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Title: Relationships between stability, structure, and energy density of nitrogen-rich Group 14 coordination compounds
Author: Campbell, Rory M.
ISNI:       0000 0004 6061 3490
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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Two established methods for the stabilisation of polyazido complexes are incorporation into salts with bulky cations, or the use of neutral Lewis base ancillary ligands. Both methods reduce the sensitivity of the compound by 'diluting' the nitrogen content and inhibiting dissociation into the sensitive binary azide. The existing synthetic principles for the preparation of group 14 azides, (PPN)2[E(N3)6] and E(N3)4(L2), (E = Si, Ge), have been adapted, and applied to the synthesis of the first charge-neutral adducts of tin tetraazide, Sn(N3)4(bpy) and Sn(N3)4(phen). The adducts Sn(N3)4(py)2 and Sn(N3)4(pic)2 have been prepared by a new synthetic route involving reaction of SnF4 with trimethylsilyl azide using the corresponding pyridine as solvent. A new method has been developed for the preparation of tin(II) azides from tin(II) fluoride and trimethylsilyl azide at ambient temperature. The adducts Sn(N3)2(py)2 and Sn(N3)2(pic)2 were obtained when the reaction was carried out in the corresponding pyridine solution, and guanidinium triazidostannate was obtained in the presence of guanidinium azide in acetonitrile. This triazidostannate salt has an extensively hydrogen bonded structure, and the simple pyridine adducts suggest the kinetic stabilisation afforded by bulky ligands employed previously is not mandatory for isolation of tin(II) azides. The nitrogen-rich salt guanidinium triazidostannate, and charge-neutral monodentate pyridinebased adducts of tin(II) azide have been fully characterised. A new method for synthesis of tin(II) azide from Sn(N3)2(py)2 enables a tenfold reduction of reaction time, and avoids the use of silver azide or the need for anhydrous ammonia as solvent, which posed additional hazards in the recently published redox-based synthesis of tin(II) azide. The material afforded by the new method was crystalline, whereas only amorphous Sn(N3)2 was obtained previously. This enabled investigation of the solid state structure of the highly sensitive explosive, tin diazide, by a combination of Rietveld refinement of powder XRD data and complementary DFT calculations. The tin centre is pentacoordinate with a 3D framework given by 1,1-bridging azide ligands between adjacent Sn(N3)2 units in the c-axis direction, and a longer 1,3-bridging in the b-axis direction. The preparation of an array of nitrogen-rich tin polyazides, and their thermal characterisation has shown that tin(IV) azides are significantly more stable than their analogous tin(II) complexes, and confirmed the correlation of energy content with nitrogen content. To understand whether hydrogen bonds can confer a similar level of stability upon polyazido complexes, the syntheses of some main group polyazido complexes with guanidinium counter ions were investigated. The lack of information available on the nitrogen-rich guanidinium azide and aminoguanidinium azide precursors prompted investigation of their syntheses, and the compounds were fully characterised by infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies, and thermal analyses (DSC and TGA), and their structures were determined by single crystal X-ray diffraction (XRD). New salt-like compounds, (G)2[Sn(N3)6] and (PPN)2[Sn(N3)6], G = guanidinium, PPN = bis(triphenylphosphine)iminium, have been prepared and fully characterised, enabling the comparison of the structure and properties of the hexaazidostannate anion in the presence and absence of hydrogen bonds. Preparation of other nitrogen-rich salts (AG)2[E(N3)6], AG = aminoguanidinium, E = Si, Sn, and (G)2[Si(N3)6], and (G)[P(N3)6] was attempted by extension of established procedures for the corresponding PPN salts. FTIR spectroscopic evidence for the formation of these nitrogen-rich polyazido complexes in solution are presented. The crystal structures of the side products guanidinium sodium azide, Na0.33(G)0.67N3, and diazido(guanidinyl)(oxido)phosphorus(V), [P(=O)(N3)2{NC(NH2)2}], were determined by single crystal X-ray diffraction. Guanidinium tetrazolate was synthesised for the first time from guanidinium carbonate and 1H-tetrazole, as it is a precursor to nitrogen-rich main group polytetrazolato complexes, and its crystal structure was determined by single crystal XRD. First and second level graph sets have been assigned to the complex 3D networks of hydrogen bonds in the structures of these nitrogen-rich salts. In an effort to go a step beyond intermolecular forces, the synthesis of 2,4,6-tris(tetrazol-1-yl)-1,3,5-triazine (TTT) was attempted, as it is a potential precursor to a novel 'polymeric' energetic compound by pressure-induced polymerisation.
Supervisor: Portius, Peter Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available