Bimetallic calcium and zinc complexes with bridged β-diketiminate ligands: Investigations on epoxide/CO2 copolymerization
Piesik DFJ, Range S, Harder S (2008)
Publication Type: Journal article, Original article
Publication year: 2008
Journal
Original Authors: Piesik D.F.-J., Range S., Harder S.
Publisher: American Chemical Society
Book Volume: 27
Pages Range: 6178-6187
Journal Issue: 23
DOI: 10.1021/om800597f
Abstract
A series of dinucleating bis(β-diketiminate) ligands with rigid bridges has been prepared. In all cases the β-diketiminate unit is 2,6-iPrCHNC(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-H, META-H and PYR-H, 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(SiMe) · THF]. 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(SiMe)], META-[ZnN(SiMe )], and PYR-[ZnN(SiMe) ]. Analogue ethylzinc complexes have been prepared in quantitative yields: PARA-(ZnEt), META-(ZnEt), and PYR-(ZnEt). Reactions of the ethylzinc complexes with SO gave access to the ethylsulfinate complexes META-(ZnO2SEt)2 and PYR-(ZnO SEt); for the para-phenylene bridged ligand no products could be isolated. Crystal structures of the following complexes are presented: (META-Ca), PARA-[CaN(SiMe) ⋯ THF], PARA[ZnN(SiMe)], META-[ZnN(SiMe)], PYR-(ZnEt), and PYR-(ZnOSEt). All heteroleptic complexes have been tested for activity in me copolymerization of cyclohexene oxide (CHO) and CO. The bimetallic complex PARA-[CaN(SiMe) · THF] 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) and META-(ZnEt)2 initiate CHO/CO 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 (M > 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 )] shows a significantly higher activity than a comparable monometallic model system. © 2008 American Chemical Society.
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APA:
Piesik, D.F.-J., Range, S., & Harder, S. (2008). Bimetallic calcium and zinc complexes with bridged β-diketiminate ligands: Investigations on epoxide/CO2 copolymerization. Organometallics, 27(23), 6178-6187. https://doi.org/10.1021/om800597f
MLA:
Piesik, Dirk F. -J., Sven Range, and Sjoerd Harder. "Bimetallic calcium and zinc complexes with bridged β-diketiminate ligands: Investigations on epoxide/CO2 copolymerization." Organometallics 27.23 (2008): 6178-6187.
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