Ansa-metallocene

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Structure of a generic ansa-metallocene. X is the linker group, often (CH2)n or R2Si (R = alkyl). Ln refers to additional ligands that may reside on the metal M.

An ansa-metallocene is a type of organometallic compound containing two

Ziegler-Natta catalysis, although none are used commercially.[1] The term ansa-metallocene (ansa being Greek for "handle") was coined by Lüttringhaus and Kullick to describe alkylidene-bridged ferrocenes, which were developed in the 1950s.[2]

Often ansa-metallocenes are described in terms of the angle defined by the two Cp rings. In titanocene dichloride, this angle is 58.5° whereas in the ansa-titanocene Me2Si(C5H4)2TiCl2 the angle is 51.2°.[3]

The prototypical linker groups are of the type (CH2)n where n = 1, 2, and 3. More easily installed are linker groups consisting of heteroatoms, e.g. (CH3)2Si.

One potential application of ansa-metallocenes is in polymerisation reactions. Specifically, the ansa bridge allows greater control of the stereochemistry at the metal centre. For example, three distinct ansa-zirconocene dichloride catalysts used in the synthesis of polypropylene in the presence of methylaluminoxane (MAO) can be used to direct whether the product is syndiotactic, isotactic, or atactic.[4]

  • Three views of ansa-dimethylsilylhafnocene dichloride (from X-ray coordinates).

References

  1. doi:10.1016/j.ccr.2005.05.012
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  2. ISBN 958-07-0140-7
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  3. doi:10.1021/ic00210a015
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  4. ISBN 978-0-387-09847-0
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