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.