Simple chance, coupled with astute observations, has been responsible for the
discovery of many new compounds. Often, in the course of what is thought to be a
straight forward synthesis or investigation. Something completely unexpected happens.
Perhaps a precipitate forms, a gas is evolved, a reaction mixture turns an unusual color, or
a yield of expected product is very low. Some curious chemist tries to find out what
“went wrong” and in that process usually makes a significant, sometimes a spectacular
discovery.
The discovery and characterisation of the structure of ferrocene, Fe(C5H5)2 in the
early 1950's, led to an explosion of interest in d-block metal carbon bonds and brought
about development and the now flourishing study of organometallic chemistry.
Prior to the 1950's few d-block organometallics were synthesized and
characterized. The first one, an ethylene complex of platinum(II), was prepared by W.C.
Zeise in 1827. In 1890's Ludwig Mond and co-workers synthesized the first metal
carbonyl, tetracarbonylnickel. However, the structures of such complexes were difficult to
deduce using chemical methods, and thus it wasn't until the 1950's when NMR and single
crystalX-ray diffraction could be used to solve the structures of these complexes in
solution and solid state respectively.
The serendipitous discovery in 1951 of ferrocene, the first recognized sandwich
compound, opened an entire new era of research, and since that time numerous
cyclopentadienyl derivatives of various metals and metalloids have been reported. Today
such compounds have not only contributed to our theoretical knowledge of chemical
bonding but also have found industrial applications ranging from antiknock additive to
polymerization catalysts. The development of the chemistry of sandwich compounds of
transition metals, which revolutionized organometallic chemistry and had a significant
impact on the broader fields of inorganic, organic and theoretical chemistry. The rapid
growth in the study of organometallic compounds by research groups around the world
led to the Nobel Prize awarded in 1974 to Ernst Fisher and Geoffrey Wilkinson for their
contribution to the field.
In 1950, R. D. Brown of the university of Melbourne had predicted that
hypothetical hydrocarbon fulvalene should be a non-benzenoid molecule. The molecule
is shown below.
In order to test Brown’s prediction, Pauson assigned the synthesis of this
compound to his new research student Thomas J. Kealy. But in 1939 Gilman and his
worker had described the synthesis of biphenyl in almost quantitative yield by treating the
Grignard reagent phenylmagnesium bromide with iron, cobalt, and nickel chlorides.
C6H5MgX + MX2 ------- C6H5C6 H5 + MgX2 + MX[or M]
The first
reported experiment for the coupling of phenyl was done with
phenylmagnesium bromide and chromic chloride. The results for the Gilman experiment
are shown below.
Metallic halide
FeCl2
CoBr2
NiBr2
RuCl3
RhCl3
PdCl2
Mole
0.01
.01
.03
.0036
.0036
.00566
Mole(C6H6MgI)
.03
.03
.095
.0108
.013
.0163
% Yield
98
98
100
99
97
98
Considering the results obtained above by different chemists, Pauson attempted to
synthesize dihydrofulvalene by refluxing cycopenatadienylmagnesium bromide with
anhydrous iron(III) chloride in an anhydrous ether and decomposing the reaction mixture
with iced ammonium chloride solution. The normal reaction leads to reduction of the
ferric salt and with excess reagent results in the formation of metallic iron according to
the equation shown below. Similar reaction showed biphenyl as product in the an earlier
experiment in god yield.
Although CoCl2 is more effectively used for such coupling reactions, Pauson
chose FeCl3 because it was available in anhydrous form and because it might oxidize
dihydrofulvalene directly to fulvalene. Instead he discovered the most famous and a new
type of compound known as ferrocene, it was also the first sandwich compound ever
discovered. The synthesis method is as follows.
Procedure1.
To a solution of a Grignard reagent ( from 18 gm ethyl bromide and 4 gm magnesium
bromide with 11 gm. Cyclopentadiene) was added a 9.05 gm of ferric chloride dissolved
in anhydrous ether. The mixture was allowed to stand at room temperature for 12 hours
and then refluxed for 1 hour, cooled and decompsed with ice cold ammonium chloride
solution, evaporation of the dried organic layer yielded an orange solid ( 3.5 gm). it was
readily soluble in ether and benzene.
