Electronic and Steric Effects on C–H Activation at Ir and Rh Half-Sandwich Complexes

A range of cyclometallated phenylpyridine, phenylNHC and benzylNHC Ir(III) and Rh(III) half-sandwich complexes and their respective ligands have been synthesised. All new compounds were characterised by 1H and 13C NMR spectroscopy, mass spectrometry and several complexes have been structurally characterised by X-ray crystallography. DFT calculations (carried out by the group of Prof. S. A. Macgregor, Heriot-Watt University) are also discussed in Chapter three.

Chapter one introduces mechanisms of C–H activation particularly key mechanistic studies on carboxylate-assisted AMLA and CMD mechanisms with Pd(II), and Ir(III) and Rh(III) half-sandwich complexes. Chapters two, three and four describe studies of steric and electronic effects on cyclometallation via AMLA C–H activation at Ir(III) and Rh(III) and Chapter five summarises the overall conclusions. Intermolecular competition experiments with phenylpyridines showed that cyclometallation reactions were faster with electron-donating substituents (Chapter two). However, the thermodynamic selectivity favoured electron withdrawing substituents. Intramolecular competition experiments with phenylimidazolium salts (Chapter three) were difficult to interpret. Under most of the conditions studied, imidazolium C–H activation is rate limiting and phenyl C–H activation was not under kinetic control. DFT calculations suggest that HOAc/chloride exchange as well as the C–H activation step could be contributing to the selectivity. The use of benzylimidazolium salts (Chapter four) which form six-membered metallacycles allowed observation of kinetic selectivity which showed very little electronic preference with no clear trend. For all ligands studied the thermodynamically more stable complexes are formed by cyclometallation on the ring substituted with the more electron-withdrawing group. The meta-substituted ligands are sensitive to steric effects with a thermodynamic preference for the para-isomers except for R = F. All of the experimental work and full characterisation of ligands and complexes are given in Chapter six.