Molecular and Silica-Supported Molybdenum Alkyne Metathesis Catalysts: Influence of Electronics and Dynamics on Activity Revealed by Kinetics, Solid-State NMR, and Chemical Shift Analysis

Molybdenum-based
molecular alkylidyne complexes of the type [MesCMo­{OC­(CH3)3–x(CF3)x}3] (MoF0, x = 0; MoF3, x = 1; MoF6, x = 2; MoF9, x = 3; Mes = 2,4,6-trimethylphenyl)
and their silica-supported analogues are prepared and characterized
at the molecular level, in particular by solid-state NMR, and their
alkyne metathesis catalytic activity is evaluated. The 13C NMR chemical shift of the alkylidyne carbon increases with increasing
number of fluorine atoms on the alkoxide ligands for both molecular
and supported catalysts but with more shielded values for the supported
complexes. The activity of these catalysts increases in the order MoF0 < MoF3 < MoF6 before
sharply decreasing for MoF9,
with a similar effect for the supported systems (MoF0 ≈ MoF9 < MoF6 < MoF3). This is consistent with the
different kinetic behavior (zeroth order in alkyne for MoF9 derivatives instead of first order for
the others) and the isolation of stable metallacyclobutadiene intermediates
of MoF9 for both molecular and
supported species. Detailed solid-state NMR analysis of molecular
and silica-supported metal alkylidyne catalysts coupled with DFT/ZORA
calculations rationalize the NMR spectroscopic signatures and discernible
activity trends at the frontier orbital level: (1) increasing the
number of fluorine atoms lowers the energy of the π*­(MC)
orbital, explaining the more deshielded chemical shift values; it
also leads to an increased electrophilicity and higher reactivity
for catalysts up to MoF6, prior
to a sharp decrease in reactivity for MoF9 due to the formation of stable metallacyclobutadiene intermediates;
(2) the silica-supported catalysts are less active than their molecular
analogues because they are less electrophilic and dynamic, as revealed
by their 13C NMR chemical shift tensors.