Cationic R-Substituted-Indenyl Nickel(II) Complexes of Arsine and Stibine Ligands: Synthesis, Characterization and Catalytic Behavior in the Oligomerization of Styrene

A series of new cationic Rn-substituted-indenyl nickel(II) complexes containing arsine or stibine ligands were synthesized in moderate to very high yields by the protonation of the corresponding bisindenyl nickel derivatives [Ni(η-Rn-Ind)2] with HBF4, in the presence of 2 equivalents of AsPh3 or SbPh3 donor ligands. These complexes, with the general formula [Ni(η-Rn-Ind)(EPh3)2]BF4 (E= As, Sb), were structurally characterized by NMR and X-ray diffraction, and subsequently tested as single-component catalysts for the oligomerization of styrene, leading to the formation of very low molecular weight head-to-tail oligomers (typically consisting of dimers, trimers and tetramers). The new 1- or 2-monosubstituted-indenyl Ni(II) catalyst precursors exhibit extremely high catalytic activities, considerably higher than those observed for the symmetrical unsubstituted- and the 1,3-disubstituted-indenyl nickel analogues (i.e. monosubstituted >> non-substituted > 1,3-disubstituted), their reactivity pattern showing similarities with that of the corresponding allyl derivatives. A simpler and more straightforward experimental procedure for the high yield preparation of [NiBr2(DME)], an important nickel starting material, which was used in the preparation of the bisindenyl nickel precursors of this work, is also described.

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