Organokovinska kemija

n-butillitij, organokovinska spojina. Štirje atomi litija (v vijolični barvi) tvorijo tetraeder s štirimi butilnimi skupinami pritrjenimi na ploskve (ogljik je črn, vodik je bel).

Organokovinska kemija se ukvarja s preučevanjem organokovinskih spojin. To so kemijske spojine, ki vsebujejo vsaj eno kemijsko vez med ogljikovim atomom organske spojine in kovino. Ta kovina je lahko alkalijska, zemljoalkalijska ali prehodna, včasih je lahko tudi polkovina (npr. bor, silicij in selen). Template:Sfn<ref name=":0">Template:GoldBookRef</ref> Aside from bonds to organyl fragments or molecules, bonds to 'inorganic' carbon, like carbon monoxide (metal carbonyls), cyanide, or carbide, are generally considered to be organometallic as well. Some related compounds such as transition metal hydrides and metal phosphine complexes are often included in discussions of organometallic compounds, though strictly speaking, they are not necessarily organometallic. The related but distinct term "metalorganic compound" refers to metal-containing compounds lacking direct metal-carbon bonds but which contain organic ligands. Metal β-diketonates, alkoxides, dialkylamides, and metal phosphine complexes are representative members of this class. The field of organometallic chemistry combines aspects of traditional inorganic and organic chemistry.Template:Sfn

Organometallic compounds are widely used both stoichiometrically in research and industrial chemical reactions, as well as in the role of catalysts to increase the rates of such reactions (e.g., as in uses of homogeneous catalysis), where target molecules include polymers, pharmaceuticals, and many other types of practical products.

Organometallic compounds

File:Magnesium bis-cyclopentadienyl bottle.jpg

A stainless bottle containing MgCp₂ (magnesium bis-cyclopentadienyl), a hazardous substance like most other organometallics. The text states "Federal law forbids transportation, if refilled penalty up to $25,000 fine and 5 year imprisonment."

Organometallic compounds are distinguished by the prefix "organo-" (e.g., organopalladium compounds), and include all compounds which contain a bond between a metal atom and a carbon atom of an organyl group.<ref name=":0" /> In addition to the traditional metals (alkali metals, alkali earth metals, transition metals, and post transition metals), lanthanides, actinides, semimetals, and the elements boron, silicon, arsenic, and selenium are considered to form organometallic compounds.<ref name=":0" /> Examples of organometallic compounds include Gilman reagents, which contain lithium and copper, and Grignard reagents, which contain magnesium. Tetracarbonyl nickel and ferrocene are examples of organometallic compounds containing transition metals. Other examples of organometallic compounds include organolithium compounds such as n-butyllithium (n-BuLi), organozinc compounds such as diethylzinc (Et₂Zn), organotin compounds such as tributyltin hydride (Bu₃SnH), organoborane compounds such as triethylborane (Et₃B), and organoaluminium compounds such as trimethylaluminium (Me₃Al).

A naturally occurring organometallic complex is methylcobalamin (a form of Vitamin B₁₂), which contains a cobalt-methyl bond. This complex, along with other biologically relevant complexes are often discussed within the subfield of bioorganometallic chemistry.Template:Sfn

• Representative Organometallic Compounds
• Ferrocene.svg

  Ferrocene is an archetypal organoiron complex. It is an air-stable, sublimable compound.

• Cobaltocene.svg

  Cobaltocene is a structural analogue of ferrocene, but is highly reactive toward air.

• HRh(CO)P3again.png

  Tris(triphenylphosphine)rhodium carbonyl hydride is used in the commercial production of many aldehyde-based fragrances.

• Zeise'sSalt.png

  Zeise's salt is an example of a transition metal alkene complex.

• Trimethylaluminium-from-xtal-3D-bs-17.png

  Trimethylaluminium is an organometallic compound with a bridging methyl group. It is used in the industrial production of some alcohols.

• Dimethylzinc-3D-balls.png

  Dimethylzinc has a linear coordination. It is a volatile pyrophoric liquid that is used in the preparation of semiconducting films.

