Transition metal allyl complex

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Structure of allylpalladium chloride dimer

Transition-metal allyl complexes are coordination complexes with allyl and its derivatives as ligands. The allyl group, the connectivity CH2=CHCH2−, binds to metal predominantly to form π-allyl complexes.

π-Allyl complexes

In π-allyl complexes, all three carbon atoms bind to the metal. This bonding mode is also referred to as η3-allyl, meaning three contiguous atoms form bonds to the metal. π-Allyl is classified as an LX-type ligand in the LXZ ligand classification scheme, serving as a 3e– donor using neutral electron counting and 4e– donor using ionic electron counting. Characteristically, the central carbon binds more tightly to the metal, as verified by X-ray crystallography. In bis(allyl)nickel, the Ni-C(central) distance is 198 picometer and the two flanking Ni-C distances are 202 pm.[1]

Substituents on the allyl group are also common:

  • 2-methallyl.[2]
  • 1-methylallyl (crotyl)

Homoleptic complexes

Mixed ligand complexes

General structure of a chelating bis(allyl) complex of Ru (L = alkene, phosphine)
Co(1,5-cyclooctadiene)(cyclooctenyl).

Many mixed ligand complexes are known: (η3-allyl)Mn(CO)4 and CpPd(allyl).

Synthetic methods

A variety of routes, some serendipitous, have been reported.

Oxidative addition of allyl halides

Allyl complexes are often generated by oxidative addition of allylic halides to low-valent metal complexes. This route is used to prepare (allyl)2Ni2Cl2:[2][5]

2 Ni(CO)4 + 2 ClCH2CH=CH2 → Ni2(μ-Cl)2(η3-C3H5)2 + 8 CO

A similar oxidative addition involves the reaction of allyl bromide to diiron nonacarbonyl.[6] The oxidative addition route has also been used to prepare Mo(II) allyl complexes:[7]

Mo(CO)3(pyridine)3 + BrCH2CH=CH2 → Mo(CO)2(Br)(C3H5)(pyridine)2 + pyridine + CO

The oxidative addition route proceeds via complexes with η1-allyl ligands. Also called σ-allyl, it is classified as an X-type ligand. One example is CpFe(CO)2(η1-C3H5), in which only the methylene group is attached to the Fe centre (i.e., it has the connectivity [Fe]–CH2–CH=CH2). Decarbonylation of CpFe(CO)2(η1-C3H5) gives CpFe(CO)(η3-C3H5).[8]

η4-Diene ligands are viable precursors to π-allyl complexes. For example, cationic butadiene complexes are susceptible to hydride reduction to give complexes of crotyl (η3-C3H4CH3). Complementarily, anionic diene complexes are known to protonate at carbon to also give crotyl derivatives.[9]

The addition of butadiene adds to hydrido cobalt tetracarbonyl to give the crotyl complex:[10]

C4H6r + HCo(CO)4 → (C3H4Me)Co(CO)3 + CO

This complex exists as a mixture of syn and anti isomers, depending on the location of methyl (Me) substituent.

1,3-Dienes such as butadiene and isoprene dimerize in the presence of some metals, giving chelating bis(allyl) complexes. Chelating bis(allyl) complexes are intermediates in the metal-catalyzed dimerization of butadiene to give vinylcyclohexene and cycloocta-1,5-diene. Such complexes also arise from ring-opening of divinylcyclobutane. [11]

Allene is another source of allyl complexes.

Reduction of cobalt(II) chloride with sodium in the presence of 1,5-cyclooctadiene gives Co(cyclooctadiene)(cyclooctenyl). In this case one nonconjugated diene serves as a source of a cyclic allyl ligand.[12]

From alkenes

Hydride abstraction from alkene complexes represents yet another route to π-allyl complexes.[4][13]

Salt metathesis reactions

Salt metathesis reactions are important routes to allyl complexes:[10]

2 C3H5MgBr + NiBr2 → Ni(C3H5)2 + 2 MgBr2
3 C3H5MgBr + Co(C5H5O2)3 → Co(C3H5)3 + 3 MgBr(C5H5O2 (where C5H5O2 is acetylacetonate)

Benzyl complexes

Structure of tetrabenzylzirconium with H atoms omitted for clarity.[14]

Benzyl and allyl ligands often exhibit similar chemical properties. Benzyl ligands commonly adopt either η1 or η3 bonding modes. The interconversion reactions parallel those of η1- or η3-allyl ligands:

