.
4

5. Group 2 Organometallic Compounds
•The structure of dimethylberyllium in the solid
state is similar to that of trimethylaluminum
•Exception: methylberyllium forms chains,
whereas the trimethylaluminium forms dimers
•In the solid state, Be(CH
3)
2 forms polymeric
chains whereby the bridging methyl groups
form 3 centre, 2 electron (3c,2e) bridging
bonds.
5

6. Group 2 Organometallic Compounds
Polymeric structure of Methyl beryllium
6
•Bulkier alkyl groups lead to lower degree
of polymerization;
•For instance, ethyl beryllium is a dimer
whiles tert-butylberyllium is a monomer.

7. Group 2 Organometallic Compounds
•An interesting organoberyllium compound is
beryllocene, (C
5H
5)
2Be.
•This can be prepared from the reaction of
potassium cyclopentadienyl (K(C
5H
5) with
BCl
2 in a toluene/diethyl ether mixture.
•The methyl substituted derivatives
(C
5Me
5)
2Be are also prepared in a similar
manner.
7

8. Group 2 Organometallic Compounds
•The solid state structure of (C
5Me
5)
2Be
suggests that the two rings are bound to the
Be differently such that 1 is designated η
5
and
the other η
1
.
•Solution state
1
H NMR analysis reveals that
all the protons are magnetically equivalent at
163 K.
8

9. Group 2 Organometallic Compounds
•Assignment: Read the following article for discussion in
our next lecture:
Synthesis, solid-state structure, and bonding analysis of
the beryllocenes [Be(C
5Me
4H)
2], [Be(C
5Me
5)
2], and
[Be(C
5Me
5)(C
5Me
4H)] (DOI: 10.1002/chem.200304876)
Solid state structure of
beryllocene (SC-XRD)
9
Solution state structure
of beryllocene (
1
H
NMR)

10. Group 2 Organometallic Compounds
•Alkyl- and aryl magnesium halides are very well
known as Grignard reagents and are widely used in
synthetic organic chemistry where they behave as a
source of R (alkyl or aryl groups).
•They are prepared from magnesium metal and an
organohalides.
Mg(s) + RBr (sol) RMgBr (sol)
•The reaction is carried out in ether or
tetrahydrofuran.
10

11. Group 2 Organometallic Compounds
•As the surface of magnesium metal is covered
by a passivating oxide film, the magnesium
must be activated before the reaction can
proceed.
•A trace of iodine is usually added to the
reactants, forming magnesium iodide.
•This compound is soluble in the solvent used
and dissolves to expose an activated
magnesium surface.
11

12. Group 2 Organometallic Compounds
•The structure of Grignard reagents is far from
simple.
•The metal atom has a coordination number of 2
only in solution when the alkyl group is bulky.
•Otherwise, it is solvated with a tetrahedral
arrangement of solvent molecules around the
Mg atom.
12

13. Group 2 Organometallic Compounds
•In addition, complex equilibria in solution known
as Schlenk equilibria, lead to the presence of
several species.
•The exact nature of which depend on
temperature, concentration and solvent.
•For example, R
2Mg, RMgX, and MgX
2 have all
been detected:
13

14. Organometallic Chemistry of Group 2 elements
•For example, R
2Mg, RMgX, and MgX
2 have all
been detected:
14
J. Phys. Chem. B2017, 121, 16, 4226-4237

15. Group 2 Organometallic Compounds
•Grignard reagents are widely used in the
synthesis of organometallic compounds of
other metals, as in the formation of
alkylberyllium compounds mentioned earlier.
• They are also widely used in organic
synthesis.
15

16. Group 2 Organometallic Compounds
•Grignard reagents undergo side reactions such as
Wurtz coupling to form a C-C bond:
•Reaction with carbonyl compounds:
•Reaction with alkylhalides:
R
1
MgX
(sol) + R
2
X
(sol) R
1
R
2

(sol) + MgX
2
(sol)
16

17. Group 2 Organometallic Compounds
•Organometallic compounds of other group 2
metals are known.
•The organometallic compounds of calcium,
strontium, and barium are generally ionic and very
unstable.
•They all form analogues of Grignard reagents by
direct interaction of the finely divided metal with
organohalide.
17

18. Group 3 Organometallic Compounds
•The most important organometallic compounds
of the Group 3 elements are those of boron and
aluminium.
•Organoboron compounds are commonly
treated as organometallic compounds even
though boron is not a real metal.
18

19. Group 3 Organometallic Compounds
Organoboron compounds
•Organoborons of the type BR
3 can be prepared by
hydroboration of an alkene with diborane.
B
2H
6 + 6 CH
2= CH
2 2 B(CH
2CH
3)
3
•Alternatively, they can be produced from a
Grignard reagent
(C
2H
5)
2O:BF
3 + 3 RMgF BR
3 + 3 MgF
2 + (C
2H
5)
2O
19