• Lithium-diphenylcuprate-etherate-dimer-from-xtal-2D-skeletal.png

  Lithium diphenylcuprate bis(diethyl etherate) is an example of a Gilman reagent, a type of organocopper complex frequently employed in organic synthesis.

• AdoCbl-ColorCoded.png

  Adenosylcobalamin is a cofactor required by several crucial enzymatic reactions that take place in the human body. It is a rare example of a metal (cobalt) alkyl in biology.

• IronPentacarbonylStructure.png

  Iron(0) pentacarbonyl is a red-orange liquid prepared directly from the union of finely divided iron and carbon monoxide gas under pressure.

• Tc99 sestamibi 2D structure.svg

  Technetium[^99mTc] sestamibi is used to image the heart muscle in nuclear medicine.

Distinction from coordination compounds with organic ligands

Many complexes feature coordination bonds between a metal and organic ligands. Complexes where the organic ligands bind the metal through a heteroatom such as oxygen or nitrogen are considered coordination compounds (e.g., heme A and Fe(acac)₃). However, if any of the ligands form a direct metal-carbon (M-C) bond, then the complex is considered to be organometallic. Although the IUPAC has not formally defined the term, some chemists use the term "metalorganic" to describe any coordination compound containing an organic ligand regardless of the presence of a direct M-C bond.<ref>Template:Cite book</ref>

The status of compounds in which the canonical anion has a negative charge that is shared between (delocalized) a carbon atom and an atom more electronegative than carbon (e.g. enolates) may vary with the nature of the anionic moiety, the metal ion, and possibly the medium. In the absence of direct structural evidence for a carbon–metal bond, such compounds are not considered to be organometallic.<ref name=":0" /> For instance, lithium enolates often contain only Li-O bonds and are not organometallic, while zinc enolates (Reformatsky reagents) contain both Zn-O and Zn-C bonds, and are organometallic in nature.

Structure and properties

The metal-carbon bond in organometallic compounds is generally highly covalent.Template:Sfn For highly electropositive elements, such as lithium and sodium, the carbon ligand exhibits carbanionic character, but free carbon-based anions are extremely rare, an example being cyanide.

Most organometallic compounds are solids at room temperature, however some are liquids such as methylcyclopentadienyl manganese tricarbonyl, or even volatile liquids such as nickel tetracarbonyl.Template:Sfn Many organometallic compounds are air sensitive (reactive towards oxygen and moisture), and thus they must be handled under an inert atmosphere.Template:Sfn Some organometallic compounds such as triethylaluminium are pyrophoric and will ignite on contact with air.<ref></ref>

Concepts and techniques

As in other areas of chemistry, electron counting is useful for organizing organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of organometallic complexes, for example metal carbonyls and metal hydrides. The 18e rule has two representative electron counting models, ionic and neutral (also known as covalent) ligand models, respectively.<ref name=":02">Template:Cite book</ref> The hapticity of a metal-ligand complex, can influence the electron count.<ref name=":02" /> Hapticity (η, lowercase Greek eta), describes the number of contiguous ligands coordinated to a metal.<ref name=":02" /> For example, ferrocene, [(η⁵-C₅H₅)₂Fe], has two cyclopentadienyl ligands giving a hapticity of 5, where all five carbon atoms of the C₅H₅ ligand bond equally and contribute one electron to the iron center. Ligands that bind non-contiguous atoms are denoted the Greek letter kappa, κ.<ref name=":02" /> Chelating κ2-acetate is an example. The covalent bond classification method identifies three classes of ligands, X,L, and Z; which are based on the electron donating interactions of the ligand. Many organometallic compounds do not follow the 18e rule. The metal atoms in organometallic compounds are frequently described by their d electron count and oxidation state. These concepts can be used to help predict their reactivity and preferred geometry. Chemical bonding and reactivity in organometallic compounds is often discussed from the perspective of the isolobal principle.

