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Transition metal alkene complex

February 20, 2024

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]

Monoalkenes edit

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]

Dienes, trienes, polyenes, keto-alkenes, and other complex alkene ligands edit

See also: (Diene)iron tricarbonyl

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 edit

 
Structure of (acac)Rh(C2H4)(C2F4), distances (red) in picometers.[3]

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]

  • Bonding images
  •  
    Orbital interactions in a metal-ethylene complex, as described by the Dewar–Chatt–Duncanson model
  •  
    Two extreme depictions of M---C2H4 interactions.

Rotational barrier edit

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 edit

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 edit

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.

Separations edit

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 edit

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

References edit

  1. ^ a b c 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. ^ a b 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

February 20, 2024
transition, metal, alkene, complex, organometallic, chemistry, transition, metal, alkene, complex, coordination, compound, containing, more, alkene, ligands, inventory, large, such, compounds, intermediates, many, catalytic, reactions, that, convert, alkenes, . 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 1 Monoalkenes 2 Dienes trienes polyenes keto alkenes and other complex alkene ligands 3 Bonding 3 1 Rotational barrier 4 Reactions and applications 4 1 Catalysis 4 2 Separations 5 Natural occurrence 6 ReferencesMonoalkenes editThe 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 Dienes trienes polyenes keto alkenes and other complex alkene ligands editSee also Diene iron tricarbonyl 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 Metal alkene complexes nbsp Bis cyclooctadiene nickel 0 a catalyst and source of naked nickel nbsp The first alkene complex the anion in Zeise s salt nbsp Chlorobis cyclooctene rhodium dimer source of RhCl nbsp Crabtree s catalyst a very active catalyst for hydrogenation nbsp Benzylideneacetone iron tricarbonyl source of Fe CO 3 nbsp Et3P 2Pt 6 h2 h2 h2 h2 h2 h2 C60 a fullerene complex nbsp Mo C7H8 CO 3 a complex of cycloheptatriene nbsp Fe C8H8 2 a complex of cyclooctatetraene nbsp Norbornadiene molybdenum tetracarbonyl a source of Mo CO 4 nbsp Xylylene Fe CO 3 illustrating the stabilization of a labile alkene bby complexationBonding edit nbsp Structure of acac Rh C2H4 C2F4 distances red in picometers 3 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 Bonding images nbsp Orbital interactions in a metal ethylene complex as described by the Dewar Chatt Duncanson model nbsp Two extreme depictions of M C2H4 interactions Rotational barrier edit 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 editAlkene 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 edit Metal alkene complexes are intermediates in many or most transition metal catalyzed reactions of alkenes polymerization hydrogenation hydroformylation and many other reactions 5 nbsp The mechanism of the Wacker process involves Pd alkene complex intermediates Separations edit 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 editMetal alkene complexes are uncommon in nature with one exception Ethene affects the ripening of fruit and flowers by complexation to a Cu I center in a transcription factor 8 References edit a b c Elschenbroich C Organometallics 2006 Wiley VCH Weinheim ISBN 3 527 29390 6 John Hartwig Organotransition Metal Chemistry From Bonding to Catalysis University Science Books ISBN 978 1 891389 53 5 a b 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 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 Piet W N M van Leeuwen Homogeneous Catalysis Understanding the Art 2004 Wiley VCH Weinheim ISBN 1 4020 2000 7 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 Boryana Nikolova Damyanova Principles of Silver Ion Complexation with Double Bonds Jose M Alonso Anna N Stepanova The Ethylene Signaling Pathway Science 2004 Vol 306 pp 1513 1515 doi 10 1126 science 1104812 Retrieved from https en wikipedia org w index php title Transition metal alkene complex amp oldid 1205577968, wikipedia, wiki, book, books, library,

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