Structural studies of novel molecular systems

Abstract

This work investigates two main areas of interest. The first is the synthesis and study of a variety of Lewis acid-Lewis base adducts of general formula X(_3)B-PR(_3) (X = CI, Br; R= alkyl or aryl). The (^11)B-P and (^10)B-P NMR coupling phenomena are investigated. The partial dissociation of some of these compounds in solution is probed by (^31)P and (^11)B NMR. The crystal structures of many of these compounds are presented and analysed for correlations between the properties of the phosphines and B P bond length. It was found that only the Tolman cone angle gave a good correlation to B P bond length. A compound containing a novel B2O2 ring was synthesised during these studies, and theoretical calculations performed to gain an insight into the planarity of this unusual 4- membered ring. The first crystallographically characterised example of the tri(o-toly)phosphonium cation is also presented. The related compounds X(_3)B pyridine, X(_3)B-2-picoline and X(_3)B 4-picohne (X = CI, Br) are investigated by various analytical methods. Crystal structures of these B-N adducts are presented. It is found that the structure of CI3B 4-picoline exhibits an interesting phase transition from orthorhombic Pnma at higher temperatures to monoclinic space group P2i/n below ca. 170K.The second area of work involves crystal structure analyses of a variety of co-crystals of his(phenylethynyl)benzene compounds, of general type R-Ph-CC-Ph-CC-Ph-R, with R = H, F, MeO, MeS, CH3 and CF3 and a varying fluorinadon pattern on the phenyl rings. The effect of these para substituents on the arene-peifluoroarene interaction was investigated. The effect of each substituent was found to be subtle. In most cases, the co-crystallisation induced an arene-perfluoroarene interaction that was not present in the individual components. Starting materials for the bis(phenylethynyl)benzene compounds often include halogenated tolans of the form Ph-CC-Ph-X (X = CI, Br, I), with either the halogenated or the non-halogenated ring fluorinated. Crystal structures of PhF(_5)-CC-Ph-X (X = I, Br, CI), Ph-CC-PhF4-I and Ph-CC-PhF4-CC-SiMe3 are presented and the arene- perfluoroarene interaction investigated. It is found that the PhPg-CC-Ph-X (X = I and Br) compounds show no evidence of the interaction. The dominating intermolecular force is a Type n halogen -halogen short contact. In contrast, the structure of the chloro analogue is dominated by the arene-perfluoroarene interaction, with no halogen -halogen short contacts. Neither intermolecular force is present in the Ph-CC- PhF(_4)-I or Ph-CC-PhF(_4)-CC-SiMe(_3) structures.