Transition metal alkene complex

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In organometallic chemistry, a transition metal alkene complex is a coordination compound containing one or more alkene ligands. The inventory is large. [1] Such compounds are intermediates in many catalytic reactions that convert alkenes to other organic products. [2]

Contents

Monoalkenes

The simplest monoalkene is ethene. Many complexes of ethene are known, including Zeise's salt (see figure), Rh2Cl2(C2H4)4, Cp*2Ti(C2H4), and the homoleptic Ni(C2H4)3. Substituted monoalkene include the cyclic cyclooctene, as found in chlorobis(cyclooctene)rhodium dimer. Alkenes with electron-withdrawing groups commonly bind strongly to low-valent metals. Examples of such ligands are TCNE, tetrafluoroethylene, maleic anhydride, and esters of fumaric acid. These acceptors form adducts with many zero-valent metals. [1]

Butadiene, cyclooctadiene, and norbornadiene are well-studied chelating agents. Trienes and even some tetraenes can bind to metals through several adjacent carbon centers. Common examples of such ligands are cycloheptatriene and cyclooctatetraene. The bonding is often denoted using the hapticity formalism. Keto-alkenes are tetrahapto ligands that stabilize highly unsaturated low valent metals as found in (benzylideneacetone)iron tricarbonyl and tris(dibenzylideneacetone)dipalladium(0).

Bonding

Structure of (acac)Rh(C2H4)(C2F4), distances (red) in picometers. AcacRh(C2H4)(C2F4).svg
Structure of (acac)Rh(C2H4)(C2F4), distances (red) in picometers.

The bonding between alkenes and transition metals is described by the Dewar–Chatt–Duncanson model, which involves donation of electrons in the pi-orbital on the alkene to empty orbitals on the metal. This interaction is reinforced by back bonding that entails sharing of electrons in other metal orbitals into the empty pi-antibonding level on the alkene. Early metals of low oxidation state (Ti(II), Zr(II), Nb(III) etc.) are strong pi donors, and their alkene complexes are often described as metallacyclopropanes. Treatment of such species with acids gives the alkanes. Late metals (Ir(I), Pt(II)), which are poorer pi-donors, tend to engage the alkene as a Lewis acid–Lewis base interaction. Similarly, C2F4 is a stronger pi-acceptor than C2H4, as reflected in metal-carbon bond distances. [3]

Rotational barrier

The barrier for the rotation of the alkene about the M-centroid vector is a measure of the strength of the M-alkene pi-bond. Low symmetry complexes are suitable for analysis of these rotational barriers associated with the metal-ethene bond.In CpRh(C2H4)(C2F4), the ethene ligand is observed to rotate with a barrier near 12 kcal/mol but no rotation is observed for about the Rh-C2F4 bond. [4]

Reactions and applications

Alkene ligands lose much of their unsaturated character upon complexation. Most famously, the alkene ligand undergoes migratory insertion, wherein it is attacked intramolecularly by alkyl and hydride ligands to form new alkyl complexes. Cationic alkene complexes are susceptible to attack by nucleophiles. [1]

Catalysis

Metal alkene complexes are intermediates in many or most transition metal catalyzed reactions of alkenes: polymerization., hydrogenation, hydroformylation, and many other reactions. [5]

The mechanism of the Wacker process involves Pd-alkene complex intermediates. WackerMechWiki3.gif
The mechanism of the Wacker process involves Pd-alkene complex intermediates.

