Investigation of Metal-Catalyzed Epoxide Polymerisation and Phosphanyl Transition-Metal Complexes by Electron Paramagnetic Resonance

Abstract

The first part of this thesis is concerned with the reaction mechanism of activation of H₂O by titanocene(III) chloride. Two important aspects of Cp₂TiCl(H₂O) complexes are investigated: (1) does the oxygen from water directly bind to titanium? and (2) is there a hydrogen bond between water and chloride?<br /> Experimental and computational studies have been carried out to describe the correct structures and revised mechanism of water binding. In the computational study, calculations of the bond dissociation energies for each molecules and the complexation of Cp₂TiCl(H₂O) with THF and without THF are performed. In the experimental study, the EPR and CV measurements provide direct evidence for the cationic species as elucidated from the magnetic properties of the Ti center. The calculations are in a good agreement with the experimental observations. <br /> The second part of the thesis concentrates on the reaction mechanism of reductive epoxide opening. This part is composed of two sections that deal with the binding of epoxide to titanocene(III) chloride with and without spin trapping. During epoxide ring opening, the spin trapping method has been carried out to detect the presence of a carbon radical. <br /> The calculations on the formation of the Cp₂TiCl-epoxide show that, in the presence of chloride, epoxide does not coordinate to titanium. In agreement with the calculations, the EPR spectra of this complex reveal rhombic symmetry and show dissociation of chloride ligand. <br /> With respect to the spin trapping experiments, DFT studies of Cp₂TiCl(DMPO)-Epoxide indicate that the epoxide opens reductively upon the binding to Ti(III). Further EPR measurements show a signal coming from a DMPO radical. The third part of the thesis focuses on the characterization of the paramagnetic Ti species in terms of electronic, structural, chemical and magnetic features. A novel concept for catalytic radical 4-exo cyclizations is studied, which does not require the assistance of the <em>gem</em>-dialkyl effect. The computational study shows that the formation of the corresponding cis-products is thermodynamically unfavorable and hence their ring opening is too fast to allow the pivotal radical reduction. <br /> The last part of the thesis concerns the oxidation of a Li/X (X = Cl^–,F^–) phosphinidenoid complex. To obtain insight into the mechanistic aspects of this oxidation reaction, the reactivity of a Li/X phosphinidenoid complex is investigated by using the two tritylium salts [Ph₃C]BF₄ and [(<em>p</em>-Tol)₃C]BF₄. The EPR investigations of this reaction show that the unpaired electron occupies a pure 3p orbital at P without admixture of the 3s orbital.