Diffusion Nuclear Magnetic Resonance Measurements on Cationic Gold (I) Complexes in Catalytic Conditions: Counterion...

Area of Science:

• Organometallic Chemistry
• Catalysis
• Solution Chemistry

Background:

• Gold(I) complexes with N-heterocyclic carbenes (NHCs) are effective catalysts for alkoxylation.
• Understanding the speciation of ions and ion pairs is crucial for optimizing catalytic performance.
• Previous studies suggested a link between catalytic activity and ion pair dissociation, but experimental evidence was limited.

Purpose of the Study:

• To experimentally determine the concentration of free ions, ion pairs, and aggregates in gold(I)-catalyzed alkoxylation.
• To investigate the influence of counterion nature and solvent dielectric constant on ion speciation and catalytic activity.
• To correlate the degree of ion dissociation (α) with the catalytic efficiency (Turnover Frequency, TOF).

Main Methods:

• Diffusion Ordered Spectroscopy (DOSY) NMR (¹H and ¹⁹F) was employed under catalytic conditions.
• Measurements were performed in various solvents, including chloroform and green solvents with high dielectric constants.
• The dissociation degree (α) of the ion pair/free ion equilibrium was calculated based on NMR data.

Main Results:

• Counterion identity significantly impacts α in chloroform: OTs^- and OTf^- yield low α (high ion pairing), while BF₄^- and BArF₄^- show higher α (more free ions).
• Catalytic activity generally increases with lower α, attributed to the anion's role in activating methanol.
• In polar green solvents, high α values were observed, indicating solvent-specific activation mechanisms involving polar functionalities.

Conclusions:

• Experimental DOSY NMR data confirm that lower ion pair dissociation (lower α) enhances gold(I)-catalyzed alkoxylation activity.
• The choice of counterion and solvent polarity are critical factors influencing the catalytic performance.
• Polar functionalities in green solvents play a key role in methanol activation, complementing the counterion's effect.