A wide variety of physical techniques are used to determine the structure, composition, and properties of organometallic compounds. X-ray diffraction is a particularly important technique that can locate the positions of atoms within a solid compound, providing a detailed description of its structure.Template:SfnTemplate:Sfn Other techniques like infrared spectroscopy and nuclear magnetic resonance spectroscopy are also frequently used to obtain information on the structure and bonding of organometallic compounds.Template:SfnTemplate:Sfn Ultraviolet-visible spectroscopy is a common technique used to obtain information on the electronic structure of organometallic compounds. It is also used monitor the progress of organometallic reactions, as well as determine their kinetics.Template:Sfn The dynamics of organometallic compounds can be studied using dynamic NMR spectroscopy.Template:Sfn Other notable techniques include X-ray absorption spectroscopy,<ref>Template:Cite journal</ref> electron paramagnetic resonance spectroscopy, and elemental analysis.Template:SfnTemplate:Sfn

Due to their high reactivity towards oxygen and moisture, organometallic compounds often must be handled using air-free techniques. Air-free handling of organometallic compounds typically requires the use of laboratory apparatuses such as a glovebox or Schlenk line.Template:Sfn

History

Early developments in organometallic chemistry include Louis Claude Cadet's synthesis of methyl arsenic compounds related to cacodyl, William Christopher Zeise's<ref>Template:Cite journal</ref> platinum-ethylene complex,<ref>Template:Cite journal</ref> Edward Frankland's discovery of diethyl- and dimethylzinc, Ludwig Mond's discovery of Ni(CO)₄,Template:Sfn and Victor Grignard's organomagnesium compounds. (Though not always acknowledged as an organometallic compound, Prussian blue, a mixed-valence iron-cyanide complex, was first prepared in 1706 by paint maker Johann Jacob Diesbach as the first coordination polymer and synthetic material containing a metal-carbon bond.Template:Sfn) The abundant and diverse products from coal and petroleum led to Ziegler–Natta, Fischer–Tropsch, hydroformylation catalysis which employ CO, H₂, and alkenes as feedstocks and ligands.

Recognition of organometallic chemistry as a distinct subfield culminated in the Nobel Prizes to Ernst Fischer and Geoffrey Wilkinson for work on metallocenes. In 2005, Yves Chauvin, Robert H. Grubbs and Richard R. Schrock shared the Nobel Prize for metal-catalyzed olefin metathesis.<ref>Template:Cite journal</ref>

Organometallic chemistry timeline

• 1760 Louis Claude Cadet de Gassicourt investigates inks based on cobalt salts and isolates cacodyl from cobalt mineral containing arsenic
• 1827 William Christopher Zeise produces Zeise's salt; the first platinum / olefin complex
• 1848 Edward Frankland discovers diethylzinc
• 1863 Charles Friedel and James Crafts prepare organochlorosilanes
• 1890 Ludwig Mond discovers nickel carbonyl
• 1899 Introduction of Grignard reaction
• 1899 John Ulric Nef discovers alkynylation using sodium acetylides.
• 1900 Paul Sabatier works on hydrogenation organic compounds with metal catalysts. Hydrogenation of fats kicks off advances in food industry, see margarine
• 1909 Paul Ehrlich introduces Salvarsan for the treatment of syphilis, an early arsenic based organometallic compound
• 1912 Nobel Prize Victor Grignard and Paul Sabatier
• 1930 Henry Gilman works on lithium cuprates, see Gilman reagent
• 1951 Walter Hieber was awarded the Alfred Stock prize for his work with metal carbonyl chemistry.
• 1951 Ferrocene is discovered
• 1956 Dorothy Crawfoot Hodgkin determines the structure of vitamin B₁₂, the first biomolecule found to contain a metal-carbon bond, see bioorganometallic chemistry
• 1963 Nobel prize for Karl Ziegler and Giulio Natta on Ziegler–Natta catalyst
• 1965 Discovery of cyclobutadieneiron tricarbonyl
• 1968 Heck reaction is developed
• 1973 Nobel prize Geoffrey Wilkinson and Ernst Otto Fischer on sandwich compounds
• 1981 Nobel prize Roald Hoffmann and Kenichi Fukui for creation of the Woodward-Hoffman Rules
• 2001 Nobel prize W. S. Knowles, R. Noyori and Karl Barry Sharpless for asymmetric hydrogenation
• 2005 Nobel prize Yves Chauvin, Robert Grubbs, and Richard Schrock on metal-catalyzed alkene metathesis
• 2010 Nobel prize Richard F. Heck, Ei-ichi Negishi, Akira Suzuki for palladium catalyzed cross coupling reactions

