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Combining spectroelectrochemistry and theory in photo-assisted electrocatalytic carbon dioxide reduction by Group-6 and -7 metal carbonyl complexes

Skelson, D. J. (2025) Combining spectroelectrochemistry and theory in photo-assisted electrocatalytic carbon dioxide reduction by Group-6 and -7 metal carbonyl complexes. PhD thesis, University of Reading

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To link to this item DOI: 10.48683/1926.00123591

Abstract/Summary

Anthropogenic emissions of CO2 contributing to global warming and their detrimental effects are of concern for the future. This, in combination with depleted fossil fuel resources have led to development of recycling and closing the carbon cycle, to provide a sustainable source of chemical feedstocks and fuels. Transition metal complexes have revealed their potential to catalytically convert CO2 by electrochemical techniques to desirable products, including CO and formate. Significant research into the application, catalytic mechanisms and activities, followed by subsequent optimisation have resulted in a vast collection of viable electrocatalysts. Diversifying the metal centre and modification of the ligand assembly from the extensive library available, has shown to exhibit high activities and tuneable selectivities, where the cooperation between the metal(s) and ligands is necessary to efficiently reduce CO2. During the initial development of this field, expensive scarce transition metals such as Re, Ru, Os, Rh and Ir were relied on, however in recent years, focus has shifted to cheaper Earth-abundant transitional metals including Mn, Fe, Ni and Cu. In the wider community the Group-6 triad (Cr, Mo, W) has received little attention, leading to one of the two main aims of this Thesis; the investigation of a series of novel complexes and their redox chemistry. Chapter 1 provides an overview of the field and the challenges faced, as well as the significant landmarks of promising catalysts explored. Chapter 2, provides an introduction to the experimental techniques employed in this work. The novel research, introduced in Chapter 3, reports on cyclic voltammetry (CV) and spectroelectrochemistry (SEC) of a series of complexes; [M(CO)4(6,6’- dmbpy)] (M = Cr, W; 6,6’-dimethyl-2,2’-bipyridine) and [M(CO)4(tBu-DAB)] (M = Cr, Mo, W; tBu-DAB = 1,4-di-tert-butyl-1,4-diazabuta-1,3-diene). This provided an insight into the contribution of the metal centre upon the redox pathways and bonding properties. Favouring a low-energy pathway probed by changes of the electrode and solvent, unlocked generation of the active catalyst at less negative overpotentials. The combination of a Au cathodic surface and NMP (N-methyl-2- pyrrolidone) solvent, exhibit synergy by facilitating this lower cathodic route. The synergy between electrochemistry and photochemistry termed the photo-assisted (PA) technique unlocks the catalyst close to the first electrochemical reduction, drastically lowering the energy cost required. In the literature, [Mo(CO)4(6,6’-dmbpy)] was among the first complexes studied by PA; however, this study applies this method on an entire series of fresh complexes where the metal centre and ligand variation were examined. Chapter 4 reveals a pioneering marriage between the established redox-active and photoreactive Group-7 carbonyl complexes and Room-Temperature Ionic Liquids (RT-ILs), investigated by both CV and SEC methods. In recent years, the RT-ILs have earned significant interest due to their promising catalytic abilities while offering a green alternative to traditional volatile organic solvents. By acting as both a solvent and electrolyte, they can act to lower the overpotential to generate the active species, as well as heavily influence the cathodic pathway. The cationic component of [BMIM][OTf] (1-butyl-3-methylimidazolium trifluoromethanesulphonate), fragments can associate with the parent [M(CO)3(α-diimine)X] (M = Mn, Re) converting to the cationic [M(CO)3(α-diimine)(1-mIm)]+ , lowering the initial reduction energy cost. This dramatic effect inspired the inclusion of 1-mIm (1-methylimidazole) as a simple additive to standard THF measurements, providing a median viewpoint between pure organic and RT-IL conditions. In the presence of CO2, the RT-IL facilitates CO2 reduction by the complex at low overpotentials, but can also independently reduce CO2 at the necessary potential. In the future, this important approach of combining RT-ILs and electrocatalysts may aid upcoming investigations. Chapters 5 and 6 describe the electrochemical and photochemical properties, respectively, of a representative series of seven [MoII(η3-allyl)(CO)2(NCS)(P∩P)] (P∩P = 1,2-bis(di-R-phosphino)-R’) complexes. These ligands include incremental changes to the backbone forming 4- to 7-membered metallocycles as well as variation of the diphosphino substituents introducing both electron-donating and -withdrawing effects. These compounds were synthesised and characterised, and their cathodic paths explored by CV and SEC, supported by DFT calculations. The simplified reduction pathway generates the supposed active 2-electron reduced 5-coordinate [Mo(η3-allyl)(CO)2(P∩P)]– at the first reduction wave, via dissociation of NCS– via an ECE mechanism. Interestingly and rather unusually, these complexes (differently from their α-diimine analogues) exhibit no catalytic activity towards CO2 reduction. Instead, they favour the formation of a dinuclear CO2-bridged adduct. UV photoirradiation of the parent complex triggers photoisomerisation to a remarkable trans(CO)-[Mo(η3-allyl)(CO)2(NCS)(P∩P)] species that in the absence of excitation, undergoes thermal isomerisation to regenerate the parent. This new phosphine series serves as an expansion to the collection of studied Group-6 electrocatalysts of this structure type, from the commonly employed α-diimine ligands, and although they have preliminarily been found catalytically inactive, they contribute interesting redox and photochemical behaviour to this field.

Item Type:Thesis (PhD)
Thesis Supervisor:Hartl, F.
Thesis/Report Department:Department of Chemistry
Identification Number/DOI:10.48683/1926.00123591
Divisions:Life Sciences > School of Chemistry, Food and Pharmacy > Department of Chemistry
ID Code:123591

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