Synthesis, structure and dynamics of NHC-based palladium macrocycles

Area of Science:

• Organometallic Chemistry
• Coordination Chemistry
• Macrocyclic Chemistry

Background:

• CNC pincer ligands are versatile scaffolds in coordination chemistry.
• Macrocyclic complexes offer unique structural and dynamic properties.
• Palladium complexes are crucial in catalysis and materials science.

Purpose of the Study:

• To synthesize and characterize novel macrocyclic palladium(II) complexes with varying alkyl spacer lengths.
• To investigate the influence of spacer length on the coordination geometry and dynamic behavior of these complexes.
• To compare the structural and spectroscopic properties of palladium(II) chloride and fluoride complexes.

Main Methods:

• Synthesis of bis(imidazolium)lutidine-based macrocyclic pro-ligands.
• Transmetallation strategy using silver(I) oxide for palladium complex formation.
• Characterization by NMR spectroscopy (1H, 13C, 19F), ESI-MS, and X-ray crystallography.
• Variable temperature NMR studies to probe dynamic behavior.

Main Results:

• Successful synthesis of cationic palladium(II) chloride and fluoride complexes with octa-, deca-, and dodecamethylene spacers.
• Smaller macrocyclic complexes (n=8, 10) exhibit dynamic ring-flipping in solution and skewed geometries in solid-state.
• The dodecamethylene complex displays a static C2 symmetric structure.
• Fluoride complexes show unique low-frequency 19F NMR shifts and C-H···F interactions.
• [PdF{CNC-(CH2)8}][BAr(F)4] exhibits exceptional C2v symmetry due to dual fluxional processes.

Conclusions:

• Spacer length significantly impacts the conformational dynamics and solid-state structures of macrocyclic palladium complexes.
• The choice of halide ligand (Cl vs. F) influences complex stability and intermolecular interactions.
• These findings provide a deeper understanding of structure-property relationships in macrocyclic organometallic compounds.