Structural and Spectroscopic Studies into Spin Crossover Complexes and Anils

Abstract

This posthumous thesis combines two literature reviews written by Helen Mason and eight papers arising from data collected by Helen during her PhD studies. Helen also synthesised most of the compounds she studied and which are reported herein. The thesis uses single crystal X-ray diffraction data combined with spectroscopic data to study spin crossover complexes and anils, Schiff bases of salicylaldehyde derivatives with aniline derivatives. The thesis is divided into two main sections.

The first section examines spin crossover complexes initially through a summary of the reported literature (Chapter 1). The remainder of the section focuses on experimental studies into complexes containing (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) ligands. Chapters 2 and 3 examine the spin crossover behaviour of Fe(abpt)2(NCS)2 polymorphs A, B and D. The spin crossover behaviour of the three polymorphs is significantly different, which is particularly interesting given that their structures are very similar. Polymorph A has a thermal spin crossover, displays LIESST (Light Induced Excited Spin-State Trapping) and undergoes pressure induced spin crossover. Polymorph B does not undergo a thermal spin crossover at ambient pressure, however under pressure it undergoes a phase transition followed by a spin crossover. Polymorph D contains two unique Fe(II) centres in the asymmetric unit one of which undergoes a thermal spin crossover and displayed LIESST, upon the application of pressure a stepped spin crossover was observed at both unique Fe(II) centres. Structural and spectroscopic data are reported. The differences in structure and intermolecular interactions are examined and set in the context of similar structures in the literature. The thermal spin crossover and LIESST behaviour of Fe(abpt)2(NCSe)2 polymorph A, which is isostructural with Fe(abpt)2(NCS)2 polymorph A, is also reported and examined both structurally and spectroscopically (Chapter 4). The concluding chapter in this section, Chapter 5, reports the structures of two polymorphs Co(abpt)2(NCS)2 and Ni(abpt)2(NCS)2 polymorph A and B, which are isostructural with the corresponding Fe(abpt)2(NCS)2 polymorphs A or B respectively.

The second section begins with a literature review of selected photochromic systems which have been structurally characterised, Chapter 6. The rest of the section reports experimental studies into some anil compounds. Twenty seven N-(methoxysalicylidene)-haloaniline (halo = F, Cl or Br) derivatives were synthesised and their crystal structures determined, Chapter 7. New polymorphs were identified for two of the compounds, 4-(methoxysalicylidene)-4-fluroaniline and 3-(methoxysalicylidene)-4-chloroaniline and a temperature induced phase transition for 5-(methoxysalicylidene)-4-bromoaniline. Visual observations of their thermochromism alongside diffuse reflectance spectra are reported and linked to the observed structure, with those showing smaller interplanar angles between the ring generally tending to be more strongly thermochromic. Neutron diffraction data are also reported for two of the strongly thermochromic compounds, 5-(methoxysalicylidene)-2-chloroaniline and 5-(methoxysalicylidene)-2-bromoaniline. In Chapter 8, the structures of three di-tertiary butyl substituted anils, E)-2,4-di-tert-butyl-6-(((4-fluorophenyl)imino)methyl)phenol, (E)-2,4-di-tert-butyl-6-(((4-chlorophenyl)imino)methyl)phenol, and (E)-2,4-di-tert-butyl-6-(((4-bromophenyl)imino)methyl)phenol, are reported. Two polymorphs were identified for (E)-2,4-di-tert-butyl-6-(((4-fluorophenyl)imino)methyl)phenol, while the other two structures underwent a thermally induced phase transition. The thermochromic and photochromic behaviour of the compounds were examined using diffuse reflectance and Raman spectroscopy. In Chapter 9, the synthesis and structures of three isoxazole containing Schiff bases, (E)-2-((isoxazol-3-ylimino)methyl)phenol, (E)-2-(((5-methylisoxazol-3-yl)imino)methyl)phenol and (E)-2,4-di-tert-butyl-6-((isoxazol-3-ylimino)methyl)phenol, are reported. None of these were found to be strongly thermochromic but all three were found to be photochromic. Finally, Chapter 10 reports the structure of a new polymorph of (E)-4-bromo-2-((phenylimino)¬methyl)¬phenol. The colours of the two polymorphs at room temperature were found to be different, and both polymorphs were found to display thermochromism.