![Metallocene](https://upload.wikimedia.org/wikipedia/commons/e/e4/Metallocene.svg)
A metallocene is a compound typically consisting of two
cyclopentadienyl anion
In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of and abbreviated as Cp−. It is formed from the deprotonation of the molecule cyclopentadiene.
Properties
The cyclopentadienyl anion i ...
s (, abbreviated Cp) bound to a
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
center (M) in the
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
II, with the resulting general formula Closely related to the metallocenes are the metallocene derivatives, e.g.
titanocene dichloride
Titanocene dichloride is the organotitanium compound with the formula ( ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowl ...
,
vanadocene dichloride
Vanadocene dichloride is an organometallic complex with formula (hapticity, ''η''5-cyclopentadienyl, C5H5)2VCl2 (commonly abbreviated as Cp2VCl2). It is a structural analogue of titanocene dichloride but with vanadium(IV) instead of titanium(IV). ...
. Certain metallocenes and their derivatives exhibit
catalytic
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to
2ZrCH3">p2ZrCH3sup>+ catalyze
olefin polymerization.
Some metallocenes consist of metal plus two
cyclooctatetraenide anion
In chemistry, the cyclooctatetraenide anion or cyclooctatetraenide, more precisely cyclooctatetraenediide, is an aromatic species with a formula of 8H8sup>2− and abbreviated as COT2−. It is the dianion of cyclooctatetraene. Salts of ...
s (, abbreviated cot
2−), namely the lanthanocenes and the
actinocene
Actinocenes are a family of organoactinide compounds consisting of metallocenes containing elements from the actinide series. They typically have a sandwich structure with two dianionic cyclooctatetraenyl ligands (COT2-, which is ) bound to an ...
s (
uranocene
Uranocene, U(C8H8)2, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraenide rings. It was one of the first organoactinide compounds to be synthesized. It is a green air-sensitive solid that dissolves in ...
and others).
Metallocenes are a subset of a broader class of compounds called
sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic, covalent bonds to two arene (ring) ligands. The arenes have the formula , substituted derivatives (for example ) and heterocyclic deriv ...
s.
In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are
aromatically stabilized. Here they are shown in a
staggered conformation
In organic chemistry, a staggered conformation is a chemical conformation of an ethane-like moiety abcX–Ydef in which the substituents a, b, and c are at the maximum distance from d, e, and f; this requires the torsion angles to be 60°. It ...
.
History
![Ferrocene](https://upload.wikimedia.org/wikipedia/commons/e/e9/Ferrocene.svg)
The first metallocene to be classified was
ferrocene, and was discovered simultaneously in 1951 by Kealy and Pauson,
and Miller et al.
Kealy and Pauson were attempting to synthesize
fulvalene
Fulvalene (bicyclopentadienylidene) is the member of the fulvalene family with the molecular formula C10H8. It is of theoretical interest as one of the simplest non-benzenoid conjugated hydrocarbons. Fulvalene is an unstable isomer of the mor ...
through the oxidation of a
cyclopentadienyl salt with anhydrous FeCl
3 but obtained instead the substance C
10H
10Fe
At the same time, Miller ''et al'' reported the same iron product from a reaction of
cyclopentadiene with iron in the presence of aluminum, potassium, or molybdenum oxides.
The structure of "C
10H
10Fe" was determined by
Geoffrey Wilkinson
Sir Geoffrey Wilkinson FRS (14 July 1921 – 26 September 1996) was a Nobel laureate English chemist who pioneered inorganic chemistry and homogeneous transition metal catalysis.
Education and early life
Wilkinson was born at Springside, Todm ...
et al.
and by
Ernst Otto Fischer
Ernst Otto Fischer (; 10 November 1918 – 23 July 2007) was a German chemist who won the Nobel Prize for pioneering work in the area of organometallic chemistry.
Early life
He was born in Solln, a borough of Munich. His parents were Karl T. Fi ...
et al. These two were awarded the
Nobel Prize in Chemistry
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
in 1973 for their work on sandwich compounds, including the structural determination of ferrocene.
They determined that the carbon atoms of the cyclopentadienyl (Cp)
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
contributed equally to the bonding and that bonding occurred due to the metal and the in the of the Cp ligands. This complex is now known as ferrocene, and the group of
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...
dicyclopentadienyl compounds is known as metallocenes. Metallocenes have the general formula Fischer et al. first prepared the ferrocene derivatives involving Co and Ni. Often derived from substituted derivatives of
cyclopentadienide
In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of and abbreviated as Cp−. It is formed from the deprotonation of the molecule cyclopentadiene.
Properties
The cyclopentadienyl anion i ...
