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

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, 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.

Classifying and understanding the reactivities of Mo based alkyne metathesis catalysts from 95Mo NMR chemical shift descriptors

“…We found that the 95 Mo δ iso are slightly but significantly different in the solid state compared to in solution, particularly for the silanolate complexes, indicating the substantial influences of dynamic and/or conformational effects (vide infra). 34 The solid-state 95 Mo δ iso are correlated (SI Section S3, Figures S43-S44) with the 13 C δ iso values, but one can readily distinguish the (fluoro)alkoxide series from the silanolate series as the latter exhibit 95 Mo chemical shifts significantly more deshielded than expected based on their 13 C chemical shifts. The 95 Mo quadrupolar coupling constants (C Q ) were also estimated to be between 2.0 and 4.5 MHz, typically significantly lower that the values calculated from first principles (SI Table S4.1), likely due to dynamic effects, present even under the low temperature measurement conditions.…”

“…n SiO] 3 MoC(Ar) (n=0-3), the 95 Mo δ ⊥ increases linearly with the number of phenyl substituents from 785 to 950 ppm indicating the significant influence of the ligand π system, which could be modulated by intramolecular dispersion interactions that were recently evidenced by UV-visible spectroscopy. 13 Both these examples highlight the utility of 95 Mo chemical shifts as a highly resolved probe of electron delocalization through the ligand π system due to their direct dependence on the energy of the Mo≡C π bond and the Mo d orbitals.…”

“…[6][7][8][9][10] Alkyne metathesis in particular has seen major recent progress with the introduction of metal alkylidyne catalysts bearing tripodal silanolate ligands ('canopy catalysts'). [11][12][13][14][15][16] Despite significant synthetic advances, there is a growing need for and interest in spectroscopic descriptors that provide information about electronic structure and relate to catalytic performances.…”

“…12,29 Direct analysis of catalytic centers through NMR of metal nuclei such as 95 Mo is an appealing and potentially more powerful alternative. The high potential resolution of this approach is manifested by the extraordinarily large 95 Mo chemical shift range, known to span >5500 ppm 30,31 compared to ~300 ppm for typical 13 C chemical shifts. However, the origins of 95 Mo NMR signatures and their relations to structure and reactivity remain mostly unexplored , 32,33 due to the low intrinsic signal sensitivities and the complex lineshapes in solid-state NMR, which arise from the low gyromagnetic ratio and quadrupolar nature (I = 5/2) of the 95 Mo nucleus.…”

“…However, the origins of 95 Mo NMR signatures and their relations to structure and reactivity remain mostly unexplored , 32,33 due to the low intrinsic signal sensitivities and the complex lineshapes in solid-state NMR, which arise from the low gyromagnetic ratio and quadrupolar nature (I = 5/2) of the 95 Mo nucleus. Recent 95 Mo NMR studies of Mo alkylidyne catalysts in solution 12 indicate that more complex information is encoded in 95 Mo chemical shifts compared to 13 C. For instance, while a weak global correlation can be seen between 95 Mo and 13 C isotropic chemical shifts (δ iso ) of the alkylidyne carbon (Figure 1a), complexes bearing different types of monoanionic ligands, e.g. silanolate vs. (fluoro)alkoxide, seem to belong to different catalyst classes.…”