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] x 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] x [L]AlMe2 (7). Similarly, the 1:1 reaction of ZnEt2 with 1 gives [LH] x 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 x 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(mu-OEt) (12), which was alternatively prepared from the reaction of 10 and ZnEt(OEt). Zinc chloride complexes [LH] x 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 degrees C in the presence of zinc ethoxide complex 12 to yield atactic polymers with controlled molecular masses and relatively narrow polydispersities.
Lian
Bing
B
Catalyse et Organométalliques, UMR 6226, University of Rennes 1, 35042 Rennes Cedex, France.
Thomas
Christophe M
CM
Casagrande
Osvaldo L
OL
Lehmann
Christian W
CW
Roisnel
Thierry
T
Carpentier
Jean-François
JF
D000535:Aluminum / Q000737:chemistry*; D008024:Ligands; D020366:Methylmethacrylate / Q000737:chemistry*; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011098:Polyglactin 910 / Q000031:analogs & derivatives* / Q000737:chemistry*; D011108:Polymers / Q000138:chemical synthesis* / Q000737:chemistry; D015032:Zinc / Q000737:chemistry*
D008024:Ligands; D009942:Organometallic Compounds; D011108:Polymers; D020366:Methylmethacrylate; D011098:Polyglactin 910; D000535:Aluminum; D015032:Zinc