Bent Metallocene

In organometallic chemistry, bent metallocenes are a subset of metallocenes. In bent metallocenes, the ring systems coordinated to the metal are not parallel, but are tilted at an angle. A common example of a bent metallocene is Cp₂TiCl₂. Several reagents and much research is based on bent metallocenes.

Synthesis

Like regular metallocenes, bent metallocenes are synthesized by a variety of methods but most typically by reaction of sodium cyclopentadienide with the metal halide. This method applies to the synthesis of the bent metallocene dihalides of titanium, zirconium, hafnium, and vanadium:
:2 NaC₅H₅ + TiCl₄ → (C₅H₅)₂TiCl₂ + 2 NaCl
In the earliest work in this area, Grignard reagents were used to deprotonate the cyclopentadiene.

Niobocene dichloride, featuring Nb(IV), is prepared via a multistep reaction that begins with a Nb(V) precursor:
:NbCl₅ + 6 NaC₅H₅ → 5 NaCl + (C₅H₅)₄Nb + organic products
:(C₅H₅)₄Nb + 2 HCl + 0.5 O₂) → sub>2Ol₂ + 2 C₅H₆
:2 HCl + sub>2Ol₂ + SnCl₂ → 2 (C₅H₅)2NbCl₂ + SnCl₄ + H₂O
Bent metallocene dichlorides of molybdenum and tungsten are also prepared via indirect routes that involve redox at the metal centres.

TebbeRgt.png, Tebbe's reagent
Schwartz's Reagent.png, Schwartz's reagent.
Ansa-Metallocene.svg, Ansa-metallocene, X is the linker group, often (CH₂)_n or R₂Si (R = alkyl)
Cp2Ti(CO)2.png, Titanocene dicarbonyl.

Structure and bonding

Bent metallocenes have idealized C_2v symmetry. The non-Cp ligands are arrayed in the wedge area. For bent metallocenes with the formula Cp₂ML₂, the L-M-L angle depends on the electron count. In the d²-complex molybdocene dichloride (Cp₂MoCl₂) the Cl-Mo-Cl angle is 82°. In the d¹ complex niobocene dichloride, this angle is more open at 85.6°. In the d⁰-complex zirconocene dichloride the angle is even more open at 92.1°. This trend reveals that the frontier orbital, which is d_z2, is oriented in the MCl₂ plane but does not bisect the MCl₂ angle.

Reactivity

Salt metathesis reactions

Since bent metallocenes typically have other ligands, often halides, these additional sites are centers of reactivity. For example, reduction of zirconocene dichloride gives the corresponding hydrido chloride called Schwartz's reagent:
:(C₅H₅)₂ZrCl₂ + 1/4 LiAlH₄ → (C₅H₅)₂ZrHCl + 1/4 "LiAlCl₄"
This hydride reagent is useful for organic synthesis. Related titanium-based complexes Petasis reagent and Tebbe's reagent also feature bent metallocenes. Titanocene pentasulfide is used in research on polysulfur rings. Alkyne and benzyne derivatives of titanocene are reagents in organic synthesis.

Reactions of Cp rings

Although the Cp ligands are generally safely considered spectator ligands, they are not completely inert. For example, attempts to prepare titanocene by reduction of titanocene dichloride affords complexes of fulvalene ligands.

Bent metallocenes derived from pentamethylcyclopentadiene can undergo reactions involving the methyl groups. For example, decamethyltungstocene dihydride undergoes dehydrogenation to give the tuck-in complex.

The original example proceeded via sequential loss of two equivalents of H₂ from decamethyltungstocene dihydride, Cp*₂WH₂. The first dehydrogenation step affords a simple tuck-in complex:
:(C₅Me₅)₂WH₂ → (C₅Me₅)(C₅Me₃(CH₂)₂)W + 2 H₂

Redox

When the non-Cp ligands are halides, these complexes undergo reduction to give carbonyl, alkene, and alkyne complexes that are useful reagents. A well-known example is titanocene dicarbonyl:
:Cp₂TiCl₂ + Mg + 2 CO → Cp₂Ti(CO)₂ + MgCl₂
Reduction of vanadocene dichloride gives vanadocene.

Olefin polymerization catalysis

Although bent metallocenes are of no commercial value as olefin polymerization catalysts, studies on these compounds were highly influential on the industrial processes. Already in 1957 there were reports on the polymerization of ethylene using a catalyst prepared from Cp₂TiCl₂ and trimethyl aluminium. Reactions involving the related Cp₂Zr₂Cl₂/Al(CH₃)₃ system revealed the beneficial effects of trace amounts of water for ethylene polymerization. It is now known that the partially hydrolyzed organoaluminium reagent methylaluminoxane ("MAO") gives rise to families of highly active catalysts. Work in this are led to constrained geometry complexes, which are not bent metallocenes, but exhibit related structural features.

References

Further reading

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*{{cite journal , title =Landmarks in Organotransition Metal Chemistry , author= Helmut Werner , journal = Profiles in Inorganic Chemistry , year = 2009 , volume = 1 , pages = 129–175 , doi=10.1007/b136581

Metallocenes
Catalysts