, metallocenes of many elements have been prepared.
One of the very earliest commercial manufacturers of metallocenes was Arapahoe Chemicals in Boulder, Colorado
Definition
![Ferrocene-from-xtal-3D-balls](https://upload.wikimedia.org/wikipedia/commons/7/7a/Ferrocene-from-xtal-3D-balls.png)
The general name metallocene is derived from
ferrocene, (C
5H
5)
2Fe or Cp
2Fe, systematically named According to the
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
definition, a metallocene contains a
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...
and two cyclopentadienyl ligands coordinated in a sandwich structure, i.e., the two cyclopentadienyl anions are on parallel
planes with equal
bond length
In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...
s and strengths. Using the nomenclature of "
hapticity
In coordination chemistry, hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η2 describes a l ...
", the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted as ''η''
5, pronounced "pentahapto". There are exceptions, such as
uranocene
Uranocene, U(C8H8)2, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraenide rings. It was one of the first organoactinide compounds to be synthesized. It is a green air-sensitive solid that dissolves in ...
, which has two
cyclooctatetraene
1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of ...
rings sandwiching a
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
atom.
In metallocene names, the prefix before the ' ending indicates what
metallic element is between the Cp groups. For example, in ferrocene, iron(II), ferrous iron is present.
In contrast to the more strict definition proposed by International Union of Pure and Applied Chemistry, which requires a d-block metal and a sandwich structure, the term metallocene and thus the denotation ', is applied in the chemical literature also to non-transition metal compounds, such as
barocene (Cp
2Ba), or structures where the aromatic rings are not parallel, such as found in
manganocene or
titanocene dichloride
Titanocene dichloride is the organotitanium compound with the formula ( ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowl ...
(Cp
2TiCl
2).
Some metallocene complexes of
actinide
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The info ...
s have been reported where there are three cyclopentadienyl ligands for a monometallic complex, all three of them bound η
5.
Classification
There are many (''η''
5-C
5H
5)–metal complexes and they can be classified by the following formulas:
Metallocene complexes can also be classified by type:
#Parallel
#Multi-decker
#
Half-sandwich compound
Half sandwich compounds, also known as piano stool complexes, are organometallic complexes that feature a cyclic polyhapto ligand bound to an MLn center, where L is a unidentate ligand. Thousands of such complexes are known. Well-known examples in ...
#
Bent metallocene or tilted
#More than two Cp ligands
Synthesis
Three main routes are normally employed in the formation of these types of compounds:
Using a metal salt and cyclopentadienyl reagents
Sodium cyclopentadienide (NaCp) is the preferred reagent for these types of reactions. It is most easily obtained by the reaction of molten sodium and dicyclopentadiene. Traditionally, the starting point is the cracking of
dicyclopentadiene
Dicyclopentadiene, abbreviated DCPD, is a chemical compound with formula C10H12. At room temperature, it is a white brittle wax, although lower purity samples can be straw coloured liquids. The pure material smells somewhat of soy wax or camphor ...
, the dimer of cyclopentadiene. Cyclopentadiene is deprotonated by strong bases or alkali metals.
:MCl
2 + 2 NaC
5H
5 → (C
5H
5)
2M + 2 NaCl (M = V, Cr, Mn, Fe, Co; solvent = THF, DME, NH
3)
:CrCl
3 + 3 NaC
5H
5 →
5H5)2Cr">C5H5)2Cr+ "C
10H
10" + 3 NaCl
NaCp acts as a reducing agent and a ligand in this reaction.
Using a metal and cyclopentadiene
This technique provides using metal atoms in the gas phase rather than the solid metal. The highly reactive atoms or molecules are generated at a high temperature under vacuum and brought together with chosen reactants on a cold surface.
:M + C
5H
6 → MC
5H
5 + H
2 (M = Li, Na, K)
:M + 2 C
5H
6 →
5H5)2M">C5H5)2M+ H
2 (M = Mg, Fe)
Using cyclopentadienyl reagents
A variety of reagents have been developed that transfer Cp to metals. Once popular was
thallium cyclopentadienide
Cyclopentadienylthallium, also known as thallium cyclopentadienide, is an organothallium compound with formula C5H5Tl. This light yellow solid is insoluble in most organic solvents, but sublimes readily. It is used as a precursor to transition m ...
. It reacts with metal halides to give thallium chloride, which is poorly soluble, and the
cyclopentadienyl complex
A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (, abbreviated as Cp−). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (''η''5-) bonding mode. The metal–cyclopentadien ...
. Trialkyl
tin
Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal.
Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
derivatives of Cp
− have also been used.
Many other methods have been developed.
