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

Abstract

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.

Bis(salicylamidine) Ligands (FAlen): A Variant of Salen with “à la Carte” Denticity

“…Their steric and electronic characteristics can indeed be easily tuned by choosing the appropriate diimine bond and phenyl ring substituents. More drastic modifications of the ligand were also made by replacing either one or both imine functions with an amine or with an iminophosphorane . These systems were found to be very efficient, sometimes even outperforming the parent Salen ligands .…”

Comparison of Imine- and Phosphinimine-Supported Indium Complexes: Tuning the Reactivity for the Sequential and Simultaneous Copolymerization of Lactide and ε-Caprolactone

“…While some of these differences can be explained by mechanistic considerations, the lack of direct structure/function studies hinders a deeper understanding of these systems. For example, it is generally recognized that phosphinimino (PN) groups are both σ and π donors and therefore more electron-donating compared to imino groups (CN), but it is not clear to what extent this influences the Lewis acidity, and thus the catalytic activity, of indium catalysts.…”

“…20 While some of these differences can be explained by mechanistic considerations, the lack of direct structure/function studies hinders a deeper understanding of these systems. For example, it is generally recognized that phosphinimino (PN) groups are both σ and π donors and therefore more electron-donating compared to imino groups (CN), 32 Employing these and other highly active indium complexes for the ROP of lactide in combination with other cyclic esters is an attractive route for the controlled synthesis of oxygenated block copolymers. 33 The simplest route to the formation of block copolymers is the sequential addition of monomers; it is significantly more challenging to generate them in a controlled fashion through the simultaneous addition of monomers.…”