In
organometallic chemistry, a sandwich compound is a
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
featuring 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 ...
bound by
haptic,
covalent bonds to two
arene
Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past groupin ...
(ring)
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 ...
s. The arenes have the formula , substituted derivatives (for example ) and
heterocyclic
A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and ...
derivatives (for example ). Because the metal is usually situated between the two rings, it is said to be "sandwiched". A special class of sandwich complexes are the
metallocenes
A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are the metallocene de ...
.
The term ''sandwich compound'' was introduced in organometallic nomenclature in 1956 in a report by J. D. Dunitz, L. E. Orgel and R. A. Rich, who confirmed the structure of
ferrocene by
X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
. The correct structure, in which the molecule features an iron atom ''sandwiched'' between two parallel
cyclopentadienyl rings, had been proposed several years previously by
Robert Burns Woodward
Robert Burns Woodward (April 10, 1917 – July 8, 1979) was an American organic chemist. He is considered by many to be the most preeminent synthetic organic chemist of the twentieth century, having made many key contributions to the subject, e ...
and, separately, 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 ...
. The structure helped explain puzzles about ferrocene's
conformers
In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds (refer to figure on single bond rotation). While any two arrangements of atoms in a molec ...
. This result further demonstrated the power of
X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
and accelerated the growth of
organometallic chemistry.
Classes
:
122px, (troticene) is an unsymmetrical sandwich complex. ">(Cycloheptatrienyl)(cyclopentadienyl)titanium (troticene) is an unsymmetrical sandwich complex.
The best known members are the
metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are the metallocene d ...
s of the formula where M =
Cr,
Fe,
Co,
Ni,
Pb,
Zr,
Ru,
Rh,
Os,
Sm,
Ti,
V,
Mo, W, Zn. These species are also called bis(cyclopentadienyl)metal complexes. Other arenes can serve as ligands as well.
*Mixed cyclopentadienyl complexes: . Some examples are where the
fullerene ligand is acting as a cyclopentadienyl analogue.
*Bis(benzene) complexes: , the best known example being
bis(benzene)chromium
Bis(benzene)chromium is the organometallic compound with the formula Cr( η6-C6H6)2. It is sometimes called dibenzenechromium. The compound played an important role in the development of sandwich compounds in organometallic chemistry and is th ...
.
*Bis(cyclooctatetraenyl) complexes: , such as and (both
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).
*Metal–
carborane
Carboranes are electron-delocalized (non-classically bonded) clusters composed of boron, carbon and hydrogen atoms.Grimes, R. N., ''Carboranes 3rd Ed.'', Elsevier, Amsterdam and New York (2016), . Like many of the related boron hydrides, these c ...
complexes (metallacarboranes), a very large and diverse family in which main-group or transition metal ions are coordinated to carborane ligands to form
polyhedral cages ranging in size from 6 to 15 vertices. Examples include bis(
dicarbollide) complexes,
such as and , and small-carborane sandwiches such as and where M is a transition metal and R and R′ are methyl or ethyl.
![KIWJOP](https://upload.wikimedia.org/wikipedia/commons/7/71/KIWJOP.png)
Closely related are the metal complexes containing (
diborolyl) ligands.
In addition to these, other sandwich complexes containing purely inorganic ligands are known, such as and .
Half-sandwich compounds
Monometallic half-sandwich compounds
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are the metallocene d ...
s including just one facially-bound planar organic
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 ...
instead of two gives rise to a still larger family of half-sandwich compounds. The most famous example is probably
methylcyclopentadienyl manganese tricarbonyl
Methylcyclopentadienyl manganese tricarbonyl (MMT or MCMT) is an organomanganese compound with the formula (C5H4CH3)Mn(CO)3. Initially marketed as a supplement for use in leaded gasoline, MMT was later used in unleaded gasoline to increase the o ...
. Such species are occasionally referred to as piano-stool compounds, at least when there are three diatomic ligands in addition to the hydrocarbon "seat" of the piano stool. The name derives from the similarity of the structure to such a "stool" with the seat being a facial planar
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...
, such as
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
or
cyclopentadiene, and the legs being ligands such as
CO or
allyl.
Dimetallic half-sandwich
Compounds such as the
cyclopentadienyliron dicarbonyl dimer
Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula ''η''5-C5H5)Fe(CO)2sub>2, often abbreviated to Cp2Fe2(CO)4, pFe(CO)2sub>2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crysta ...
and
cyclopentadienylmolybdenumtricarbonyl dimer can be considered a special case of half-sandwiches, except that they are dimetallic.
A structurally related species is .
Multidecker sandwiches
The first isolated multidecker sandwich was the tris(
cyclopentadienyl)di
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
triple-decker complex , a highly air- and water-sensitive compound reported in 1972, with X-ray crystallographic confirmation in 1974.
In 1973 the electrically neutral air-stable triple-decker cobaltacarborane sandwiches 1,7,2,3- and 1,7,2,4- (where R = H, Me) were isolated and characterized by multinuclear
NMR
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 ...
and
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
studies
(the structure of the 1,7,2,3 isomer is shown).
![Co2C2B3_triple-decker1](https://upload.wikimedia.org/wikipedia/commons/9/94/Co2C2B3_triple-decker1.jpg)
Since then many three-, four-, five-, and six-decker sandwich complexes have been described. The largest structurally characterized multidecker sandwich monomer is the hexadecker shown at lower right.
An extensive family of multidecker sandwiches incorporating planar (diborolyl) ligands has also been prepared.
Numerous multidecker sandwich compounds featuring hydrocarbon bridging rings have also been prepared, especially triple deckers. A versatile method involves the attachment of
Cp*Ru
+ to preformed sandwich complexes.
Linked sandwiches
Monomeric double-decker and multidecker sandwiches have been used as building blocks for extended systems, some of which exhibit electron delocalization between metal centers.
An example of a cyclic poly(metallacarborane) complex is the octahedral "carbon-wired" system shown below, which contains a planar macrocycle.
Inverse sandwiches
In these ''anti''-bimetallic compounds, the metals are found to be bridged by a single carbocyclic ring. Examples include and .
Double- and multimetallic sandwich compounds
Another family of sandwich compound involves more than one metal sandwiched between two carbocyclic rings. Examples of the double sandwich include , and . Depicted at right is an example of a multimetallic sandwich compound, which has four
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
atoms joined in a chain sandwiched between two
perylene
Perylene or perilene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12, occurring as a brown solid. It or its derivatives may be carcinogenic, and it is considered to be a hazardous pollutant. In cell membrane cytochemistry, ...
units. The
counterions are bulky
tetraarylborates.
Applications
Ferrocene and
methylcyclopentadienyl manganese tricarbonyl
Methylcyclopentadienyl manganese tricarbonyl (MMT or MCMT) is an organomanganese compound with the formula (C5H4CH3)Mn(CO)3. Initially marketed as a supplement for use in leaded gasoline, MMT was later used in unleaded gasoline to increase the o ...
have been used as
antiknock agent
An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating by raising the temperature and pressure at which auto-ignition occurs. The mixture known as gasoline or petrol, when used in high comp ...
s. Certain bent metallocenes of zirconium and hafnium are effective precatalysts for the polymerization of
propylene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petro ...
. Many half sandwich complexes of ruthenium, such as those derived from
(cymene)ruthenium dichloride dimer catalyse
transfer hydrogenation
In chemistry, transfer hydrogenation is a chemical reaction involving the addition of hydrogen to a compound from a source other than molecular . It is applied in laboratory and industrial organic synthesis to saturate organic compounds and redu ...
, a useful reaction in
organic synthesis.
:
References
{{organometallics