Iridium-phosphine ligand complexes as an alternative to rhodium-based catalysts for the efficient hydroformylation of propene

Expensive rhodium (Rh)-based catalysts have been widely used for the hydroformylation of propene. To find a cheaper and effective alternative to these Rh-based catalysts, herein, a series of phosphine ligands were used to coordinate with iridium, and their catalytic reactivities for the hydroformylation of propene were systematically investigated in this study. The effects of different phosphine ligands, pressures, temperatures, and catalyst dosages on the hydroformylation of propene were investigated. Tripyridyl phosphine iridium Ir-2(cod)(2)Cl-2-P(3-py)(3) (Ir(i)-L5) and its derivatives exhibit the highest catalytic reactivity. Surprisingly, the catalytic reactivity of Ir(i)-L5 is higher than that of Rh-2(cod)(2)Cl-2-P(3-py)(3) (Rh(i)-L5). When the Ir(i)-L5 complex is used as the catalyst, reactions performed in a polar solvent gave higher turnover number (TON) values than those in a non-polar solvent. Up to a TON of 503 can be obtained. Different n-butyraldehyde/iso-butyraldehyde (n/i) ratios can be obtained by adjusting the phosphine ligands or the proportion of gas pressure. The catalyst showed good reusability in five recycling experiments. Furthermore, based on DFT theoretical calculations, a probable reaction mechanism was proposed. It is reliable that an Ir-based catalyst can be considered as a highly effective catalyst for the hydroformylation of propylene with CO.

Automated summary

In this study, a series of phosphine ligands were used to coordinate with iridium to find a cheaper and effective alternative to expensive rhodium-based catalysts for the hydroformylation of propene. Tripyridyl phosphine iridium Ir-2(cod)(2)Cl-2-P(3-py)(3) (Ir(i)-L5) and its derivatives showed the highest catalytic reactivity, surpassing that of Rh-2(cod)(2)Cl-2-P(3-py)(3) (Rh(i)-L5). The Ir(i)-L5 complex exhibited higher turnover number (TON) values when reactions were performed in polar solvents, and different n-butyraldehyde/iso-butyraldehyde (n/i) ratios can be obtained by adjusting the phosphine ligands or gas pressure proportions. The catalyst also demonstrated good reusability and a probable reaction mechanism was proposed based on DFT theoretical calculations.