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Molecular Catalysis for Electro- and Photochemical CO2 Reductive Functionalization

項目計劃:
優配研究金
項目年份:
2018/2019
項目負責人:
梁致輝博士
(科學與環境學系)
Molecular Catalysis for Electro- and Photochemical CO2 Reductive Functionalization

It is proposed in this project to study the electro- and photocatalytic CO2 reductive coupling with a series of nucleophilic organic substrates bearing varied (C, N and S) donor atoms by using molecular first-row transition metal catalysts, which contain tetradentate polypyridine-based ligands and their functionalised derivatives.

To tackle the issues of global warming and fossil fuel depletion, much effort have been devoted to develop chemical processes for tapping CO2 as a renewable resource. One of the approaches is to use CO2 as a single-carbon (C1) feedstock by the coupling the green-house gas with organic co-reactants. Conventionally, the active substrates, e.g. alkene, alkyne, aziridine, epoxide, or organozinc, of high free energy is employed to react with CO2, forming lower-energy products, e.g. carboxylic acids, ester, carbonate, lactone, carbamate or oxazolidinone, in which the formal oxidation state of the CO2 carbon (4+) remains unchanged. Thus, the variety of products obtained by this non-reductive approach remains limited. As a result of the stability and kinetic inertness of CO2, high reaction pressure and temperature are very often required, even in the presence of a catalyst. On the other hand, reactions of amine-type substrates with CO2 have also been carried out in the presence of reducing agents, e.g. H2 and hydrides (hydrosilane and hydroborane), with the help of catalysts to give such products as formamide, formamidine and methylamines in which the CO2 carbon is reduced. Given the higher free energy of these products bearing reduced carbon, use of excessive chemical reductants and similar drastic conditions are often needed to overcome the kinetic barrier. So far, efficient catalysts made of economic and earth-abundant materials, as well as the scope of reactivity remain limited for such reductive approach. Reported catalytic processes for CO2 reductive coupling are so far confined to amine-type co-reactants and rely mostly on precious metals such as ruthenium, though some less efficient systems with first-row transition metals or organocatalysts have also been reported recently. Therefore, catalytic systems which functionalise CO2 reductively and sustainably, i.e. driven by renewable energy, mediated by earth-abundant catalysts and without using chemical reductants, will be highly desirable.

 

It is proposed in this project to study the electro- and photocatalytic CO2 reductive coupling with a series of nucleophilic organic substrates bearing varied (C, N and S) donor atoms by using molecular first-row transition metal catalysts, which contain tetradentate polypyridine-based ligands and their functionalised derivatives, i.e. 2,2:6,2:6,2-quaterpyridine (qpy), 6,6-bis(oxazolinyl or benzimidazolyl or imidazolin-2-ylidenyl)-2,2-bipyridine. Our preliminary studies demonstrate that [Ni(qpy)]2+ is active for mediating the coupling of CO2 with secondary amine and the subsequent electro- and photocatalytic reduction to produce the corresponding formamide. The reaction of CO2 with various nucleophilic co-reactants, e.g. active-CH2 or CH compounds, amines and thiols, will be similarly studied using an electro- and photochemical approach. Products and the reaction pathway will be characterised using such techniques as chromatography, mass spectrometry and spectroscopy. By using cyclic voltammetry and controlled-potential electrolysis, intrinsic catalytic properties, e.g. rate constant (kcat), turnover frequency-overpotential relationship (TOF vs η), and their variation with metal and ligand functionality will be evaluated. Relevant photocatalytic reactions will also be studied using these catalysts with common photosensitisers and sacrificial donors. The product nature, reaction order, kcat and TOF for the photocatalysis will be examined. The photoreaction will also be characterised using spectrophotometric methods, e.g. Stern-Volmer plot and transient absorption. The above investigations will also be complemented with Density Functional Theory (DFT) calculation to reveal the reaction pathway and key intermediates in both the electro- and photochemical processes. With these efforts, our proposed studies shall contribute to advancing CO2 reductive functionalisation as a means for exploiting CO2, a green-house gas, as a renewable material feedstock.