Electronic Effect on C-H Activation at Half Sandwich Complexes of Ir, Rh and Ru

A range of cyclometallated half-sandwich complexes have been synthesised in good yields by reactions of [MCl2Cp*]2 (M = Ir, Rh) or [RuCl2(p-cymene)]2 with different para-, meta- (NMe2, OMe, Me, F, CF3 and NO2) and ortho- ( R= Me and NO2) substituted 1-phenylpyrazole ligands in the presence of NaOAc. All complexes were characterised by using 1H, 13C, 2 D NMR spectroscopy, ESI-MS and several compounds 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. Chapter one gives an overview of the mechanisms of C-H activation including AMLA and CMD and the synthesis of cyclometallated complexes, in particular of Ir, Rh and Ru. Catalytic C-H functionalisation by AMLA/CMD C-H activation is introduced. Chapter two describes the synthesis and characterisation of half-sandwich cyclometallated Ir(III), Rh(III) and Ru(II) complexes from different para- and ortho-substituted 1-phenylpyrazoles. Chapter three provides an introduction to the regioselectivity of acetate assisted cyclometallation in the literature and the synthesis and characterisation of a range of half-sandwich Ir(III), Rh(III) and Ru(II) cyclometallated complexes with different meta-substituted 1-phenylpyrazoles. NMR spectroscopy was used to determine isomer ratios and deuteration experiments and computational studies were used to evaluate whether the selectivity is kinetic or thermodynamic in origin. Chapter four gives a brief introduction on the use of Hammett plots to investigate reaction mechanisms. Competition reactions between different para- and meta- substituted 1-phenylpyrazoles were used to study the electronic effects of the substituents on the cyclometallation. The results show that ligands with electron donating groups react faster whilst those with electron withdrawing groups give more thermodynamically stable products. The experimental results agree well with those from DFT calculations. Chapter five includes all the experimental work and characterisation data of discussed in Ch.2, Ch.3 and Ch.4.