Metallocene and related catalysts for olefin, alkyne and silane dimerization and oligomerization

Abstract

This review summarizes the use of metallocene complexes and related compounds as catalysts in the dimerization or oligomerization of olefins (alkenes) or terminal acetylenes (alkynes) and in the dehydrocoupling/dehydrooligomerization of silanes. Metallocene complexes of group-III metals (scandocenes, yttrocenes, lanthanocenes), lanthanoids (neodymocenes) and group-IV metals (titanocenes, zirconocenes, hafnocenes) have been utilized in the selective (co-/hydro-)oligomerization of ethene, of the α-olefins propene, 1-butene, 1-pentene, 1-hexene, 1-heptene, and 1-octene, of branched olefins, e.g. methyl-butenes, methyl-pentenes and styrene, of cycloolefins, e.g. cyclopentene and norbornene and of α, ω-dienes, e.g. 1,5-hexadiene and 1,7-octadiene. Group-III metallocenes are often active in the C–C coupling without a cocatalyst; group-IV metallocenes require the help of a cocatalyst, such as methylalumoxane, MAO, aluminum alkyls, e.g. Al i Bu 3, or perfluorated boranes, e.g. B(C 6F 5) 3. The actinoid metallocenes (C 5M 5) 2AnMe 2 with An = thorium, uranium allow for the dimerization and oligomerization of terminal acetylenes. The dehydrooligomerization of (hydro)silanes is typically achieved by group-IV metallocene chlorides together with n-butyl lithium. Also included in this review are related sandwich and half-sandwich (mono-cyclopentadienyl) complexes used for olefin oligomerization. The related sandwich complexes feature phospholyl, boratabenzene or carboranate ligands. Methods of oligo–olefin analyses by gel permeation chromatography (GPC), 1H NMR spectroscopy, gas chromatography (GC) or viscosity measurements for molecular weight determinations and by 1H and 13C NMR spectroscopy or MALDI–TOF mass spectrometry for end group structure determinations are summarized. Possible applications of olefin oligomers, in particular oligopropenes are presented. The functionality of a double bond at the end of each chain (for further modifications) together with the product homogeneity are the advantages of oligomers from metallocene catalysis. In addition, olefin oligomerization is used to study mechanistic aspects and to obtain a better insight into the reaction mechanism of metallocene polymerization catalysis because of the homogeneity of the reaction mixture and because certain mechanistic aspects are easier to investigate in oligomeric products than in high-molar-mass polymers.