Synthesis of Boron Porphyrinoids

Abstract

The coordination of boron, a small non-metal, to porphyrinoid ligands is a steadily developing area of research. Largely overshadowed by the highly fluorescent BODIPY compounds, boron porphyrinoids are beginning to emerge as interesting compounds with unique chemical properties and potential applications from sensors to electronic materials. In the past two decades, the Brothers research group has contributed to this field via the synthesis of boron porphyrins and corroles, achieved by insertion of boron into their N4 cores using boron trihalides and their derivatives. This has yielded rich chemistry for both the boron and the ligands such as the presence of mono and di-boron complexes, several unique coordination modes for the boron such as B-O-B, B-H-B and B-B bonding as well as tetragonal distortion of the ligands. This thesis will describe the coordination of boron to three novel porphyrinoid ligands; calixphyrins, porphyrazines and phthalocyanines. Chapter 1 gives an overview of the porphyrinoid ligands used to coordinate boron and a brief description of their synthesis and properties. This is followed by a summary of current literature on boron porphyrinoid chemistry including potential applications for the synthesised compounds. Chapter 2 reports the synthesis of mono-boron calixphyrin complexes using BF3 .Et2O as the boron source. These compounds were synthesised as potential BODIPY analogues, and their full structural characterisation is given and their photophysical properties are assessed. A minor product that was isolated was shown to isomerise into the major product in solution. The kinetics of this isomerism was also investigated Chapter 3 describes the unusual reduction of calixphyrin ligands when a combination of PhBCl2 and N(iPr)2Et is used. Insights into a potential reaction mechanism are provided based on experiments using other bases and similar boron reagents with the formula PhBX2. During this process, several other mono-boron products were synthesised and their characterisations are presented here. Chapter 4 reports the synthesis of di-boron calixphyrins that can only be made using higher reaction temperatures. In the case of B2OF2(Calix), two isomers are formed. The mechanism of the isomer formation is investigated using a combination of experimental work and DFT calculations. Chapter 5 describes the synthesis of the first examples of boron porphyrazine compounds. All compounds are fully structurally characterised. The reaction of PhBCl2 with H2(OEPz) yielded two structural isomers of PhBOBPh(OEPz) with cisoid and transoid geometry; presenting the first opportunity to directly compare the properties of isomers of boron porphyrinoids with different geometries. Chapter 6 reports the first known synthesis of boron phthalocyanine complexes along with their characterisation which indicates that their properties could be different compared to four-coordinated metallophthalocyanines.