20. Group 3 Organometallic Compounds
Organoboron compounds
•Alkylborons are not hydrolyzed but are
pyrophoric.
•The aryl species are more stable.
•They are all monomeric and planar.
•Like other boron compounds, the organoboron
species are electron deficient and consequently
act as Lewis acids and form adducts easily.
20

21. Group 3 Organometallic Compounds
Organoboron compounds
•An important anion is the tetraphenylborate ion,
[B(C
6H
5)
4]
-
, more commonly written BPh
4
-.
•An analogous to the tetrahydroborate ion, BH
4
-
.
•The sodium salt can be obtained by a simple
addition reaction:
BPh
3 + NaPh Na
+
[BPh
4]
-
21

22. Group 3 Organometallic Compounds
Organoboron compounds
•The sodium salt of [B(C
6H
5)
4]
-
is used as a
counterion for cationic coordination
compounds.
•An analogue is sodium tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate, popularly
known as NaBArF.
22

23. Group 3 Organometallic Compounds
Organoboron compounds
•These are soluble in polar organic solvents such as
methanol.
•Consequently, the anion is useful as a precipitating
agent and can be used in gravimetric analysis.
•They are mostly used as halide abstracting agents in
coordination compounds.
23

24. Group 3 Organometallic Compounds
Organoaluminium compounds
•Alkylaluminium compounds can be prepared on a
laboratory scale by transmetallation of alkylmercury
compounds:
2 Al + 3 Hg(CH
3)
2 Al
2(CH
3)
6 + 3Hg
•Trimethylaluminium is prepared commercially by
the reaction of aluminium metal with chloromethane
to give Al
2Cl
2(CH
3)
4.
•This intermediate is then reduced with sodium and
the Al
2(CH
3)
6 is removed by fractional distillation.
24

25. Group 3 Organometallic Compounds
Organoaluminium compounds
❖The resulting compound is a dimer:2Al + 6MeCl + 6Na Al
2Me
6 + 6NaCl
Al
2Cl
2(CH
3)
4 + 2CH
3Cl + 4Na Al
2(CH
3)
6 + 4NaCl
25
Structure of Trimethylaluminium

26. Group 3 Organometallic Compounds
Organoaluminium compounds
•Alkylaluminium dimers are similar in structure to the
analogous dimeric halides but the bonding is
different. In the halides, the bridging Al-Cl-Al bonds
are 2c,2e bonds; that is, each Al-Cl bond involves
an electron pair.
•In the alkylaluminium dimers, the Al-C-Al bonds are
longer than the terminal Al-C bonds, which suggests
that they are 3c,2e bonds, with one bonding pair
shared across the Al-C-Al unit, somewhat
analogous to the bonding in diborane, B
2H
6.
26

27. Organometallic Chemistry of Group 13
elements
Organoaluminium compounds
•Triethylaluminium and higher alkyl
compounds are prepared from the metal, an
appropriate alkene and hydrogen gas at
elevated temperatures and pressures.
•This route is relatively cost-effective and, as a
result, alkylaluminium compounds have found
many commercial applications.
27

28. Organometallic Chemistry of Group 13
elements
Organoaluminium compounds
•Alternative route is:
•This is followed by insertion of an alkene.
28

29. Group 3 Organometallic Compounds
Organoaluminium compounds
•Triethylaluminium, often written as the
monomer, Al(C
2H
5)
3, is an organometallic
complex of aluminium of major industrial
importance.
➢It is used as a co-catalyst in the Ziegler-Natta
ethylene polymerization.
➢It is also used as co-catalyst in ethylene
polymerization and oligomerization using
homogeneous metal catalysts.
29

30. Group 3 Organometallic Compounds
Organoaluminium compounds
•Steric factors have a powerful effect on the
structures of alkylaluminium compounds.
•Where dimers are formed, the long weak
bridging bonds are easily broken.
•This tendency increases with the bulkiness
of the ligand.
–Example, triphenylaluminium is a dimer but
trimesitylaluminium (where trimesityl is 2,4,6-
(CH
3)
3C
6H
2) is a monomer.
30

31. Group 3 Organometallic Compounds
Organoaluminium compounds
31
Monomer
formula
Al(CH
3)
3 Al(C
2H
5)
3 Al(C
3H
7)
3 Al(i-C
4H
9)
3
Boiling point126.0
o
C 186.6
o
C 192.8
o
C 214.1
o
C
Structure in
liquid state
Dimers Dimers Dimers +
monomers
Dimers +
monomers
Structure in
vapour state
Dimers +
monomers
Monomers Monomers Monomers