Separations

Since alkenes are mainly produced as mixtures with alkanes, the separation of alkanes and alkenes is of commercial interest. Separation technologies often rely on facilitated transport membranes containing Ag+ or Cu+ salts that reversibly bind alkenes. [6]

In argentation chromatography, stationary phases that contain silver salts are used to analyze organic compounds on the basis of the number and type of alkene (olefin) groups. This methodology is commonly employed for the analysis of the unsaturated content in fats and fatty acids. [7]

Natural occurrence

Metal-alkene complexes are uncommon in nature, with one exception. Ethylene affects the ripening of fruit and flowers by complexation to a Cu(I) center in a transcription factor. [8]

Related Research Articles

<span class="mw-page-title-main">Alkene</span> Hydrocarbon compound containing one or more C=C bonds

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<span class="mw-page-title-main">Diene</span> Covalent compound that contains two double bonds

In organic chemistry, a diene ; also diolefin, dy-OH-lə-fin) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alkene units, with the standard prefix di of systematic nomenclature. As a subunit of more complex molecules, dienes occur in naturally occurring and synthetic chemicals and are used in organic synthesis. Conjugated dienes are widely used as monomers in the polymer industry. Polyunsaturated fats are of interest to nutrition.

<span class="mw-page-title-main">Ethylene</span> Hydrocarbon compound (H₂C=CH₂)

Ethylene is a hydrocarbon which has the formula C2H4 or H2C=CH2. It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene.

In organic chemistry, hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes from alkenes. This chemical reaction entails the net addition of a formyl group and a hydrogen atom to a carbon-carbon double bond. This process has undergone continuous growth since its invention: production capacity reached 6.6×106 tons in 1995. It is important because aldehydes are easily converted into many secondary products. For example, the resultant aldehydes are hydrogenated to alcohols that are converted to detergents. Hydroformylation is also used in speciality chemicals, relevant to the organic synthesis of fragrances and pharmaceuticals. The development of hydroformylation is one of the premier achievements of 20th-century industrial chemistry.

In chemistry, π backbonding is a π-bonding interaction between a filled (or half filled) orbital of a transition metal atom and a vacant orbital on an adjacent ion or molecule. In this type of interaction, electrons from the metal are used to bond to the ligand, which dissipates excess negative charge and stabilizes the metal. It is common in transition metals with low oxidation states that have ligands such as carbon monoxide, olefins, or phosphines. The ligands involved in π backbonding can be broken into three groups: carbonyls and nitrogen analogs, alkenes and alkynes, and phosphines. Compounds where π backbonding is prominent include Ni(CO)4, Zeise's salt, and molybdenum and iron dinitrogen complexes.

<span class="mw-page-title-main">Wilkinson's catalyst</span> Chemical compound

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<span class="mw-page-title-main">Rhodium(III) chloride</span> Chemical compound

Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)n, where n varies from 0 to 3. These are diamagnetic red-brown solids. The soluble trihydrated (n = 3) salt is the usual compound of commerce. It is widely used to prepare compounds used in homogeneous catalysis.

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<span class="mw-page-title-main">Zeise's salt</span> Chemical compound

Zeise's salt, potassium trichloro(ethylene)platinate(II) hydrate, is the chemical compound with the formula K[PtCl3(C2H4)]·H2O. The anion of this air-stable, yellow, coordination complex contains an η2-ethylene ligand. The anion features a platinum atom with a square planar geometry. The salt is of historical importance in the area of organometallic chemistry as one of the first examples of a transition metal alkene complex and is named for its discoverer, William Christopher Zeise.

<span class="mw-page-title-main">Cyclooctadiene rhodium chloride dimer</span> Chemical compound

Cyclooctadiene rhodium chloride dimer is the organorhodium compound with the formula Rh2Cl2(C8H12)2, commonly abbreviated [RhCl(COD)]2 or Rh2Cl2(COD)2. This yellow-orange, air-stable compound is a widely used precursor to homogeneous catalysts.

<span class="mw-page-title-main">Dewar–Chatt–Duncanson model</span> Model in organometallic chemistry

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

In coordination chemistry, the bite angle is the angle on a central atom between two bonds to a bidentate ligand. This ligand–metal–ligand geometric parameter is used to classify chelating ligands, including those in organometallic complexes. It is most often discussed in terms of catalysis, as changes in bite angle can affect not just the activity and selectivity of a catalytic reaction but even allow alternative reaction pathways to become accessible.

