Molecular and Silica-Supported Molybdenum Alkyne Metathesis Catalysts: Influence of Electronics and Dynamics on...

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.