Английская Википедия:Half sandwich compound

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Шаблон:Short description Half sandwich compounds, also known as piano stool complexes, are organometallic complexes that feature a cyclic polyhapto ligand bound to an ML_n center, where L is a unidentate ligand. Thousands of such complexes are known.Шаблон:Page needed Well-known examples include cyclobutadieneiron tricarbonyl and (C₅H₅)TiCl₃. Commercially useful examples include (C₅H₅)Co(CO)₂, which is used in the synthesis of substituted pyridines, and methylcyclopentadienyl manganese tricarbonyl, an antiknock agent in petrol.

• MMT is a commercially useful antiknock compound.

  MMT is a commercially useful antiknock compound.

• CpCo(CO)2 is a catalyst for the synthesis of pyridines.

  CpCo(CO)₂ is a catalyst for the synthesis of pyridines.

• (C4H4)Fe(CO)3.

  (C₄H₄)Fe(CO)₃.

• CpFe(CO)2I is an example of a piano stool complex with two different monodentate ligands.

  CpFe(CO)₂I is an example of a piano stool complex with two different monodentate ligands.

• The diruthenium of cymene is readily cleaved by ligands to give monoRu half-sandwich derivatives.

  The diruthenium of cymene is readily cleaved by ligands to give monoRu half-sandwich derivatives.

• Cycloheptatriene molybdenum tricarbonyl

  Cycloheptatriene molybdenum tricarbonyl

• Cp2V2(CO)5 featuring a pair of semi-bridging CO ligands.[2]

  Cp₂V₂(CO)₅ featuring a pair of semi-bridging CO ligands.

(η⁵-C₅H₅) piano stool compounds

Half sandwich complexes containing cyclopentadienyl ligands are common. Well studied examples include (η⁵-C₅H₅)V(CO)₄, (η⁵-C₅H₅)Cr(CO)₃H, (η⁵-CH₃C₅H₄)Mn(CO)₃, (η⁵-C₅H₅)Cr(CO)₃H, [(η⁵-C₅H₅)Fe(CO)₃]⁺, (η⁵-C₅H₅)V(CO)₄I, and [[Tris(acetonitrile)cyclopentadienylruthenium hexafluorophosphate|(η⁵-C₅H₅)Ru(NCMe)Шаблон:Su]]. (η⁵-C₅H₅)Co(CO)₂ is a two-legged piano stool complex. Bulky cyclopentadienyl ligands such as 1,2,4-C₅H₂(tert-Bu)₃⁻ form unusual half-sandwich complexes.

(η⁶-C₆H₆) piano stool compounds

Файл:Piano Stool Complex.png

In organometallic chemistry, (η⁶-C₆H₆) piano stool compounds are half-sandwich compounds with (η⁶-C₆H₆)ML₃ structure (M = Cr, Mo, W, Mn(I), Re(I) and L = typically CO). (η⁶-C₆H₆) piano stool complexes are stable 18-electron coordination compounds with a variety of chemical and material applications. Early studies on (η⁶-C₆H₆)Cr(CO)₃ were carried out by Natta, Ercoli and Calderazzo, and Fischer and Ofele, and the crystal structure was determined by Corradini and Allegra in 1959. The X-ray data indicate that the plane of the benzene ring is nearly parallel to the plane defined by the oxygen atoms of the carbonyl ligands, and so the structure resembles a benzene seat mounted on three carbonyl legs tethered by the metal atom.

Cr and Mn(I) (η⁶-C₆H₆) piano stool complexes

Piano stool complexes of the type (η⁶-C₆H₆)M(CO)₃ are typically synthesized by heating the appropriate metal carbonyl compound with benzene. Alternately, the same compounds can be obtained by carbonylation of the bis(arene) sandwich compounds, such as (η⁶-C₆H₆)₂M compound with the metal carbonyl compound. This second approach may be more appropriate for arene ligands containing thermally fragile substituents.

Файл:Piano Synthesis 2.png

Reactivity of (η⁶-C₆H₆)Cr(CO)₃

The benzene ligand in (η⁶-C₆H₆)Cr(CO)₃Mi is prone to deprotonation. For example, Organolithium compounds form adducts featuring cyclohexadienyl ligands. Subsequent oxidation of the complex results in the release of a substituted benzene. Oxidation of the chromium atom by I₂ and other iodine reagents has been shown to promote exchange of arene ligands, but the intermediate chromium iodide species has not been characterized.

Файл:Arene-Cr(CO)3 Reaction with RLi.png

(η⁶-C₆H₆)Cr(CO)₃ complexes exhibit "cine" and "tele" nucleophilic aromatic addition. Processes of this type involve reaction of (η⁶-C₆H₆)Cr(CO)₃ with an alkyl lithium reagent. Subsequent treatment with an acid results in the addition of a nucleophile to the benzene ring at a site ortho ("cine"), meta or para ("tele") to the ipso carbon (see Arene substitution patterns).

Файл:(Benzene)chromiumtricarbonyl electrophile nucleophilic carbonylation.png

Reflecting its increased acidity, the benzene ligand can be lithiated with n-butyllithium. The resulting organolithium compound serves as a nucleophile in various reactions, for example, with trimethylsilyl chloride:Шаблон:Citation needed

Файл:(Benzene)chromiumtricarbonyl lithiation TMS.png

(η⁶-C₆H₆)Cr(CO)₃ is a useful catalyst for the hydrogenation of 1,3-dienes. The product alkene results from 1,4-addition of hydrogen. The complex does not hydrogenate isolated double bonds.Шаблон:Citation needed

A variety of arenes ligands have been installed aside from benzene. Weakly coordinating ligands may be employed to improve ligand exchange and thus the turnover rates for (η⁶-C₆H₆)M(CO)₃ complexes.Шаблон:Rp(η⁶-C₆H₆)M(CO)₃ complexes have been incorporated into high surface area porous materials.

(η⁶-C₆H₆)M(CO)₃ complexes serve as models for the interaction of metal carbonyls with graphene and carbon nanotubes. The presence of M(CO)₃ on extended π-network materials has been shown to improve electrical conductivity across the material.

Reactivity of [(η⁶-C₆H₆)Mn(CO)₃]⁺

Typical arene tricarbonyl piano stool complexes of Mn(I) and Re(I) are cationic and thus exhibit enhanced reactivity toward nucleophiles. Subsequent to nucleophilic addition, the modified arene can be recovered from the metal.

Файл:Mn Piano Stool Reaction.png

(η⁶-C₆H₆)Ru complexes

Half-sandwich compounds employing Ru(II), such as (cymene)ruthenium dichloride dimer, have been mainly investigated as catalysts for transfer hydrogenation. These complexes feature three coordination sites that are susceptible to substitution, while the arene ligand is tightly bonded and protects the metal against oxidation to Ru(III). They are prepared by reaction of RuCl₃·x(H₂O) with 1,3-cyclohexadienes. Work is also conducted on their potential as anticancer drugs.

Файл:Synthesis of benzeneRu dimer2.png

(η⁶-C₆H₆)RuCl₂ readily undergoes ligand exchange via cleavage of the chloride bridges, making this complex a versatile precursor to Ru(II) piano stool derivatives.