X-ray absorption spectroscopy for the study of a homogeneous catalysis system based upon chromium

The selective trimerisation and tetramerisation of ethane to 1-hexene and 1-octene is of great interest due to the importance of these co-monomers in the production of linear low-density polyethylene. A route based upon a cyclic trimerisation and using a chromium catalyst has achieved extremely high selectivity for a trimer. This mechanism is thought to be the basis of newer reactions based upon Cr(III) precursors, using methylaluminoxane (MAO) as a co-catalyst. The body of this work was commenced by investigating the nature of the active species when the chromium(III) complex [CrCl3{HN(CH2CH2SC10H2}], known to trimerise ethylene to 1-hexene, is treated with an aluminium reagent. Studies were carried out via a wide range of techniques, such as UV/visible and EPR spectroscopy as well as X-ray absorption spectroscopy, to validate or otherwise the first species of the catalytic cycle. Similar investigations were carried out on the chromium(III) complex [CrCl3{PNP}(THF)], known to tetramerise ethylene to 1-octene. The work was then focused on the synthesis of novel S-, N- and O-donor tridentate ligands with three geometries; the open-chains S(CH2CH2SC10H21)2, O(CH2CH2SC10H21)2, the tripods MeC(CH2SC4H9)3, MeC(CH2SC10H21)3 and the macrocycles (C10H21)[9]aneN3, (C10H21)3[9]aneN3. Their coordination chemistry with Cr(III) is investigated via spectroscopic methods and the ability of these complexes to trimerise, oligomerise and polymerise is studied with catalytic testings. Partial structure of the active species is proposed after analysis of EXAFS data. The coordination chemistry of N- and S-donor ligands S(CH2CH2SCH3)2, MeC(CH2SCH3)3, [9]aneN3, Me3[9]aneN3, Me3[9]aneN3 and (isoPr)2[9]aneN3 with Cr(III) is investigated.