On the generation and modulation of electron spin density in open-shell phosphorus compounds

Abstract

This Ph.D. thesis consists of a systematic study on the accessibility of open shell phosphorus compounds having a P–O bond. For this two concepts for the stabilisation of such radicals were probed. Firstly, metal complexes of these radicals were investigated including synthesis of precursors of tungsten and iron complexes and thermal stability studies (I). Secondly, a new approach using N-heterocyclic carbenes (NHCs) instead of coordinated metals was tested, following the isolobal concept (II). Oxophosphonium NHC adducts C were synthesized and studied as precursors for metal free stable radicals. Experimental results were supported by theoretical calculations applying density functional theory (DFT). These calculations provided insights into formation and allowed for a comparison of spin density distributions of such radicals. <br /> In part I the focus was on compounds with cyclic diheteroatom substituted phosphorus centres which led to the isolation of new aminoxyl phosphane tungsten complexes thus expending this rare class of these compounds. This approach allowed also for the synthesis of the first iron tetracarbonyl complex derivatives. Notably, the chemistry of the latter appeared to be quite different from reactivity reported for tungsten derivatives and therefore , was investigated thoroughly. The fate of a potential (stable) radical derived from this complex was analysed by various techniques, e.g., NMR, EPR, IR and 57Fe Mößbauer spectroscopy together with cyclic voltammetry (CV). To enable a broader perspective tungsten, iron and manganese complexes were investigated by DFT calculations, thus exploring the formation of Gibbs free energies, bond dissociation enthalpies (BDGs) and differences in spin density distribution s . Comparison to experimental observations allowed prediction of suitable candidates for stable metal complex precursors and, hence, were targeted experimentally. A purely DFT based study was performed (in collaboration with Frontera) on the effects of co-ligands , focusing on changes in the spin density distribution for tungsten, iron and nickel complexes. A strong(er) impact on the spin density distribution was found in a DFT study (in collaboration with Hui and Qu) analysing the effect upon substitution of P–O to P–E (E = S, NMe, PMe) in tungsten and iron complexes. <br /> In part II metal fragments were formally substituted by NHCs and pyridine. The synthesis of such adducts with sterically demanding (IMes) and less demanding (IMe2) NHCs is presented. The accessibility of sterically demanding NHC adducts is discussed in depth having the background that such NHC adducts have previously been only obtained (mostly) by chance. Their potential and that of pyridine adducts as suitable precursors for open shell species was probed, and CV and DFT calculations were performed to corroborate the experimentally observed data. Based on promising CV results, the chemistry of a suitable oxophosphonium NHC adduct was explored in solution.