CpFe(CO)2(η1-CH2Ph) → CpFe(CO)(η3-CH2Ph) + CO (Ph = phenyl)

In all bonding modes, the benzylic carbon atom is more strongly attached to the metal as indicated by M-C bond distances, which differ by ca. 0.2 Å in η3-bonded complexes.[15] X-ray crystallography demonstrates that the benzyl ligands in tetrabenzylzirconium are flexible. One polymorph features four η2-benzyl ligands, whereas another polymorph has two η1- and two η2-benzyl ligands.[14]

Reactions

The behavior of π-allyl complexes has attracted the considerable attention. The reactivity depends strongly on the ancillary ligands.[16][17]

Protonolysis can afford the free propene:[18]

2 L2Rh(C3H5) + 2 HCl → [L2RhCl]2 + 2 CH3CH=CH2 (L = phosphine ligand)

Applications

π-Allyl complexes are often discussed in mechanistic organometallic chemistry. For example, the formation of allyl-metal-hydride intermediates are often invoked to explain the isomerization of alkene complexes.

Many homogeneous catalysts have been developed from allyl complexes, but few have commercial applications. In the area of organic synthesis, a popular allyl complex is allyl palladium chloride.[19] [20][21][22][23]

Further reading

  • Powell, P. (1982). "Synthesis of η3-allyl complexes". In Hartley, Frank R.; Patai, Saul (eds.). The Chemistry of the Metal–Carbon Bond. Vol. 1: The structure, preparation, thermochemistry and characterization of organometallic compounds. Chichester, UK: Interscience (published April 1987). pp. 326–8. ISBN 0471100587.

