Olefin Ligands

Introduction

Olefin are an important class of unsaturated ligands that bind to a metal by σ−donating its C=C π−electrons and also accepts electrons from the metal in its π* orbital of C=C bond. These symbiotic σ−donation and π−back donation in metal bound olefin complexes have a significant impact on their structure and reactivity properties. The olefin-metal bonding interaction is best explained by the Dewar−Chatt model, that takes into account two mutually opposing electron donation involving σ−donation of the olefinic C=C π−electrons to an empty d_π metal orbital followed by π−back donation from a filled metal dπ orbital into the unoccupied C=C π* orbital. The first metal olefin complex dates back a long time to the beginning of 19th century, in 1827, the Danish chemist Zeise synthesized the famous Zeise's salt K[PtCl₃(C₂H₄)]∙H₂O containing a Pt bound ethylene moiety, which is first metal−olefin complex. Nowadays, a large number of complexes using alkenes as ligands have been developed and have a wide range of applications.

Fig.1 The 3D ball structure of Zeise's salt

Classifications

Although there are many classification way of olefin ligands, the most common and most clear way is based on their structure, based on this, olefin ligands can be divided into alkene ligands, alkyne ligands, polyene ligands and cycloalkene ligands. The details are as follows:

• Alkene ligands: An alkene ligand contains a π bond between carbon atoms, C=C, which can serve as an electron pair donor in a metal complex. The alkene ligands bond to the metal centre by both electron donation and acceptance, similar to the situation with carbon monoxide. The allyl ligands with structure of -CH₂-CH=CH₂ is a one of representative alkene ligands, can bind to a metal atom in either of two configurations: as an η¹-ligand or an η³-ligand (as shown in Fig.2).

Fig.2 Examples of η¹- and η³-allyl complexes

• Alkyne ligands: Alkyne, R-C≡C-R, has two π bonds and hence is a potential four-electron donor, for this reason, substituted alkyne can form very stable polymetallic complexes. The coordination of alkynes to transition metals is similar to that of alkenes and can be described by the Dewar−Chatt model.
• Polyene ligands: Polyenes are poly-unsaturated organic compounds that contain at least two alternating double and single carbon–carbon bonds. Larger polyene ligands present the possibility of several points of attachment to a metal atom, the resulting polyene complexes are usually more stable than the equivalent monohapto complexes with individual ligands.
• Cycloalkene ligands: A cycloalkene is a type of alkene hydrocarbon which contains a closed ring of carbon atoms and either one or more double bonds, but has no aromatic character. These rings, which have alternating double and single bonds, are among the most important ligands in organometallic chemistry; the most common members of this group range from cyclobutadiene (C₄H₄) to cyclooctatetraene (C₈H₈). Cycloalkene ligands are also known to form complexes in which they bind to a metal atom through some but not all of their carbon atoms.

Applications

The main application of olefin ligands is catalysis, and the most popular and extensive is in asymmetric catalysis. During the last century, in addition to many coordination studies, olefin complexes involving late-transition metals such as Rh, Ir, Pt, Pd or Ni are mostly employed as catalyst precursors in catalysis. Today, chiral olefins have recently emerged as among the most promising ligands for asymmetric catalysis, which exhibit higher activity and selectivity than the traditional nitrogen, phosphine, oxide ligands in some reactions, and solved some problems that other ligands could not do.

The types of chiral olefins ligands commonly used in asymmetric catalysis are bicyclic dienes, chain diene ligands, phosphorus/terminal-olefin hybrid ligands, and sulfur/olefin hybrid ligands. They have been successfully utilized in a series of transition-metal-catalyzed asymmetric reactions, such as iridium-catalyzed asymmetric hydrogenation of imines, allylic substitution; rhodium-catalyzed conjugate addition of organoboron reagents to α, β-unsaturated compounds, 1,2-addition of organoboron reagents to imines/carbonyl compounds, intramolecular hydroacylation; and palladium-catalyzed asymmetric allylic alkylation/amination/etherification of allylic esters, as well as Suzuki-coupling reactions. In many cases, the reactions occur with high enantioselectivities, allows for access to a broad range of valuable chiral products^[1].

Fig.3 The asymmetric catalysis by coordination compound based on chiral-chain diene ligand

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Reference

• [1] Yu Y N.; et al. Chiral phosphorus-olefin ligands for asymmetric catalysis[J]. Acta Chim. Sinica, 2017, 75(7), 655-670.