Organozirconium and organohafnium chemistry explained

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.[1] Organozirconium compounds have been widely studied, in part because they are useful catalysts in Ziegler-Natta polymerization.

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 [CpZrCl<sub>3</sub>]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.[2] In 1966, the dihydride Cp2ZrH2 was obtained by the reaction of Cp2Zr(BH4)2 with triethylamine.[3] 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).[4] [5] [6] The development of organozirconium reagents was recognized by a Nobel Prize in Chemistry to Ei-Ichi Negishi.[7] [8]

Zirconocene chemistry

See main article: Zirconocene. The foremost applications of zirconocenes involve their use as catalysts for olefin polymerization.[9] [10]

Schwartz's reagent ([Cp<sub>2</sub>ZrHCl]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,[11] 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*<sub>2</sub>Zr]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.[12] [13]

[14]

Alkyl and CO complexes

The simplest organozirconium compounds are the homoleptic alkyls. Salts of [Zr(CH<sub>3</sub>)<sub>6</sub>]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).

In addition to mixed Cp2Zr(CO)2, zirconium forms the binary carbonyl [Zr(CO)<sub>6</sub>]2-.[15]

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).

Cationic hafnocene complexes, post-metallocene catalysts, are used on an industrial scale for the polymerization of alkenes.[16] [17]

Additional reading

Notes and References

  1. 10.1021/cen-v082n015.p035. Organozirconium Chemistry Arrives. A. Maureen Rouhi. 19 April 2004. 82. 16. 36–39. 0009-2347. Chemical & Engineering News.
  2. G. Wilkinson. P. L. Pauson. J. M. Birmingham. F. A. Cotton. Bis-cyclopentadienyl derivatives of some transition elements. Journal of the American Chemical Society . 75. 4 . 1011–1012 . 1953 . 10.1021/ja01100a527.
  3. James . B. D. . Nanda . R. K. . Walbridge . M. G. H. . 10.1021/ic50057a009 . Reactions of Lewis bases with tetrahydroborate derivatives of the Group IVa elements. Preparation of new zirconium hydride species . Inorganic Chemistry . 6 . 11 . 1979–1983 . 1967 .
  4. Wailes . P. C. . Weigold . H. . 10.1016/S0022-328X(00)80281-8 . Hydrido complexes of zirconium I. Preparation . Journal of Organometallic Chemistry . 24 . 2 . 405–411 . 1970 .
  5. Wailes . P. C. . Weigold . H. . 10.1016/S0022-328X(00)80282-X . Hydrido complexes of zirconium II. Reactions of dicyclopentadienylzirconium dihydride with carboxylic acids . Journal of Organometallic Chemistry . 24 . 2 . 413–417 . 1970 .
  6. Wailes . P. C. . Weigold . H. . Bell . A. P. . 10.1016/S0022-328X(00)82168-3 . Hydrido complexes of zirconium . Journal of Organometallic Chemistry . 27 . 3 . 373–378 . 1971 .
  7. 10.1055/s-1988-27453. Organozirconium Compounds in Organic Synthesis. Synthesis. 1988. 1–19. 1988. Negishi. Ei-Ichi. Takahashi. Tamotsu.
  8. 10.3998/ark.5550190.0012.803. Discovery of ZACA reaction − Zr-catalyzed asymmetric carboalumination of alkenes. Arkivoc. 2011. 8. 34. 2010. free. 2027/spo.5550190.0012.803. free.
  9. 10.1021/cr940442r. 11848972. Group 4ansa-Cyclopentadienyl-Amido Catalysts for Olefin Polymerization. Chemical Reviews. 98. 7. 2587–2598. 1998. McKnight. Andrew L.. Waymouth. Robert M..
  10. 10.1021/cr9804700. 11749264. 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. 2000. Alt. Helmut G.. Köppl. Alexander.
  11. Allylic Alcohols by Alkene Transfer from Zirconium to Zinc: 1-[(''tert''-Butyldiphenylsilyl)oxy]-dec-3-en-5-ol. Organic Syntheses. 1997. 10.15227/orgsyn.071.0083. Ruen Chu. Sun . Masami . Okabe. David L. . Coffen . Jeffrey . Schwartz. 74. 205.
  12. 69. 3. 633–638. Fillery. Shaun F.. Richard J. Whitby . George J. Gordon . Tim Luker . Tandem reactions on a zirconocene template. Pure and Applied Chemistry. 1997. 10.1351/pac199769030633. free.
  13. Book: 10.1016/B978-0-08-052349-1.00149-9 . Zirconium-promoted Bicyclization of Enynes . Comprehensive Organic Synthesis . 1163–1184 . 1991 . 978-0-08-052349-1 . Negishi . Ei-Ichi .
  14. Thomas . E. . Dixon . S. . Whitby . R. J. . 10.1002/anie.200602822 . A Rearrangement to a Zirconium–Alkenylidene in the Insertion of Dihalocarbenoids and Acetylides into Zirconacycles . Angewandte Chemie International Edition . 45 . 42 . 7070–7072 . 2006 . 17009379. free .
  15. Metal Carbonyl Anions: from [Fe(CO)<sub>4</sub>]2− to [Hf(CO)<sub>6</sub>]2− and Beyond. Ellis, J. E.. Organometallics. 2003 . 22 . 3322–3338. 10.1021/om030105l.
  16. 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 . 10.1016/j.progpolymsci.2008.05.003 .
  17. Klosin, J.. Fontaine, P. P.. Figueroa, R. . Development of Group Iv Molecular Catalysts for High Temperature Ethylene-Α-Olefin Copolymerization Reactions. Accounts of Chemical Research. 2015. 48. 7. 2004–2016. 10.1021/acs.accounts.5b00065. 26151395. free.

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