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Organometallic chemistry

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n-Butyllithium,an organometallic compound. Four lithium atoms (in purple) form atetrahedron,with fourbutylgroups attached to the faces (carbon is black, hydrogen is white).

Organometallic chemistryis the study oforganometallic compounds,chemical compoundscontaining at least onechemical bondbetween acarbonatom of anorganic moleculeand ametal,includingalkali,alkaline earth,andtransition metals,and sometimes broadened to includemetalloidslike boron, silicon, and selenium, as well.[1][2]Aside from bonds toorganylfragments or molecules, bonds to 'inorganic' carbon, likecarbon monoxide(metal carbonyls),cyanide,orcarbide,are generally considered to be organometallic as well. Some related compounds such astransition metal hydridesandmetal phosphine complexesare 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 traditionalinorganicandorganic chemistry.[3]

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 ofhomogeneous catalysis), where target molecules include polymers, pharmaceuticals, and many other types of practical products.

Organometallic compounds

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A steel bottle containing MgCp2(magnesium bis-cyclopentadienyl),which, like several other organometallic compounds, is pyrophoric in air.

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 anorganyl group.[2]In addition to the traditional metals (alkali metals,alkali earth metals,transition metals,andpost transition metals),lanthanides,actinides,semimetals, and the elementsboron,silicon,arsenic,andseleniumare considered to form organometallic compounds.[2]Examples of organometallic compounds includeGilman reagents,which containlithiumandcopper,andGrignard reagents,which containmagnesium.Boron-containing organometallic compounds are often the result ofhydroborationandcarboborationreactions.Tetracarbonyl nickelandferroceneare examples of organometallic compounds containingtransition metals.Other examples of organometallic compounds includeorganolithiumcompounds such asn-butyllithium(n-BuLi),organozinccompounds such asdiethylzinc(Et2Zn),organotincompounds such astributyltin hydride(Bu3SnH),organoboranecompounds such astriethylborane(Et3B), andorganoaluminiumcompounds such astrimethylaluminium(Me3Al).[3]

A naturally occurring organometallic complex ismethylcobalamin(a form ofVitamin B12), which contains acobalt-methylbond. This complex, along with other biologically relevant complexes are often discussed within the subfield ofbioorganometallic chemistry.[4]

Distinction from coordination compounds with organic ligands

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Manycomplexesfeaturecoordination bondsbetween a metal and organicligands.Complexes where the organic ligands bind the metal through aheteroatomsuch as oxygen or nitrogen are considered coordination compounds (e.g.,heme AandFe(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.[5]

The status of compounds in which thecanonical anionhas a negative charge that is shared between (delocalized) a carbon atom and an atom moreelectronegativethan 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.[2]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.[3]

Structure and properties

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The metal-carbon bond in organometallic compounds is generally highlycovalent.[1]For highly electropositive elements, such as lithium and sodium, the carbon ligand exhibitscarbanioniccharacter, but free carbon-based anions are extremely rare, an example beingcyanide.

a single crystal of a Mn(II) complex, [BnMIm]4[MnBr4]Br2. Its bright green color originates from spin-forbidden d-d transitions

Most organometallic compounds are solids at room temperature, however some are liquids such asmethylcyclopentadienyl manganese tricarbonyl,or evenvolatileliquids such asnickel tetracarbonyl.[1]Many organometallic compounds areair sensitive(reactive towards oxygen and moisture), and thus they must be handled under aninert atmosphere.[1]Some organometallic compounds such astriethylaluminiumarepyrophoricand willigniteon contact with air.[6]

