Bicyclic phosphines as ligands for cobalt catalysed hydroformylation. Crystal structures of [Co(Phoban[3.3.1]-Q)(CO)3]2 (Q = C2H5, C5H11, C3H6NMe2, C6H11)

Abstract

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 PBu₃ 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 = CH₂CH₃, (CH₂)₄CH₃, (CH₂)₉CH₃, (CH₂)₁₉CH₃, (CH₂)₃N(CH₃)₂, C₆H₁₁ and C₆H₅). 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 ∼85–90% from Phoban-Ph to Phoban-Cy respectively. Hydrogenation of the alkene substrate varied from ∼9–15% for Phoban-Ph and Phoban-Cy respectively the least and most electron donating derivatives. The two phosphine isomers were separated for Phoban-C₂ 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, PBu₃ 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]-C₂)]₂, [Co(CO)₃(Phoban[3.3.1]-C₅)]₂, [Co(CO)₃(Phoban[3.3.1]-C₃NMe₂)]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) Å respectively. The cone angles of the Phoban ligands were calculated from the crystallographic data, according to the Tolman model, and ranges from 159–165°.