An NMR study of cobalt-catalyzed hydroformylation using para-hydrogen induced polarisation

Abstract

The syntheses of Co(η³-C₃H₅)(CO)₂PR₂R′ (R, R′ = Ph, Me; R, R′ = Me, Ph; R = R′ = Ph, Cy, CH₂Ph) and Co(η³-C₃H₅)(CO)(L) (L = dmpe and dppe) are described, and X-ray structures for Co(η³-C₃H₅)(CO)(dppe) and the PPh₂Me, PCy₃ derivatives reported. The relative ability of Co(η³-C₃H₅)(CO)₂(PR₂R′) to exchange phosphine for CO follows the trend PMe₂Ph < PPh₂Me < PCy₃ < P(CH₂Ph)₃ < PPh₃. Reactions of the allyl complexes with para-hydrogen (p-H₂) lead to the observation of para-hydrogen induced polarisation (PHIP) in both liberated propene and propane. Reaction of these complexes with both CO and H₂ leads to the detection of linear acyl containing species Co(COCH₂CH₂CH₃)(CO)₃(PR₂R′) and branched acyl complexes Co(COCH(CH₃)₂)(CO)₃(PR₂R′) via the PHIP effect. In the case of PPh₂Me, additional signals for Co(COCH₂CH₂CH₃)(CO)₂(PPh₂Me)(propene) and Co(COCH(CH₃)₂)(CO)₂(PPh₂Me)(propene) are also detected. When the reactions of H₂ and diphenylacetylene are studied with the same precursor, Co(CO)₃(PPh₂Me)(CHPhCH₂Ph) is seen. Studies on how the appearance and ratio, of the PHIP enhanced signals vary as a function of reaction temperature and H₂ : CO ratio are reported. These profiles are used to learn about the mechanism of catalysis and reveal how the rates of key steps leading to linear and branched hydroformylation products vary with the phosphine. These data also reveal that the PMe₂Ph and PPh₂Me based systems yield the highest selectivity for linear hydroformylation products.