Concepts and techniques

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As in other areas of chemistry,electron countingis useful for organizing organometallic chemistry. The18-electron ruleis helpful in predicting the stabilities of organometallic complexes, for examplemetal carbonylsandmetal hydrides.The 18e rule has two representative electron counting models, ionic and neutral (also known as covalent) ligand models, respectively.[7]The hapticity of a metal-ligand complex, can influence the electron count.[7]Hapticity(η, lowercase Greek eta), describes the number of contiguous ligands coordinated to a metal.[7]For example,ferrocene,[(η5-C5H5)2Fe], has twocyclopentadienyl ligandsgiving a hapticity of 5, where all five carbon atoms of the C5H5ligand bond equally and contribute one electron to the iron center. Ligands that bind non-contiguous atoms are denoted the Greek letter kappa, κ.[7]Chelatingκ2-acetate is an example. Thecovalent bond classification methodidentifies 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 theird electron countandoxidation state.These concepts can be used to help predict their reactivity and preferredgeometry.Chemical bonding and reactivity in organometallic compounds is often discussed from the perspective of theisolobal principle.

A wide variety of physical techniques are used to determine the structure, composition, and properties of organometallic compounds.X-ray diffractionis a particularly important technique that can locate the positions of atoms within a solid compound, providing a detailed description of its structure.[1][8]Other techniques likeinfrared spectroscopyandnuclear magnetic resonance spectroscopyare also frequently used to obtain information on the structure and bonding of organometallic compounds.[1][8]Ultraviolet-visible spectroscopyis 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 theirkinetics.[8]The dynamics of organometallic compounds can be studied usingdynamic NMR spectroscopy.[1]Other notable techniques includeX-ray absorption spectroscopy,[9]electron paramagnetic resonance spectroscopy,andelemental analysis.[1][8]

Due to their high reactivity towards oxygen and moisture, organometallic compounds often must be handled usingair-free techniques.Air-free handling of organometallic compounds typically requires the use of laboratory apparatuses such as agloveboxorSchlenk line.[1]

History

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Early developments in organometallic chemistry includeLouis Claude Cadet's synthesis of methyl arsenic compounds related tocacodyl,William Christopher Zeise's[10]platinum-ethylene complex,[11]Edward Frankland's discovery ofdiethyl-anddimethylzinc,Ludwig Mond's discovery ofNi(CO)4,[1]andVictor Grignard's organomagnesium compounds. (Although not always acknowledged as an organometallic compound,Prussian blue,a mixed-valence iron-cyanide complex, was first prepared in 1706 by paint makerJohann Jacob Diesbachas the firstcoordination polymerand synthetic material containing a metal-carbon bond.[12]) The abundant and diverse products from coal and petroleum led toZiegler–Natta,Fischer–Tropsch,hydroformylationcatalysis which employ CO, H2,and alkenes as feedstocks and ligands.

Recognition of organometallic chemistry as a distinct subfield culminated in the Nobel Prizes toErnst FischerandGeoffrey Wilkinsonfor work onmetallocenes.In 2005,Yves Chauvin,Robert H. GrubbsandRichard R. Schrockshared the Nobel Prize for metal-catalyzedolefin metathesis.[13]

Organometallic chemistry timeline

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Scope

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Subspecialty areas of organometallic chemistry include:

Industrial applications

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Organometallic compounds find wide use in commercial reactions, both ashomogenous catalystsand asstoichiometric reagents.For instance,organolithium,organomagnesium,andorganoaluminium compounds,examples of which are highly basic and highly reducing, are useful stoichiometrically but also catalyze many polymerization reactions.[14]

Almost all processes involving carbon monoxide rely on catalysts, notable examples being described ascarbonylations.[15]The production of acetic acid from methanol and carbon monoxide is catalyzed viametal carbonyl complexesin theMonsanto processandCativa process.Most synthetic aldehydes are produced viahydroformylation.The bulk of the synthetic alcohols, at least those larger than ethanol, are produced byhydrogenationof hydroformylation-derived aldehydes. Similarly, theWacker processis used in the oxidation ofethylenetoacetaldehyde.[16]

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

Almost all industrial processes involvingalkene-derived polymers rely on organometallic catalysts. The world's polyethylene and polypropylene are produced via bothheterogeneouslyviaZiegler–Nattacatalysis and homogeneously, e.g., viaconstrained geometry catalysts.[17]

Most processes involving hydrogen rely on metal-based catalysts. Whereas bulkhydrogenations(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.[18]Organometallic complexes allow these hydrogenations to be effected asymmetrically.