In organometallic chemistry, a migratory insertion is a type of reaction wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products. However, often the two are used interchangeably because the mechanism is sometimes unknown. Therefore, migratory insertion reactions or insertion reactions, for short, are defined not by the mechanism but by the overall regiochemistry wherein one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:

<span class="mw-page-title-main">Organoiridium chemistry</span> Chemistry of organometallic compounds containing an iridium-carbon bond

Organoiridium chemistry is the chemistry of organometallic compounds containing an iridium-carbon chemical bond. Organoiridium compounds are relevant to many important processes including olefin hydrogenation and the industrial synthesis of acetic acid. They are also of great academic interest because of the diversity of the reactions and their relevance to the synthesis of fine chemicals.

Hydroacylation is a type of organic reaction in which an electron-rich unsaturated hydrocarbon inserts into a formyl C-H bond. With alkenes, the product is a ketone:

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">Organorhodium chemistry</span> Field of study

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<span class="mw-page-title-main">Cyclooctadiene iridium chloride dimer</span> Chemical compound

Cyclooctadiene iridium chloride dimer is an organoiridium compound with the formula [Ir(μ2-Cl)(COD)]2, where COD is the diene 1,5-cyclooctadiene (C8H12). It is an orange-red solid that is soluble in organic solvents. The complex is used as a precursor to other iridium complexes, some of which are used in homogeneous catalysis. The solid is air-stable but its solutions degrade in air.

<span class="mw-page-title-main">Chlorobis(cyclooctene)rhodium dimer</span> Chemical compound

Chlorobis(cyclooctene)rhodium dimer is an organorhodium compound with the formula Rh2Cl2(C8H14)4, where C8H14 is cis-cyclooctene. Sometimes abbreviated Rh2Cl2(coe)4, it is a red-brown, air-sensitive solid that is a precursor to many other organorhodium compounds and catalysts.

<span class="mw-page-title-main">Chlorobis(ethylene)rhodium dimer</span> Chemical compound

Chlorobis(ethylene)rhodium dimer is an organorhodium compound with the formula Rh2Cl2(C2H4)4. It is a red-orange solid that is soluble in nonpolar organic solvents. The molecule consists of two bridging chloride ligands and four ethylene ligands. The ethylene ligands are labile and readily displaced even by other alkenes. A variety of homogeneous catalysts have been prepared from this complex.

References

  1. 1 2 3 Elschenbroich, C. "Organometallics" (2006) Wiley-VCH: Weinheim. ISBN   3-527-29390-6
  2. ↑ John Hartwig. Organotransition Metal Chemistry: From Bonding to Catalysis. University Science Books. ISBN   978-1-891389-53-5.
  3. 1 2 Evans, J. A.; Russell, D. R. (1971). "The Crystal Structures of Ethylene and Tetrafluoroethylene Complexes of Rhodium(I)". Journal of the Chemical Society D: Chemical Communications (4): 197. doi:10.1039/C29710000197.
  4. ↑ Cramer, Richard; Kline, Jules B.; Roberts, John D. (1969). "Bond Character and Conformational Equilibration of Ethylene- and Tetrafluoroethylenerhodium Complexes from Nuclear Magnetic Resonance Spectra". Journal of the American Chemical Society. 91 (10): 2519–2524. doi:10.1021/ja01038a021.
  5. ↑ Piet W. N. M. van Leeuwen "Homogeneous Catalysis: Understanding the Art", 2004, Wiley-VCH, Weinheim. ISBN   1-4020-2000-7
  6. ↑ Azhin, Maryam; Kaghazchi, Tahereh; Rahmani, Mohammad (2008). "A Review on Olefin/Paraffin Separation Using Reversible Chemical Complexation technology". Journal of Industrial and Engineering Chemistry. 14 (5): 622–638. doi:10.1016/j.jiec.2008.04.014.
  7. ↑ Boryana Nikolova-Damyanova. "Principles of Silver Ion Complexation with Double Bonds".
  8. ↑ Jose M. Alonso, Anna N. Stepanova "The Ethylene Signaling Pathway" Science 2004, Vol. 306, pp. 1513-1515. doi : 10.1126/science.1104812
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