Cobalt and chromium amino-bis(phenolate) complexes for epoxide homopolymerization and copolymerization with carbon dioxide

Ambrose, Kenson (2019) Cobalt and chromium amino-bis(phenolate) complexes for epoxide homopolymerization and copolymerization with carbon dioxide. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Chemical transformations of CO₂ into useful materials have received significant attention in the past as it is a potential C1 feedstock of very high abundance. This attention originates from the increased abundance of CO₂ in the earth’s atmosphere due to anthropogenic processes such as fossil fuel combustion. The use of CO₂ as a C1 feedstock will not lead to a decrease in atmospheric CO₂ concentrations but rather as an alternative starting material to petroleum derived small molecules. Metal-catalyzed reactions of CO₂ with epoxides into either polycarbonate via copolymerization or cyclic carbonates via cycloaddition are potentially lucrative methods for CO₂ use. In this work, Co(II)/(III) complexes bearing tripodal amino-bis(phenolate) ligands were synthesized and characterized by NMR, MALDI-TOF mass spectrometry, elemental analysis and X-ray diffraction, then investigated for catalytic activity toward CO2 and epoxide coupling in the presence of the following co-catalysts: TBAB (tetrabutylammonium bromide), PPNCl (bis(triphenylphosphine)iminium chloride) or DMAP (4-dimethylaminopyridine). Co(III) complexes also contained ancillary ligands such as [2,4-DNP]⁻ (2,4-dinitrophenolate), OAc⁻ (acetate) and ⁻O₂CCF₃ (trifluoroacetate) and showed selectivity toward cyclic carbonate production. A Co(II) complex, however, contained coordinated KOAc (observed via X-ray diffraction and elemental analyses) and was selective toward polycarbonate production where OAc⁻ served as a minor initiator during the copolymerization process, as determined by end-group analysis using MALDI-TOF mass spectrometry. The coordinated KOAc is believed to play an important role in overall catalyst activity as other reported catalysts with similar structure, but not containing KOAc, showed little to no copolymerization. Cr(III) complexes bearing slightly different amino-bis(phenolate) ligands were also synthesized and investigated as catalyst for reactions of CO₂ with epoxides. A Cr(III) complex with a ligand containing tert-butyl groups in the ortho and para positions of the phenolate O-donors, was highly active toward CHO (cyclohexene oxide) ring-opening homopolymerization to produce high molecular weight PCHO (poly(cyclohexene oxide)) without the use of a co-catalyst and even in the presence of CO₂. Modification of this complex by introducing methyls into the ortho and para positions of the phenolate donors switched activity to CHO/CO₂ copolymerization, with high selectivity and CO2 incorporation. This complex showed the fastest rate of CO₂/epoxide copolymerization with PPNN₃ as a co-catalyst, when monitored using in situ FTIR spectroscopy. Kinetics studies of the CHO/CO₂ copolymerization process showed a first order dependence on catalyst concentration.

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