Manysemiconductorsare produced fromtrimethylgallium,trimethylindium,trimethylaluminium,andtrimethylantimony.These volatile compounds are decomposed along withammonia,arsine,phosphineand related hydrides on a heated substrate viametalorganic vapor phase epitaxy(MOVPE) process in the production oflight-emitting diodes(LEDs).

Organometallic reactions

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Organometallic compounds undergo several important reactions:

The synthesis of many organic molecules are facilitated by organometallic complexes.Sigma-bond metathesisis a synthetic method for forming new carbon-carbonsigma bonds.Sigma-bond metathesis is typically used with early transition-metal complexes that are in their highest oxidation state.[19]Using transition-metals that are in their highest oxidation state prevents other reactions from occurring, such asoxidative addition.In addition to sigma-bond metathesis,olefin metathesisis used to synthesize various carbon-carbonpi bonds.Neither sigma-bond metathesis or olefin metathesis change the oxidation state of the metal.[20][21]Many other methods are used to form new carbon-carbon bonds, includingbeta-hydride eliminationandinsertion reactions.

Catalysis

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Organometallic complexes are commonly used in catalysis. Major industrial processes includehydrogenation,hydrosilylation,hydrocyanation,olefin metathesis,alkene polymerization,alkene oligomerization,hydrocarboxylation,methanol carbonylation,andhydroformylation.[16]Organometallic intermediates are also invoked in manyheterogeneous catalysisprocesses, analogous to those listed above. Additionally, organometallic intermediates are assumed forFischer–Tropsch process.

Organometallic complexes are commonly used in small-scale fine chemical synthesis as well, especially incross-coupling reactions[22]that form carbon-carbon bonds, e.g.Suzuki-Miyaura coupling,[23]Buchwald-Hartwig aminationfor producing aryl amines from aryl halides,[24]andSonogashira coupling,etc.

Environmental concerns

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Roxarsoneis an organoarsenic compound used as an animal feed.

Natural and contaminant organometallic compounds are found in the environment. Some that are remnants of human use, such as organolead and organomercury compounds, are toxicity hazards.Tetraethylleadwas prepared for use as agasolineadditive but has fallen into disuse because of lead's toxicity. Its replacements are other organometallic compounds, such asferroceneandmethylcyclopentadienyl manganese tricarbonyl(MMT).[25]Theorganoarsenic compoundroxarsone is a controversial animal feed additive. In 2006, approximately one million kilograms of it were produced in the U.S alone.[26]Organotin compoundswere once widely used inanti-fouling paintsbut have since been banned due to environmental concerns.[27]

