Typically, metallacycles are cyclic compounds with two metal carbon bonds. 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 center is the metallacyclopropane.
Metallacyclobutanes
The parent metallacyclobutane has the formula LnM3 where L is a ligand attached to M. A stable example is 2Pt3. The first example was prepared by oxidative addition of cyclopropane to platinum. 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 and metallabenzenes
The parent metallacyclopentadiene has the formula LnM4. Most arise from the coupling of two alkynes at a low valent metal centers such as derivatives of Co and Zr. Late metal derivatives 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 Fe26. They are derived from coupling of alkynes as well as from the desulfurization of thiophenes. The parent metallacyclobenzenes have the formula LnM5. They can be viewed as derivatives of benzene wherein a CH center has been replaced by a transition metal complex.
Metallacyclopentanes
The parent metallacyclopentane has the formula LnM4. Such compounds are intermediates in the metal catalysed dimerization, trimerization, and tetramerization of ethylene to give 1-butene, 1-hexene, and 1-octene, respectively. 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.
Ortho-metalation
Metallacycles often arise by cyclization of arene-containing donor ligands, e.g. aryl phosphines and amines. An early example is the cyclization of IrCl3 to give the corresponding Ir hydride containing a four-membered IrPCC ring. Palladium and platinum have long been known to ortho-metalate 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-metalation.
