organochromium chemistry
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Organochromium chemistry is a branch of
organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
that deals with
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. The ...
s containing a
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...
to
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
bond and their reactions. The field is of some relevance to
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
. The relevant oxidation states for organochromium complexes encompass the entire range of possible oxidation states from –4 (d10) in Na4 r–IV(CO)4to +6 (d0) in oxo-alkyl complexes like Cp*CrVI(=O)2Me.


History

The first organochromium compound was described in 1919 by
Franz Hein Franz Hein (30 June 1892 – 26 February 1976) was a German scientist and artist. History Franz Hein was born in Grötzingen (Baden), Germany. His high school years were spent in Leipzig, as well as, his college years at the University of Leipzig. ...
. He treated
phenylmagnesium bromide Phenylmagnesium bromide, with the simplified formula , is a magnesium-containing organometallic compound. It is commercially available as a solution in diethyl ether or tetrahydrofuran (THF). Phenylmagnesium bromide is a Grignard reagent. It is o ...
with chromium(III) chloride to give a new product (after hydrolysis) which he incorrectly identified as pentaphenyl chromium bromide (Ph5CrBr). Years later, in 1957 H.H. Zeiss et al. repeated Hein's experiments and correctly arrived at a cationic bisarene chromium sandwich compound (ArH2Cr+).
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 ...
itself was discovered around the same time in 1956 by Ernst Otto Fischer by reaction of chromium(III) chloride,
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, ...
and aluminum chloride. The related compound chromocene had been discovered a few years earlier in 1953 also by Fischer. Anet and Leblanc also in 1957 prepared a benzyl chromium solution from
benzyl bromide Benzyl bromide is an organic compound with the formula . The molecule consists of a benzene ring substituted with a bromomethyl group. It is a colorless liquid with lachrymatory properties. The compound is a reagent for introducing benzyl groups ...
and
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...
(II)
perchlorate A perchlorate is a chemical compound containing the perchlorate ion, . The majority of perchlorates are commercially produced salts. They are mainly used as oxidizers for pyrotechnic devices and to control static electricity in food packaging. Per ...
. This reaction involves one-electron oxidative addition of the carbon-bromine bond, a process which was shown by Kochi to be a case of double single electron transfer, first to give the benzyl free radical and then to the benzyl
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
. : G. Wilke et al. introduced tris-(η-allyl)chromium in 1963 as an early Ziegler–Natta catalyst, albeit of limited commercial success. Chromocene compounds were first employed in
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
in 1972 by
Union Carbide Union Carbide Corporation is an American chemical corporation wholly owned subsidiary (since February 6, 2001) by Dow Chemical Company. Union Carbide produces chemicals and polymers that undergo one or more further conversions by customers befor ...
and continue to be used today in the industrial production of
high-density polyethylene High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, ...
. The organochromium compound (phenylmethoxycarbene)pentacarbonylchromium, Ph(OCH3)C=Cr(CO)5 was the first
carbene In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. The term "carbene" ma ...
complex to be crystallographically characterized by Fischer in 1967 (now called a Fischer carbene). The first ever
carbyne In organic chemistry, a carbyne is a general term for any compound whose structure consists of an electrically neutral carbon atom connected by a single covalent bond and has three non-bonded electrons. The carbon atom has either one or three ...
, this one also containing chromium, made its debut in 1973. : The first example of a proposed metal-metal quintuple bond is found in a compound of the type
rAr RAR or Rar may refer to: * Radio acoustic ranging, a non-visual technique for determining a ship's position at sea * "rar", the ISO 639-2 code for the Cook Islands Māori language * RAR (file format), a proprietary compressed archive file format in ...
sub>2, where Ar is a bulky aryl ligand.


Applications in organic synthesis

Although organochromium chemistry is heavily employed in industrial catalysis, relatively few reagents have been developed for applications in
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
. Two are the Nozaki-Hiyama-Kishi reaction (1977) (transmetallation with organonickel intermediate) and the
Takai olefination Takai olefination in organic chemistry describes the organic reaction of an aldehyde with a diorganochromium compound to form an alkene. It is a name reaction, referencing Kazuhiko Takai, who first reported it in 1986. In the original reaction, ...
(1986)(oxidation of Cr(II) to Cr(III) while replacing halogens). In a niche exploit, certain tricarbonyl(arene)chromium complexes display benzylic activation.


Organochromium compounds

Organochromium compounds can be divided into these broad compound classes: * Sandwich compounds: chromocene Cp2Cr and
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 ...
derivatives (ArH)2Cr. More commonly studied are
half-sandwich complex 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 i ...
es such as 6-C6H5OMe)Cr(CO)3. * Chromium
carbene In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. The term "carbene" ma ...
s (R1)(R2)C::CrLn and
carbyne In organic chemistry, a carbyne is a general term for any compound whose structure consists of an electrically neutral carbon atom connected by a single covalent bond and has three non-bonded electrons. The carbon atom has either one or three ...
s (RC:::CrLn) * Chromium(III) complexes RCrL5. *Complexes of chromium carbonyl anion and cation (e.g Na4Cr(CO)4).


Ethylene polymerization and oligomerization

Chromium catalysts are important in
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
. The Phillips catalyst is prepared by impregnating
chromium(VI) oxide Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple s ...
on
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
followed activation in dry air at high temperatures. The bright yellow catalyst becomes reduced by the ethylene to afford a probable Cr(II) species that is catalytically active. A related catalytic systems developed by
Union Carbide Union Carbide Corporation is an American chemical corporation wholly owned subsidiary (since February 6, 2001) by Dow Chemical Company. Union Carbide produces chemicals and polymers that undergo one or more further conversions by customers befor ...
and DSM are also based on silica with chromocene and other chromium complexes. How these catalysts work is unclear. One model system describes it as
coordination polymerization Coordination polymerisation is a form of polymerization that is catalyzed by transition metal salts and complexes. Types of coordination polymerization of alkenes Heterogeneous Ziegler–Natta polymerization Coordination polymerization started in t ...
: : With two
THF Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...
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 electr ...
s the catalyst is stable but in
dichloromethane Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...
one ligand is lost to form a 13 electron chromium intermediate. This enables side-on addition of an ethylene unit and a polymer chain can grow by migratory insertion. Chromium compounds also catalyse the trimerization of ethylene to produce the monomer 1-hexene.


References

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