A palladium thiolate-SNS complex-catalyst for Mizoroki-Heck cross-coupling

Area of Science:

• Organometallic Chemistry
• Catalysis
• Synthetic Chemistry

Background:

• Mizoroki-Heck cross-coupling reactions are vital in organic synthesis.
• Phosphine ligands in palladium catalysts can be air and moisture sensitive.
• Developing phosphine-free alternatives enhances catalyst stability and handling.

Purpose of the Study:

• To synthesize and evaluate novel phosphine-free palladium(II) complexes for Mizoroki-Heck catalysis.
• To compare the catalytic activity and stability of two SNS pincer ligand-based complexes.
• To investigate the deactivation pathway of an inactive palladium complex.

Main Methods:

• Synthesis of two palladium(II) complexes, Pd(κ²-SNS^Me)₂ (Pd-1) and PdI(κ³-SNS^Me) (Pd-2), featuring a thiolate-imine-thioether SNS pincer ligand.
• Comparative catalytic testing of Pd-1 and Pd-2 in Mizoroki-Heck cross-coupling reactions.
• Structural analysis and investigation of catalyst deactivation pathways under reaction conditions.

Main Results:

• Pd-2 exhibited excellent catalytic activity in Mizoroki-Heck reactions, achieving high yields with low catalyst loading (1.5 mol%) and short reaction times (5 hours) across diverse substrates.
• Pd-1 demonstrated no catalytic activity, undergoing isomerization to a Pd(II)-N₂S₂ complex via imine C-C bond formation at elevated temperatures.
• Heating Pd-2 with excess triethylamine led to selective C-S bond activation, forming a palladium dithiolate tetramer, [Pd(μ-κ³-SNS)]₄.

Conclusions:

• The SNS pincer ligand effectively stabilizes palladium complexes for Mizoroki-Heck catalysis.
• Pd-2 represents a highly active and robust phosphine-free catalyst for cross-coupling reactions.
• Understanding catalyst deactivation mechanisms, such as isomerization and C-S bond activation, is crucial for catalyst design.