Organozirconium and organohafnium chemistry

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A zirconocene Ewen-style catalyst for producing syndiotactic polypropylene. Syndiotactic-zirconocene-front-3D-balls.png
A zirconocene Ewen-style catalyst for producing syndiotactic polypropylene.

Organozirconium chemistry is the science of exploring the properties, structure, and reactivity of organozirconium compounds, which are organometallic compounds containing chemical bonds between carbon and zirconium. [2] Organozirconium compounds have been widely studied, in part because they are useful catalysts in Ziegler-Natta polymerization.

Contents

Comparison with organotitanium chemistry

Many organozirconium compounds have analogues on organotitanium chemistry. Zirconium(IV) is more resistant to reduction than titanium(IV) compounds, which often convert to Ti(III) derivatives. By the same token, Zr(II) is a particularly powerful reducing agent, forming robust dinitrogen complexes. Being a larger atom, zirconium forms complexes with higher coordination numbers, e.g. polymeric [CpZrCl3]n vs monomeric CpTiCl3 (Cp = C5H5).

History

Zirconocene dibromide was prepared in 1953 by a reaction of the cyclopentadienyl magnesium bromide and zirconium(IV) chloride. [3] In 1966, the dihydride Cp2ZrH2 was obtained by the reaction of Cp2Zr(BH4)2 with triethylamine. [4] In 1970, the related hydrochloride (now called Schwartz's reagent) was obtained by reduction of zirconacene dichloride (Cp2ZrCl2) with lithium aluminium hydride (or the related LiAlH(t-BuO)3). [5] [6] [7] The development of organozirconium reagents was recognized by a Nobel Prize in Chemistry to Ei-Ichi Negishi. [8] [9]

Zirconocene chemistry

The structure of Schwartz's reagent. Cp4Zr2H2Cl2.png
The structure of Schwartz's reagent.

The foremost applications of zirconocenes involve their use as catalysts for olefin polymerization. [11] [12]

Schwartz's reagent ([Cp2ZrHCl]2) participates in hydrozirconation, which enjoys some use in organic synthesis. Substrates for hydrozirconation are alkenes and alkynes. Terminal alkynes give vinyl complexes. Secondary reactions are nucleophilic additions, transmetalations, [13] conjugate additions, coupling reactions, carbonylation, and halogenation.

Extensive chemistry has also been demonstrated from decamethylzirconocene dichloride, Cp*2ZrCl2. Well-studied derivatives include Cp*2ZrH2, [Cp*2Zr]2(N2)3, Cp*2Zr(CO)2, and Cp*2Zr(CH3)2.

Zirconocene dichloride can be used to cyclise enynes and dienes to give cyclic or bicyclic aliphatic systems. [14] [15]

Whitby Zirconocyclisation reaction.png [16]

Alkyl and CO complexes

The simplest organozirconium compounds are the homoleptic alkyls. Salts of [Zr(CH3)6]2- are known. Tetrabenzylzirconium is a precursor to many catalysts for olefin polymerization. It can be converted to mixed alkyl, alkoxy, and halide derivatives, Zr(CH2C6H5)3X (X = CH3, OC2H5, Cl).

Structure of tetrabenzylzirconium with H atoms omitted for clarity. QIGLEZ.svg
Structure of tetrabenzylzirconium with H atoms omitted for clarity.

In addition to mixed Cp2Zr(CO)2, zirconium forms the binary carbonyl [Zr(CO)6]2-. [18]

Organohafnium chemistry

Organohafnium compounds behave nearly identically to organozirconium compounds, as hafnium is just below zirconium on the periodic table. Many Hf analogues of Zr compounds are known, including bis(cyclopentadienyl)hafnium(IV) dichloride, bis(cyclopentadienyl)hafnium(IV) dihydride, and dimethylbis(cyclopentadienyl)hafnium(IV).

Generic structure of a post-metallocene catalyst based on Dow's pyridyl-amido design. VersifyCats.png
Generic structure of a post-metallocene catalyst based on Dow's pyridyl-amido design.

Cationic hafnocene complexes, post-metallocene catalysts, are used on an industrial scale for the polymerization of alkenes. [19] [20]

Additional reading

Related Research Articles

<span class="mw-page-title-main">Zirconium</span> Chemical element, symbol Zr and atomic number 40

Zirconium is a chemical element with the symbol Zr and atomic number 40. The name zirconium is derived from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian zargun. It is a lustrous, grey-white, strong transition metal that closely resembles hafnium and, to a lesser extent, titanium. Zirconium is mainly used as a refractory and opacifier, although small amounts are used as an alloying agent for its strong resistance to corrosion. Zirconium forms a variety of inorganic and organometallic compounds such as zirconium dioxide and zirconocene dichloride, respectively. Five isotopes occur naturally, four of which are stable. Zirconium compounds have no known biological role.

