Application of Iron(II) β-Diketiminate Catalysis in Silicon-Heteroatom Polymerisations and Depolymerisations

Abstract

The application of iron catalysis has garnered attention as an alternative to precious metal catalysis in synthetic transformations. Chapter 1 details the emergence of iron(II) β-diketiminate catalysts and introduces silicon-based polymers. The foundation of this research is based on the discussion of why a cyclic approach to the synthesis and chemical degradation of silicon-based polymers, mediated by iron catalysis would be attractive.

Investigations begin in Chapter 2, with the application of an iron β-diketiminate catalyst in the synthesis of fourteen different Si–O-based polymers and oligomers. This is achieved via an optimised dehydrocoupling procedure between silanes and diols, mediated by an iron β-diketiminate complex. The polymers are subjected to thorough characterisation by structural and thermal analytical methods. Studies are also extended to exploit the release of H₂ in these reactions as a driving force to push polymerisations further.

Prior to depolymerisation studies, desilylations of silazane and silyl ether monomers are studied with two hydroborane reductants, to glean insight into the mechanistic transformations and capabilities of iron mediated Si–E (E = O or N) bond breaking. Therefore, Chapter 3 begins by detailing an intensive study of the Si–N bond breaking mechanism and highlights the importance of hydroborane selection in these reactions. A scope of monomers bearing Si–O motifs is then selected to demonstrate the efficacy of iron β-diketiminate catalysis in Si–O bond breaking as a preparatory study for depolymerisation by desilylation.

In Chapter 4, a selection of commercially available and synthetically obtained Si–E polymers (including examples synthesised in Chapter 2) are subjected to depolymerisation reactions under similar conditions to the monomer desilylations discussed in Chapter 3. This chapter highlighted the recyclability potential for Si–E based polymers where polymerisation and depolymerisation may be achieved by iron catalysis. Additionally, some shortcomings of this depolymerisation procedure were found, where not all substrates tested were susceptible to reactivity.

Finally, a large portion of the studies discussed in this report relied on NMR spectroscopy for in situ reaction monitoring of solutions containing paramagnetic iron(II). Solutions containing paramagnetic species are notorious for their broad and highly upfield or downfield shifted signals, causing difficulties during NMR spectroscopy studies. In Chapter 5, a bold effort is made to investigate the possibility of in situ reaction monitoring via observation of extremely paramagnetically shifted signals (referred to as “superparamagnetic”) of ¹H and ¹¹B nuclei directly bound, or in close proximity to the paramagnetic iron(II) centre. Major challenges come from the novelty of studying the paramagnetic regions and difficulty with assigning newly detected signals.

Date of Award	13 Nov 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	Michael Whittlesey (Supervisor), Matthew Jones (Supervisor), Ruth L. Webster (Supervisor) & Sara H. Kyne (Supervisor)