Tripodal phosphine ligands have been widely used in coordination chemistry and catalysis, yet,
of the many derivatives reported, the nitrogen-centred tripodal phosphine ligand (N-triphos) has received little attention. However, the unique inclusion of the apical nitrogen centre, with a
chemically accessible lone pair, sets N-triphos apart from other ligand systems, leaving room for
further exploration of the coordination chemistry and applications of this ligand motif. One particular application with significant implications towards sustainable food production is nitrogen fixation. Replacing the energy intensive Haber-Bosch process for other, more efficient routes towards ammonia formation is a problem that has been addressed by chemists for over 50 years. Such alternatives include the preparation of novel metal-dinitrogen complexes that are able to catalyse the reductive protonation of dinitrogen to form ammonia, under ambient conditions. This thesis therefore explores the use of the N-triphos ligand to develop transition metal dinitrogen complexes that could act as catalysts for ambient N2 fixation.
A range of novel molybdenum and cobalt N-triphos dinitrogen complexes were prepared,
and the N2 activation and reactivity of these systems were investigated. The denticity of the
N-triphos ligand in both cases was dependent on the phosphine substituents i.e. more electron-donating ligands increased the degree of dinitrogen activation but displayed only bidentate coordination behaviour. The phenyl N-triphos ligand provided the most stability in both Mo and Co systems, and these dinitrogen complexes were investigated for further reactivity. The molybdenum complex did not show any N2 functionalisation, but reactivity instead occurred at the apical N-triphos amine, and an unusual alkali-metal mediated rearrangement was observed. In contrast, the cobalt complex showed a high degree of N2 activation, and subsequent dinitrogen
functionalisation was achieved by silylation. The cobalt systems show significant potential for
future catalytic applications in nitrogen fixation.
Novel, nitrogen-centred tripodal N-heterocyclic carbene ligands were also envisioned, as
derivatives of the N-triphos ligand with stronger ligand donor properties. The pro-ligand salts
were synthesised, but deprotonation to achieve the free carbenes was not achieved, nor was
complexation of the tripodal system to group 11 metals.