Metallocene
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A metallocene is a compound typically consisting of two cyclopentadienyl anions (, 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 typi ...
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. ...
II, with the resulting general formula Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride. 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 recycl ...
properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to p2ZrCH3sup>+ catalyze
olefin polymerization In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
. Some metallocenes consist of metal plus two cyclooctatetraenide anions (, abbreviated cot2−), namely the lanthanocenes and the actinocenes ( uranocene and others). Metallocenes are a subset of a broader class of compounds called sandwich compounds. 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.


History

The first metallocene to be classified was
ferrocene Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, ...
, and was discovered simultaneously in 1951 by Kealy and Pauson, and Miller et al. Kealy and Pauson were attempting to synthesize fulvalene through the oxidation of a cyclopentadienyl salt with anhydrous FeCl3 but obtained instead the substance C10H10Fe At the same time, Miller ''et al'' reported the same iron product from a reaction of
cyclopentadiene Cyclopentadiene is an organic compound with the formula C5H6.LeRoy H. Scharpen and Victor W. Laurie (1965): "Structure of cyclopentadiene". ''The Journal of Chemical Physics'', volume 43, issue 8, pages 2765-2766. It is often abbreviated CpH beca ...
with iron in the presence of aluminum, potassium, or molybdenum oxides. The structure of "C10H10Fe" was determined by Geoffrey Wilkinson et al. and by Ernst Otto Fischer 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 ele ...
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 c ...
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, metallocenes of many elements have been prepared. One of the very earliest commercial manufacturers of metallocenes was Arapahoe Chemicals in Boulder, Colorado


Definition

The general name metallocene is derived from
ferrocene Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, ...
, (C5H5)2Fe or Cp2Fe, 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 c ...
and two cyclopentadienyl ligands coordinated in a sandwich structure, i.e., the two cyclopentadienyl anions are on parallel
planes Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * ''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 ...
s and strengths. Using the nomenclature of " hapticity", 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, which has two cyclooctatetraene 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 weakly ...
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 (Cp2Ba), or structures where the aromatic rings are not parallel, such as found in manganocene or titanocene dichloride (Cp2TiCl2). 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 inf ...
s have been reported where there are three cyclopentadienyl ligands for a monometallic complex, all three of them bound η5.


Classification

There are many (''η''5-C5H5)–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 #
Bent metallocene In organometallic chemistry, bent metallocenes are a subset of metallocenes. In bent metallocenes, the ring systems coordinated to the metal are not parallel, but are tilted at an angle. A common example of a bent metallocene is Cp2TiCl2. Several ...
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. :MCl2 + 2 NaC5H5 → (C5H5)2M + 2 NaCl (M = V, Cr, Mn, Fe, Co; solvent = THF, DME, NH3) :CrCl3 + 3 NaC5H5C5H5)2Cr+  "C10H10" + 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 + C5H6 → MC5H5 +  H2 (M = Li, Na, K) :M + 2 C5H6C5H5)2M+ H2 (M = Mg, Fe)


Using cyclopentadienyl reagents

A variety of reagents have been developed that transfer Cp to metals. Once popular was thallium cyclopentadienide. It reacts with metal halides to give thallium chloride, which is poorly soluble, and the cyclopentadienyl complex. Trialkyl tin derivatives of Cp have also been used. Many other methods have been developed. Chromocene 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; 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 C5H6 → Cr(C5H5)2 + 6 CO + H2 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 Sublimation or sublimate may refer to: * ''Sublimation'' (album), by Canvas Solaris, 2004 * Sublimation (phase transition), directly from the solid to the gas phase * Sublimation (psychology), a mature type of defense mechanism * Sublimate of mer ...
. 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 MCp2 involves the variation of the M-C bonds, which elongate as the valence electron count deviates from 18. In metallocenes of the type (C5R5)2M, the cyclopentadienyl rings rotate with very low barriers. Single crystal X-ray diffraction studies reveal both eclipsed or
staggered Stagger or staggered may refer to: * Stagger (aeronautics), the horizontal positioning of a plane's wings *Stagger, a motorsport term for the difference in size between right and left tires * ''Stagger'' (EP), by Poppy, 2022 * ''Staggered'' (f ...
rotamers. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few
kJ/mol The joule per mole (symbol: J·mol−1 or J/mol) is the unit of energy per amount of substance in the International System of Units (SI), such that energy is measured in joules, and the amount of substance is measured in moles. It is also an SI ...
. 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 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 aroun ...
and
Raman Raman may refer to: People * Raman (name) *C. V. Raman (1888–1970), Indian Nobel Prize-winning physicist Places * Raman, Punjab (India) * Raman, Rawalpindi, Pakistan * Raman District, Yala Province, Thailand ** Raman Railway Station * Ra ...
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 Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...
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 Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is u ...
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 number ...
distribution of the metal. The three major fragments observed in mass spectrometry are the molecular ion peak, 10H10Msup>+, and fragment ions, 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 polymerizations to give soluble high molecular weight polymers with transition metals in the polymer backbone. Ansa-metallocenes 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 Valence or valency may refer to: Science * Valence (chemistry), a measure of an element's combining power with other atoms * Degree (graph theory), also called the valency of a vertex in graph theory * Valency (linguistics), aspect of verbs rel ...
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. * Ruthenocene 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 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 ...
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 f ...
ions p2Rhsup>+ are very stable, unlike the neutral monomers Cp2Rh which dimerize immediately at room temperature and they have been observed in matrix isolation.


Multi-decker sandwich compounds

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, , 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 In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
. Unlike traditional and still dominant heterogeneous Ziegler–Natta catalysts, metallocene catalysts are homogeneous. Early metal metallocene derivatives, e.g. Tebbe's reagent, Petasis reagent, 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 Princet ...
are useful in specialized organic synthetic operations.


Potential applications

The ferrocene/ ferrocenium
biosensor A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physical chemistry, physicochemical detector. The ''sensitive biological element'', e.g. tissue, microorganisms, or ...
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 (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
cycles. Metallocene dihalides p2MX2(M = Ti, Mo, Nb) exhibit anti-tumor properties, although none have proceeded far in clinical trials.


See also

* Jemmis ''mno'' rules * Actinocenes *
f-block metallocene In organometallic chemistry, f-block metallocenes are a class of sandwich compounds consisting of an Block (periodic table)#f-block, f-block metal and a set of electron-rich ligands such as the cyclopentadienyl anion. History The first prepar ...


References


Additional references

* * * * * {{Coordination complexes