Metallacycle

In organometallic chemistry, a metallacycle is a derivative of a carbocyclic compound wherein a metal has replaced at least one carbon center. 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.

Nomenclature

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

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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.

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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 L_nM(CH₂)₃ where L is a ligand attached to M. A stable example is (PPh₃)₂Pt(CH₂)₃.

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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. This work helped to validate the Chauvin mechanism for olefin metathesis.

Metallacyclopentadienes

The parent metallacyclopentadiene has the formula L_nM(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. For example, the zirconacyclopentadiene Cp₂ZrC₄Me₄ is a useful carrier for C₄Me₄²⁻. 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.

Metallacyclopentanes

The parent metallacyclopentane has the formula L_nM(CH₂)₄. Such compounds are intermediates in the metal catalysed dimerization, trimerization, and tetramerization of ethylene to give 1-butene, 1-hexene, and 1-octene, respectively.

Metallabenzenes

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Interactions between these orbitals give rise to a cyclically delocalized pi electronic structure.

The parent metallacyclobutane has the formula L_nM(CH)₅. Since the discovery of the structure of benzene, many analogues of this iconic structure have been described. Examples include pyridine, phosphabenzene, arsabenzene, and pyrylium. Included in this group are the metallabenzenes.

Metallabenzene complexes have been classified into three varieties; in such compounds the parent acyclic hydrocarbon ligand is viewed as the anion C₅H₅⁻. The 6 π electrons in the metallacycle conform to the Hückel (4n+2) theory.

Three classes of stable metallabenzenes.

The first reported stable metallabenzene was the osmabenzene Os(C₅H₄S)CO(PPh₃)₂. Characteristic of other metallaarenes, the Os-C bonds are about 0.6 Å longer than the C-C bonds (in benzene these are 1.39 Å), resulting in a distortion of the hexagonal ring. ¹H NMR signals for the ring protons are downfield, consistent with aromatic "ring current." Osmabenzene and its derivatives can be regarded as an Os(II), d⁶ octahedral complex.

Isolated and characterized metallabenzenes have been also been reported with metals ruthenium, iridium, platinum and rhenium

Ortho-metallation

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. Palladium(II) and platinum(II) have long been known to ortho-metallate aromatic ligands such as azobenzene, benzylamines, and 2-phenylpyridines. These reactions are strongly influenced by substituent effects, including the Thorpe-Ingold Effect. Ligands that lack aryl substituents will sometimes cyclometalate via activation of methyl groups, an early example being the internal oxidative addition of methylphosphine ligands. Metallacycle formation interferes with intermolecular C-H activation processes. For this reason, specialized "pincer ligands" ligands have been developed that resist ortho-metallation.