Metallacycle

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Structure of the platinacyclobutane complex PtC3H6(bipy) derived from activation of cyclopropane. PtC3H6(bipy).png
Structure of the platinacyclobutane complex PtC3H6(bipy) derived from activation of cyclopropane.

In organometallic chemistry, a metallacycle is a derivative of a carbocyclic compound wherein a metal has replaced at least one carbon center; [2] [3] this is to some extent similar to heterocycles. Metallacycles appear frequently as reactive intermediates in catalysis, e.g. olefin metathesis and alkyne trimerization. In organic synthesis, directed ortho metalation is widely used for the functionalization of arene rings via C-H activation. One main effect that metallic atom substitution on a cyclic carbon compound is distorting the geometry due to the large size of typical metals.

Contents

Nomenclature

Typically, metallacycles are cyclic compounds with two metal carbon bonds. [4]

Structure of a carbocycle (cyclopentane), a metallacycle (a metallacyclopentane), and a metal chelated to ethylenediamine, a metal-containing ring that is not classified as a metallacycle NotaMetallacycle.png
Structure of a carbocycle (cyclopentane), a metallacycle (a metallacyclopentane), and a metal chelated to ethylenediamine, a metal-containing ring that is not classified as a metallacycle

Many compounds containing metals in rings are known, for example chelate rings. Usually, such compounds are not classified as metallacycles, but the naming conventions are not rigidly followed. Within the area of coordination chemistry and supramolecular chemistry, examples include metallacrowns, metallacryptands, metallahelices, and molecular wheels.

Classes of metallacycles

Metal-alkene complexes can be viewed as the smallest metallacycles, but they usually are not classified as metallacycles. In the Dewar–Chatt–Duncanson model, one resonance structure for the M(η2-alkene) center is the metallacyclopropane.

Representative metallacycles. From the left: a ferrole, a cobaltacyclopentadiene (a trapped intermediate from alkyne trimerization), zirconacyclopentadiene, chromacycloheptane (intermediate in trimerization of ethylene, L is unspecified), a molybdacyclobutane, a platinacyclopentane, and an osmabenzene MetallacycleVarPack2013lessGlitch.png
Representative metallacycles. From the left: a ferrole, a cobaltacyclopentadiene (a trapped intermediate from alkyne trimerization), zirconacyclopentadiene, chromacycloheptane (intermediate in trimerization of ethylene, L is unspecified), a molybdacyclobutane, a platinacyclopentane, and an osmabenzene

Metallacyclobutanes

The parent metallacyclobutane has the formula LnM(CH2)3 where L is a ligand attached to M. A stable example is (PPh3)2Pt(CH2)3. The first example was prepared by oxidative addition of cyclopropane to platinum.

The Chauvin mechanism for olefin metathesis Chem507f092 metathesis.png
The Chauvin mechanism for olefin metathesis

Metallacyclobutane intermediates are involved in the alkene metathesis and in the oligomerization and dimerization of ethylene. In alkene metathesis, the Chauvin mechanism invokes the attack of an alkene at an electrophilic metal carbene catalyst. [5] [6] [7] This work helped to validate the Chauvin mechanism for olefin metathesis.

Metallacyclopentadienes and metallabenzenes

The parent metallacyclopentadiene, or metallole, has the formula LnM(CH)4. Most arise from the coupling of two alkynes at a low valent metal centers such as derivatives of Co(I) and Zr(II). Late metal derivatives (Co, Ni) are intermediates in the metal-catalysed trimerization of alkynes to arenes. Early metal derivatives, i.e. derivatives of Ti and Zr, are used stoichiometrically. [4] For example, the zirconacyclopentadiene Cp2ZrC4Me4 is a useful carrier for C4Me42−. [8] Some of the oldest metallacycles are the ferroles, which are dimetallacyclopentadiene complexes of the formula Fe2(C2R4)(CO)6. They are derived from coupling of alkynes as well as from the desulfurization of thiophenes. [9]

The parent metallacyclobenzenes have the formula LnM(CH)5. They can be viewed as derivatives of benzene wherein a CH center has been replaced by a transition metal complex. [10]

Metallacyclopentanes

The parent metallacyclopentane has the formula LnM(CH2)4. Such compounds are intermediates in the metal catalysed dimerization, trimerization, and tetramerization of ethylene to give but-1-ene, hex-1-ene and oct-1-ene, respectively. [11] Metallacyclopentanes are invoked as intermediates in the evolution of heterogeneous alkene metathesis catalysts from ethylene and metal oxides. Metallacyclopentane intermediates are proposed to isomerize to metallacyclobutanes, which then eliminate propylene giving the alkylidene. [12]

Ortho-metalation

Structure of a palladacycle. HerrmannCat.png
Structure of a palladacycle.

