Structures and magnetic properties of iron(III) spin-crossover compounds

Abstract

Transition metal fragments displaying switching behaviour are appealing materials, which may be used in a functional way in research and technology. Some molecular species containing transitionmetal ions may exhibit a crossover between states having a different magnetic moment, the magnetic interconversion between the low-spin and high-spin state in FeIII systems which can be triggered by a change in temperature, pressure or by light irradiation.The research presented in this thesis focuses on using substituted derivatives of R-salicylaldehyde 4R’-thiosemicarbazone (H2L, H2-R-thsa-R’) for generating FeIII spin crossover. The aim was to design mononuclear FeIII compounds with a view of studying their structural features and magnetic properties. The results present the full structural analysis of differently charged FeIII-bis(ligand) complexes, these include: (i) (cation+)[FeIII(L2‒)2]·x(solvent), (ii) [FeIII(HL‒)(L2‒)]·x(solvent) and(iii) [FeIII(HL‒)2](anion‒)·x(solvent). The studies discuss several influences on the structural features and the magnetic properties of the reported FeIII compounds, these include: the nature of the anionor cation associated with the FeIII complex, the degree of solvation of the complex and the variations in the ligand substituents.The magnetic studies of the (cation+)[FeIII(L2‒)2]·x(solvent) compounds have presented one high-spin compound ((CH3)2NH2[Fe(3-OEt-thsa)2]) and four low-spin compounds (Cs[Fe(3-OEt-thsa-Me)2]·CH3OH, Cs[Fe(5-Br-thsa)2], NH4[Fe(thsa)2] and NH4[Fe(5-Br-thsa)2]), of which NH4[Fe(5-Br-thsa)2] revealed a fraction of the FeIII ions convert into the high-spin state. Three [FeIII(HL‒)(L2‒)]·x(solvent) compounds have been studied, of which a rare two-step spin transition has been observed for [Fe(H-5-Cl-thsa-Me)(5-Cl-thsa-Me)]·H2O, while the isostructural compounds [Fe(Hthsa-Me)(thsa-Me)]·H2O undergoes an incomplete spin transition and [Fe(H-3-OEt-thsa-Me)(3-OEt-thsa-Me)]·H2O remains in the high-spin state. It was recognised that the steric and electronic features imposed by the R,R’-substituents may have an impact on the spin state of FeIII cations, and affect at which temperature the spin transition occurs. Furthermore, the FeIII cation of both [FeIII(HL‒)2](anion‒)·x(solvent) compounds, [Fe(H-5-Br-thsa-Et)2](NO3)·H2O and [Fe(H-4-OHthsa)2]4·(SO4)2·9H2O, were found to be in the high-spin state. Collectively, the described research has provided new insight into this family of FeIII bis(Rsalicylaldehyde4R’-thiosemicarbazone) compounds and provides a strong foundation for further studies.