Amidine functionalized phosphines: tuneable ligands for transition metals

Area of Science:

• Organometallic Chemistry
• Ligand Synthesis
• Coordination Chemistry

Background:

• Development of novel ligands is crucial for advancing coordination chemistry and catalysis.
• Amidine-phosphine ligands offer unique electronic and steric properties for metal complexation.

Purpose of the Study:

• To synthesize and characterize a novel amidine-phosphine ligand and its derivatives.
• To investigate the donating ability of the synthesized ligands.
• To explore the coordination behavior of these ligands with Rh(I), Au(I), and Pt(II) metal centers.

Main Methods:

• Selective phosphination of a bicyclic amidine with 1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphatricyclo[3.3.1.1^3,7]decane (α,β-CgP).
• Methylation to form the corresponding amidinium salt.
• Synthesis and characterization of Rh(I), Au(I), and Pt(II) complexes.
• Spectroscopic (IR) and computational (DFT) analyses to assess ligand donating ability.
• Single-crystal X-ray diffraction for structural determination of selected complexes.

Main Results:

• A novel amidine-phosphine ligand (α,β-CgPAm) and its amidinium salt ([α,β-CgPAmMe]BF₄) were successfully synthesized as diastereomeric mixtures.
• Phosphination occurred selectively at the 4-position of the amidine without migration.
• α,β-CgPAm demonstrated superior electron-donating ability compared to its cationic counterpart, confirmed by IR spectroscopy and DFT calculations.
• Attempts to resolve the diastereomeric metal complexes via fractional crystallization were unsuccessful.
• Structures of three metal complexes were determined by X-ray crystallography.

Conclusions:

• The study successfully synthesized and characterized a new class of amidine-phosphine ligands.
• The ligand's electronic properties were evaluated, confirming its behavior as a net donor.
• The inherent diastereomeric nature of the ligand complicates resolution of its metal complexes.