Mechanistic studies of first-row homogeneous transition metal catalysts

In our day-to-day lives, we are reliant on catalytic processes to produce a wide range of products. Due to the paramagnetic nature of the involved intermediates, the mechanism through which these catalysts operate is often poorly understood. In this thesis, using a variety of spectroscopic techniques (e.g. XAS and EPR), a deeper understanding of two important catalytic reactions is obtained.
The first part of this thesis focusses on the chromium-catalyzed selective oligomerization of ethene. Typically, a trivalent chromium precursor is activated with excess alkylaluminium reagent. In Chapter 3, a chromium-pyrrolyl system developed by Chevron Phillips Chemical is studied. The activation and deactivation pathways are described in detail. In Chapter 4, the oxidation state of the active species in the [(R-SN(H)S-R)CrCl₃] system is investigated. Using three different activators, the (electronic) structure of chromium after activation in the absence and presence of a suitable substrate (ethene, other alkenes and alkynes) is elucidated. Plausible models of the active species are studied through DFT calculations. In Chapter 5, the roles of the ligand and activator in the activation process for chromium complexes containing phosphine ligands are studied. Complexes are prepared with and without an ortho-methoxy substituent within the ligand framework and their activation with AlMe₃ and MMAO is studied.
The second part of this thesis focusses on the copper-catalyzed azide-alkyne cycloaddition. In Chapter 6, novel cationic copper complexes containing iminophosphorane ligands are detailed. Using a combination of spectroscopy, kinetics and DFT calculations, the resting state and the rate-determining step are identified.