Bimetallic Calcium and Zinc Complexes with Bridged β-Diketiminate Ligands: Investigations on Epoxide/CO₂ Copolymerization

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-iPr2C6H3NC(Me)C(H)C(Me)N-(bridge), and the bridges are either para-phenylene, 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(SiMe3)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-(ZnO2SEt)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 the 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 meta-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 (M n > 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(SiMe3)2]2 shows a significantly higher activity than a comparable monometallic model system.

Differences in Reactivity of Epoxides in the Copolymerisation with Carbon Dioxide by Zinc‐Based Catalysts: Propylene Oxide versus Cyclohexene Oxide

“…Limberg, Hultzsch and co-workers as well as Harder and co-workers focussed on a BDI-like structure in their dinuclear catalysts 6 and 7. [37,38] Recently we proofed the bimetallic nature of the copolymerisation of propylene oxide and carbon dioxide with dimeric salphen complexes (Scheme 4). [11,39,40] All of these catalysts finally led to the homogeneous dinuclear zinc catalyst reported by Williams et al, which is active in the copolymerisation of cyclohexene oxide and carbon dioxide even at one atmosphere of CO 2 and therefore shows the highest turnover frequency (TOF) ever reported under such conditions (25 h À1 ; Scheme 4).…”

Flexibly Tethered Dinuclear Zinc Complexes: A Solution to the Entropy Problem in CO2/Epoxide Copolymerization Catalysis?

“…However, most of the reported structures suffer from poor solubility or low polymerization activity. [16,[22][23][24][25][26][27][28][29][30] One major breakthrough aside from zinc phenoxides developed by Darensbourg is the synthesis of b-diketiminato (BDI) zinc complexes by Coates et al [21,[31][32][33][34][35][36][37][38][39][40][41] For these highly active catalysts the mechanism involving a dimetallic intermediate was unambiguously demonstrated. Kinetic investigations using in situ IR spectroscopy showed that the rate-determining step is the ring opening of the epoxide.…”

Binuclear β-diketiminate complexes of copper(i)

“…X-ray crystallography was recorded using the Agilent Xcalibur 3 E and PX Ultra A X-ray Diffraction Systems with Cu Single Wavelength Sealed Tube Source with wavelengths of 0.71 Å and 1.54 Å and at 173 K. The dinucleating ligands 1•H2 and 2•H2 were prepared by modification of the literature procedures (see supporting information). [37][38][39] The copper cluster [2•Cu2]2 was prepared by the literature procedure. 31 Synthesis of 3•H2: To a solution of 2-(2,6-di-iso-propylphenyl)imido-2-penten-4-one (2.6 g, 9.98 mmol) in DCM (15 mL) at room temperature was added a solution of [Et3O]BF4 (2.1 g, 11.03 mmol) in DCM (5 mL) to produce a brown-colored solution.…”