Tuneable Phosphaamidinate Ligands: Alkyl-Magnesium Complexes Responsive to Increased Ligand Bulk

We describe the synthesis of five new phosphaamidine ligands of formula RNC(R’)P(H)(R’’), featuring a hindered tert-butyl-bearing backbone and variable steric bulk at both the nitrogen and phosphorus centres. Crystallographic and heteronuclear NMR studies indicate the geometry of the phosphaamidine is dependent on the steric bulk of the peripheral substituents, and in one case multiple isomers were observed in solution. Ligand deprotonation by reaction with 0.5 or 1 equivalents of nBu₂Mg respectively forms heteroleptic (LMgnBu) or homoleptic (L₂Mg) complexes, with the most bulky phosphaamidinate ligand generating a monomeric, unsolvated three-coordinate magnesium centre. Three LMgnBu complexes were isolated and show excellent activity for the ring opening polymerization of rac-lactide (<99 % conversion in two minutes), with a uniform polymer chain length (Đ=1.02–1.37).