Chromocene
Chromocene is the organochromium compound with the formula r(C5H5)2 Like structurally related metallocenes, chromocene readily sublimes in a vacuum and is soluble in non-polar organic solvents. It is more formally known as bis(η5-cyclopentadieny ...
can be prepared from
chromium hexacarbonyl
Chromium carbonyl, also known as chromium hexacarbonyl, is the chemical compound with the formula Cr( CO)6. At room temperature the solid is stable to air, although it does have a high vapor pressure and sublimes readily. Cr(CO)6 is zerovalent ...
by direct reaction with cyclopentadiene in the presence of
diethylamine
Diethylamine is an organic compound with the formula (CH3CH2)2NH. It is a secondary amine. It is a flammable, weakly alkaline liquid that is miscible with most solvents. It is a colorless liquid, but commercial samples often appear brown due to im ...
; in this case, the formal deprotonation of the cyclopentadiene is followed by
reduction of the resulting protons to
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
gas, facilitating the
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
of the metal centre.
:Cr(CO)
6 + 2 C
5H
6 → Cr(C
5H
5)
2 + 6 CO + H
2
Metallocenes generally have high thermal stability. Ferrocene can be sublimed in air at over 100 °C with no decomposition; metallocenes are generally purified in the laboratory by vacuum
sublimation. Industrially, sublimation is not practical so metallocenes are isolated by crystallization or produced as part of a hydrocarbon solution. For Group IV metallocenes, donor solvents like ether or THF are distinctly undesirable for polyolefin catalysis. Charge-neutral metallocenes are soluble in common organic solvents. Alkyl substitution on the metallocene increases the solubility in hydrocarbon solvents.
Structure
A structural trend for the series MCp
2 involves the variation of the M-C bonds, which elongate as the valence electron count deviates from 18.
In metallocenes of the type (C
5R
5)
2M, the cyclopentadienyl rings rotate with very low barriers. Single crystal
X-ray diffraction studies reveal both
eclipsed or
staggered rotamers. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few
kJ/mol. Crystals of ferrocene and osmocene exhibit eclipsed conformations at low temperatures, whereas in the related bis(pentamethylcyclopentadienyl) complexes the rings usually crystallize in a staggered conformation, apparently to minimize
steric hindrance
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
between the
methyl group
In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ma ...
s.
Spectroscopic properties
Vibrational (infrared and Raman) spectroscopy of metallocenes
Infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
and
Raman spectroscopies have proved to be important in the analysis of cyclic polyenyl metal sandwich species, with particular use in elucidating covalent or ionic M–ring bonds and distinguishing between central and coordinated rings. Some typical spectral bands and assignments of iron group metallocenes are shown in the following table:
NMR (1H and 13C) spectroscopy of metallocenes
Nuclear magnetic resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
(NMR) is the most applied tool in the study of metal sandwich compounds and organometallic species, giving information on nuclear structures in solution, as liquids, gases, and in the solid state.
1H NMR chemical shifts for paramagnetic organotransition-metal compounds is usually observed between 25 and 40 ppm, but this range is much more narrow for diamagnetic metallocene complexes, with chemical shifts usually observed between 3 and 7 ppm.
Mass spectrometry of metallocenes
Mass spectrometry of metallocene complexes has been very well studied and the effect of the metal on the fragmentation of the organic moiety has received considerable attention and the identification of metal-containing fragments is often facilitated by the
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...
distribution of the metal. The three major fragments observed in mass spectrometry are the molecular ion peak,
10H10M">10H10Msup>+, and fragment ions,
5H5M">5H5Msup>+ and M
+.
Derivatives
After the discovery of ferrocene, the synthesis and characterization of derivatives of metallocene and other sandwich compounds attracted researchers’ interests.
Metallocenophanes
Metallocenophanes feature linking of the cyclopentadienyl or polyarenyl rings by the introduction of one or more heteroannular bridges. Some of these compounds undergo thermal
ring-opening polymerization
In polymer chemistry, ring-opening polymerization (ROP) is a form of chain-growth polymerization, in which the terminus of a polymer chain attacks cyclic monomers to form a longer polymer (see figure). The reactive center can be radical, anion ...
s to give soluble high molecular weight polymers with transition metals in the polymer backbone.
Ansa-metallocene
An ''ansa''-metallocene is a type of organometallic compound containing two cyclopentadienyl ligands that are linked by a bridging group such that both cyclopentadienyl groups are bound to the same metal. The link prevents rotation of the cyclopen ...
s are derivatives of metallocenes with an intramolecular
bridge
A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...
between the two cyclopentadienyl rings.
