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

Abstract

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-H₂), 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 π-interactions to the potassium centre. L^O3-H₂ also reacts with 1 equiv. of ZnEt₂ or MgⁿBu₂ to give [M₂(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 ε-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 ε-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.