See also

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References

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  1. ^abcdefghijCrabtree 2009,p.[page needed].
  2. ^abcdIUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "organometallic compounds".doi:10.1351/goldbook.O04328
  3. ^abcC. Elschenbroich (2006).Organometallics.VCH.ISBN978-3-527-29390-2.
  4. ^Lippard & Berg 1994,p.[page needed].
  5. ^Rodríguez-Reyes, J.C.F.; Silva-Quiñones, D. (2018). "Metalorganic Functionalization in Vacuum".Encyclopedia of Interfacial Chemistry.pp. 761–768.doi:10.1016/B978-0-12-409547-2.13135-X.ISBN978-0-12-809894-3.
  6. ^"Triethylaluminium – SDS"(PDF).chemBlink.24 May 2016.Retrieved3 January2021.
  7. ^abcdCrabtree, Robert H. (2014).The organometallic chemistry of the transition metals(6 ed.). Hoboken, New Jersey. pp. 43, 44, 205.ISBN978-1-118-78824-0.OCLC863383849.{{cite book}}:CS1 maint: location missing publisher (link)
  8. ^abcdShriver et al. 2014,p.[page needed].
  9. ^Nelson, Ryan C.; Miller, Jeffrey T. (2012). "An introduction to X-ray absorption spectroscopy and its in situ application to organometallic compounds and homogeneous catalysts".Catal. Sci. Technol.2(3): 461–470.doi:10.1039/C2CY00343K.
  10. ^Hunt, L. B. (1 April 1984). "The First Organometallic Compounds".Platinum Metals Review.28(2): 76–83.CiteSeerX10.1.1.693.9965.
  11. ^Zeise, W. C. (1831)."Von der Wirkung zwischen Platinchlorid und Alkohol, und von den dabei entstehenden neuen Substanzen"[About the effect between platinum chloride and alcohol, and about the new substances that are created in the process].Annalen der Physik und Chemie(in German).97(4): 497–541.Bibcode:1831AnP....97..497Z.doi:10.1002/andp.18310970402.
  12. ^Crabtree 2009,p. 98.
  13. ^Dragutan, V.; Dragutan, I.; Balaban, A. T. (1 January 2006)."2005 Nobel Prize in Chemistry".Platinum Metals Review.50(1): 35–37.doi:10.1595/147106706X94140.
  14. ^Elschenbroich 2016,p.[page needed].
  15. ^W. Bertleff; M. Roeper; X. Sava. "Carbonylation".Ullmann's Encyclopedia of Industrial Chemistry.Weinheim: Wiley-VCH.doi:10.1002/14356007.a05_217.ISBN978-3527306732.
  16. ^abLeeuwen 2005,p.[page needed].
  17. ^Klosin, Jerzy; Fontaine, Philip P.; Figueroa, Ruth (21 July 2015)."Development of Group IV Molecular Catalysts for High Temperature Ethylene-α-Olefin Copolymerization Reactions".Accounts of Chemical Research.48(7): 2004–2016.doi:10.1021/acs.accounts.5b00065.PMID26151395.
  18. ^Rylander, Paul N. "Hydrogenation and Dehydrogenation".Ullmann's Encyclopedia of Industrial Chemistry.Weinheim: Wiley-VCH.doi:10.1002/14356007.a13_487.ISBN978-3527306732.
  19. ^Waterman, Rory (23 December 2013). "σ-Bond Metathesis: A 30-Year Retrospective".Organometallics.32(24): 7249–7263.doi:10.1021/om400760k.
  20. ^"Olefin Metathesis".The Organometallic HyperTextBook.
  21. ^"Sigma Bond Metathesis".Organometallic HyperTextBook.
  22. ^Jana, Ranjan; Pathak, Tejas P.; Sigman, Matthew S. (9 March 2011)."Advances in Transition Metal (Pd,Ni,Fe)-Catalyzed Cross-Coupling Reactions Using Alkyl-organometallics as Reaction Partners".Chemical Reviews.111(3): 1417–1492.doi:10.1021/cr100327p.PMC3075866.PMID21319862.
  23. ^Maluenda, Irene; Navarro, Oscar (24 April 2015)."Recent Developments in the Suzuki-Miyaura Reaction: 2010–2014".Molecules.20(5): 7528–7557.doi:10.3390/molecules20057528.PMC6272665.PMID25919276.
  24. ^Magano, Javier; Dunetz, Joshua R. (9 March 2011). "Large-Scale Applications of Transition Metal-Catalyzed Couplings for the Synthesis of Pharmaceuticals".Chemical Reviews.111(3): 2177–2250.doi:10.1021/cr100346g.PMID21391570.
  25. ^Seyferth, D. (2003)."The Rise and Fall of Tetraethyllead. 2".Organometallics.22(25): 5154–5178.doi:10.1021/om030621b.
  26. ^Hileman, Bette (9 April 2007)."Arsenic In Chicken Production".Chemical & Engineering News.85(15): 34–35.doi:10.1021/cen-v085n015.p034.
  27. ^Lagerström, Maria; Strand, Jakob; Eklund, Britta; Ytreberg, Erik (January 2017)."Total tin and organotin speciation in historic layers of antifouling paint on leisure boat hulls".Environmental Pollution.220(Pt B): 1333–1341.doi:10.1016/j.envpol.2016.11.001.PMID27836476.

Sources

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