Reactions of Metallacyclopentadiene with Terminal Alkynes: Isolation and Characterization of Metallafulvenallene Complexes

Metallacyclopentadienes play an important role in transition-metal-catalyzed cyclotrimerization of alkynes. Isolation and characterization of these unique five-membered metallacycles could promote further understanding of related catalytic reactions. In this work, the first metallafulvenallene complexes were synthesized by the reaction of fused metallacyclopentadiene with terminal alkynes. Single-crystal X-ray diffraction and NMR spectroscopic studies on the metallafulvenallene complexes demonstrate that the bicyclic framework consists of a metallacyclopentadiene fused with a six-membered metallacycle, featuring an η1-vinylidene ligand. Deuterium labeling experiments were carried out to investigate the mechanism for the transformation of fused metallacyclopentadiene to fused metallafulvenallene. The UV–vis absorption spectra of these unique structures were measured.