1085283-56-2Relevant academic research and scientific papers
Bimetallic calcium and zinc complexes with bridged β-diketiminate ligands: Investigations on epoxide/CO2 copolymerization
Piesik, Dirk F.-J.,Range, Sven,Harder, Sjoerd
, p. 6178 - 6187 (2009/04/13)
A series of dinucleating bis(β-diketiminate) ligands with rigid bridges has been prepared. In all cases the β-diketiminate unit is 2,6-iPr2C6H3NC(Me)C(H)C(Me)N-(bridge), and the bridges are either paraphenylene, meta-phenylene or 2, 6-pyridylene (the dinucleating ligands are abbreviated as PARA-H2, META-H2 and PYR-H2, respectively). These ligands have been converted to heteroleptic bimetallic calcium and zinc complexes. For calcium, only the PARA-phenylene bridged ligand led to a heteroleptic bimetallic calcium amide complex PARA-[CaN(SiMe3)2 · THF]2. For the other ligands, homoleptic complexes have been isolated: META-Ca and PYR-Ca. For zinc, the whole range of heteroleptic amides could be isolated: PARA-[ZnN(SiMe3)2]2, META-[ZnN(SiMe 3)2]2, and PYR-[ZnN(SiMe3) 2]2. Analogue ethylzinc complexes have been prepared in quantitative yields: PARA-(ZnEt)2, META-(ZnEt)2, and PYR-(ZnEt)2. Reactions of the ethylzinc complexes with SO2 gave access to the ethylsulfinate complexes META-(ZnO2SEt)2 and PYR-(ZnO 2SEt)2; for the para-phenylene bridged ligand no products could be isolated. Crystal structures of the following complexes are presented: (META-Ca)2, PARA-[CaN(SiMe3)2 ... THF]2, PARA[ZnN(SiMe3)2]2, META-[ZnN(SiMe3)2]2, PYR-(ZnEt)2, and PYR-(ZnO2SEt)2. All heteroleptic complexes have been tested for activity in me copolymerization of cyclohexene oxide (CHO) and CO2. The bimetallic complex PARA-[CaN(SiMe3)2 · THF]2 is not active. For zinc, the PARA and META complexes were found to be active under highly concentrated conditions (Zn/CHO ratio of 1/1000; no solvent), but no significant polymer yields could be achieved with PYR complexes. This is either due to conformational changes of the complex or to coordination of the pyridylene N atom to the catalytic centers. The order of activity found in the bimetallic zinc complexes is META > PARA. This is likely due to a more advantageous Zn · Zn distance in the meto-phenylene bridged bimetallic catalysts. There are several indications for bimetallic action. First of all, the ethylzinc systems PARA-(ZnEt)2 and META-(ZnEt)2 initiate CHO/CO2 copolymerization, whereas monometallic ethylzinc catalysts are not reactive. Second, the bimetallic zinc catalysts are also highly active under diluted conditions: a low metal/CHO ratio of 1/3000 gave high-MW polymers (Mn > 100.000, PDI = 1.33) with essentially only carbonate linkages. In contrast, monometallic systems show a drastic loss of activity upon dilution. Finally, the bimetallic catalyst META-[ZnN(SiMe 3)2]2 shows a significantly higher activity than a comparable monometallic model system.
