Spectroscopic and Theoretical Study on Siloxy-Based Molybdenum and Tungsten Alkylidyne Catalysts for Alkyne Metathesis

A combined spectroscopic
and theoretical study on triphenyl- and
dimethyl-phenyl siloxy molybdenum and tungsten alkylidyne catalysts
for alkyne metathesis is reported. Using NMR, X-ray, UV–vis,
and resonance Raman spectroscopy and density functional theory calculations,
the influence of different ligand systems and metal centers on the
geometric and electronic structure and thermochemistry of different
intermediates is investigated, that is, the starting alkylidyne and
the derived metallacyclobutadiene (MCBD) and metallatetrahedrane (MTd).
This includes a mechanistic and kinetic study on the formation and
isomerization of MCBDs and MTds. Upon changing from monodentate to
tripodal siloxy ligands, higher steric strain is imposed, which modulates
the relative energies of the different intermediates. Additionally,
intramolecular dispersion interactions between the bound substrate
and the ligand can be operative. Tungsten as the central metal leads
to stronger M–C σ-bonds, which overstabilize the reaction
intermediates and preclude effective turnover. Furthermore, kinetic
modeling strongly suggests that MTds are off-cycle intermediates based
on the high barriers for direct formation but low barriers for isomerization
from MCBDs. We infer from our findings that effective catalysis can
only be achieved when factors that (over)­stabilize intermediates,
such as strong M–C bonds or large dispersion interactions,
are prevented by appropriate catalyst design.