Transition metal alkyne complex: Difference between revisions
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==Synthesis== |
==Synthesis== |
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Transition metal alkyne complexes are often formed by the displacement of labile ligands by the alkyne. For example, a variety of cobalt-alkyne complexes may be formed by reaction of the alkyne with dicobalt octacarbonyl.<ref>Kemmitt, R. D. W.; Russell, D. R.; "Cobalt" in ''Comprehensive Organometallic Chemistry I''; Abel, E.W.; Stone, F.G.A.; Wilkinson, G. eds., 1982, Pergamon Press, Oxford. ISBN 0-08-025269-9</ref> |
Transition metal alkyne complexes are often formed by the displacement of labile ligands by the alkyne. For example, a variety of cobalt-alkyne complexes may be formed by reaction of the alkyne with dicobalt octacarbonyl.<ref>Kemmitt, R. D. W.; Russell, D. R.; "Cobalt" in ''Comprehensive Organometallic Chemistry I''; Abel, E.W.; Stone, F.G.A.; Wilkinson, G. eds., 1982, Pergamon Press, Oxford. ISBN 0-08-025269-9</ref> |
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:Co<sub>2</sub>(CO)<sub>8</sub> + R<sub>2</sub>C<sub>2</sub> → Co<sub>2</sub>(C<sub>2</sub>R<sub>2</sub>)(CO)<sub>6</sub> + 2 CO |
:Co<sub>2</sub>(CO)<sub>8</sub> + R<sub>2</sub>C<sub>2</sub> → Co<sub>2</sub>(C<sub>2</sub>R<sub>2</sub>)(CO)<sub>6</sub> + 2 CO |
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Many alkyne complexes are produced by reduction of metal halides, e.g. [[titanocene dichloride]] and [[bis(triphenylphosphine)platinum dichloride]] in the presence of the alkyne: |
Many alkyne complexes are produced by reduction of metal halides, e.g. [[titanocene dichloride]] and [[bis(triphenylphosphine)platinum dichloride]] in the presence of the alkyne: |
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:Cp<sub>2</sub>TiCl<sub>2</sub> + C<sub>2</sub>R<sub>2</sub> + Mg → Cp<sub>2</sub>Ti(C<sub>2</sub>R<sub>2</sub>) + MgCl<sub>2</sub> |
:Cp<sub>2</sub>TiCl<sub>2</sub> + C<sub>2</sub>R<sub>2</sub> + Mg → Cp<sub>2</sub>Ti(C<sub>2</sub>R<sub>2</sub>) + MgCl<sub>2</sub> |
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==Structure and Bonding== |
==Structure and Bonding== |
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[[File:VarietyPackAlkyneCmpx.svg|thumb|center|620px|Structures of various metal-alkyne complexes.]] |
[[File:VarietyPackAlkyneCmpx.svg|thumb|center|620px|Structures of various metal-alkyne complexes.]] |
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The coordination of alkynes to transition metals is similar to that of alkenes. The bonding is described by the [[Dewar-Chatt-Duncanson model]]. Upon complexation the C-C bond elogates and the alkynyl carbon bends. For example in the phenylpropyne complex Pt(PPh<sub>3</sub>)<sub>2</sub>(C<sub>2</sub>)Ph(Me), the C-C distance is 1.277(25) Å vs 1.20 for a typical alkyne. |
The coordination of alkynes to transition metals is similar to that of alkenes. The bonding is described by the [[Dewar-Chatt-Duncanson model]]. Upon complexation the C-C bond elogates and the alkynyl carbon bends. For example in the phenylpropyne complex Pt(PPh<sub>3</sub>)<sub>2</sub>(C<sub>2</sub>)Ph(Me), the C-C distance is 1.277(25) Å vs 1.20 for a typical alkyne. The C-C-C angle distorts 40° from linearity.<ref>William Davies, B.; C. Payne, N., "Studies on metal-acetylene complexes: V. Crystal and molecular structure of bis(triphenylphosphine)(1-phenylpropyne)platinum(0), [P(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>]<sub>2</sub>(C<sub>6</sub>H<sub>5</sub>CCCH<sub>3</sub>)Pt<sup>0</sup>" J. Organomet. Chem. 1975, volume 99, pp. 315. {{doi|10.1016/S0022-328X(00)88462-4}}</ref> In the IR spectra, the C-C vibration of alkynes, which occurs near 2300 cm<sup>-1</sup>, shifts upon complexation to around 1800 cm<sup>-1</sup>, indicating a weakening of the C-C bond. |
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===η<sup>2</sup>-coordination to a single metal center=== |
===η<sup>2</sup>-coordination to a single metal center=== |
