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Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: This study aimed to combine the principles of crystal engineering with the properties of cyclotriphosphazene derivatives to construct supramolecular assemblies in the solid state. The ease with which the chloro substituents on cyclotriphosphazenes can be replaced makes them ideal candidates for this study. The substituents were chosen for their ability to form either hydrogen bonding interactions or halogen bonding interactions in the solid state. The cyclotriphosphazene derivatives were co-crystallised with various small organic molecules with complementary functional groups, as well as with other cyclophosphazene derivatives. The aim was to form co-crystals or solvates with these cyclophosphazene derivatives as co-crystals contain a wealth of information regarding the forces governing the aggregation of molecules in the solid state. Cyclotriphosphazenes, with their array of substituents, could broaden the range of potential interactions governing crystalline assembly. Fifteen cyclotriphosphazene derivatives were synthesised and characterised in this study. The novel crystal structures of two cyclotriphosphazene derivatives have been elucidated by single crystal X-ray diffraction. These are 2,2-bis(4-formylphenoxy)-4,4,6,6-bis[spiro(2',2"-dioxy-1',1"-biphenylyl)]cyclotriphosphazene and hexakis(4-cyano-phenoxy)cyclotriphosphazene. In the course of this study two novel polymorphs of hexakis(4-fluorophenoxy)cyclotri-phosphazene were identified and studied. The novel triclinic form undergoes an irreversible transformation to the previously reported monoclinic phase at high temperatures. The reported monoclinic phase, however, transforms to a monoclinic C phase in a single-crystal to single-crystal fashion. It is also suspected that this phase transformation is in fact reversible on cooling of the crystal to temperatures below -45 °C. One novel co-crystal structure of hexakis(4-pyridyloxy)cyclotriphosphazene with terephthalic acid was identified and characterised. However, analysis of the Cambridge Structural Database indicates that co-crystal formation with cyclophosphazenes is not a commonly occurring phenomenon. This leads to the conclusion that cyclotriphosphazenes can be used in crystal engineering as supramolecular building blocks, but their shape and size tend to inhibit the formation of co-crystals. Therefore co-crystal formers have to be chosen with great care. / AFRIKAANSE OPSOMMING: Die doel van hierdie studie was om die beginsels van kristalingenieurswese te kombineer met die eienskappe van siklotrifosfaseen afgeleides om sodoende supramolekulêre versamelings in die vastetoestand te bou. Die gemak waarmee die chloor substituente op die siklotrifosfaseenring vervang kan word, maak hierdie molekules ideaal vir hierdie studie. Die substituente is gekies op grond van hul potensiaal om waterstofbindings of intermolekulêre halogeenbindings in die vastetoestand te vorm. Ko-kristallisasie eksperimente is met die siklotrifosfaseen afgeleides en verskeie klein organiese molekules met komplementêre funksionele groepe uitgevoer, asook tussen die verskeie siklotrifosfaseen afgeleides met mekaar. Die doel was om mede-kristalle of solvate met hierdie siklotrifosfaseen afgeleides te vorm aangesien mede-kristalle ‘n magdom inligting bevat rakende die kragte wat die versameling van molekules in die vaste fase beheer. Die siklotrifosfaseen afgeleides wat ‘n wye verskeidenheid substituente kan dra, kan hierdeur die moontlike intermolekulêre interaksies wat die versameling in die kristallyne vaste fase beheer verbreed. In hierdie studie is vyftien siklotrifosfaseen afgeleides gesintetiseer en gekarakteriseer. Die voorheen onbekende kristalstrukture van twee siklotrifosfaseen afgeleides is in hierdie studie geïdentifiseer, naamlik 2,2-bis(4-formielfenoksie)-4,4,6,6-bis[spiro(2',2"-dioksie-1',1"-bifeniliel)]siklotrifosfaseen en heksa(4-sianofenoksie)siklotrifosfaseen. Die strukture is bepaal deur enkelkristal X-straaldiffraksie. In die loop van hierdie studie is twee voorheen onbekende polimorfs van heksa(4-fluorofenoksie)siklotrifosfaseen geïdentifiseer en bestudeer. Die nuwe trikliniese vorm ondergaan ‘n onomkeerbare faseverandering na die monokliniese vorm by hoë temperature. Die bekende monokliniese P fase ondergaan egter ‘n verdere faseverandering na ‘n monokliniese C fase. Hierdie geskied as ‘n enkel-kristal na ‘n enkel-kristal faseverandering. Daar word ook gespekuleer dat hierdie spesifieke faseverandering wel omkeerbaar is indien die kristal na -45 °C afgekoel word. Een nuwe mede-kristal tussen heksa(4-pyridieloksie)sikotrifosfaseen en 1,3-dibensoësuur is in hierdie studie geïdentifiseer en gekarakteriseer. ‘n Analise van die Cambridge Strukturele Databasis het egter aangedui dat die vorming van mede-kristalle nie ‘n alledaagse verskynsel is in sikotrifosfaseen afgeleides nie. Dit lei tot die gevolgtrekking dat sikotrifosfaseen molekules wel in kristalingenieurswese gebruik kan word as supramolekulêre boustene, maar dat die vorm en grootte van die molekules die kristallisering van mede-kristalle verhoed. Dus moet die molekules wat saam met die siklotrifosfaseen molekules gekristalliseer wil word, goed deurdink word.