Conformationally Rigid Binuclear Zinc(II)-Schiff Base Complexes: Synthesis, Characterization, and Efficient Catalysts for Coupling of Carbon Dioxide with Epoxides

Abstract

Carbon dioxide is considered to be one of the most important C₁ building blocks due to its inexpensive, abundant and sustainable characteristics. However, some challenges exist considering the chemical fixation of CO₂. Because CO₂ is in the most oxidized and stable state of carbon, the search for cheap, robust and effective catalysts capable of CO₂ fixation under mild conditions has attracted a lot of interest.

In Chapter 1, an introduction on various catalyst systems (including mononuclear, binuclear and multinuclear catalysts) for the generation of cyclic carbonate and polycarbonate from the coupling reaction of CO₂ with epoxides is given. Plausible mechanisms for the reactions are presented. In Chapter 2, general experimental procedures are described.

In Chapter 3, a series of new conformationally rigid binuclear Zn(II) complexes have been prepared, characterized and employed as catalysts for CO₂ fixation reactions. The strategy of modifying the structure of bimetallic catalysts, by introducing trifluoromethyl substituents and replacing the xanthene support with dibenzofuran, finally afforded high conversion capability for dibenzofuran-based catalysts for the coupling of CO₂ with epoxides. High initial turnover frequencies (up to 13750 h^–1 Zn^–1) for coupling of CO₂ with neat epoxides in conjunction with nBu4NI, and excellent conversions under mild conditions (93% conversion, 1 bar pCO₂, 55 ºC, 8 h), have been achieved for dibenzofuran-linked bimetallic catalyst featuring cofacial Zn-salphen units. The Zn···Zn distances have been probed by DFT calculations with the aim of gaining insights into the nature of these catalyst frameworks. The ^tBu groups ortho to the O(phenolate) groups are proposed to suppress facile dimerization processes (via Zn···O(salphen) interactions) as well as inhibit the aggregation, and thus prevent blockage and saturation, of the active sites. The electron-withdrawing CF₃ groups can increase the Lewis acidity of the metal centers, and hence enhance the reactivity. These results emphasize that the relative orientation of active sites, and intermetallic separations and geometry, are important for synergistic effects and enhanced reactivity.

In Chapter 4, the reactivity of dibenzofuran-based bimetallic [Zn(II)-salphen]₂ complexes for cyclic carbonate formation has been tested in detail. The kinetic studies, using in situ React-IR for CO₂/1,2-epoxybutane coupling, clearly show that the reaction follows an intramolecular, cooperative pathway for the bimetallic catalysts when compared with their monometallic analogues. However, the decreased intermolecular rate for binuclear catalysts compared with their monomers is noteworthy. The results reveal that although an intramolecular, cooperative reaction is observed for binuclear complexes, simply placing two catalysts at close proximity is not a prerequisite for an efficient rate increase. Both intermolecular and intramolecular rates are important for overall reaction rate enhancement.

In Chapter 5, new binuclear Zn(II)-salphen complexes linked by a m-terphenyl backbone, with ortho-connecting positions for the O(phenolate) groups with respect to the aryl-aryl bonds, has been prepared and characterized. The structure of an anti-complex has been elucidated by X-ray crystallography. Trifluoromethyl, methyl and tert-butyl groups have been introduced at the Schiff base moiety to give a series of complexes bearing different electronic and steric properties. The catalytic properties of the binuclear complexes have been investigated for the coupling of CO₂ with epoxides. In general, the binuclear anti-complexes show higher activities than the syn-complexes for the CO₂-epoxybutane (neat) coupling reactions. For the coupling reaction of CO₂ with epoxyhexane in MEK catalyzed by syn-complexes, a polyether was unexpectedly formed together with the production of cyclic carbonate. A possible mechanism is proposed to explain the formation of both cyclic carbonate and polyether products.

Date of Award	7 Sept 2020
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Michael Chi Wang CHAN (Supervisor)