Aluminum and Zinc Complexes Based on an Amino-Bis(pyrazolyl) Ligand: Synthesis, Structures, and Use in MMA and Lactide Polymerization

The coordination chemistry of bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine (1, LH) with aluminum- and zinc-alkyls
has been studied. Reaction of 1 with AlR3 affords the adducts [LH]·AlR3 (R = Me, 2; Et, 3), which undergo alkane
elimination upon heating to yield the amido complexes [L]AlR2 (R = Me, 4; Et, 5). Reaction of LiO(iPrO)CCMe2
with 2 proceeds via N−H deprotonation to give Li[L]AlMe3 (6), while the former enolate adds to 4 to generate
[Me2CC(OiPr)OLi]·[L]AlMe2 (7). Similarly, the 1:1 reaction of ZnEt2 with 1 gives [LH]·ZnEt2 (9), which is transformed
into [L]ZnEt (10) upon heating. When an excess of ZnEt2 was used in the latter reaction, the bimetallic complex
[L]ZnEt·ZnEt2 (11) was isolated beside 10. Performing the same reaction in the presence of O2 traces yielded
selectively the dinuclear ethyl-ethoxide complex [L]Zn2Et2(μ-OEt) (12), which was alternatively prepared from the
reaction of 10 and ZnEt(OEt). Zinc chloride complexes [LH]·ZnRCl (R = Et, 13; p-CH3C6H4CH2, 14) and [L]ZnCl
(15) were prepared in high yields following similar strategies. Ethyl abstraction from 10 with B(C6F5)3 yields
[L]Zn+EtB(C6F5)3- (16). All complexes have been characterized by multinuclear nuclear magnetic resonance (NMR),
elemental analysis, and single-crystal X-ray diffraction studies for four-coordinate Al complexes 2, 4, and 6 and Zn
complexes 9−12 and 14. Aluminate species 6 and 7 initiate the polymerization of methyl methacrylate, and the
monomer conversions are improved in the presence of neutral complexes 2 or 4, respectively; however, these
methyl methacrylate (MMA) polymerizations are uncontrolled. Polymerization of rac-lactide takes place at 20 °C in
the presence of zinc ethoxide complex 12 to yield atactic polymers with controlled molecular masses and relatively
narrow polydispersities.