Design and Synthesis of New, More Structured, Trigonal Host Molecules

This thesis presents first a personal overview of the study of clathrate phenomena, over the past two decades. This is followed by a description of attempt to design, using logical principles, new trigonal host molecules. The first line of approach was to employ as suitable building blocks para-substituted phenols sharing some common structural features with Dianin's compound (p-hydroxyphenyl-2,2,4,-trimethylchroman): using salts of such phenols six-fold nucleophilic substitution of hexaflurobenzene was directed toward the production of new, more highly structured "hexa-host" molecules. A striking example of this approach is the tight inclusion of phosgene, scavenged from a 12.5% solution in toluene, within the closed clathrate cavities of hexakis(p. -phenoxyphenoxy)benzene. An X-ray crystal structure analysis of this novel host lattice is presented. X-ray analysis of the iodomethane adduct of the same host reveals the iodine atom of the statistically disordered guest, close to the centre of the cavity. An isomorphous clathrate structure has also been defined for the CH3CCl3 adduct of hexakis (p-benzylphenoxy )benzene by X-ray methods. The similarity in the clathrate structures of the two hosts is highlighted along with their ability to include highly volatile and toxic guest molecules. In addition, novel use of solid-state CPMAS 13C NMR spectra is described; this provides evidence, independent of X-ray method, for the site-symmetry of hexak is (p-benzylphenoxy) benzene in its clathrates and its unsolvated forms. Parallel CPMAS studies are also reported for hexakis(p-cumylphenoxy)benzene and hexak is(p-cumylphenoxymethyl)benzene. A second strategy, termed the Piedfort concept (the name deriving from special coins struck at double thickness) is introduced for the first time. Two molecules of 2,4,6-tris (p-cumylphenoxy)-1,3,5-triazine are shown to undergo novel self-assembly to produce a composite unit (Piedfort unit), which acts as a single hexa-host molecule. The structure of this new unit, found both in the 1,4-dioxane inclusion compound of the above triazine and in its unsolvated crystal, has been elucidated by X-ray methods. The more highly structured 2,4,6-tris [p-(1-naphthyl) phenoxy]-1,3,5-triazine has also been synthesized; in this case the Piedfort units have almost certainly attained exact trigonal symmetry in the now rhombohedral adducts