The above orange solid that Pauson and Kealy received from their experiment was
crystallized from methanol in large needles and had melting point 173 - 174 C. they also
found molecular weight by cryoscopic determination in benzene, 186.5 ( C, 64.6 %, H,
5.4 %, Fe, 30.1 % ). They also found Iron gravimetrically as Fe2O3 after heating it with
nitric acid under a reflux condenser. The other properties that the compound have were
sublimation above 100C, insoluble and unattacked by water, 10 % caustic soda,
concentrated hydrochloric acid. It was dissolved in dilute nitric acid and conc. Sulfuric
acid with strong deep red solution with blue fluorescence.
Pauson concluded from the analytical data that the compound was dicyclopentadienyl
iron formed according to the equation.
2RMgBr + FeCl3
---------- RFeR + MgBr2 + MgCl2
OR
2[C5H5MgBr] + FeCl3 ------ (C5H5)2Fe + MgBr2 + MgCl2
They also showed the structure of the compound as shown below.
The research team of Samuel A. Miller and co-workers of the British Oxygen
Company had already antedated Kealy and Pauson’s discovery by reacting reduced iron
with cyclopentadiene vapor n a nitrogen atmosphere at 300C.(3). Their internal report
was written in 1948, but they did not submit their article until 1951 and it was not
published until the following year. Their structure was the same but their procedure was
completely different than the Pauson and his co-workers. Another interesting thing was
that, Geoffrey Wilkinson believed that workers at Union Carbide were probably the first
to prepare the compound in the 1930’s while cracking cyclopentadiene through hot iron
tubes. His suggestion was probably came after seeing the procedure used by Samuel anf
co-workers. The second group used reduced iron in the form of the well known doubly
promoted synthetic ammonia catalyst which can be made to react with cyclopentadiene in
nitrogen at 300 C and at atmospheric pressure, to give a ellow crystalline compound, of
composition C10H10Fe. They found that the iron was in its bivalent form, because the
treatment of carbon tetrachloride soluion of the material with bromine in the same solvent
gave a dark green precipitate, which dissolved in water togive a blue solution containing
ferrous and bromide ions. The compound was charred by and reduces sulfuric acid and
decolorises acid potassium permanganate. In the reaction the iron was proceeding only
10-15 minutes and after that the residual reduced iron was unchanged in regard to its
activity as a catalyst with respect to synthesis of ammonia at 550 C. The initial rate of
production of iron and the total periond of reaction before inhibation sets in
both were increased by about 3 fold by the addition of molybdenum oxide to the doubly
promoted catalyst.
Procedure2
Ferric nitrate (3 kg) and a aluminum nitrate (300 g) were dissolved in distilled
water, and treated with KOH as a 40 % solution. The ppt was separated on the centrifuge
and washed with water. It was dried at 100 C for four hours and then at 600 C in the
muffle furnace for 10 hours. It was washed and dried again and then converted into
pellets.
The pellets were charged into the 1”- diameter silica reactor tube as in the diagram and
then reduced during 10 hours at 450 C in a 50 liter per hour stream of dry hydrogen. A
short test was carried out with a mixture of nitrogen and hydrogen through the reactor to
confirm the activity of the reduced iron as a catalyst for synthesis of ammonia.
The cyclopentadiene was prepared by boiling with a little iron filling and astream
nitrogen was passed through it and the issuing mixture of monomer and dimer passed up
through the fractionating column surrounded by a reflux codenser. The nitrogen issuing
at the top of the container was regarded as being satured with cyclopentadiene at the
temperature of the coling water.
After the first reaction the reactor was purged with nitrogen and then mixtures of
nitrogen and air were passed through it. In order to avoid high temp on the surface of the
catalyst, th eair content of the mixture was increased slowly in 5 hour steps as 5, 10, 20,
40, and 100 % of air and the temperature was gradually raised. Oxidation was considered
complete when the temperatur kept quite steady wit 100 % air passing.
The product in each was washed out of the receiver withether, and the ether
evaporate off under reduced pressure. Some of the cyclopentadiene was recovered in the
cold trap, conversion of its iron compound could only be estimated approximatey but
during 15- minutes periods of activity, the conversion amounted to 40-50 %.
Experiments in the presence of Molybdenum.-A solution was prepared from ferric nitrate (650 g), aluminum nitrate 29 g,
ammonium molybdate (13 g) and 500 ml of water and then KOH in a small amount of
water was added. The solution was stirred and conc. Ammonia was added until mixture
was alkaline. Ammonia and water were evaporated to leave a pasty mess which was
heated on sand bath and the resulting residue was ignited in the muffle furnace at 600 C
for 6 hours. It was then formed into pellets and then passed into silica tune reactor. On
the passage of nitrogen saturated with cyclopentadiene over the iron at 300C.
