Martin, Craig Robert Leslie
Crystal engineering approaches to controlling the formation of molecular complexes and their polymorphs.
PhD thesis, University of Glasgow.
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This work aimed to investigate and exploit the hydrogen bonds generated between heterocyclic aromatic compounds, namely benzimidazole and imidazole, and the carboxylic acid group. The flexible but robust hydrogen bonds generated have been used to create molecular complexes, using practical and relevant co-molecules. A systematic approach has been used in the selection of co-molecules on the basis of crystal engineering principles. A library of robust hydrogen bonds and primary structural motifs has been generated, which has been used to explain the solid-state assembly of the collection of molecular complexes produced in this work and in related published structures. The similarities in hydrogen bond strength, bonding motifs and proton transfer behaviour between very dissimilar molecular complexes have been remarkable. The opposite is also true in other examples, with very similar molecular complexes showing remarkable differences, but overall, a picture is built up of predictable use of crystal engineering principles in designing molecular complexes with anticipated structural and packing features.
The phenomenon of polymorphism, widely known but poorly understood, is essential to many industrial processes. A primary aim of this work was to promote and control the formation of molecular complex polymorphs through varying crystallisation conditions. Co-crystallisations involving benzimidazole with the whole series of halo-benzoic acid molecules were scrutinised and polymorphism found to be prominent throughout. Selective growth for individual forms has been achieved, offering the potential for polymorph selection, but not fully understood.
The behaviour of the protons was investigated in the molecular complexes generated; proton transfer was prevalent. This was achieved through three methods; firstly with the use of variable temperature X-ray and neutron diffraction experiments on the product, by altering the levels of pH during the crystallisation process and lastly by introducing competing acceptor sites through co-molecule selection.
A feasibility study into the use of the relatively new solvent-free crystallisation processes was undertaken. It was shown to be a successful technique in screening for polymorphs and molecular complexes.
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