A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1-alkenes (alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility:

<span class="mw-page-title-main">Kaminsky catalyst</span> Ethylene polymerization catalyst

A Kaminsky catalyst is a catalytic system for alkene polymerization. Kaminsky catalysts are based on metallocenes of group 4 transition metals activated with methylaluminoxane (MAO). These and other innovations have inspired development of new classes of catalysts that in turn led to commercialization of novel engineering polyolefins.

<span class="mw-page-title-main">Hafnium tetrachloride</span> Chemical compound

Hafnium(IV) chloride is the inorganic compound with the formula HfCl4. This colourless solid is the precursor to most hafnium organometallic compounds. It has a variety of highly specialized applications, mainly in materials science and as a catalyst.

<span class="mw-page-title-main">Titanocene dichloride</span> Chemical compound

Titanocene dichloride is the organotitanium compound with the formula (η5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. It shows antitumour activity and was the first non-platinum complex to undergo clinical trials as a chemotherapy drug.

<span class="mw-page-title-main">Zirconium(IV) chloride</span> Chemical compound

Zirconium(IV) chloride, also known as zirconium tetrachloride, is an inorganic compound frequently used as a precursor to other compounds of zirconium. This white high-melting solid hydrolyzes rapidly in humid air.

<span class="mw-page-title-main">Schwartz's reagent</span> Chemical compound

Schwartz's reagent is the common name for the organozirconium compound with the formula (C5H5)2ZrHCl, sometimes called zirconocene hydrochloride or zirconocene chloride hydride, and is named after Jeffrey Schwartz, a chemistry professor at Princeton University. This metallocene is used in organic synthesis for various transformations of alkenes and alkynes.

<span class="mw-page-title-main">Organotitanium chemistry</span>

Organotitanium chemistry is the science of organotitanium compounds describing their physical properties, synthesis, and reactions. Organotitanium compounds in organometallic chemistry contain carbon-titanium chemical bonds. They are reagents in organic chemistry and are involved in major industrial processes.

A carbometallation is any reaction where a carbon-metal bond reacts with a carbon-carbon π-bond to produce a new carbon-carbon σ-bond and a carbon-metal σ-bond. The resulting carbon-metal bond can undergo further carbometallation reactions or it can be reacted with a variety of electrophiles including halogenating reagents, carbonyls, oxygen, and inorganic salts to produce different organometallic reagents. Carbometallations can be performed on alkynes and alkenes to form products with high geometric purity or enantioselectivity, respectively. Some metals prefer to give the anti-addition product with high selectivity and some yield the syn-addition product. The outcome of syn and anti- addition products is determined by the mechanism of the carbometallation.

Zirconocene dichloride is an organozirconium compound composed of a zirconium central atom, with two cyclopentadienyl and two chloro ligands. It is a colourless diamagnetic solid that is somewhat stable in air.

Organovanadium chemistry is the chemistry of organometallic compounds containing a carbon (C) to vanadium (V) chemical bond. Organovanadium compounds find only minor use as reagents in organic synthesis but are significant for polymer chemistry as catalysts.

<i>Ansa</i>-metallocene Organometallic compound

An ansa-metallocene is a type of organometallic compound containing two cyclopentadienyl ligands that are linked by a bridging group such that both cyclopentadienyl groups are bound to the same metal. The link prevents rotation of the cyclopentadienyl ligand and often modifies the structure and reactivity of the metal center. Some ansa-metallocenes are active in Ziegler-Natta catalysis, although none are used commercially. The term ansa-metallocene was coined by Lüttringhaus and Kullick to describe alkylidene-bridged ferrocenes, which were developed in the 1950s.

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.

<span class="mw-page-title-main">Titanium ethoxide</span> Chemical compound

Titanium ethoxide is a chemical compound with the formula Ti4(OCH2CH3)16. It is a commercially available colorless liquid that is soluble in organic solvents but hydrolyzes readily. Alkoxides of titanium(IV) and zirconium(IV) are used in organic synthesis and materials science. They adopt more complex structures than suggested by their empirical formulas.