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When bonded side-on to a single metal atom, an alkyne |
When bonded side-on to a single metal atom, an alkyne serves as a dihapto usually two-election donor. For early metal complexes, e.g., Cp<sub>2</sub>Ti(C<sub>2</sub>R<sub>2</sub>), strong π-backbonding into one of the π* antibonding orbitals of the alkyne is indicated. This complex is described as a metallacyclopropene derivative of Ti(IV). For late transition metal complexes, e.g., Pt(PPh<sub>3</sub>)<sub>2</sub>(MeC<sub>2</sub>Ph), the π-backbonding is less prominent, and the complex is assigned oxidation state (0).<ref>Hill, A.F. ''Organotransition Metal Chemistry'', 2002, Royal Society of Chemistry, ISBN 0-471-28163-8.</ref><ref name=Crabtree>Crabtree, R. H. ''Comprehensive Organometallic Chemistry V'', 2009, John Wiley & Sons, Inc. ISBN 0-470-25762-3.,</ref> |
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In some complexes, the alkyne is classified as a four-electron donor. In these cases, both pairs of pi-electrons donate to the metal. This kind of bonding was first implicated in the |
In some complexes, the alkyne is classified as a four-electron donor. In these cases, both pairs of pi-electrons donate to the metal. This kind of bonding was first implicated in complexes of the type W(CO)(R<sub>2</sub>C<sub>2</sub>)<sub>3</sub>.<ref> Joseph L. Templeton "Four-Electron Alkyne Ligands in Molybdenum(II) and Tungsten(II) Complexes" Advances in Organometallic Chemistry Volume 29, 1989, Pages 1–100.</ref> |
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===η<sup>2</sup>, η<sup>2</sup>-coordination bridging two metal centers=== |
===η<sup>2</sup>, η<sup>2</sup>-coordination bridging two metal centers=== |
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Metal alkyne species are implicated as intermediates in the [[hydrogenation]] of alkynes, e.g. to alkenes. |
Metal alkyne species are implicated as intermediates in the [[hydrogenation]] of alkynes, e.g. to alkenes. |
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Metal-alkyne complexes are intermediates in the metal-catalyzed [[alkyne trimerisation]]. With the shift away from coal-based (acetylene) to petroleum-based feedstocks (olefins), catalytic reactions with alkynes are not widely practiced industrially. Specialized variants are still exploited for certain niche uses, e.g. synthesis of substituted [[pyridine]]s. [[Cyclooctatetraene]] is produced from acetylene via the intermediacy of metal alkyne complexes. The [[Pauson-Khand reaction]] provides a route to cyclopentenones via the intermediacy of cobalt-alkyne complexes. |
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[[File:Pauson Khand reaction original.svg|center|PK reaction]] |
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==References== |
==References== |
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Revision as of 02:08, 6 April 2015
In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization.[1]
Synthesis
Transition metal alkyne complexes are often formed by the displacement of labile ligands by the alkyne. For example, a variety of cobalt-alkyne complexes may be formed by reaction of the alkyne with dicobalt octacarbonyl.[2]
- Co2(CO)8 + R2C2 → Co2(C2R2)(CO)6 + 2 CO
Many alkyne complexes are produced by reduction of metal halides, e.g. titanocene dichloride and bis(triphenylphosphine)platinum dichloride in the presence of the alkyne:
- Cp2TiCl2 + C2R2 + Mg → Cp2Ti(C2R2) + MgCl2
Structure and Bonding
The coordination of alkynes to transition metals is similar to that of alkenes. The bonding is described by the Dewar-Chatt-Duncanson model. Upon complexation the C-C bond elogates and the alkynyl carbon bends. For example in the phenylpropyne complex Pt(PPh3)2(C2)Ph(Me), the C-C distance is 1.277(25) Å vs 1.20 for a typical alkyne. The C-C-C angle distorts 40° from linearity.[3] In the IR spectra, the C-C vibration of alkynes, which occurs near 2300 cm-1, shifts upon complexation to around 1800 cm-1, indicating a weakening of the C-C bond.
