Synthesis, reactivity, and coordination chemistry relevant to the copolymerization of CO2 and epoxides by first row transition metal schiff base complexes

摘要

Excepting agricultural based products, which themselves require a great deal ofenergy to produce, our supply of natural resources such as minerals, metal ore, freshwater, coal, oil and natural gas are all limited in supply. The depletion of thesesubstances is imminent and this knowledge weighs heavily on humankind. Theutilization of CO2 for the production of polycarbonates is one attempt at exploiting aprofoundly abundant and renewable resource. The importance of research in this andsimilar fields justifies the detailed study of the chemicals and procedures involved withthis chemistry. This current work concentrates on the fundamental study of transitionmetal Schiff base complexes that have shown a great deal of promise in their ability tocatalyze the copolymerization of CO2 and epoxide to form aliphatic polycarbonates.A new chromium(III) Schiff base complex has been synthesized and evaluatedfor its ability to catalyze the formation of polymer. The ligand employed bears an N2O2coordination sphere identical to the widely utilized chromium(III) and cobalt(III) salencatalysts. This complex was shown to be active towards the copolymerization of CO2 and cyclohexene oxide. Although the activity was less than that seen withchromium(III) salen complex, the study demonstrates that new ligand systems areavailable beyond salen and deserve further attention.A class of manganese(III) Schiff base complexes was also synthesized andevaluated as catalysts. Although crystallographic data has shown that these complexesare structural analogs to chromium(III) salens, the difference in metal center leads to anearly complete elimination of catalytic activity. Such a marked difference has beentaken advantage of by using this very low activity to study the ring-opening of epoxidein the initial step of the copolymerization both mechanistically and kinetically. It hasalso been utilized in an evaluation of the coordination chemistry of the polymerizationprocess. This has led to some valuable conclusions about the nature and role of themetal center that previously have not been studied. Manganese(III) salen complexeswere also synthesized and evaluated in an effort to compare these important ligands toother Schiff bases and confirm the findings mentioned above.