Ionic Phosphines And Corresponding Rh-Complexes: Syntheses, Characterization And Applications To Hydroformylation

Hydroformylation is one of the most important homogeneously catalyzed industrial processes for the production of aldehydes from alkenes with 100% atom economy, which was found by Otto Roelen as early as 1938. High-pressure cobalt catalysts completely dominated industrial hydroformylation until the early 1970’s when low-pressure rhodium catalysts were discovered. Then Rhone-Poulenc and Ruhrchemie commercialized an aqueous-organic biphasic process based on a catalyst derived from Rh/TPPTS in 1980’s. However, especially for barely water-insoluble long-chain alkenes, the homogeneous hydroformylation catalyzed by the phosphine modified rhodium catalyst is still the most widely used process, though there are some problems unsolved yet. For example, the activities of the Rh-catalysts need to be further improved due to the high economic cost; the traditional rhodium/phosphine catalysts are sensitive to water and oxygen, which can lead to deactivation of the expensive rhodium catalysts; there exists few efficient methods to separate the catalysts from the products or to recover the catalysts for recycling uses.In this thesis, in order to solve the above problems in Rh-catalyzed hydroformylation, the following work has been carried out on the platform of the imidazolium-based ionic liquids (ILs), which are featured with the high polarity (to provide good solubility for transition metal complexes) and the flexible chemical modifications.1) A series of phosphine-functionalized ILs were synthesized, which are defined as the ionic phosphines in our work because they are all in solid state at room temperature (Scheme 1). The P(III)-atom in each of the obtained ionic phosphines is vicinal directly to the positive charged imidazolium, which makes the phosphine act as a strong π-acceptor with advantage of insensitivity to water and oxygen. The as-synthesized ionic phosphines can be divided into three types including the ionic hybrid P,N-ligands (1-4) with potential hemilability, the amphiphilic zwitterionic phosphines (5 and 6) bearing hydrophilic sulfonate (-SO3-), and the ionic phosphines (7-9) with the same phosphine-incorporated cation but different counter-anions (PF6-, BF4-, TfO-). Subsequently, the corresponding ionic phosphine-ligated rhodium complexes were synthesized, including the Rh(Ⅰ)-complexes of C1-C7, and the Rh(Ⅲ)-complex of C8 (Scheme 2). All the obtained ionic phosphines and the corresponding complexes were fully characterized by 1H/31P NMR, single crystal X-ray diffraction, FT-IR and CHN-elemental analysis.2) The ionic Rh(Ⅰ)-complexes of C1, C3 and C4 were used as the catalysts comparatively for hydroformylation of 1-octene to elucidate the effects of the π-accepting nature of the ionic phosphines and the hemilabile P,N-ligations to Rh-center on the inherent catalytic nature. It was found that C1 was the most efficient catalyst, and the formation and lifetime of the active Rh-H species (2047 cm-1) derived from Cl could be facilitated by the presence of π-accepting P,N-hemilabile ligation which was supported by the in situ high-pressure FT-IR analysis. In addition, the ionic phosphines and Rh-complexes could be applied together with the RTIL solvents as the efficient alternative to immobilize the Rh-P catalysts for recovery and recycling. Hence, the catalytic performance of C8 in the presence of the different ionic phosphine ligands was explored in the RTIL solvents. It was found that the use of the IL of [PEMIM]BF4 as a solvent not only fulfilled the efficient biphasic hydroformylation of 1-octene, but also guaranteed the successful recovery and recycling of C8.3) The stable ionic amphiphilic phosphines (2,-5,6,8) possessing the hydrophilic group of sulfonate (-SO3-) or BF4- were used for Rh-catalyzed hydroformylation of 1-octene and styrene (derivatives) in water suspension to investigate the effect of water. The results showed that the activity of the ionic amphiphilic phosphine-invovled Rh-catalytic system could be dramatically boosted up by water, in which 2 reflected the best promoting effect due to the synergetic effect coming from the favored P,N-hemilabile ligation in the cation. The in situ high pressure FT-IR spectral information verified that water as a weak ligand greatly facilitated the formation and stability of the active Rh-H species, which is in charge of the efficient hydroformylation of alkenes.4) The performance of the potential bi-functional zwitterion Rh(I)-complexes of C5 and C6 were investigated for one-pot tandem hydroformylation-acetalization of olefins in the presence (or absence) of 5 and 6 respectively. It was found that the C5 could be as a bi-functional catalyst featured with both Rh-P group and acid group (-SO3H) by reacting with H2 in situ during the process. Therefore, C5 exhibited not only high hydroformylation activity but also high acetalization activity. Furthermore, the catalytic system of C5-5 could be captured tightly in the RTIL of [BMIM]BF4 and then recycled for 4 runs.