Zinc and enolato-magnesium complexes based on bi-, tri- and tetradentate aminophenolate ligands

Abstract

The coordination chemistry of a series of potentially tetra-, tri- and bi-dentate aminophenol pro-ligands ([L¹]H–[L⁵]H) with magnesium and zinc derivatives has been studied. Reactions of the pro-ligands [L¹]H, [L²]H and [L³]H with one equiv. of ZnEt₂ or Zn(N(SiMe₃)₂)₂ in toluene at room temperature afford cleanly, viaethane or amine elimination, the ethyl- and amido-zinc complexes 1–4. Complexes [L⁴]ZnEt (5) and [L⁴]Zn(N(SiMe₃)₂)₂ (6) derived from the tridentate pro-ligand [L⁴]H were prepared in 84% and 76% yields, following similar alkane and amine elimination protocols, respectively. The 1 : 1 reactions of bidentate pro-ligand [L⁵]H with ZnEt₂ or Zn(N(SiMe₃)₂)₂ in toluene using different reaction protocols systematically yielded mixtures of the bis(ligand) complex [L⁵]₂Zn (7) and the corresponding ethyl-{[L⁵]ZnEt}_n (8; n = 1 or 2) or amido- {[L⁵]Zn(N(SiMe₃)₂)}_n complexes (9; n = 1 or 2). The synthesis of magnesium-enolate complexes 10 and 11 was carried out in a one-pot, two-step procedure by first reacting pro-ligands [L¹]H and [L⁴]H with one equiv. of Mg(n,sBu)₂ to generate the corresponding {[Lⁿ]Mg(n,sBu)}_n species, which were further reacted with one equiv. of 2,4,6-Me₃C₆H₂COMe. All complexes have been characterized by multinuclear NMR, elemental analysis, and by single-crystal X-ray diffraction studies for five-coordinate magnesium complex 10, and four-coordinate Zn complexes 3–7. Preliminary studies indicate that magnesium species 10 and 11 as well as zinc complexes 1 and 5 are not active initiators for the polymerization of methyl methacrylate, even in combination with one equiv. of Li[OC(CH₂)(2,4,6-Me₃C₆H₂)], while zinc complexes 2, 4 and 6 are effective initiators for the ring-opening polymerization of ε-caprolactone and rac-lactide at 20 °C.