Bridgehead substituted scorpionates providing helically chiral complexes
Bell, Nicola Louise
Tripodal borate ligands, including Tp and Tm, are some of the most widely used in organometallic chemistry and were originally prepared, as anions, from the reaction of the relevant heterocycle with an alkali metal borohydride. However, an alternate route, allowing access to zwitterionic, charge-neutral, scorpionates was recently developed within the Bailey group using tris(dimethylamino)borane as the boron source. This thesis describes the expansion of the borane synthetic route to create new, charge-neutral, zwitterionic, tris(methimazolyl)borate (ZTm) ligands containing B-N, B-O and B-C coordinate bonds. Unusual reactivity with isonitrile donors is also presented which has allowed access to boron substituted anionic Tm ligands from the charge-neutral starting material, (HNMe2)ZTm. Attempts to control the helical chirality of ZTm complexes, by using chiral imidazoline donors on the central boron are also described. The borane synthetic route has allowed access to the novel ligand ZThp, the first example of a tripod based on 2-hydroxypyridine ligand arms. As with Tm, this ligand exhibits helical chirality upon complexation and demonstrates how individual atom hybridisation within the ligand arms affects the helicity and thus the chirality of flexible scorpionate ligands. Coordination studies of both zwitterionic and boron-substituted anionic Tm ligands have shown a tendency for the formation of ‘sandwich’ complexes of the form L2M with some metal precursors, whilst the formation of the corresponding ‘half-sandwich’ complexes of these ligands with ruthenium and rhodium was found to be disfavoured.