Unlocking the Hidden Potential of Cobalt: Hydrodeoxygenation of Aromatic Ketones by Cobalt Clusters

  • Author / Creator
    Braziunaite, Aukse
  • Cobalt phosphoranimide clusters [Co(NPPh3)(OSiMe3)(THF)]2, [Co(NPPh3)(OtBu)(THF)]2 and [Co(NPiPr3)2]3 were synthesized. Salt metathesis between CoBr2 and NaNPPh3 and a subsequent in situ reaction with KOSiMe3 provides a convenient and scalable preparation of [Co(NPPh3)(OSiMe3)(THF)]2. The same synthetic route affords the analogous [Co(NPPh3)(OtBu)(THF)]2 from a metathesis reaction with KOtBu but the synthesis suffers from reproducibility issues. An easy, one-step reaction between CoBr2 and NaNPiPr3 gives [Co(NPiPr3)2]3 in high yield. All complexes were characterized by Elemental Analysis and Fourier Transform – Infrared Spectroscopy. Preliminary catalytic experiments with these clusters found that 4-acetylbiphenyl is deoxygenated overnight at 200 °C and 34 atm H2 in the presence of excess KOtBu as a co-catalyst and water scavenger. Aldol condensation and dimerization driven by benzylic radical intermediates occur to a significant extent. Further radical additions produce insoluble oligomers as the major product. Mercury tests revealed that the precatalysts decompose into heterogeneous cobalt nanoparticles under harsh reaction conditions. A deliberate pre-decomposition of clusters [Co(NPPh3)(OSiMe3)(THF)]2 and [Co(NPPh3)2]3 at 200 °C and 34 atm H2 yields heterogeneous catalysts that completely deoxygenate 4-acetylbiphenyl using activated 3Å molecular sieves to scavenge the resulting H2O. Aldol condensation is suppressed in the base-free reactions; only minor, radical-driven dimerization takes place. The thermally stable phosphoranimide ligands cause slow cluster decomposition but are eventually liberated to produce the active cobalt catalyst. Activation of ligand-free CoBr2 by alkali metal tert-butoxides and a subsequent in situ decomposition at 200 °C and 34 atm H2 for 1 h produces the most active catalyst. At 7 atm H2 and 200 °C, heterogeneous cobalt particles fully deoxygenate 4-acetylbiphenyl in 2.5 h using molecular sieves to scavenge water. Deoxygenation of deactivated substrates such as 4-fluoroacetophenone requires longer reaction times; the catalyst is inactive towards cleaving unactivated C–O bonds. The heterogeneous reaction exhibits a strong dependence on the reaction temperature, pressure and reaction time.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.