Synthesis and Characterization of a Series of Bis(oxo/thiophosphinic)diamido Yttrium Complexes and Their Application as Initiators for Lactide Ring-Opening Polymerization

The syntheses, structures, and lactide polymerization initiation are reported for a series of novel [{N,N‘-1,3-bis(P,P‘-di-isopropyloxophosphinic)-2,2-dimethylpropylenediamido}(amido)yttrium] complexes and
contrasted with [{N,N‘-1,3-bis(P,P‘-di-isopropylthiophosphinic)-2,2-dimethylamido}{bis(trimethylsilyl)amido}yttrium]. The syntheses of the novel [{N,N‘-1,3-bis(P,P‘-di-isopropyloxo/thiophosphinic)-2,2-dimethylpropylenediamido}(amido)yttrium] complexes were achieved in excellent yields by reaction of
the N,N‘-1,3-bis(P,P‘-di-isopropyloxo/thiophosphinic)-2,2-dimethylpropylenediamine ligands with [Y(NR2)3·xTHF] (R = SiMe3, SiMe2H, i-Pr, x = 0, 2). The new complexes were characterized by NMR and IR spectroscopies, elemental analyses, and for R = SiMe2H X-ray crystallography. The solution and solid-state
structures were compared using 2D NMR techniques and in particular using pulsed gradient spin echo
spectroscopy (PGSE) to derive the solution hydrodynamic radius and compare it to that calculated from
the X-ray crystal structure:  the [{N,N‘-bis(P,P‘-di-isopropyloxophosphinic)-2,2-dimethylpropylenediamido}(amido)yttrium] complexes were all dimeric in both the solid state and solution. They showed unusual
structures where each ligand has one oxophosphinic group bonded to a single yttrium center and one
oxophosphinic group bonded to two yttrium centers. The new complexes were tested as lactide
polymerization initiators; they all showed very high activities. The degree of polymerization control
exerted by the novel initiators was related to the size of the initiating amide group and the structure of
the complex.