Metallocycle

A metallocycle (also metallacycle) in organometallic chemistry is a cyclic compound with at least one carbon atom replaced by a metal. [cite book | last=Jolly |first = William L. |coauthors = | title=Modern Inorganic Chemistry |edition = third Edition |publisher= McCraw-Hill | year=1989 | isbn=0-07-032760-2] Metallacycles appear frequently as reactive intermediates in chemistry. Metallacyclic species are prevalent in both stoichiometric and catalytic transformations, e.g. olefin metathesis and alkyne trimerization. In organic synthesis, directed ortho metalation is widely used for the functionalization of arene rings.

Metallacycle formation often arise by cyclization of arene-containing donor ligands, e.g. aryl phosphines and amines. An early example of metallacycle formation via oxidative addition is the cyclization of IrCl(PPh₃)₃ to give the corresponding Ir(III) hydride containing a four-membered IrPCC ring. [M. A. Bennett, D. L. Milner "Chlorotris(triphenylphosphine)iridium(I) and related complexes. Oxidative addition reactions and hydrogen abstraction from the coordinated ligand"J. Am. Chem. Soc. 1969, 91, 6983-6994.DOI|10.1021/ja01053a016.] Palladium(II) and platinum(II) have long been known to ortho-metallate aromatic ligands such as [azobenzene] , benzylamines, and 2-phenylpyridines. [A. C. Cope, R. W. Siekman "Formation of Covalent Bonds from Platinum or Palladium to Carbon by Direct Substitution" J. Am. Chem. Soc. 1965, volume 87, 3272-3273. doi|10.1021/ja01092a063] These reactions are strongly influenced by substituent effects, including the Thorpe-Ingold effect. [Shaw, B. L., "Formation of large rings, internal metalation reactions, and internal entropy effects", J. Am. Chem. Soc. 1975, volume 97, 3856-3857. doi|10.1021/ja00846a072.] Ligands that lack aryl substituents will sometimes cyclometalate via activation of methyl groups, an early example being the internal oxidative addition of methylphosphine ligands. ["The tautomerism of arene and ditertiary phosphine complexes of ruthenium(0), and the preparation of new types of hydrido-complexes of ruthenium(II)" J. Chatt and J. M. Davidson, J. Chem. Soc. 1965, 843 DOI|10.1039/JR9650000843] Metallacycle formation interferes with intermolecular C-H activation processes. For this reason, specialized "pincer ligands" ligands have been developed that resist cyclization.

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