Abstract
The replacement of palladium in cross-coupling chemistry with iron has been a long-term goal in the realm of sustainable chemistry, with new methodologies opening the way for new mechanistic investigations which in turn inform the development of new, more effective, methodologies. These investigations have shown a wide range in accessible iron species, oxidation states, and ligand roles in this chemistry dependent on a wide variety of factors in the reaction.Chapter 2 reports a study of the reactivity of the homoleptic organoiron complexes Fe₂Mes₄ and [FeMes₃]⁻ with the main group salts ZnBr₂ and MgBr₂ as well as the reactivity with the boronic esters ⁿBuBPin and ᵗBuBPin, species all formed as side products in different cross-coupling reactions. The use of ¹H NMR spectroscopy enabled the identification of a heteroleptic organoiron-bromide species [FeBrMes₂]⁻ as being formed upon reaction of these complexes with the Mg, Zn or Fe bromide salts. This was then isolated and characterised by X-ray diffraction. The capacity of iron-mesityl complexes to transmetallate reversibly to these main group salts was also uncovered.
Chapter 3 details a mechanistic investigation of an iron-catalysed Kumada cross-coupling between an aryl Grignard reagent and an alkyl halide. Through the profiling of stoichiometric reactions of a series of iron-mesityl complexes with OctBr, their respective rates of reaction were determined, and a radical, Feᴵᴵ/Feᴵᴵᴵ/Feᴵⱽ-based catalytic cycle has been proposed.
In Chapter 4, another aspect of iron catalysis is explored, namely the iron-catalysed reduction of N₂ to NH₃. Taking inspiration from previously reported synthetic iron-based N₂ reduction catalysts, a series of iron complexes bearing ferrocenyl-diphosphine ligands were synthesised as potential pre-catalysts for N₂ reduction. However, this reactivity was prevented by the reduction of the ferrocenyl ligand occurring before N₂ binding occurs. As an alternative, a series of octanuclear, [Fe₈S₄] clusters supported by pyrazolate ligands were synthesised, taking a cue from the use of nitrogenase enzymes which bear a carbide core embedded in an iron-sulphur cluster. These clusters were characterised by ¹H NMR, before being reacted with aryl and acetylyl organolithium reagents, to give organometallic products.
| Date of Award | 21 Jun 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Robin B Bedford (Supervisor) & Paul J Wyatt (Supervisor) |