Cationic 5-phosphonio-substituted N-heterocyclic carbenes

Area of Science:

• Organometallic Chemistry
• Catalysis
• Materials Science

Background:

• Imidazolium salts are key precursors for N-heterocyclic carbenes (NHCs), widely used in catalysis.
• Functionalization of imidazolium scaffolds allows for tuning of electronic and steric properties of derived NHCs.
• Development of new synthetic routes to diverse NHC ligands is crucial for advancing coordination chemistry.

Purpose of the Study:

• To synthesize novel 2- and 5-phosphanyl-substituted imidazolium salts.
• To explore the reactivity of these salts towards the formation of functionalized NHCs and their metal complexes.
• To investigate the catalytic potential and coordination abilities of the newly synthesized NHC ligands.

Main Methods:

• Synthesis of 2-phosphanyl- and 5-phosphanyl-substituted imidazolium salts via reaction of phosphines with NHC precursors.
• Functionalization of imidazolium salts through methylation, oxidation, and fluoride abstraction.
• Preparation and characterization of transition metal complexes (Au, Cu, Rh, Ag) with novel NHC ligands.
• Quantum chemical studies to elucidate electronic properties and reactivity of intermediates.

Main Results:

• Successful synthesis of 2-phosphanyl- and 5-phosphanyl-substituted imidazolium salts (9a,b[OTf], 10a,b[OTf]).
• Generation of cationic 5-phosphonio-substituted NHCs (17a,b[OTf], 18a,b[OTf]) via SN2(Cl) reaction.
• Formation of various transition metal complexes, including a rare tricationic bis-carbene silver complex (22[OTf]3).
• Demonstration of NHC 17b[OTf] as an effective dehydrocoupling reagent for phosphanes.

Conclusions:

• The developed synthetic strategies provide access to a diverse range of functionalized imidazolium salts and NHCs.
• The novel NHC ligands exhibit versatile coordination behavior with transition metals.
• The synthesized compounds hold promise for applications in catalysis, particularly in phosphane coupling reactions.