Phosphasalen vs. Salen Ligands: What Does the Phosphorus Change?

Phosphasalen ligands can be viewed as the phosphorus analogues of salen, in which the imine functions were replaced by iminophosphoranes (P=N). Their divergent stereoelectronic properties compared to salen, namely larger electron-donation, higher flexibility, and larger steric hindrance due to the presence of two substituents on the tetrahedral phosphorus atoms, were exploited in coordination chemistry and catalysis. They were proven to be able to stabilize metals in unusual oxidation states; square planar copper(III) and nickel(III) complexes were isolated. Because of the redox non-innocent character of these ligands, the electronic structure of the one-electron oxidized complexes was studied in depth to determine the metallic vs. ligand contribution in the overall delocalization. In addition, the catalytic activity of different phosphasalen complexes was proven for the ring opening polymerization of lactide, and rare-earth phosphasalen complexes were shown to be powerful, rapid, controlled, and stereoselective initiators. Importantly, the stereochemical outcome of the reaction can be tuned by changing the ligand structure or the metal. In particular, efficient iso-selective initiators were developed.