<span class="mw-page-title-main">Zirconocene</span> Chemical compound

Zirconocene is a hypothetical compound with 14 valence electrons, which has not been observed or isolated. It is an organometallic compound consisting of two cyclopentadienyl rings bound on a central zirconium atom. A crucial question in research is what kind of ligands can be used to stabilize the Cp2ZrII metallocene fragment to make it available for further reactions in organic synthesis.

<span class="mw-page-title-main">(Cyclopentadienyl)zirconium trichloride</span> Chemical compound

(Cyclopentadienyl)zirconium trichloride is an organozirconium compound with the formula (C5H5)ZrCl3. It a moisture-sensitive white solid. The compound adopts a polymeric structure. The compound has been well studied spectroscopically.

<span class="mw-page-title-main">Decamethyltitanocene dichloride</span> Chemical compound

Decamethyltitanocene dichloride is an organotitanium compound with the formula Cp*2TiCl2 (where Cp* is C5(CH3)5, derived from pentamethylcyclopentadiene). It is a red solid that is soluble in nonpolar organic solvents. The complex has been the subject of extensive research. It is a precursor to many organotitanium complexes. The complex is related to titanocene dichloride, which lacks the methyl groups.

<span class="mw-page-title-main">Decamethylzirconocene dichloride</span> Chemical compound

Decamethylzirconocene dichloride is an organozirconium compound with the formula Cp*2ZrCl2 (where Cp* is C5(CH3)5, derived from pentamethylcyclopentadiene). It is a pale yellow, moisture sensitive solid that is soluble in nonpolar organic solvents. The complex has been the subject of extensive research. It is a precursor to many other complexes, including the dinitrogen complex [Cp*2Zr]2(N2)3). It is a precatalyst for the polymerization of ethylene and propylene.

<span class="mw-page-title-main">Tetrabenzylzirconium</span> Chemical compound

Tetrabenzylzirconium is an organozirconium compound with the formula Zr(CH2C6H5)4. The molecule features diamagnetic Zr(IV) bonded to four benzyl ligands. It is an orange air- and photo-sensitive solid, which is soluble in hydrocarbon solvents. The compound is a precursor to catalysts for the polymerization of olefins.

<span class="mw-page-title-main">Hafnocene dichloride</span> Chemical compound

Hafnocene dichloride is the organohafnium compound with the formula (C5H5)2HfCl2. It is a white solid that is sparingly soluble in some organic solvents. The lighter homologues zirconacene dichloride and titanocene dichloride have received much more attention. While hafnocene is only of academic interest, some more soluble derivatives are precatalysts for olefin polymerization. Moreso than the Zr analogue, this compound is highly resistant to reduction.