Obseg

Podpodročja organokovinske kemije obsegajo:

• elementi 2. periode: organolitijeva kemija, organoberilijeva kemija, organoborova kemija
• elementi 3. periode: organonatrijeva kemija, organomagnezijeva kemija, organoaluminijeva kemija, organosilicijeva kemija
• elementi 4. periode: organokalcijeva kemija, organoskandijeva kemija, organotitanova kemija, organovanadijeva kemija, organokromova kemija, organomanganova kemija, organoželezova kemija, organokobaltova kemija, organonikljeva kemija, organobakrova kemija, organocinkova kemija, organogalijeva kemija, organogermanijeva kemija, organoarzenova kemija, organoselenova kemija
• elementi 5. periode: organoitrijeva kemija, organocirkonijeva kemija, organoniobijeva kemija, organomolibdenova kemija, organorutenijeva kemija, organorodijeva kemija, organopaladijeva kemija, organosrebrova kemija, organokadmijeva kemija, organoindijeva kemija, organokositrova kemija, organoantimonova kemija, organotelurjeva kemija
• elementi 6. periode: organolantanova kemija, organocerijeva kemija, organotantalova kemija, organorenijeva kemija, organoosmijeva kemija, organoiridijeva kemija, organoplatinska kemija, organozlata kemija, organoživosrebrova kemija, organotalijeva kemija, organosvinčeva kemija, organobizmuntova kemija, organopolonijeva kemija
• elementi 7. periode: organoaktinijeva kemija, organouranova kemija, organoneptunijeva kemija

Industrial applications

Organometallic compounds find wide use in commercial reactions, both as homogenous catalysts and as stoichiometric reagents. For instance, organolithium, organomagnesium, and organoaluminium compounds, examples of which are highly basic and highly reducing, are useful stoichiometrically but also catalyze many polymerization reactions.Template:Sfn

Almost all processes involving carbon monoxide rely on catalysts, notable examples being described as carbonylations.<ref name=Ullmann>Template:Ullmann</ref> The production of acetic acid from methanol and carbon monoxide is catalyzed via metal carbonyl complexes in the Monsanto process and Cativa process. Most synthetic aldehydes are produced via hydroformylation. The bulk of the synthetic alcohols, at least those larger than ethanol, are produced by hydrogenation of hydroformylation-derived aldehydes. Similarly, the Wacker process is used in the oxidation of ethylene to acetaldehyde.Template:Sfn

File:ConstrainedGeomCmpx.png

A constrained geometry organotitanium complex is a precatalyst for olefin polymerization.

Almost all industrial processes involving alkene-derived polymers rely on organometallic catalysts. The world's polyethylene and polypropylene are produced via both heterogeneously via Ziegler–Natta catalysis and homogeneously, e.g., via constrained geometry catalysts.<ref>Template:Cite journal</ref>

Most processes involving hydrogen rely on metal-based catalysts. Whereas bulk hydrogenations (e.g., margarine production) rely on heterogeneous catalysts, for the production of fine chemicals such hydrogenations rely on soluble (homogenous) organometallic complexes or involve organometallic intermediates.<ref name=Rylander>Template:Ullmann</ref> Organometallic complexes allow these hydrogenations to be effected asymmetrically.

Many semiconductors are produced from trimethylgallium, trimethylindium, trimethylaluminium, and trimethylantimony. These volatile compounds are decomposed along with ammonia, arsine, phosphine and related hydrides on a heated substrate via metalorganic vapor phase epitaxy (MOVPE) process in the production of light-emitting diodes (LEDs).

