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 R-n-substituted-indenyl nickel(II) complexes containing arsine or stibine ligands were synthesized in moderate to very high yields by the protonation of the corresponding bis(indenyl) nickel derivatives [Ni(eta-R-n-Ind)(2)] with HBF4, in the presence of 2 equiv. of AsPh3 or SbPh3 donor ligands. These complexes, with the general formula [Ni(eta-R-n-Ind)(EPh3)(2)]BF4 (E = As, Sb), were structurally characterized by NMR spectroscopy 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 bis(indenyl) nickel precursors of this work, is also described.