Zirconocene
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| Surname | Zirconocene | |||||||||
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| Molecular formula | C 10 H 10 Zr | |||||||||
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| Molar mass | 221.40 g mol −1 | |||||||||
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | ||||||||||
Zirconocene is a hypothetical compound with 14 outer electrons , which up to now has not been directly observed or isolated. It is a metallocene , i.e. an organometallic compound with aromatic ring systems, in the center of which there is a zirconium atom . A currently crucial question in research is which ligands can be used to stabilize the zirconocene fragment Cp 2 Zr II and make it available for further reactions in organic synthesis.
structure
In contrast to the sandwich compound ferrocene with opposing pentadienyl rings , zirconocene and other group 4 metallocenes are angled metallocene compounds. Conversion into ferrocene analogs is problematic for these metallocenes. Instead, a Cp 2 Zr II fragment can be generated with the help of so-called zirconocene generators (see below). Without these ligands, a binuclear zirconium complex, a fulvalene complex , is formed.
history
The first examples of organozirconium compounds were reported in 1954 by Wilkinson and Birmingham. These were zirconocene dihalides , Cp 2 ZrX 2 , with X = Cl or Br. Systematic, extensive research into the chemistry of zirconocene compounds was carried out in the 1980s by Negishi , Takahashi and Buchwald , among others . In the 1990s, Rosenthal succeeded in synthesizing a zirconocene bis (trimethylsilyl) complex, which offers a wide range of reaction options. Since then, this research area has grown rapidly so that zirconium has become one of the most widely used transition metals in organic synthesis.
Extraction and presentation
The Cp 2 Zr II compound does not exist in isolation due to its instability with 14 external electrons, but can be generated in situ in various ways. Ligands can be used, which serve to stabilize the zirconocene fragment. One possible way is to use π-acceptor ligands such as carbon monoxide . In addition, a reaction with trimethylphosphine can take place.
Alternatively provides zirconocene dichloride in tetrahydrofuran with two equivalents of n-BuLi at -78 ° C (1-butene), zirconocene which the resonance structures A and B has. This reagent was developed by Negishi .
A higher yield than with n -BuLi can be achieved with bis (trimethylsilyl) acetylene than. Zirconocene complexes are synthesized into the so-called Rosenthal reagent with the boundary structures A and B as follows . In addition to the general reaction shown, variously substituted cyclopentadienyl ligands and additional ligands (e.g. pyridine ) are often used. This allows reactions with the metallocene complexes to be controlled more specifically and the reagent can also be stored.
Instead of zirconium, titanium can also be used as the central atom, which enables titanocene to be provided in an analogous manner .
Reactions
Since the zirconocene fragment only has 14 outer electrons, it is considered to be extremely reactive. It has a lone pair of electrons and two unoccupied valence orbitals and can therefore be compared with carbenes with regard to its reactivity . Typical reactions are coupling and insertion reactions to metallacycles, which can be observed, for example, on the basis of the addition of carbon dioxide , ketones , nitriles or alkynes .
use
Cp 2 Zr II compounds play an important role in organic and inorganic synthesis . Particularly noteworthy are novel CC coupling reactions , which open up a wide range of possibilities. In particular, the synthesis of synthetically demanding organic structures and a wide variety of heterometallacycles can be realized in this context. The use of the Rosenthal reagent has proven to be particularly efficient here , as it is characterized by high selectivity and large yields can be achieved.
Individual evidence
- ↑ This substance has either not yet been classified with regard to its hazardousness or a reliable and citable source has not yet been found.
- ↑ a b c Uwe Rosenthal, Vladimir V. Burlakov: Organometallic Chemistry of Titanocene and Zirconocene Complexes with Bis (trimethylsilyl) acetylene as the Basis for Applications in Organic Synthesis . In: Ilan Marek (Ed.): Titanium and Zirconium in Organic Synthesis . Wiley-VCH, 2002, ISBN 3-527-30428-2 , pp. 355-389 , doi : 10.1002 / 3527600671.ch10 .
- ^ A b c Ei-ichi Negishi, Jean-Luc Montchamp: Zirconocenes . In: Antonio Togni, Ronald L. Halterman (ed.): Metallocenes . Wiley-VCH, 1998, doi : 10.1002 / 9783527619542.ch5 .
- ^ Ei-ichi Negishi, Shouquan Huo: Synthesis and Reactivity of Zirconocene Derivatives . In: Ilan Marek (Ed.): Titanium and Zirconium in Organic Synthesis . Wiley-VCH, 2002, ISBN 3-527-30428-2 , pp. 1–2 , doi : 10.1002 / 3527600671.ch1 .
- ^ Ei-Ichi Negishi, Tamotsu Takahashi: Patterns of Stoichiometric and Catalytic Reactions of Organozirconium and Related Complexes of Synthetic Interest . In: Accounts of Chemical Research . tape 27 , no. 5 , May 1994, pp. 124-130 , doi : 10.1021 / ar00041a002 .
- ^ A b Jonathan R. Nitschke, Stefan Zürcher, T. Don Tilley: New Zirconocene-Coupling Route to Large, Functionalized Macrocycles . In: Journal of the American Chemical Society . tape 122 , no. 42 , October 2000, p. 10345-10352 , doi : 10.1021 / ja0020310 .
- ↑ a b Uwe Rosenthal, Vladimir V. Burlakov, Perdita Arndt, Wolfgang Baumann, Anke Spannenberg: The Titanocene Complex of Bis (trimethylsilyl) acetylene: Synthesis, Structure, and Chemistry † . In: Organometallics . tape 22 , no. 5 , March 2003, p. 884-900 , doi : 10.1021 / om0208570 .
- ↑ Lisanne Becker, Uwe Rosenthal: Five-membered all-C- and hetero-metallacycloallenoids of group 4 metallocenes . In: Coordination Chemistry Reviews . tape 345 , August 2017, p. 137–149 , doi : 10.1016 / j.ccr.2016.07.008 .
- ↑ U. Rosenthal: Reactions of group 4 metallocene complexes of bis (trimethylsilyl) acetylene with nitriles and isonitriles . In: Angewandte Chemie . 23 August 2018, doi : 10.1002 / anie.201805157 .