Versatile coordination chemistry of a bis(methyliminophosphoranyl)pyridine ligand on copper centres

Abstract

The coordination of a bis(methyliminophosphoranyl)pyridine ligand (L) to copper centres was studied. The use of copper(I) bromide precursors gave access to [LCuBr] (2) in which only one iminophosphorane arm is coordinated to the metal, as observed by X-ray crystallography and MAS ³¹P NMR. Its fluxional behaviour in solution was demonstrated by VT-³¹P NMR, and investigated by DFT calculations. On the other hand, coordination of L to [Cu(CH₃CN)₄]PF₆ gave a dimer [L₂Cu₂](PF₆)₂ (3) in which the two copper centres do not have the same coordination sphere as shown by X-ray crystallography. Addition of a strong ligand such as PEt₃ allows the preparation of a cationic monomeric copper complex (4) in which L has a behaviour similar to that observed for 2. Synthesis of copper(II) complexes was also achieved by chemical oxidation of 2, which shows an irreversible oxidation at −0.36 vs. Fc⁺/Fc, or directly via the coordination of L to CuBr₂. In [LCuBr₂] (5), L adopts a pincer coordination. Finally, the catalytic behaviour of copper(I) complexes 2 and 3 was investigated in cyclopropanation reactions and [3 + 2] cycloadditions.