Ring-Opening Polymerization of Cyclic Esters by Pincer Complexes Derived from Alkaline Earth Metals

We report the synthesis and characterization of new Ca(II) and Mg(II) complexes of the type [(Ph- PPP)MR] (M = Ca, R = N(SiMe3)2 (1); M = Mg, R = nBu (2)) where Ph-PPP is a tridentate monoanionic phosphido ligand (Ph-PPP-H =bis(2-diphenylphosphinophenyl)phosphine). Reaction of the opportune metal precursor (Ca[N(SiMe3)2]2(THF)2 or MgnBu2) with one equivalent of the pro-ligand PPP-H produces the corresponding calcium amido (1) or magnesium butyl (2) complex in high yield. Solution NMR studies show monomeric structures and kinetically stable for both species. The obtained complexes efficiently mediate the ring-opening polymerization of ɛcaprolactone showing a TOF up to 40000 h-1. In the presence of a hexogen alcohol, up to 2000 equivalents of monomer are converted by using a low loading of catalyst (5 μmol). Kinetic studies show a first order reaction in monomer concentration.