Pincer-type Heck catalysts and mechanisms based on Pd(IV) intermediates: a computational study

Area of Science:

• Organometallic Chemistry
• Catalysis
• Computational Chemistry

Background:

• Pincer-type palladium complexes are highly active Heck catalysts.
• Proposed catalytic mechanisms often involve controversial Pd(II)/Pd(IV) cycles.
• Pd(IV) intermediates in these systems have not been experimentally observed.

Purpose of the Study:

• To computationally investigate the feasibility of Pd(II)/Pd(IV) catalytic cycles with pincer-type Heck catalysts.
• To explore potential reaction mechanisms involving Pd(IV) intermediates.
• To compare these mechanisms with palladium nanoparticle formation.

Main Methods:

• Density functional theory (DFT) calculations were employed.
• Calculations were performed on aminophosphine-, phosphine-, and phosphite-based pincer-type Heck catalysts.
• The Heck reaction with styrene and phenyl bromide, forming (E)-stilbene, was studied in DMF solvent.

Main Results:

• Potential-energy surfaces indicate that Pd(II)/Pd(IV) mechanisms are thermally accessible.
• The study identified key intermediates, including cationic three-coordinate Pd(II) species and hexacoordinate Pd(IV) complexes.
• Cationic styrene adducts were found to be the resting states of the catalytic reaction.

Conclusions:

• Pd(II)/Pd(IV) catalytic cycles are a viable alternative to palladium nanoparticle formation.
• The computational findings provide theoretical support for proposed Pd(II)/Pd(IV) mechanisms.
• This work elucidates the detailed steps of the Heck reaction catalyzed by pincer-type palladium complexes.