Werner Janse van Rensburg

Bicyclic phosphines as ligands for cobalt-catalysed hydroformylation

A range of tertiary bicyclic phosphine ligands derived from cis, cis-1,5-cyclooctadiene (Phoban family) was studied by batch autoclave reactions during the hydroformylation of a mixture of linear internal decenes using a cobalt catalyst system. Comparative runs were performed with PBu(3) as representative of standard trialkyl phosphine behaviour. The Phoban ligands comprise of a cyclooctyl bicycle with a mixture of the [3.3.1] and [4.2.1] isomers where the third substituent was systematically varied, Phoban-Q (Q = CH(2)CH(3), (CH(2))(4)CH(3), (CH(2))(9)CH(3), (CH(2))(19)CH(3), (CH(2))(3)N(CH(3))(2), C(6)H(11) and C(6)H(5)). An increase in ligand concentration resulted in a decrease in the reaction rate while the selectivity towards the n-alcohol product increased in accordance with a move from more unmodified catalysis to more modified catalysis. Alcohol yields of 77-85% were obtained at rates of 1.8-2.4 h(-1) for highly modified catalysis. Under highly modified conditions the linearity of the alcohol ranges in a narrow band from approximately 85-90% from Phoban-Ph to Phoban-Cy respectively. Hydrogenation of the alkene substrate varied from approximately 9-15% for Phoban-Ph and Phoban-Cy respectively the least and most electron donating derivatives. The two phosphine isomers were separated for Phoban-C(2) and the hydroformylation activity were re-evaluated for each isomer. The less electron donating [4.2.1] isomer required slightly higher ligand concentrations to achieve fully modified catalysis and gave rates and linearities comparable to the [3.3.1] isomer but giving slightly higher yields due to less hydrogenation of the olefin. In comparison, at fully modified conditions, PBu(3) gave a rate of 0.6 h(-1), alcohol yield of 77%, linearity of 81% and 17% hydrogenation. The crystal structures of the cobalt dimers [Co(CO)(3)(Phoban[3.3.1]-C(2))](2), [Co(CO)(3)(Phoban[3.3.1]-C(5))](2), [Co(CO)(3)(Phoban[3.3.1]-C(3)NMe(2))](2), and [Co(CO)(3)(Phoban[3.3.1]-Cy)](2) have been determined and indicated very similar geometries with Co-Co and Co-P bond distances ranging from 2.6526(10)-2.707(3) and 2.1963(8)-2.2074(9) A respectively. The cone angles of the Phoban ligands were calculated from the crystallographic data, according to the Tolman model, and ranges from 159-165 degrees.

On the Kinetic Interpretation of DFT-Derived Energy Profiles: Cu-Catalyzed Methanol Synthesis

A mean field microkinetic evaluation of previously reported DFT-derived Gibbs free energy profiles for CO and CO2 hydrogenation to methanol on Cu(111), Cu(211) and Zn-modified Cu(211) is presented. It is demonstrated that explicit consideration of the effect of surface coverages of reaction intermediates on rates is needed in order to arrive at a realistic evaluation of the activity and selectivity. In particular, both the methanol formation rate and the CO/CO2 selectivity for methanol production are demonstrated to be highly sensitive to the saturation coverage of formate at steady state. In general, the study emphasises the importance of including explicit kinetic analyses when mechanistic DFT-derived energy profiles are interpreted for catalytic processes.Graphical Abstract