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

Abstract

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.