Organokovinske reakcije

Organokovinske spojine so podvržene številnim pomembnim reakcijam:

• asociativna in disociativna substitucija
• oksidativna adicija in reduktivna eliminacija
• transmetilacija
• migracijski vložek (ang. migratory insertion)
• β-hidrid eliminacija
• prenos elektrona
• aktivacija vezi ogljik-ogljik
• ogljikometalacija (ang. carbometalation)
• hidrometalacija (ang. hydrometalation)
• ciklometalacija
• nukleofilna abstrakcija

Organokovinski kompleksi olajšajo sintezo mnogih organskih spojin. Metateza sigma vezi je način tvorjenja novih ogljik-ogljik sigma vezi. Običajno se uporablja pri kompleksih prehodnih kovin leve polovice d-bloka, ki so v svojem najvišjem oksidacijskem stanju.<ref>Template:Cite journal</ref> Uporaba prehodnih kovin, ki so v najvišjih možnih oksidacijskih stanjih prepreči, da potečejo druge reakcije, kot je recimo oksidativna adicija. Poleg metateze sigma vezi, se metateza alkenov oz. olefinska metateza uporablja za tvorbo raznih ogljik-ogljik pi vezi. Nobena od teh metatez ne spremeni oksidacijskega stanja kovine.<ref></ref><ref></ref> Za tvorbo novih ogljik-ogljik vezi se uporabljajo tudi številne druge metode, kot sta beta-hidrid eliminacija in reakcija vstavljanja (ang. insertion reaction).

Kataliza

Organokovinski kompleksi se običajno uporabljajo za katalize. Glavni industrijski procesi vključujejohidrogenacijo, hidrosililacijo, hidrocianacijo, olefinsko metatezo, alkensko polimerizacijo, alkensko oligomerizacijo, hidrokarboksilacijo, karbonilacijo metanola in hidroformilacijo.Template:Sfn Organokovinski intermediati so tudi vključeni v številne heterogene katalize, analogne tem, ki so zgoraj naštete. Prav tako predvidevajo, da so uporabni za Fischer-Tropschev proces.

Organokovinski kompleksi se pogosto uporabljajo pri finih kemijskih sintezah v mikromerilu, posebej v "cross-coupling" reakcijahs<ref>Template:Cite journal</ref> , ki tvorijo vezi ogljik-ogljik, npr. Suzuki-Miyaura spajanje,<ref>Template:Cite journal</ref> Buchwald-Hartwigova aminacija za tvorbo aril aminov iz aril halidov,<ref>Template:Cite journal</ref> and Sonogashira spajanje, itd.

Tveganje za okolje

File:Roxarsone.png

Roxarson je organoarzenova spojine, ki se uporablja kot krma za živali.

V okolju najdemo organokovinske spojine, ki so naravne in nevarne za okolje. Nekatere organokovinske spojine v okolju so posledica človeške rabe. To so na primer organosvinčeve in organoživosrebrove spojine, ki so toksične. Tetraetilsvinec je bil pripravljen kot dodatek k bencinu, a se zaradi svinčeve toksičnosti ne uporablja več. Njegov nadomestek so druge organokovinske spojine, kot je npr. ferocen in metilciklopentadienil manganov trikarbonil (MMT).<ref name="Seyferth">Template:Cite journal</ref> Organoarzenova spojina roxarson je sporen dodatek h krmi za živali. Leta 2006 bi se ga naj samo v ZDA proizvedlo približno milijon kilogramov.<ref>Template:Cite journal</ref> Organokositrove spojine so se široko uporabljale v barvah proti obraščanju, ampak so jih zaradi tveganja za okolje prepovedali.<ref>Template:Cite journal</ref>

See also

• Bioorganokovinska kemija
• Kovinski kompleks ogljikovega dioksida

References

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Sources

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External links

• MIT OpenCourseWare: Organometallic Chemistry
• Rob Toreki's Organometallic HyperTextbook
• web listing of US chemists who specialize in organometallic chemistry