Spin crossover in chalcogen-containing heterocyclic metal–organic frameworks

Abstract

The work presented in this thesis primarily leverages a uniquely tuneable chalcogenophene core to create a series of varying angled bis(4-pyridyl) ligands.

Chapter 3 describes the structural characterisation of a 3D SCO framework of the form [Fe(bpop)(Au(CN)2)2] (bpop = 2,5-bis(4-pyridyl)furan). While displaying the same coordination sphere typical of Hofmann-like frameworks, the small bite angle of the bpop ligand results in a frustrated, weakly cooperative framework with antagonistic host-guest interactions, leading to a trapped intermediate spin state upon cooling from a high spin (HS) phase. This was found to result from the constrictive nature of the pore environment and size constraints of guest species that prevent the structural deformation concurrent with SCO.

Chapter 4 details a further extension of the chalcogenophene series through incorporation of S, Se, and Te analogues of the bpop ligand. These systems, of the form [Fe(bpxp)(Au(CN)2)2] (bpxp = 2,5-bis(4-pyridyl)chalcogenophene), all display an expected Hofmann-like topology with varying degrees of SCO and structural deformation caused by differing ligand bite angles.

Chapter 5 details the utilisation of neutron diffraction measurements to probe the complex spin state ordering previously shown in the SCO MOF [Fe(dps)2(Ag(CN)2](ClO4). New theoretical calculations suggest the existence of a new phase of matter, spin state ice (SSI), may be present in the Kagome-like topology of the dps framework. Possible evidence of the signature pinch points in the diffuse neutron scattering which would provide experimental proof of SSI was observed. However, definitive determination of an SSI phase was ambiguous.

Chapter 6 details the partial structural solution of a highly disordered SCO framework of the general form [Fe(dpa)2(M(CN)2] which has a cristobalite-like topology. Both a large amount of diffuse scattering and apparent pseudo-symmetry was observed, resulting in only a partial structural solution obtained.