Metallacycles often arise by cyclization of arene-containing donor ligands, e.g. aryl phosphines and amines. An early example is the cyclization of IrCl(PPh3)3 to give the corresponding Ir(III) hydride containing a four-membered IrPCC ring. [14] Palladium(II) and platinum(II) have long been known to ortho-metalate aromatic ligands such as azobenzene, benzylamines, and 2-phenylpyridines. [15] These reactions are strongly influenced by substituent effects, including the Thorpe-Ingold effect. [16] Ligands that lack aryl substituents will sometimes cyclometalate via activation of methyl groups, an early example being the internal oxidative addition of methylphosphine ligands. [17] Metallacycle formation interferes with intermolecular C-H activation processes. For this reason, specialized "pincer ligands" ligands have been developed that resist ortho-metalation.

Related Research Articles

<span class="mw-page-title-main">Organometallic chemistry</span> Study of organic compounds containing metal(s)

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 sometimes broadened to include metalloids like boron, silicon, and selenium, as well. Aside from bonds to organyl fragments or molecules, bonds to 'inorganic' carbon, like carbon monoxide, 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.

In organic chemistry, an alkyne trimerisation is a [2+2+2] cycloaddition reaction in which three alkyne units react to form a benzene ring. The reaction requires a metal catalyst. The process is of historic interest as well as being applicable to organic synthesis. Being a cycloaddition reaction, it has high atom economy. Many variations have been developed, including cyclisation of mixtures of alkynes and alkenes as well as alkynes and nitriles.

Organopalladium chemistry is a branch of organometallic chemistry that deals with organic palladium compounds and their reactions. Palladium is often used as a catalyst in the reduction of alkenes and alkynes with hydrogen. This process involves the formation of a palladium-carbon covalent bond. Palladium is also prominent in carbon-carbon coupling reactions, as demonstrated in tandem reactions.

<span class="mw-page-title-main">Olefin metathesis</span>

Olefin metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Because of the relative simplicity of olefin metathesis, it often creates fewer undesired by-products and hazardous wastes than alternative organic reactions. For their elucidation of the reaction mechanism and their discovery of a variety of highly active catalysts, Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock were collectively awarded the 2005 Nobel Prize in Chemistry.

<span class="mw-page-title-main">Alkyne metathesis</span>

Alkyne metathesis is an organic reaction that entails the redistribution of alkyne chemical bonds. The reaction requires metal catalysts. Mechanistic studies show that the conversion proceeds via the intermediacy of metal alkylidyne complexes. The reaction is related to olefin metathesis.

<span class="mw-page-title-main">Organotitanium chemistry</span>

Organotitanium chemistry is the science of organotitanium compounds describing their physical properties, synthesis, and reactions. Organotitanium compounds in organometallic chemistry contain carbon-titanium chemical bonds. They are reagents in organic chemistry and are involved in major industrial processes.

Hydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds. Ordinarily the reaction is conducted catalytically and usually the substrates are unsaturated organic compounds. Alkenes and alkynes give alkyl and vinyl silanes; aldehydes and ketones give silyl ethers. Hydrosilylation has been called the "most important application of platinum in homogeneous catalysis."

<span class="mw-page-title-main">Organonickel chemistry</span>

Organonickel chemistry is a branch of organometallic chemistry that deals with organic compounds featuring nickel-carbon bonds. They are used as a catalyst, as a building block in organic chemistry and in chemical vapor deposition. Organonickel compounds are also short-lived intermediates in organic reactions. The first organonickel compound was nickel tetracarbonyl Ni(CO)4, reported in 1890 and quickly applied in the Mond process for nickel purification. Organonickel complexes are prominent in numerous industrial processes including carbonylations, hydrocyanation, and the Shell higher olefin process.

