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

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 (31)P NMR. Its fluxional behaviour in solution was demonstrated by VT-(31)P 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.