Considerable formation of the yellow crystals were obtained but after 15 minutes the rate
of production dropped rapidly. The total solid obtained in 25 minutes was 3.0 g,
compared with average 1g in the absence of molybdenum.
The procedure was also carried on with elevated pressure but there was no case
that the product could be isolated.
In the above experiments both chemists got the same product but there was only
one problem, the structure they proposed was wrong. This started a debate and a new
structure was proposed shortly after their paper got published. The new structure was
published in April of 1952, and supported by the results, and they was proposed by R. B.
Woodward, G. Wilkinson and Co-workers. They said because of the equal unsaturation
of each of the carbon atoms of the cyclopentadienyl anions suggested the two such unots
might form covalent bonds to ferrous ion symmetrically(4). They proposed two
structures shown below as (II) and (III).
Woodward and his Co-workers gave following sets of data in the support of their
conclusion about their structure.
 Iron biscyclopentadienyl is diamagnetic.
It has 25mole = -125 x 10-6
 IR absorption between 3 -4 .
 Sharp band at 3.25, indicates only one type of C-H bonds.
 Ultraviolet shows maxima at 326 m (  = 50 ) and 440 m ( = 87 ).
 The dipole moment is effectively zero ( 0.05 D ).
 It has detectable vapor pressure at 0.
 It resists pyrolysis at 470.
 It readily oxidises to blue cation [ Fe( C6H5)2]+.
 Polarographic studies show an oxidation potential of -0.05
 The perchlorate in 1N perchloric acid showed max. 253 m (  = 13,300).
The cation isolated as the crystalline tetrachlorogallate and picrate.
The cation isolated as the crystalline tetrachlorogallate and picrate.
Analytical calculations
Carbon
30.25
30.47
46.35
45.99
Gallate
Found
Picrate
Found
Hydrogen
2.54
2.77
2.92
3.27
Iron
14.02
13.97
13.48
13.43
Galium
17.52
17.52
N =10.13
N= 10.05
Chlorine
35.76
35.70
---
In July 5, 1952 a new article appeared in the journal of American chemical society
in which Woodward, Rosenblum and whitting gave the name ferrocene to iron
biscyclopentadienyl and also discussed more properties of this new compound. They
proposed the name ferrocene because in experiments, it demonstrated the aromatic
properties and also it had two rings each of five equivalent C-H groupings. The new
experiments showed that it did not react with maleic anhydride and was not hydrogenated
under normal conditions over reduced platinum oxide. It gave red diacetyl derivative in
the presence of aluminum chloride, m.p. 130 - 131C.
Derivatives and their melting points
Derivative
Dioxime
bis--chloropropionylferrocene
bisacrylolferrocene
dibenzyl ester
dimethyl ester
m. p. (  C ).
decomposes > 200
117 - 121
71 - 71.5
144 - 144.5
114 - 115
Ferrocene dicarboxylic acid
pK1 3.1x10-7
pK2 2.7x10-8
pK 2.4x10-7
Benzoic acid
The acidity constants are great interest and they are usually measured in ethanol or
water.
The very small difference between the two dissociation constants of ferrocene
dicarboxylic acids indicate that the carboxyl group interact very little, and must be very
far apart, while the near identity of the first constant with that benzoic acid demonstrates
that the ring carbon atoms of ferrocene and necessarily the central atom as well are
substantially neutral. This observation is very important in respect to the detailed
electronic structure of ferrocene.
Infrared spectrum
H-R
R = ferrocene, 
3.26
CH3 C = O R
CH3O C = O R
CH3O O CC6H4C = O R
5.97
5.82
6.02
Other bands, 
3.28
3.31
5.93
5.81
5.97
The next step in proving the structure comes from the article written by Dunitz
and Orgel from Nature journal which was published in January 17, 1953. In that journal
the author showed X-ray evidence for the correctness of structure ( II ), which was
suggested by Woodward and co-workers. Dunitz and Orgel showed that the structure of
ferrocene was monoclinic with cell dimensions as shown.