Hydroacylation is a type of organic reaction in which an alkene is inserted into the a formyl C-H bond. The product is a ketone. The reaction requires a metal catalyst. It is almost invariably practiced as an intramolecular reaction using homogeneous catalysts, often based on rhodium phosphines.

Organoplatinum chemistry is the chemistry of organometallic compounds containing a carbon to platinum chemical bond, and the study of platinum as a catalyst in organic reactions. Organoplatinum compounds exist in oxidation state 0 to IV, with oxidation state II most abundant. The general order in bond strength is Pt-C (sp) > Pt-O > Pt-N > Pt-C (sp3). Organoplatinum and organopalladium chemistry are similar, but organoplatinum compounds are more stable and therefore less useful as catalysts.

<span class="mw-page-title-main">Organomolybdenum chemistry</span>

Organomolybdenum chemistry is the chemistry of chemical compounds with Mo-C bonds. The heavier group 6 elements molybdenum and tungsten form organometallic compounds similar to those in organochromium chemistry but higher oxidation states tend to be more common.

Transition metal carbyne complexes are organometallic compounds with a triple bond between carbon and the transition metal. This triple bond consists of a σ-bond and two π-bonds. The HOMO of the carbyne ligand interacts with the LUMO of the metal to create the σ-bond. The two π-bonds are formed when the two HOMO orbitals of the metal back-donate to the LUMO of the carbyne. They are also called metal alkylidynes—the carbon is a carbyne ligand. Such compounds are useful in organic synthesis of alkynes and nitriles. They have been the focus on much fundamental research.

Metal carbon dioxide complexes are coordination complexes that contain carbon dioxide ligands. Aside from the fundamental interest in the coordination chemistry of simple molecules, studies in this field are motivated by the possibility that transition metals might catalyze useful transformations of CO2. This research is relevant both to organic synthesis and to the production of "solar fuels" that would avoid the use of petroleum-based fuels.

A metal carbido complex is a coordination complex that contains a carbon atom as a ligand. They are analogous to metal nitrido complexes. Carbido complexes are a molecular subclass of carbides, which are prevalent in organometallic and inorganic chemistry. Carbido complexes represent models for intermediates in Fischer–Tropsch synthesis, olefin metathesis, and related catalytic industrial processes. Ruthenium-based carbido complexes are by far the most synthesized and characterized to date. Although, complexes containing chromium, gold, iron, nickel, molybdenum, osmium, rhenium, and tungsten cores are also known. Mixed-metal carbides are also known.

In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization.

<span class="mw-page-title-main">Organotantalum chemistry</span> Chemistry of compounds containing a carbon-to-tantalum bond

Organotantalum chemistry is the chemistry of chemical compounds containing a carbon-to-tantalum chemical bond. A wide variety of compound have been reported, initially with cyclopentadienyl and CO ligands. Oxidation states vary from Ta(V) to Ta(-I).

Organoniobium chemistry is the chemistry of compounds containing niobium-carbon (Nb-C) bonds. Compared to the other group 5 transition metal organometallics, the chemistry of organoniobium compounds most closely resembles that of organotantalum compounds. Organoniobium compounds of oxidation states +5, +4, +3, +2, +1, 0, -1, and -3 have been prepared, with the +5 oxidation state being the most common.

In organic chemistry, hydrovinylation is the formal insertion of an alkene into the C-H bond of ethylene. The more general reaction, hydroalkenylation, is the formal insertion of an alkene into the C-H bond of any terminal alkene. The reaction is catalyzed by metal complexes. A representative reaction is the conversion of styrene and ethylene to 3-phenybutene:

<span class="mw-page-title-main">Metallacyclopentanes</span>

In organometallic chemistry, metallacyclopentanes are compounds with the formula LnM(CH2)4 (Ln = ligands, and M = metal). They are a type of metallacycle. Metallacyclopentanes are intermediates in some metal-catalysed reactions in homogeneous catalysis.

Metal arene complexes are organometallic compounds of the formula (C6R6)xMLy. Common classes are of the type (C6R6)ML3 and (C6R6)2M. These compounds are reagents in inorganic and organic synthesis. The principles that describe arene complexes extend to related organic ligands such as many heterocycles (e.g. thiophene) and polycyclic aromatic compounds (e.g. naphthalene).

