M. Carmen Puerta

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(η‐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(η‐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.

A variety of Ni^II complexes with a wide range of electronic and steric properties, bearing picolylimidazolidene ligands (a–g) and Cp (Cp = η⁵‐C₅H₅; 2a–f) or Cp* (Cp* = η⁵‐C₅Me₅; 3a, c, g) groups, have been synthesised and characterised by using NMR spectroscopy and single‐crystal X‐ray crystallography. The complexes have been used as precatalysts for a wide range of catalytic transformations, which most likely involve a Ni⁰/Ni^II catalytic cycle. In particular, the new well‐defined 2a, 2c, 3a and 3c complexes have demonstrated great efficiency and versatility towards Suzuki–Miyaura coupling reactions, hydroamination of activated olefins and C–S cross‐coupling reactions of aryl halides and thiols under mild conditions.

New indenyl nickel(ii) complexes bearing As or Sb ligands are efficient catalysts for the oligomerisation of styrene.

The complex [Cp*RuCl(ⁱPr₂PNHPy)] (1) reacts with 1-alkynes HC≡CR (R = COOMe, C₆H₄CF₃) in dichloromethane furnishing the corresponding vinylidene complexes [Cp*Ru═C═CHR(ⁱPr₂PNHPy)]Cl (R = COOMe (2a-Cl), C₆H₄CF₃ (2b-Cl)), whereas reaction of 1 with NaBPh₄ in MeOH followed by addition of HC≡CR (R = COOMe, C₆H₄CF₃) yields the metastable π-alkyne complexes [Cp*Ru(η²-HC≡CR)(ⁱPr₂PNHPy)][BPh₄] (R = COOMe (3a-BPh₄), C₆H₄CF₃ (3b-BPh₄)). The transformation of 3a-BPh₄/3b-BPh₄ into their respective vinylidene isomers in dichloromethane is very slow and requires hours to its completion. However, this process is accelerated by addition of LiCl in methanol solution. Reaction of 1 with HC≡CR (R = COOMe, C₆H₄CF₃) in MeOH goes through the intermediacy of the π-alkyne complexes [Cp*Ru(η²-HC≡CR)(ⁱPr₂PNHPy)]Cl (R = COOMe (3a-Cl), C₆H₄CF₃ (3b-Cl)), which rearrange to vinylidenes in minutes, i.e., much faster than their counterparts containing the [BPh₄]⁻ anion. The kinetics of these isomerizations has been studied in solution by NMR. With the help of DFT studies, these observations have been interpreted in terms of chloride- and methanol-assisted hydrogen migrations. Calculations suggest participation of a hydrido–alkynyl intermediate in the process, in which the hydrogen atom can be transferred from the metal to the β-carbon by means of species with weak basic character acting as proton shuttles.