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.