Synthesis and structural exploration of 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole based ligands and their transition metal complexes

Abstract

This thesis focuses on the synthesis and modification of substituted 1,2,4-triazole systems, along with structural investigations of their complexation to metal ion species. To begin, a high-pressure study was conducted on polymorph D of the literature system [Fe(abpt)2(NCS)2] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole). Crystals of this form display a phenomenon known as spin crossover, in which the spin state of the metal ions can switch upon exposure to external stimuli. In the case of polymorph D, where two crystallographically independent metal centres exist in the high spin state under ambient conditions, thermal switching of only one of the metal centres has previously been shown. Nevertheless, this work expands upon prior publications and demonstrates that, using high pressure as the external stimulus, it is possible to access the phase space in which both metal centres are switched to the low spin state. To expand on the exploration of this interesting phenomenon and using the knowledge of the accessibility of variable spin states for ‘abpt’ based complexes, thirteen new substituted 1,2,4-triazoles were prepared. The preparation of these novel systems was approached using a synthetic route with a common backbone, requiring only minor modifications, to produce the desired product. This work was conducted alongside the preparation of eight known literature compounds. The resultant 1,2,4-triazoles were coordinated to various metal salts and crystallised to enable interrogation of their solid-state structures. The analyses of any resultant crystals were performed through the use of single crystal X-ray diffraction, allowing detailed investigation of even subtle structural properties, particularly those that are known to be connected to the spin crossover phenomenon. Five new organometallic systems were discovered which demonstrated interesting structural properties, including spin crossover, solvent mediated spin crossover and unexpected migration characteristics. Additionally, many other metal complexes produced during this research were structurally characterised with these systems exposed to the same level of structural interrogation. This has built a body of work leading to a greater understanding of the driving forces involved in the generation of exotic materials that have useful characteristics.