Inclusion Studies of Metal-Organic Hosts
Thesis (PhD)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: The primary objective of the work presented in this thesis was to prepare and investigate the properties of novel inclusion complexes of metal-organic host materials. Two types of host materials were studied, namely 0D metallocycles and 3D metal-organic frameworks (MOFs). The first section details the synthesis and characterisation of six ditopic imidazole derived ligands. These conformationally flexible ligands were reacted with a variety of transition metal salts in a systematic manner with a view to synthesising 0D metallocycles. The structures of 29 novel coordination complexes were elucidated by single-crystal X-ray diffraction and five of these were 0D metallocycles. The thermal stability of the five metallocyclic inclusion complexes was investigated and it was found that all five of the metallocycles have remarkably high thermal stability with respect to the loss of their included volatile solvents. Indeed, in some cases the solvent is only lost when the host decomposes. This is a relatively rare phenomenon, with only a few examples of such systems reported in the literature. A computational study was undertaken in an attempt to rationalise this high thermal stability in terms of guest-guest and host-guest interaction energies. Furthermore, although the single crystals do not survive the desolvation process, all five of these systems exhibit Class II porosity (i.e. the included solvent could readily be exchanged for a different solvent guest without significant disruption to the host packing arrangement). This exchange process was monitored as a single-crystal to single-crystal transformation. The final section reports the attempted gas-phase diffusion of two organic radicals into four porous crystalline materials. The radicals diffused into only one of the four possible host systems, thereby indicating a selective inclusion process. The structures of the two new inclusion complexes were elucidated from powder X-ray diffraction data and it was demonstrated that the properties of both the organic radical as well as the host framework are modified in the new material. Furthermore, the host framework acts as a “vessel” in which the organic radicals, which are normally highly reactive species, can be stored under ambient conditions within the channels of the host framework for a period of three months without degradation. The host framework can thus be used for controlled release of the reactive species by exposure to a suitable solvent.