Polynuclear and heterobimetallic metallocenes
*Ferrocene derivatives: biferrocenophanes have been studied for their mixed
valence properties. Upon one-electron oxidation of a compound with two or more equivalent ferrocene moieties, the electron vacancy could be localized on one ferrocene unit or completely
delocalized
In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref>
The term delocalization is general and can have slightly dif ...
.
*
Ruthenocene
Ruthenocene is an organoruthenium compound with the formula (C5H5)2Ru. This pale yellow, volatile solid is classified as a sandwich compound and more specifically, as a metallocene.
Structure and bonding
Ruthenocene consists of a ruthenium ion s ...
derivatives: in the solid state biruthenocene is disordered and adopts the transoid conformation with the mutual orientation of Cp rings depending on the intermolecular interactions.
*
Vanadocene
Vanadocene, bis(η5-cyclopentadienyl) vanadium, is the organometallic compound with the formula V(C5H5)2, commonly abbreviated Cp2V. It is a violet crystalline, paramagnetic solid. Vanadocene has relatively limited practical use, but it has been ex ...
and
rhodocene
Rhodocene is a chemical compound with the formula . Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compo ...
derivatives: vanadocene complexes have been used as starting materials for the synthesis of heterobimetallic complexes. The 18
valence electron
In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair form ...
ions
2Rh">p2Rhsup>+ are very stable, unlike the neutral monomers Cp
2Rh which
dimerize
A dimer () ('' di-'', "two" + ''-mer'', "parts") is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, covalent or intermolecular. Dimers also have significant implications in polymer chemistry, inorganic ch ...
immediately at room temperature and they have been observed in
matrix isolation
Matrix isolation is an experimental technique used in chemistry and physics. It generally involves a material being trapped within an unreactive matrix. A ''host'' matrix is a continuous solid phase in which ''guest'' particles (atoms, molecules, i ...
.
Multi-decker sandwich compounds
![Ni-triple-sandwich](https://upload.wikimedia.org/wikipedia/commons/c/cc/Ni-triple-sandwich.png)
Triple-decker complexes are composed of three Cp anions and two metal cations in alternating order. The first triple-decker sandwich complex, , was reported in 1972. Many examples have been reported subsequently, often with
boron-containing rings.
Metallocenium cations
The most famous example is
ferrocenium
Ferrocenium tetrafluoroborate is an organometallic compound with the formula e(C5H5)2F4. This salt is composed of the cation e(C5H5)2sup>+ and the tetrafluoroborate anion (). The related hexafluorophosphate is also a popular reagent with simi ...
, , the blue iron(III) complex derived from oxidation of orange iron(II) ferrocene (few metallocene anions are known).
Applications
Many derivatives of early metal metallocenes are active catalysts for
olefin polymerization. Unlike traditional and still dominant heterogeneous
Ziegler–Natta catalysts, metallocene catalysts are homogeneous.
Early metal metallocene derivatives, e.g.
Tebbe's reagent
Tebbe's reagent is the organometallic compound with the formula (C5H5)2TiCH2ClAl(CH3)2. It is used in the methylenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative. It ...
,
Petasis reagent
The Petasis reagent, named after Nicos A. Petasis, is an organotitanium compound with the formula Cp2Ti(CH3)2. It is an orange-colored solid.
Preparation and use
The Petasis reagent is prepared by the salt metathesis reaction of methylmagnesium ...
, and
Schwartz's reagent
Schwartz's reagent is the common name for the organozirconium compound with the formula (C5H5)2ZrHCl, sometimes called zirconocene hydrochloride or zirconocene chloride hydride, and is named after Jeffrey Schwartz, a chemistry professor at Prince ...
are useful in specialized organic synthetic operations.
Potential applications
The ferrocene/
ferrocenium
Ferrocenium tetrafluoroborate is an organometallic compound with the formula e(C5H5)2F4. This salt is composed of the cation e(C5H5)2sup>+ and the tetrafluoroborate anion (). The related hexafluorophosphate is also a popular reagent with simi ...
biosensor
A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector.
The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell rece ...
has been discussed for determining the levels of glucose in a sample electrochemically through a series of connected
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
cycles.
Metallocene dihalides
2MX2">p2MX2(M = Ti, Mo, Nb) exhibit anti-tumor properties, although none have proceeded far in clinical trials.
See also
*
Jemmis ''mno'' rules
*
Actinocenes
Actinocenes are a family of organoactinide compounds consisting of metallocenes containing elements from the actinide series. They typically have a sandwich structure with two dianionic cyclooctatetraenyl ligands (COT2-, which is ) bound to an a ...
*
f-block metallocene
References
Additional references
*
*
*
*
*
{{Coordination complexes