Transition metal allyl complex

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File:-AllPdCl-2.png
Structure of allylpalladium chloride dimer

Transition-metal allyl complexes are coordination complexes with allyl and its derivatives as ligands. Allyl is the radical with the connectivity CH2CHCH2, although as a ligand it is usually viewed as an allyl anion CH2=CH−CH2−, which is usually described as two equivalent resonance structures.

Examples

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The allyl ligand is commonly in organometallic chemistry. Usually, allyl ligands bind to metals via all three carbon atoms, the η3-binding mode. The η3-allyl group is classified as an LX-type ligand in the Green LXZ ligand classification scheme, serving as a 3e– donor using neutral electron counting and 4e– donor using ionic electron counting.

Scope

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Commonly, allyl ligands occur in mixed ligand complexes. Examples include (η3-allyl)Mn(CO)4 and CpPd(allyl).

Substituents on the allyl group are also common, e.g. 2-methallyl.[1]

Homoleptic complexes

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Chelating bis(allyl) complexes

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File:ChelBis(allyl)cmpx.png
General structure of a chelating bis(allyl) complex of Ru (L = alkene, phosphine)

1,3-Dienes such as butadiene and isoprene dimerize in the coordination spheres of some metals, giving chelating bis(allyl) complexes. Such complexes also arise from ring-opening of divinylcyclobutane. Chelating bis(allyl) complexes are intermediates in the metal-catalyzed dimerization of butadiene to give vinylcyclohexene and cycloocta-1,5-diene.[4]

Allyl σ ligands

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Complexes with η1-allyl ligands (classified as X-type ligands) are also known. 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). As is the case for many other η1-allyl complexes, the monohapticity of the allyl ligand in this species is enforced by the 18-electron rule, since CpFe(CO)2(η1-C3H5) is already an 18-electron complex, while an η3-allyl ligand would result in an electron count of 20 and violate the 18-electron rule. Such complexes can convert to the η3-allyl derivatives by dissociation of a neutral (two-electron) ligand L. For CpFe(CO)2(η1-C3H5), dissociation of L = CO occurs under photochemical conditions:[5]

CpFe(CO)2(η1-C3H5) → CpFe(CO)(η3-C3H5) + CO

Synthetic methods

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Allyl complexes are often generated by oxidative addition of allylic halides to low-valent metal complexes. This route is used to prepare (allyl)2Ni2Cl2:[1][6]

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.[7] The oxidative addition route has also been used to prepared Mo(II) allyl complexes:[8]

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

Other methods of synthesis involve addition of nucleophiles to η4-diene complexes and hydride abstraction from alkene complexes.[3] For example, palladium(II) chloride attacks alkenes to give first an alkene complex, but then abstracts hydrogen to give a dichlorohydridopalladium alkene complex, and then eliminates hydrogen chloride:[9]

PdCl2 + >C=CHCH< → Cl2Pd–(η2-(>CCHCH<)) → Cl2Pd(H)⚟(>CCHC<) → ClPd⚟(>CCHC<) + HCl

One allyl complex can transfer an allyl ligand to another complex.[10] An anionic metal complex can displace a halide, to give an allyl complex. However, if the metal center is coordinated to 6 or more other ligands, the allyl may end up "trapped" as a σ (η1-) ligand. In such circumstances, heating or irradiation can dislocate another ligand to free up space for the alkene-metal bond.[11]

In principle, salt metathesis reactions can adjoin an allyl ligand from an allylmagnesium bromide or related allyl lithium reagent.[3] However, the carbanion salt precursors require careful synthesis, as allyl halides readily undergo Wurtz coupling. Mercury and tin allyl halides appear to avoid this side-reaction.[12]

Benzyl complexes

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File:QIGLEZ.svg
Structure of tetrabenzylzirconium with H atoms omitted for clarity.[13]

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

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.[14] X-ray crystallography demonstrate that the benzyl ligands in tetrabenzylzirconium are highly flexible. One polymorph features four η2-benzyl ligands, whereas another polymorph has two η1- and two η2-benzyl ligands.[13]

Applications

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Allyl complexes are often discussed in academic research,[15][16][17][18] but few have commercial applications. A popular allyl complex is allyl palladium chloride.[19]


The reactivity of allyl ligands depends on the overall complex, although the influence of the metal center can be roughly summarized as[20]

(more reactive) Fe ≫ Pd > Mo > W (less reactive)

Such complexes are usually electrophilic (i.e., react with nucleophiles), but nickel allyl complexes are usually nucleophilic (resp. with electrophiles).[21] In the former case, the addition may occur at unusual locations, and can be useful in organic synthesis.[22]

References

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  10. ^ Powell 1982, p. 328.
  11. ^ Powell 1982, p. 329.
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  19. ^ Tatsuno, Y.; Yoshida, T.; Otsuka, S. "(η3-allyl)palladium(II) Complexes" Inorganic Syntheses, 1990, volume 28, pages 342-345. Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
  20. ^ Pearson 1987, pp. 932–933.
  21. ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
  22. ^ Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
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