Transition metal allyl complex

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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.

Structure of allylpalladium chloride dimer

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

bis(allyl)nickel^[3]
bis(allyl)palladium^[3]
bis(allyl)platinum^[3]
tris(allyl)chromium^[3]
tris(allyl)rhodium^[4]
tris(allyl)iridium^[4]

Mixed ligand complexes

General structure of a chelating bis(allyl) complex of Ru (L = alkene, phosphine)

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 + BrCH_₂CH=CH_₂ → 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]

From dienes and related compounds

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]

References

[1]
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[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.
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[6]
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[13]
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[14]
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[15]
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[16]
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[17]
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[19]
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[20]
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[21]
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[22]
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[23]
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