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Abstract:
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, Resonance Raman spectroscopy and DFT 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 and metallatetrahedrane. This includes a mechanistic and kinetic study on the formation and isomerization of metallacyclobutadienes and metallatetrahedranes. 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 bound substrate and 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 metallatetrahedranes are off-cycle intermediates based on the high barriers for direct formation but low barriers for isomerization from metallacyclobutadienes. 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.
