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

The coordination chemistry of a series of potentially tetra-, tri- and bi-dentate aminophenol proligands ([L-1]H-[L-5]H) with magnesium and zinc derivatives has been studied. Reactions of the pro-ligands [L-1]H, [L-2]H and [L-3]H with one equiv. of ZnEt2 or Zn(N(SiMe3)(2))(2) in toluene at room temperature afford cleanly, via ethane or amine elimination, the ethyl- and amido-zinc complexes 1-4. Complexes [L-4]ZnEt (5) and [L-4]Zn(N(SiMe3)(2))(2) (6) derived from the tridentate pro-ligand [L-4]H were prepared in 84% and 76% yields, following similar alkane and amine elimination protocols, respectively. The 1 : 1 reactions of bidentate pro-ligand [ L 5] H with ZnEt2 or Zn(N(SiMe3)(2))(2) in toluene using different reaction protocols systematically yielded mixtures of the bis(ligand) complex [L-5](2)Zn (7) and the corresponding ethyl-{[L-5]ZnEt}(n) (8; n = 1 or 2) or amido-{[L-5]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-s tep procedure by first reacting proligands [L-1]H and [L-4]H with one equiv. of Mg(n, sBu)(2) to generate the corresponding {[ L n] 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 epsilon-caprolactone and rac-lactide at 20 degrees C.