Transition-metal allyl complexes arecoordination complexes withallyl and its derivatives asligands. 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.
The allylligand is commonly inorganometallic chemistry. Usually, allyl ligands bind to metals via all three carbon atoms, theη3-binding mode. The η3-allyl group is classified as an LX-type ligand in the Green LXZligand classification scheme, serving as a 3e– donor using neutralelectron counting and 4e– donor using ionic electron counting.
General structure of a chelating bis(allyl) complex of Ru (L = alkene, phosphine)
1,3-Dienes such asbutadiene andisoprene 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 givevinylcyclohexene andcycloocta-1,5-diene.[4]
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 the18-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 the18-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]
Allyl complexes are often generated byoxidative 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 todiiron 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 eliminateshydrogen chloride:[9]
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 anallylmagnesium bromide or related allyl lithium reagent.[3] However, the carbanion salt precursors require careful synthesis, as allyl halides readily undergoWurtz coupling. Mercury and tin allyl halides appear to avoid this side-reaction.[12]
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 intetrabenzylzirconium are highly flexible. Onepolymorph features four η2-benzyl ligands, whereas another polymorph has two η1- and two η2-benzylligands.[13]
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^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.
^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.ISBN978-0-470-77177-8.
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^abRong, Yi; Al-Harbi, Ahmed; Parkin, Gerard (2012). "Highly Variable Zr–CH2–Ph Bond Angles in Tetrabenzylzirconium: Analysis of Benzyl Ligand Coordination Modes".Organometallics. 31pages=8208–8217 (23):8208–8217.doi:10.1021/om300820b.
^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.PMID24554487.
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^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.PMID11749315.
^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.PMID11848759.
^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.
^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.ISBN0471905577.
^Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010.ISBN189138953X
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