a = 10.50 A
c = 5.95 A
b = 7.63 A
 = 121
space-group P2/a
The measured density of 1.516 g/cc showed 2 molecules per cell, and thus
required the two iron atoms to be at the cell corner and body center. The cell symmetry
requires that the molecule is centro-symmetric, with the iron atom at its center. Each
molecule possessed a center of symmetry occupied by the iron atom which contributes
either +2Fe or zero to the structure factors according as h + k is odd or even. They
showed the main features of the structure from the initial projections of the electron
density on the ( 010) and (001) crystals planes. The function (xz) computed with all
positive signs and was shown and it will be seen on the next page as fig1. In that figure
they resolved four of the five atoms in each cyclopentadienyl and the fifth was obscured
by the heavy iron peak at the origin. Similarly (xy) in which all terms with h + k = 2n
were taken as positive and the functions (xy) will be shown as fig2. The plane of the
cyclopentadienyl ring was normal to the projection plane and the individual atoms were
not resolved but they showed the general shape of the molecule which was obvious.
Fig. 1. Plot of function (xz) shown by Dunitz. Contour lines were
drawn at equal arbitrary intervals except for the iron atom, where the
intervals had been increased by a factor of four for the sake of clarity.
Fig. 2. Plot of function (xy) shown by Dunitz and Orgel. Contour
lines were drawn at equal arbitrary intervals except for the iron atom, where
the intervals had been increased by a factor of four for the sake of clarity.
Fig. 3. The three dimensional Fourier analysis completed by Dunitz
and Orgel and an electron density map of the complete molecule obtained
from their work is shown here.
The above three projections considered together leave no doubt that
the sandwich structure proposed was correct.
They also suggested that the set of d-orbitals transforms in the group D5d =
C5v  , according to the following scheme : dz2  A 1g
dxz , dyz  E 1g
dx2-y2,dxy  E2g.
The - orbital of the cyclopentadieny radical, according to simple
molecular orbital theory, are A1, E1, and E2 in order of decreasing stability.
For two cyclopentadienyl radicals oriented as in this molecule, each pair of
orbital can combine to give two orbital transforming as even ( gerade) or
odd ( ungerade) representations.
The viberational spectra of ferrocene was obtained. IR spectra in CCl 4, CS2
and Nujol solutions were obtained in the region from 2-25 and also
Raman spctras were shown. The excitation was accomplished with the Hg
5770 - 5790A doublet as well as Hg 5461A line using the appropriate
filters. It was found that intensity of the Raman scattering for ferrocene was
rather sharply dependent on the concentration and that for a given
wavelength there existed an optimum concentration for maximum intensity.
Evidence for equivalent multiple bonds resulting from delocalization of  electron of the cyclopentadienyl rings is the appearance of multiple bond
frequencies near 1410 cm-1.
Table. The Raman and Infrared spectra of ferrocene ( cm-1).
Raman
Infrared
303 m
388 w
478 s
492 s
782 w
811 s
864 w
1002 s
1051 w
1108 s
1188 w
1010 w
1050 w
1105 s
1178 m
1356 w
1408 m
1411 s
1620 m
1650 m
1684 m
1720 m
1758 m
3085 m
3099 s
3085 s
A polarogram of the ferricinium salt solution resulting from the
electrolysis, showed a well defined cathodic wave with a half-wave
potential of + 0.30 v. versus the S. C. E. agreeing, within the limits of
experimental error, with the half - wave potential for the oxidation of
ferrocine.
The
ferrocene-ferricinium
ion
couple
is
hence
a
thermodynamically reversible system in the alcoholic supporting electrolyte.
(C5H5)2Fe = [(C5H5)2Fe]+ + e- ;
Eo = -0.56v.
In Nov, 1952 some other properties of ferrocene were published, in which they
measured vapor pressure and vapor densities of the compound. They used two samples
such that one was completely vaporized at 190C, and the other at 290 C. The vapor was
found to obey the perfect gas law up to 400C. A molecular weight of 186 was calculated
for the vapor, proving it to be monomeric and un-dissociated over the temperature range
studied. The vapor pressures of the solid were represented by the equation.
log P mm = 7.615 - ( 2470 / T )
For the liquid.
log P mm = 10.27 - ( 3680 / T )
From the equations they calculated the following constants.
Kcal / mole
heat of sublimation of solid
16.81
heat of vaporization of liquid
11.30
heat of fusion
5.50
triple point
183
normal boiling point
249
Trouton’s constant
21.2
Fig1-Vapor pressure of ferrocene shown below.
