Steric and electronic investigations into Samarium (II) dinitrogen, heteroalkyne and heterocycle reductions

This thesis describes the synthesis, characterisation and reactivity of a range of samarium(II) and samarium(III) complexes of a modified porphyrinogen, trans-N,Nʹ-dimethyl-meso-octaethylporphyrinogen, `Et_8N_4Me_2H_2`. Various related reactivity themes include the sterically rigid porphyrinogen and the `Sm^(II)``/``Sm^(III)` redox couple. Chapter 1 provides a broad introduction and relevant background on the properties of samarium with respect to the other lanthanoids. General aspects of the organometallic chemistry of the lanthanoid elements are discussed, including: sterically induced reduction, relevant and related chemistry of the modified porphyrinogen, and general considerations of computational techniques employed in this thesis. Chapter 2 outlines the synthesis of a Lewis base free (unsolvated) samarium(II) porphyrinogen complex, [`{(Et_8N_4Me_2)Sm}_n`], and its subsequent unprecedented reactivity with dinitrogen to provide the first example of end-on binding of dinitrogen to a lanthanoid element, [`{(Et_8N_4Me_2)Sm}_2(μ-η^1:η^1-N_2)`]. The unique binding mode is rationalised by steric and electronic arguments and subsequently probed by theoretical investigations which provide a richer understanding of the degree of activation of the dinitrogen moiety. A number of other `Sm^(II)` complexes are also reported. The reactions of the samarium(II) complex, [`(Et_8N_4Me_2)Sm(THF)_2`] and simple N-heteroalkynes are described in Chapter 3. At room temperature, simple 2:1 and 1:1 adducts of nitriles are observed, while isonitriles and nitriles at higher temperature are reduced by the samarium(II) complex resulting initially in C−N and C−C bond cleavage, respectively, with catalytic oligomerisation of nitriles and t- Butyl radical trapping chemistry observed as subsequent steps. A systematic investigation of the interaction between substituted pyridines, benzannulated pyridines, 2,2ʹ-bipyridine and substituted 1,10-phenanthrolines by X-ray crystallography is detailed in Chapter 4. Reaction outcomes including mono and bis- adduct formation, single electron ligand reduction, and reductive dimerisations are reported. Computational techniques are employed to analyse unpaired electron populations and thus determine oxidation states and rationalise reactivity observations regarding ligand reduction versus reductive dimerisation across this series. Concluding remarks and comments regarding further work are made in Chapter 5; all references are catalogued in Chapter 6. General experimental information is provided in Appendix A. A rigorous trigonometric definition of the molecular geometry is provided in Appendix B. The X-ray crystal structure data, by way of crystallographic information file (.CIF), is provided electronically in Appendix C. Computational output files (.OUT) are provided electronically in Appendix D