η2-coordination to a single metal center
When bonded side-on to a single metal atom, an alkyne serves as a dihapto usually two-election donor. For early metal complexes, e.g., Cp2Ti(C2R2), strong π-backbonding into one of the π* antibonding orbitals of the alkyne is indicated. This complex is described as a metallacyclopropene derivative of Ti(IV). For late transition metal complexes, e.g., Pt(PPh3)2(MeC2Ph), the π-backbonding is less prominent, and the complex is assigned oxidation state (0).[4][5]
In some complexes, the alkyne is classified as a four-electron donor. In these cases, both pairs of pi-electrons donate to the metal. This kind of bonding was first implicated in complexes of the type W(CO)(R2C2)3.[6]
η2, η2-coordination bridging two metal centers
Because alkynes have two π bonds, alkynes can form stable complexes in which they bridge two metal centers. The alkyne donates a total of four electrons, with two electrons donated to each of the metals. And example of a complex with this bonding scheme is η2-diphenylacetylene-(hexacarbonyl)dicobalt(0).[5]
Benzyne complexes
Transition metal benzyne complexes represent a special case of alkyne complexes since the free benzynes are not stable in the absence of the metal.[7]
Applications
Metal alkyne species are implicated as intermediates in the hydrogenation of alkynes, e.g. to alkenes.
Metal-alkyne complexes are intermediates in the metal-catalyzed alkyne trimerisation. With the shift away from coal-based (acetylene) to petroleum-based feedstocks (olefins), catalytic reactions with alkynes are not widely practiced industrially. Specialized variants are still exploited for certain niche uses, e.g. synthesis of substituted pyridines. Cyclooctatetraene is produced from acetylene via the intermediacy of metal alkyne complexes. The Pauson-Khand reaction provides a route to cyclopentenones via the intermediacy of cobalt-alkyne complexes.
References
- ^ Elschenbroich, C. ”Organometallics” 2006 Wiley-VCH: Weinheim. ISBN 3-527-29390-6.
- ^ Kemmitt, R. D. W.; Russell, D. R.; "Cobalt" in Comprehensive Organometallic Chemistry I; Abel, E.W.; Stone, F.G.A.; Wilkinson, G. eds., 1982, Pergamon Press, Oxford. ISBN 0-08-025269-9
- ^ William Davies, B.; C. Payne, N., "Studies on metal-acetylene complexes: V. Crystal and molecular structure of bis(triphenylphosphine)(1-phenylpropyne)platinum(0), [P(C6H5)3]2(C6H5CCCH3)Pt0" J. Organomet. Chem. 1975, volume 99, pp. 315. doi:10.1016/S0022-328X(00)88462-4
- ^ Hill, A.F. Organotransition Metal Chemistry, 2002, Royal Society of Chemistry, ISBN 0-471-28163-8.
- ^ a b Crabtree, R. H. Comprehensive Organometallic Chemistry V, 2009, John Wiley & Sons, Inc. ISBN 0-470-25762-3.,
- ^ Joseph L. Templeton "Four-Electron Alkyne Ligands in Molybdenum(II) and Tungsten(II) Complexes" Advances in Organometallic Chemistry Volume 29, 1989, Pages 1–100.
- ^ William M. Jones, Jerzy Klosin "Transition-Metal Complexes of Arynes, Strained Cyclic Alkynes, and Strained Cyclic Cumulenes" Advances in Organometallic Chemistry 1998, Volume 42, Pages 147–221. doi:10.1016/S0065-3055(08)60543-2