References

  1. ↑ R. J. Klingler; J. C. Huffman; J. K. Kochi (1982). "Synthesis, structure, and electrochemistry of platinum(II) metallacyclobutanes". J. Am. Chem. Soc. 104 (8): 2147–2157. doi:10.1021/ja00372a010.
  2. ↑ Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN   978-3-527-29390-2
  3. ↑ Jolly, William L. (1989). Modern Inorganic Chemistry (third ed.). McCraw-Hill. ISBN   0-07-032760-2.
  4. 1 2 Rosenthal, Uwe; Burlakov, Vladimir V.; Bach, Marc A.; Beweries, Torsten (2007). "Five-membered metallacycles of titanium and zirconium – attractive compounds for organometallic chemistry and catalysis". Chem. Soc. Rev. 36 (5): 719–728. doi:10.1039/b605734a. PMID   17471397.
  5. ↑ "Press Release: The Nobel Prize in Chemistry 2005" . Retrieved 3 December 2009.
  6. ↑ Hérisson, J.-L.; Chauvin, Y. (1971). "Catalyse de transformation des oléfines par les complexes du tungstène. II. Télomérisation des oléfines cycliques en présence d'oléfines acycliques". Makromol. Chem. 141: 161–176. doi:10.1002/macp.1971.021410112.
  7. ↑ Schrock, R.R. (1986). "On the trail of metathesis catalysts". J. Organomet. Chem. 300 (1–2): 249–262. doi:10.1016/0022-328X(86)84064-5.
  8. ↑ Paul J. Fagan, William A. Nugent (1998). "1-Phenyl-2,3,4,5-Tetramethylphosphole". Organic Syntheses .; Collective Volume, vol. 9, p. 653
  9. ↑ G. Dettlaf, E. Weiss "Kristallstruktur, 1H-NMR- und Massenspektrum von Tricarbonylferracyclopentadien-Tricarbonyleisen, C4H4Fe2(CO)6 Journal of Organometallic Chemistry 1976, vol. 108, pp. 213-223.
  10. ↑ Bleeke, J.R. (2001). "Metallabenzenes". Chem. Rev. 101 (5): 1205–27. doi:10.1021/cr990337n. PMID   11710218.
  11. ↑ Dixon, John T.; Green, Mike J.; Hess, Fiona M.; Morgan, David H. (2004). "Advances in selective ethylene trimerisation – a critical overview". Journal of Organometallic Chemistry. 689 (23): 3641–3668. doi:10.1016/j.jorganchem.2004.06.008.
  12. ↑ Schrock, R. R.; Coperet, C. (2017). "Formation of High-Oxidation-State Metal-Carbon Double Bonds". Organometallics. 36 (10): 1884–1892. doi:10.1021/acs.organomet.6b00825. hdl: 1721.1/115114 .
  13. ↑ Herrmann, W. A.; Brossmer, C.; Reisinger, C.-P.; Riermeier, T. H.; Öfele, K.; Beller, M. (1997). "Palladacycles: Efficient New Catalysts for the Heck Vinylation of Aryl Halides". Chemistry – A European Journal. 3 (8): 1357–1364. doi:10.1002/chem.19970030823.
  14. ↑ Bennett, M. A.; Milner, D. L. (1969). "Chlorotris(triphenylphosphine)iridium(I) and related complexes. Oxidative addition reactions and hydrogen abstraction from the coordinated ligand". J. Am. Chem. Soc. 91 (25): 6983–6994. doi:10.1021/ja01053a016.
  15. ↑ Cope, A. C.; Siekman, R. W. (1965). "Formation of Covalent Bonds from Platinum or Palladium to Carbon by Direct Substitution". J. Am. Chem. Soc. 87 (14): 3272–3273. doi:10.1021/ja01092a063.
  16. ↑ Shaw, B. L. (1975). "Formation of large rings, internal metalation reactions, and internal entropy effects". J. Am. Chem. Soc. 97 (13): 3856–3857. doi:10.1021/ja00846a072.
  17. ↑ Chatt, J.; Davidson, J. M. (1965). "The tautomerism of arene and ditertiary phosphine complexes of ruthenium(0), and the preparation of new types of hydrido-complexes of ruthenium(II)". J. Chem. Soc. : 843. doi:10.1039/JR9650000843.
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