Mesoionic Triazolylidene Nickel Complexes: Synthesis, Ligand Lability, and Catalytic C–C Bond Formation Activity

A set of triazolylidene
(trz) nickel­(II) complexes [NiCpX­(trz)] was synthesized by a direct
metalation of the corresponding triazolium salt with nickelocene,
NiCp2. While at short reaction times and in the presence
of a coordinating anion X the mono-carbene complex is preferably formed,
long reaction times induce the gradual transformation of [NiCpX­(trz)]
to the bis-carbene complexes [Ni­(Cp)­(trz)2]+. Kinetic analyses lend strong support to a consecutive pathway involving
triazolylidene dissociation from [NiCpX­(trz)] en route to the bis-carbene
complex. Similar carbene transfer is observed in a solid-state reaction
upon heating the complex [NiCpI­(trz)] in vacuo, which induces disproportionation
to [NiI2(trz)2] and NiCp2, confirming
that the Ni–C­(trz) bond is kinetically labile. The complexes
[Ni­(Cp)­(trz)2]+ and [NiCpX­(trz)] were both efficient
catalyst precursors for Suzuki–Miyaura cross-coupling of aryl
bromides and phenylboronic acid, with turnover frequencies exceeding
228 h–1. Complex degradation after short reaction
times, identified in separate experiments, prohibits high turnover
numbers, and for high conversions, repetitive additions of triazolylidene
nickel complex and phenylboronic acid are necessary.