Potassium, zinc, and magnesium complexes of a bulky OOO-tridentate bis(phenolate) ligand: synthesis, structures, and studies of cyclic ester polymerisation

Reaction of the OOO-coordinating tridentate bis(phenolate) protio-ligand 2,2'-{oxybis(methylene)}bis-{4,6-di(1-methyl-1-phenylethyl) phenol} (L-O3-H-2), with 1 equiv. of KN(SiMe3)(2) in toluene or THF yielded [K(L-O3-H)] (1) or [K(L-O3-H)(THF)] (2), respectively. Single-crystal X-ray diffraction studies of 1 and 2 revealed mononuclear structures with the phenyl rings of the bulky ligand displaying stabilising pi-interactions to the potassium centre. L-O3-H-2 also reacts with 1 equiv. of ZnEt2 or MgnBu(2) to give [M-2(L-O3)(2)] (M = Zn (3) or Mg (4)) in good yield. The molecular structures of complex 3 and 4 reveal dinuclear species in which the metal centres are tetra-coordinated to the three oxygen atoms of one L-O3 ligand, and to the bridging oxygen atom of one phenolate group of another. Complexes 1-4 are catalysts for ring-opening polymerisation of epsilon-caprolactone and L- and rac-lactide in the presence of benzyl alcohol (BnOH) and also other initiators to give the corresponding polyesters. Kinetic studies for the ROP of epsilon-caprolactone using 3 and BnOH gives an unusual rate expression R-p = -d[CL]/dt = k(p)[BnOH](0)[3](0)(0.5) for which a tentative kinetic model is proposed.