Iron and nickel complexes as catalysts in CO₂/Epoxide coupling, radical polymerisation and hydrocyanation reactions
Item statusRestricted Access
Embargo end date01/07/2020
Transition metal complexes are essential for a wide range of homogeneously catalysed processes. Along with the constant optimisation of ligand design, the utilisation of cheap and easily available metals has become increasingly important. This work explores the synthesis of novel complexes comprising some of the most abundant first-row transition metals, as well as their application as catalysts in small molecule activation and polymerisation reactions. The application of novel/underexplored bidentate ligands was investigated the in Ni-catalysed hydrocyanation of alkenes. A tetraphenol-based diphosphite ligand, TP2, and the corresponding Ni⁰ complex were synthesised and characterised via X-ray crystallography. Contrary to previously reported patents, the complex was shown to isomerise 2-methyl-3-butenenitrile. Another diphosphite ligand, Biphephos, was employed in the hydrocyanation of styrene as a benchmark substrate. This allowed the optimisation of important reaction parameters such as the pre-formation of the Ni complex or the continuous addition of the HCN source. Biphephos was also studied in the hydrocyanations of butadiene and 3-pentenenitrile, but most importantly, it showed remarkably high activity in the isomerisation of 2-methyl-3-butenenitrile. A series of eight half salen ligands and the corresponding novel Feᴵᴵᴵ chloride complexes were synthesised. Characterisation via X-ray crystallography revealed monometallic structures with trigonal bipyramidal geometries around the metal centre. The air- and moisture-stable complexes were applied as catalysts in the coupling of CO₂ and epoxides which led to the exclusive formation of cyclic carbonates. Remarkably high activities were achieved, moreover, some structure-activity relationships were uncovered. Notably, the catalysts were extremely robust and showed efficiency in the presence of oxygen, water and a large loading of unpurified substrate. The substrate scope of the reaction was extended, including the sterically challenging internal epoxide, cyclohexene oxide. The activity of the half salen Feᴵᴵᴵ complexes was further explored investigating polymerisation reactions. The complexes were employed as mediators in the reverse atom transfer radical polymerisation of styrene and methyl methacrylate. Using AIBN or V-70 initiators, the complexes exerted moderate to good control over the molecular weights and dispersities of the obtained polymers. Some structure-reactivity relationships were discovered, along with the presence of competing mechanisms. The complexes were also applied in the ring opening polymerisation of raclactide using propylene oxide to generate the active Fe-alkoxide species in situ. Importantly, high conversions could be achieved with the addition of a low ratio of epoxides. However, GPC and MALDIToF mass spectrometry analyses of the obtained polymers revealed poor control over the molecular weights and dispersities, indicating a strong presence of transesterification side-reactions.
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