They also showed the ultraviolet absorption spectrum in hexane and it
shows maxima at 325 and 440 m in agreement with the old values. The
spcctra in ethanol and methanol were practically the same with that hexane,
indicating little if any solvation by alcohols. In carbon tetrachloride the 440
m peak was little changed but there was very marked increase in
absorption below 400 m, as compare with solutions in the other solvents.
The spectra obeyed Beer’s law over a 50- fold concentration range and was
unchnaged after standing several week in dark.
Fig 2. shows the absorption spectra.
In January, 1953 the electronic structure of ferrocene was considered
by constructing the molecular orbitals from the atomic orbitals of the iron
atom and the molecular orbitals of the cyclopentadienyl radicals.
The
structure proposed for ferrocene belonged to the symmetry group C5v. The
AO’s of iron and the MO’s of the cyclopenadienyl can be divided in groups
according to their symmetry with respect to z-axis of iron.
Since the
molecular orbitals cyclopenadienyl with one or more nodes occur in
degenerate pairs, linear combinations of such molecular orbital pairs can
always be constructed.
The available orbitals with their symmetry
classifications are listed in table (III).
The orbitals of separate
cyclopenadienyl
by
radicals
were
replaced
linear
combinations
coresponding orbitals from both radicals. From these orbitals the following
MO’s for ferrocene were constructed:
1 = ( s Cosa + dz Sina) Cos1 + 2-1/2 ( 1+ 1 ) Sin1
2 = pz Cos2 + 2-1/2 ( 1- 1 ) Sin2
3 = px Cos3 + 2-1/2 ( 2+ 2 ) Sin3
4 = d x+z Cos4 + 2-1/2 ( 2- 2 ) Sin4
5 = py Cos5 + 2-1/2 ( 3+ 3 ) Sin5
6 = dy+z Cos6 + 2-1/2 ( 3- 3 ) Sin6
7 = dx+y Cos7 + 2-1/2 ( 4+ 4 ) Sin7
8 = dxy Cos8 + 2-1/2 ( 5+ 5 ) Sin8
9 = s Sina - dz Cosa
10 = 2-1/2 ( 4- 4 )
11 = 2-1/2 ( 5- 5 )
12 = dxy Sin8 + 2-1/2 ( 5+ 5 ) Cos8.
The orbitals 13 to 19 followed as negative combinations
corresponding to 1 to 7.
1 to 7 are bonding orbitals, and all are doubly occupied, 9 to 11
are nonbonding, and 12 to 19 are antibonding orbitals. The value of the i
may be determined by solution of the secular equations and will depend on
the energies of the component orbitals and the overlap and resonance
integrals between them. Since 4 and 5 are normally unoccupied high
energy orbitals of cyclopntadienyl, and s and d z are noramally occupied low
energy orbitals of iron, it is reasonable to assume that the two electrons
which remain after orbitals 1 to 8 are filled will occupy 9. Consequently
these electrons were paired and the compound was diamagnetic.
If
ferrocene would be paramagnetic then these electrons were to occupy the
degenerate orbitals 10 and 11.
The great stability of ferrocene is the result of two factors. The rare
gas electron configuration of the iron atom and the existence of eight
bonding molecular orbitlas. Table (iii) is shown below with their symmetry
classifications.
Iron
s, pz, dz
Cyclopentadienyl
2-1/2 ( 1 1 )
Symmetry
a1
pz , dx+z
2-1/2 ( 2 2 )
b1
py , dy+z
2-1/2 ( 3 3 )
b2
dx+y , dxy
2-1/2 ( 4 4 ) ;
a2
2-1/2 ( 5 5 )
The interpretation of the ultraviolet absorption spectrum of ferrocene
was also discussed. The longest wavelength absorption band (  = 440 m )
can be attributed to a charge transfer transition, 9 10. This transition is
forbidden by symmetry and can occur only since symmetry is distorted by
vibrational motion. The second absorption band ( = 326 m, max = 50 )
must be also a forbidden transition. In this case the upper level arising from
a splitting of the degenerate orbitals 10 and 11, or it may be a transition to
the to lowest antibonding orbital (9 12). The intense far ultraviolet
absorption (( = 225 m,  = 50 ) must arise from excitation of one of the
bonding electrons to a nonbonding or anti bonding molecular orbitals (12
or 8 10,11 ). Since the orbitals 8 and 10 to 12 all belong to class a2,
either transition is allowed and therefore the observed high intensity is
consistent with the proposed structure.
