Rhodium-Catalyzed Hydroformylation of 1-Hexene in an Ionic Liquid: A Molecular Dynamics Study of the Hexene/[BMI][PF6] Interface

Abstract

We report a molecular dynamics study of biphasic systems involved in the rhodium-catalyzed hydroformylation of 1-hexene in the 1-butyl-3-methyl-imidazolium hexafluorophosphate ionic liquid ([BMI][PF(6)] IL). We first describe the neat [BMI][PF(6)] interfaces with hexene (the substrate) and heptanal (the linear reaction product) as organic phases. The former interface is molecularly sharp with BMI+ cations preferentially oriented "perpendicular" (i.e., pointing their butyl chains toward the organic phase), whereas hexene molecules tend to be somewhat parallel to the interface. The interface with heptanal is approximately twice as broad, due to BMI+...O(heptanal) attractions, and the solvent molecules are disordered at the interface. No IL ions solubilize in the organic phase(s) whereas ca. 2-3 hexene or heptanal molecules diffused into the IL phase. The presence of the CO and H2 gases does not modify the nature of the hexene/IL interface, as these gases are mainly solubilized in the organic phase, respectively, as diluted species and in the form of a "gaseous" droplet. In the IL phase, one finds a few CO monomers, whereas the less soluble H2 molecules spend only transient excursions. We next simulate the phase separation of "randomly mixed" IL/hexene liquids with the [RhH(CO)L(3)] precatalyst as a solute, comparing the PPh(3) to the TPPTS(3-) ligands (L). The phases separate much more slowly than in the case of classical liquids, and the neutral complex with PPh(3) ligands solubilizes in the hexene phase, displaying loose dynamical contacts with the IL interface. This contrasts with the -9 charged [RhH(CO)(TPPTS)(3)](9-) complex that sits "immobilized" on the IL side of the interface and is mainly solvated by BMI+ cations. Finally, we characterize the solvation of -6 charged [RhH(CO)(TPPTS)(2)](6-), [RhH(CO)(2)(TPPTS)(2)](6-), and [RhH(CO)(TPPTS)(2)(hexene)](6-) complexes involved as reaction intermediates in the hydroformylation reaction and of the free TPPTS(3-) ligand itself in the bulk IL.