Organozirconium chemistry


Organozirconium compounds are organometallic compounds containing a carbon to zirconium chemical bond. Organozirconium chemistry is the corresponding science exploring properties, structure and reactivity of these compounds. 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 is more resistant to reduction than titanium compounds, which often convert to Ti derivatives. By the same token, Zr is a particularly powerful reducing agent, forming robust dinitrogen complexes. Being a larger atom, zirconium forms complexes with higher coordination numbers, e.g. polymeric n vs monomeric CpTiCl3.

History

Zirconocene dibromide was prepared in 1953 by a reaction of the cyclopentadienyl magnesium bromide and zirconium chloride. In 1966 the dihydride Cp2ZrH2 was obtained by the reaction of Cp2Zr2 with triethylamine. In 1970 the related hydrochloride was obtained by reduction of zirconacene dichloride with lithium aluminium hydride. The development of organozirconium reagents was recognized by a Nobel Prize in Chemistry to Ei-Ichi Negishi.

Zirconocene chemistry

The foremost applications of zirconocenes involve their use as catalysts for olefin polymerization.
Schwartz's reagent participate 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, conjugate additions, coupling reactions, carbonylation and halogenation.
Extensive chemistry has also been demonstrated from decamethylzirconocene dichloride, Cp*2ZrCl2. Well-studied derivatives include Cp*2ZrH2, 23, Cp*2Zr2, and Cp*2Zr2.
Zirconocene dichloride can be used to cyclise enynes and dienes to give cyclic or bicyclic aliphatic systems.

Alkyl and CO complexes

The simplest organozirconium compounds are the homoleptic alkyls. Salts of 2- are known. Tetrabenzylzirconium is a precursor to many catalysts for olefin polymerization. It can be converted to mixed alkyl, alkoxy, and halide derivatives, Zn3X.
with H atoms omitted for clarity.
In addition to mixed Cp2Zr2, zirconium forms the binary carbonyl 2-.

Organohafnium chemistry

Organohafnium compounds behave nearly identically to organozirconium compounds, as hafnium, which is just below zirconium on the periodic table. Many Hf analogues of Zr compounds are known, including bishafnium dichloride, bishafnium dihydride, and dimethylbishafnium.
Cationic hafnocene complexes, post-metallocene catalysts, are used on an industrial scale for the polymerization of alkenes.

Additional reading