The structure of ferrocene was also confirmed by electron diffraction
in the 1955. The results obtained were in excellent agreement with the
highly symmetrical “ sandwich” structure proposed by wilkinson.
The
experimental results were best satisfied by a molecular scattering intensity
curve which is the average of intensity curves calculated for staggered and
eclipsed configurations of the cyclopentadienyl rings. The Scalc / Sobs ratio
for the best staggered, eclipsed, and average configurations are
(1.0000.003, 1.0000.004, and 1.0000.001) respectively. This may be
deemed to indicate that at the vapor temperature ( 400C ) the rings rotate
freely about the common orthogonal axis. This result may be compared
with the earlier observation of a staggered configuration in the crystal state,
of a restricted rotation for ferrocene in solution (both presumably at room
temperature). All the studies so far had stated that no major barrier exists
against free rotation of the cyclopentadienyl rings around the orthogonal
axis. According to Moffit, the electron densities of the bonding iron orbitals
are axially directional but rotationally non directional. A small barrier to
rotation may arise from the H-H, H-C, and C-C interactions between the
cyclopentadienyl rings which are not considered in these theoretical
descriptions. However, it is noteworthy that the separation of the rings (
3.25A ) is also very similar to that in graphite ( 3.35A ). Therefore, there is
little direct binding between the rings.
Table for Bond length results for ferrocene.
Bond
Fe - C
C-C
C-H
Length and error in A
2.030.02
1.430.03
1.090.01
The NMR spectra of ferrocene showed a single, sharp resonance peak
and the variation with temperature of the width of the single resonance line
has been interpreted in terms of the orientation of the cyclopentadienyl rings
about their five fold symmetry axes. The resonance shows a progressive
narrowing when the temperature is decreased from -48C to -158C. This has
been attributed to the conversion of molecules in the eclipsed to the
staggered
configuration stable at low temperatures. The interaction between protons
is larger in the eclipsed form than in the staggered configuration.
The most striking feature of the IR spectra is its simplicity arising
from the high symmetry of molecules. In addition to C-H stretching
frequency at 3075 cm-1, in the region typical for aromatic C-H bonds, there
are four other strong bands. Two of these, at 811and 1002 cm-1 arise from
C-H bending vibration and one at 1108cm-1 is an antisymmetrical ring
breathing and one at 1411cm-1 an antisymmetrical C-C stretching vibration.
The IR spectra of ferrocene is shown bwlow.
It has been observed that the four strong bands are retained at the
same positions in all monosubstituted ferrocene derivatives, but all four
strong bands disappear when buth rings are substituted. The IR spectra of
ferrocene and acetylferrocene are shown. note that the carbonyl vibration is
observed at 1650 cm-1. This band is much lower than the carbonyl vibration
of acetylbenzene at 1686 cm-1, which reflects the difference in electron
releasing ability of the phenyl and ferrocenyl groups.
Two of the most detailed accounts of bonding in ferrocene have been
given by Shustorovich and Dahl. Both treatments SCF-LCAO-MO method
for calculating the energy levels but differ in that one uses Slater function
and the other uses Hatree-Fock orbitals instead. This difference leads to
some differences in the ordering of bonding and anti-boning orbitals. In
both cases, however, the three bonding orbitals of the lowest energy are A 1g,
A2u, E1u. These are formed from empty metal and filled ring orbitals and
contain 8 electrons. The E1g level, formed by overlap of the half filled metal
and ring E1g orbitals, contain 4 electrons and the remaining 6 electrons
occupy the weakly bonding E2g levels and the nonbonding A1g level. The
bonding between iron and the ring as described previously is now regarded
as correct.
Fig. Energy level diagram for ferrocene according to Dahl and Ballhausen
is shown below.
Fig2. Energy level diagram for ferrocene according to Shustorovich and
Dyatkina is shown below.
Some of the uses of ferrocene are given below.
Ferrocene, or his (Cyclopentadieny) iron, is an organometallic
compound. Formula: Fe (C5H6)2. It is orange-yellow flake crystals with the
smell of camphor at room temperature. Melting point: 172 ~ 174 °C.
Insoluble in water. Soluble in benzene, ether, gasoline, diesel and other
organic solvents. It is chemically stable and does not react with acid, base
and ultraviolet. It does not decompose below 400°C and is harmless to man.