References

  1. ↑ Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. (1988). "Syndiospecific propylene polymerizations with Group IVB metallocenes". Journal of the American Chemical Society. 110 (18): 6255–6256. doi:10.1021/ja00226a056. PMID   22148816.
  2. ↑ A. Maureen Rouhi (19 April 2004). "Organozirconium Chemistry Arrives". Chemical & Engineering News. 82 (16): 36–39. doi:10.1021/cen-v082n015.p035. ISSN   0009-2347.
  3. ↑ G. Wilkinson; P. L. Pauson; J. M. Birmingham; F. A. Cotton (1953). "Bis-cyclopentadienyl derivatives of some transition elements". Journal of the American Chemical Society. 75 (4): 1011–1012. doi:10.1021/ja01100a527.
  4. ↑ James, B. D.; Nanda, R. K.; Walbridge, M. G. H. (1967). "Reactions of Lewis bases with tetrahydroborate derivatives of the Group IVa elements. Preparation of new zirconium hydride species". Inorganic Chemistry. 6 (11): 1979–1983. doi:10.1021/ic50057a009.
  5. ↑ Wailes, P. C.; Weigold, H. (1970). "Hydrido complexes of zirconium I. Preparation". Journal of Organometallic Chemistry. 24 (2): 405–411. doi:10.1016/S0022-328X(00)80281-8.
  6. ↑ Wailes, P. C.; Weigold, H. (1970). "Hydrido complexes of zirconium II. Reactions of dicyclopentadienylzirconium dihydride with carboxylic acids". Journal of Organometallic Chemistry. 24 (2): 413–417. doi:10.1016/S0022-328X(00)80282-X.
  7. ↑ Wailes, P. C.; Weigold, H.; Bell, A. P. (1971). "Hydrido complexes of zirconium". Journal of Organometallic Chemistry. 27 (3): 373–378. doi:10.1016/S0022-328X(00)82168-3.
  8. ↑ Negishi, Ei-Ichi; Takahashi, Tamotsu (1988). "Organozirconium Compounds in Organic Synthesis". Synthesis. 1988: 1–19. doi:10.1055/s-1988-27453.
  9. ↑ "Discovery of ZACA reaction − Zr-catalyzed asymmetric carboalumination of alkenes". Arkivoc. 2011 (8): 34. 2010. doi: 10.3998/ark.5550190.0012.803 . hdl: 2027/spo.5550190.0012.803 .
  10. ↑ Jones, Christopher G.; Asay, Matthew; Kim, Lee Joon; Kleinsasser, Jack F.; Saha, Ambarneil; Fulton, Tyler J.; Berkley, Kevin R.; Cascio, Duilio; Malyutin, Andrey G.; Conley, Matthew P.; Stoltz, Brian M.; Lavallo, Vincent; Rodríguez, José A.; Nelson, Hosea M. (6 September 2019). "Characterization of Reactive Organometallic Species via MicroED". ACS Central Science. 5 (9): 1507–1513. doi: 10.1021/acscentsci.9b00403 . PMC   6764211 . PMID   31572777.
  11. ↑ McKnight, Andrew L.; Waymouth, Robert M. (1998). "Group 4ansa-Cyclopentadienyl-Amido Catalysts for Olefin Polymerization". Chemical Reviews. 98 (7): 2587–2598. doi:10.1021/cr940442r. PMID   11848972.
  12. ↑ Alt, Helmut G.; Köppl, Alexander (2000). "Effect of the Nature of Metallocene Complexes of Group IV Metals on Their Performance in Catalytic Ethylene and Propylene Polymerization". Chemical Reviews. 100 (4): 1205–1222. doi:10.1021/cr9804700. PMID   11749264.
  13. ↑ Sun, Ruen Chu; Okabe, Masami; Coffen, David L.; Schwartz, Jeffrey (1997). "Allylic Alcohols by Alkene Transfer from Zirconium to Zinc: 1-[(tert-Butyldiphenylsilyl)oxy]-dec-3-en-5-ol". Organic Syntheses. 74: 205. doi:10.15227/orgsyn.071.0083.
  14. ↑ Fillery, Shaun F.; Richard J. Whitby; George J. Gordon; Tim Luker (1997). "Tandem reactions on a zirconocene template". Pure and Applied Chemistry. 69 (3): 633–638. doi: 10.1351/pac199769030633 .
  15. ↑ Negishi, Ei-Ichi (1991). "Zirconium-promoted Bicyclization of Enynes". Comprehensive Organic Synthesis. pp. 1163–1184. doi:10.1016/B978-0-08-052349-1.00149-9. ISBN   978-0-08-052349-1.
  16. ↑ Thomas, E.; Dixon, S.; Whitby, R. J. (2006). "A Rearrangement to a Zirconium–Alkenylidene in the Insertion of Dihalocarbenoids and Acetylides into Zirconacycles". Angewandte Chemie International Edition. 45 (42): 7070–7072. doi:10.1002/anie.200602822. PMID   17009379.
  17. ↑ Rong, Yi; Al-Harbi, Ahmed; Parkin, Gerard (2012). "Highly Variable Zr–CH2–Ph Bond Angles in Tetrabenzylzirconium: Analysis of Benzyl Ligand Coordination Modes". Organometallics. 31 (23): 8208–8217. doi:10.1021/om300820b.
  18. ↑ Ellis, J. E. (2003). "Metal Carbonyl Anions: from [Fe(CO)4]2− to [Hf(CO)6]2− and Beyond". Organometallics. 22: 3322–3338. doi:10.1021/om030105l.
  19. ↑ Chum, P. S.; Swogger, K. W. (2008). "Olefin Polymer Technologies-History and Recent Progress at the Dow Chemical Company". Progress in Polymer Science. 33: 797–819. doi:10.1016/j.progpolymsci.2008.05.003.
  20. ↑ Klosin, J.; Fontaine, P. P.; Figueroa, R. (2015). "Development of Group Iv Molecular Catalysts for High Temperature Ethylene-Α-Olefin Copolymerization Reactions". Accounts of Chemical Research. 48 (7): 2004–2016. doi: 10.1021/acs.accounts.5b00065 . PMID   26151395.
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