References

  1. Goddard, R.; Krueger, C.; Mark, F.; Stansfield, R.; Zhang, X. (1985). "Effect upon the hydrogen atoms of bonding an allyl group to a transition metal. A theoretical investigation and an experimental determination using neutron diffraction of the structure of bis(.eta.3-allyl)nickel". Organometallics. 4 (2): 285–290. doi:10.1021/om00121a014.
  2. Semmelhack, Martin F.; Helquist, Paul M. (1972). "Reactions of Aryl Halides with π-Allylnickel Halides: Methallylbenzene". Organic Syntheses. 52: 115. doi:10.15227/orgsyn.052.0115.
  3. O'Brien, S.; Fishwick, M.; McDermott, B.; Wallbridge, M. G. H.; Wright, G. A. (1972). "Isoleptic Allyl Derivatives of Various Metals". Inorganic Syntheses. Vol. 13. pp. 73–79. doi:10.1002/9780470132449.ch14. ISBN 978-0-470-13244-9.
  4. Kevin D. John; Judith L. Eglin; Kenneth V. Salazar; R. Thomas Baker; Alfred P. Sattelberger (2014). "Tris(Allyl)Iridium and -Rhodium". Inorganic Syntheses: Volume 36. Vol. 36. p. 165. doi:10.1002/9781118744994.ch32. ISBN 978-1-118-74499-4.
  5. Craig R. Smith; Aibin Zhang; Daniel J. Mans; T. V. RajanBabu (2008). "(R)-3-methyl-3-phenyl-1-pentene Via Catalytic Asymmetric Hydrovinylation". Org. Synth. 85: 248–266. doi:10.15227/orgsyn.085.0248. PMC 2723857. PMID 19672483.
  6. Putnik, Charles F.; Welter, James J.; Stucky, Galen D.; d'Aniello, M. J.; Sosinsky, B. A.; Kirner, J. F.; Muetterties, E. L. (1978). "Metal clusters in catalysis. 15. A Structural and Chemical Study of a Dinuclear Metal Complex, Hexacarbonylbis(η3-2-propenyl)diiron(Fe-Fe)". Journal of the American Chemical Society. 100 (13): 4107–4116. doi:10.1021/ja00481a020.
  7. Pearson, Anthony J.; Schoffers, Elke (1997). "Tricarbonyltris(pyridine)molybdenum: A Convenient Reagent for the Preparation of (π-Allyl)molybdenum Complexes". Organometallics. 16 (24): 5365–5367. doi:10.1021/om970473n.
  8. Fish, R. W.; Giering, W. P.; Marten, D.; Rosenblum, M. (1976-01-27). "Thermal and photochemical interconversions of isomeric monocarbonyl η5-Cyclopentadienyl(η3-allyl)iron complexes". Journal of Organometallic Chemistry. 105 (1): 101–118. doi:10.1016/S0022-328X(00)91977-6. ISSN 0022-328X.
  9. Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. ISBN 189138953X
  10. C. Elschenbroich (2006). Organometallics. VCH. p. 436. ISBN 978-3-527-29390-2.
  11. Hirano, Masafumi; Sakate, Yumiko; Komine, Nobuyuki; Komiya, Sanshiro; Wang, Xian-qi; Bennett, Martin A. (2011). "Stoichiometric Regio- and Stereoselective Oxidative Coupling Reactions of Conjugated Dienes with Ruthenium(0). A Mechanistic Insight into the Origin of Selectivity". Organometallics. 30 (4): 768–777. doi:10.1021/om100956f.
  12. Gosser, L. W.; Cushing, M. A. Jr. (1977). "Π-Cyclooctenyl-π-L,5-Cycloocta-Dienecobalt". π-Cyclooctenyl-π-1,5-cyclooctadienecobalt. Inorganic Syntheses. Vol. 17. pp. 112–15. doi:10.1002/9780470132487.ch32. ISBN 978-0-470-13248-7.
  13. Pearson, A. J. (1987). "Transition metal-stabilized carbocations in organic synthesis". In Hartley, Frank R. (ed.). The Chemistry of the Metal–Carbon Bond. Vol. 4. John Wiley & Sons. p. 911. doi:10.1002/9780470771778. ISBN 978-0-470-77177-8.
  14. Rong, Yi; Al-Harbi, Ahmed; Parkin, Gerard (2012). "Highly Variable Zr–CH2–Ph Bond Angles in Tetrabenzylzirconium: Analysis of Benzyl Ligand Coordination Modes". Organometallics. 31 (23): 8208–8217. doi:10.1021/om300820b.
  15. Trost, Barry M.; Czabaniuk, Lara C. (2014). "Structure and Reactivity of Late Transition Metal η3-Benzyl Complexes". Angew. Chem. Int. Ed. 53 (11): 2826–2851. doi:10.1002/anie.201305972. PMID 24554487.
  16. Pearson 1987, pp. 932–933.
  17. Chiusoli, G. P.; Salerno, G.; Tsuji J.; Sato F. (1985). "Carbon-carbon bond formation using η3-allyl complexes". In Hartley, Frank R.; Patai, Saul (eds.). The Chemistry of the Metal–Carbon Bond. Vol. 3: Carbon-carbon bond formation using organometallic compounds. Chichester, UK: John Wiley & Sons. pp. 143–205. ISBN 0471905577.
  18. Nixon, J.F.; Wilkins, B. (1972). "One the π-allylmetal hydride mechanism for the isomerisation of olefins catalysed by transition metal complexes". Journal of Organometallic Chemistry. 44: C25–C28. doi:10.1016/0022-328X(72)80038-X.
  19. Tatsuno, Y.; Yoshida, T.; Otsuka, S. "(η3-allyl)palladium(II) Complexes" Inorganic Syntheses, 1990, volume 28, pages 342-345. ISBN 0-471-52619-3
  20. Consiglio, Giambattista; Waymouth, Robert M. (1989). "Enantioselective homogeneous catalysis involving transition-metal-allyl intermediates". Chemical Reviews. 89: 257–276. doi:10.1021/cr00091a007.
  21. Li, Chien-Le; Liu, Rai-Shung (2000). "Synthesis of Heterocyclic and Carbocyclic Compounds via Alkynyl, Allyl, and Propargyl Organometallics of Cyclopentadienyl Iron, Molybdenum, and Tungsten Complexes". Chemical Reviews. 100 (8): 3127–3162. doi:10.1021/cr990283h. PMID 11749315.
  22. Ley, Steven V.; Cox, Liam R.; Meek, Graham (1996). "(π-Allyl)tricarbonyliron Lactone Complexes in Organic Synthesis: A Useful and Conceptually Unusual Route to Lactones and Lactams". Chemical Reviews. 96: 423–442. doi:10.1021/cr950015t. PMID 11848759.
  23. Welker, Mark E. (1992). "3 + 2 Cycloaddition reactions of transition-metal 2-alkynyl and .eta.1-allyl complexes and their utilization in five-membered-ring compound syntheses". Chemical Reviews. 92: 97–112. doi:10.1021/cr00009a004.