2. Applications
(1) Fuel additive for burning-promoting, smoke-abatement and energysaving: It can be used in various fuels, such as diesel gasoline, heavy oil and
coal, Addition of 0.1% ferrocene into engine diesel leads to fuel saving by
10~14 %, increase of vehicle speed by 10%, power output by 10 ~13 % and
reduction of smoke in tail gas by 30~80 %. Furthermore, addition of 0.03%
in heavy oil and 0.02% in coal can also cause reduction of fuel requirement
and smoke (by over 30%).
(2) Additive in synthetic gasoline and synthetic LNG: Addition of 0.01
~0.50% ferrocene and related additives in synthetic gasoline results in 80#,
85# and 90# synthetic gasoline reformulations; Addition of 0.03% in
methanol can reformulate synthetic LNG with fuel value of 33472~ 38656
KJ/Kg; Addition of 0.005~0.080% in methanol- ethanol mixture can
reformulate a new highly-efficient civil fuel.
Addition of ferrocene in various fuel considerable benefits energy-saving,
smoke abatement, pollution control, inhibiting mechanical abrasion and
prolonging life of the machines.
(3) Antidetonator:
Ferrocene can be used as gasoline antidetonator instead of tetraethyl lead to
get rid of the contamination and poisoning to man by the lead in tail gas and
form high-grade lead-free gasoline. For example, addition of 0.0166~0.0332
g/L ferrocene and 0.05~0.10 g/L tert-butyl acetate can increase octane
number by 4.6~6.0.
(4) Polymerization and Ammonia synthesis catalysts and silicone resin and
rubber curing agent: Some derivatives of ferrocene can inhibit the
degradation of polyethylene by light. When used in agricultural film, they
can promote its auto-degradation and self -destruction within certain period
so that farming and fertiliser application are not influenced. More over,
ferrocene can also be used as the protecting agent of polyethylene,
polypropylene and polyester fibres to improve the thermal stability of
plastics, rubber and fibres.
(5) Ferrocene can be used as burning rate catalyst of rocket propellant in
cosmonantic industry.
(6) Ferrocene can be used as the raw material of antibiotic and bloodnourishing agents.
3. Principle of Action
When ferrocene is burned together with various fuels mentioned above over
400°C, active iron ions are released which fiercely react with the oxygen in
air to form active a - FezOs molecules, a - FezOs, as a burning-promoting
catalyst for various fuels, can speed up the burning and make the
combustion complete. More over, Addition of ferrocens in various fuel oils
can increase the fraction of lower molecules, lower their fire point, complete
the combustion, enhance combustion efficiency, elevate burst pressure of
engines and improve dynamic performance of the vehicles. Ferrocene is also
a lubricant at higher temperature and thus inhibits mechanical abrasion.
Ferrocene is both a chemical catalyst and a high-temperature lubricant. With
combustion takes place in engine, combination of all these actions results in
fuel oil saving, smoke abatement, power increase and speed enhancement.
4. How to use
The amount of ferrocene used depends on specific fuel oil. Ferrocene is
added to the fuel oil and stirred until it is completely dissolved and then the
oil can be filled into fuel tank and is ready for used. Alternatively, ferrocene
and a certain amount of oil are mixed to form a highly-concentrated
mixture, which is diluted to required concentration before use.
When used in solid fuel, it can be mixed with the fuel mechanically or
dissolved in some solvent ( i, g, diesel, ethanel) and sprayed onto the solid
fuel, which is then thoroughly mixed and ready for use.
The chief interest in the chemistry of ferrocene after its discovery lies
in the substitution reactions which the cyclopentadienyl rings can undergo,
among these is the Friedal-crafts reaction in which hydrogen is replaced by
an acyl or alkyl group. The electrophilic substitution of ferrocene appears
to proceed by the initial formation of a charge complex between the
electrophile, E. and the orbital electrons of the metal. Support for this
mechanism comes from the discovery that ferrocene can be protonated. The
protination occurs at the metal and it is observed by the very field proton
magnetic resonance. This provides clear evidence for the accessibility of
electrons for bonding, the electrons concerned occupying the highest filled
E2g levels. The intermediate charge transfer complex rearranges, resulting
in partial transfer of charge to the ring undergoing substitution. Finally, loss
of proton gives the substituted ferrocene.
The mechanism for electrophilic substitution is shown below.
The structure of ferrocene was proposed by Wilkinson and coworkers and then independently E. O. Fischer by X-ray diffraction studies,
arrived at the antiprismatic structure.
The X-ray diffraction studies of
Eiland and Dunitz were also important in determining the structure of
ferrocene.
References
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