Coinage metal complexes bearing mono-supported borohydride ligands based on scorpionate scaffolds
Singly supported borohydride ligands, in comparison to scorpionate ligands are scarcely reported within the chemical literature. This thesis looks at select examples of the coordination of such ligands and compares the different supporting scaffolds and structural motifs between these ligands. It also draws comparisons with their neutral analogues, the supported borane ligand. Both supported borohydride and borane ligands can observe a range of coordination modes which are discussed. Using the available spectroscopic and diffraction data Chapter 2 explores the coordination of such ligands across a range of select examples throughout the chemical literature. The coordination of singly supported borohydride ligands was then investigated. This thesis presents the first example of the coordination of a singly supported borohydride ligand to copper complexes in Chapter 3. The ligands utilised were based on known scorpionate scaffolds and were compared to their bis- and tris- counterparts. The coordination of the mono- ligands allows for the [BHn]− unit to come into closer proximity to the metal centre. The addition of substitutions at the [BHn]− moiety have also been shown to influence the coordination of the bridging B-H bonds. For example, the addition of a phenyl ring results in the bridging hydrogens being more protic in character and conversely a methyl substituent gives hydrogens that are more hydridic in character. Using the same metal, the nature of the supporting scaffolds is also investigated and compared. Chapter 4 extends the investigation and reports the synthesis of silver complexes and the first examples of gold complexes of singly supported borohydride ligands. Such complexes were previously thought to be unachievable due to the reducing power of the [BHn]− group. Comparisons between the supporting scaffolds and the addition of substituents to the borohydride group support the conclusions made in the previous chapter. In particular the coordination of the 7-azaindol-7-ylborate (Mai) scaffold demonstrates an unexpected coordination mode which is consistent across the copper and silver complexes. Comparisons of complexes down the coinage metal group reveals that coordination of the [BHn]− group becomes weaker with the heavier metal centres. The final results chapter, Chapter 5 explores the potential for the development of new supporting scaffolds for singly supported borohydride ligands, in particular those which are supported by carbenes. Carbenes were selected for their notorious election donating properties. This chapter explores the synthesis of carbenes and their pro-ligands which contain a vinyl group as a potential site for hydroboration. The synthesis of the precursors was found to be best achieved using the addition of BrCH2BPin to functionalised imidazoles. The pro-ligands were found to be uncharacteristically stubborn to deprotonation and did not form carbenes. Furthermore, the synthesis of the respective CAAC iminium precursor again did not deprotonate.
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