Shape-Persistent Binuclear Zinc- and Cobalt-Schiff Base Complexes: Synthesis, Characterization and Catalytic Reactivity for Fixation of Carbon Dioxide
Student thesis: Doctoral Thesis
The use of transition metal complexes to catalyze the transformation of CO2 into useful products has been extensively developed due to their inexpensive, efficient and robust nature. This thesis concerns the design, synthesis and characterization of shape-persistent binuclear zinc(II) and cobalt(III) Schiff base complexes, and their catalytic reactivity for coupling CO2 with epoxides.
Chapter 1 gives an introduction to various catalyst systems for the synthesis of cyclic carbonates and polycarbonates from CO2 and epoxides, with a focus on zinc and cobalt catalysts, as well as a brief review of the recent progress in metal-catalyzed CO2 fixation. Chapter 2 describes general experimental procedures.
In Chapter 3, a new class of conformationally rigid bis-Zn(II) complexes bearing substituted Schiff base ligands, with rigid xanthene or dibenzofuran linkers, has been prepared and employed as catalysts for CO2/epoxide coupling reactions. The complexes have been characterized by multiple spectroscopic techniques, and the X-ray structures of two derivatives have been determined. All complexes are active in the catalytic coupling reactions of CO2 with epoxide under mild conditions. By changing the anchoring position, nature of Schiff base and (bulky) substituents, and replacing the xanthene linker with dibenzofuran, high conversion levels was finally obtained for the coupling of CO2 with epoxide. High initial turnover frequencies (up to 12800 h–1) for coupling of CO2 with neat epoxides in conjunction with nBu4NI, and excellent conversions under mild conditions (1 bar pCO2, 45 °C), have been achieved for dibenzofuran-linked bimetallic catalysts featuring cofacial Zn-salphen units. The nature of the binuclear frameworks has been analyzed using DFT calculations, and comparisons with the crystal structures have been made. These results emphasize the importance of the relative orientation of active sites, and intermetallic separations and geometry, for synergistic effects and enhanced reactivity. A plausible binuclear catalytic mechanism is presented.
The employment of a rigid m-terphenyl linker was investigated in Chapter 4. A new series of binuclear Zn(II)-salphen complexes, based on m-mesityl linkers with ortho-, meta- or para-connecting positions for the O(phenolate) groups with respect to the aryl-aryl bond, has been synthesized and characterized. The catalytic properties of the binuclear complexes, and a mononuclear relative, for cyclic carbonate formation from coupling of CO2 with epoxides were investigated. In general, the activities of the binuclear complexes based on m-terphenyl linkers were similar or lower than the mononuclear relative. A polyether was unexpectedly produced by a binuclear Zn(II)-salphen complex based on the ortho-connecting m-terphenyl linker. This unusual reactivity is tentatively ascribed to the relative geometry and positioning of the two Zn-salphen moieties in this catalyst, and a plausible mechanism is presented to explain the formation of both cyclic carbonate and polyether products. DFT calculations were performed, confirming the existence of syn- and anti-configurations for the ortho-connected complex.
Following the synthetic work and results for the bis-(Zn(II)-salphen) systems in Chapters 3 and 4, a series of shape-persistent binuclear Co(III)-salphen complexes with rigid linkers are developed in Chapter 5. The bis-(Co(III)-salphen) complexes tethered by xanthene, dibenzofuran, or m-mesityl linker groups have been synthesized through transmetalation and oxidation from the binuclear Zn(II) congeners. These catalysts are active for the coupling of CO2 with epoxide to give cyclic carbonate and polycarbonate products. The catalyst with the m-terphenyl linker showed the highest selectivity for the synthesis of polycarbonates (> 90%) with a TOF of 35 h-1 Co-1, which can be compared with the results (selectivity of PPC > 97%, a TOF of 40 h-1 Co-1) for the mononuclear analogue under identical conditions. Plausible mechanisms for mono- and bimetallic catalysts are presented, and the differences in the selectivity of these binuclear Co(III) complexes towards cyclic carbonate or polycarbonate is attributed to the competition between propagation of the polymer chain and the backbiting formation of cyclic carbonate.