Copper(III) Metallacyclopentadienes via Zirconocene Transfer and Reductive Elimination to an Isolable Phenanthrocyclobutadiene

Despite the widespread use of copper catalysis for the formation of C-C bonds, debate about the mechanism persists. Reductive elimination from Cu(III) is often invoked as a key step, yet examples of its direct observation from isolable complexes remain limited to only a few examples. Here, we demonstrate that incorporation of bulky mesityl (Mes) groups into the α-positions of a phenanthrene-appended zirconacyclopentadiene, Cp₂Zr(2,5-Mes₂-phenanthro[9,10]C₄), enables efficient oxidative transmetalation to the corresponding, formal Cu(III) metallacyclopentadiene dimer. The dimer was quantitatively converted to a structurally analogous anionic monomer [^<i>n</i>Bu₄N]{Cl₂Cu(2,5-Mes₂-phenanthro[9,10]C₄)} upon treatment with [^<i>n</i>Bu₄N][Cl]. Both metallacycles undergo quantitative reductive elimination upon heating to generate phenanthrocyclobutadiene and a Cu(I) species. Due to the steric protection provided by the mesityl groups, this cyclobutadiene was isolated and thoroughly characterized to reveal antiaromaticity comparable to that of free cyclobutadiene, which imbues it with a small highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap of 1.85 eV and accessible reduced and oxidized electronic states.