Rare-Earth Complexes Supported by Tripodal Tetradentate Bis(phenolate) Ligands: A Privileged Class of Catalysts for Ring-Opening Polymerization of Cyclic Esters

Tripodal dianionic diamino- or amino-alkoxy-bis(phenolate) ligands {ONXOR1,R2} (X = NR2, OR) constitute a privileged class of ligands which has found much interest in combination with group 3 and group 4 metals for generating highly efficient polymerization catalysts. This account article describes the structural variety of and synthetic routes toward trivalent rare-earth complexes incorporating such {ONXOR1,R2} ancillaries along with reactive amido, alkyl, alkoxide, amidinate, guanidinate, halide, and borohydride groups. The chemistry of related divalent rare-earth complexes is also included. This class of Ln{ONXOR1,R21}(R) complexes features outstanding performance in the ring-opening polymerization (ROP) of cyclic esters. Examples of their high reactivity, which has allowed enlarging the scope of ROP reactions to monomers that are difficult to ring-open, and of their ability to control reactions are provided. A particular emphasis is given on their propensity to fine-tune the stereoselectivity of ROP reactions involving chiral cyclic esters, by modifying the ortho substituents on the phenolate rings. The associated mechanistic issues which have evidenced steric and much less common electronic interactions are discussed.