Silver(I) complexes as model catalysts in olefin hydroformylation

[摘要] English: The aim of this study was to synthesize Ag(I) complexes of the type [AgXLn] (L = tertiary phosphine; n = 1-4; X = coordinating or non-coordinating anion) and explore the olefin hydroformylation activity and ligand exchange rates of these complexes. Tertiary phosphine complexes of Ag(I) of the type [AgXLn] (L = P(p-tol)3; n = 1-4; X = Br-, ClO4 -, PF6 -) were synthesized and characterized through X-ray crystallography. Selected crystal data is shown in Table 1. Table 1 Selected crystal data as obtained for the three Ag(I) crystal structures solved in this study. These complexes are comparable to similar complexes containing transition metals, other phosphine ligands or different counterions. Occurrences of similar structures in literature, however, were limited, indicating a field open to study. The behavior of Ag/PX3 complexes in solution are not yet explored, due to, amongst others, rapid and complex kinetics, and could be expanded on in future. The coordination of CO to these complexes for application as hydroformylation catalysts were investigated through high-pressure infrared spectroscopy. No evidence could be obtained through high-pressure infrared of coordination of CO to Ag(I) complexes. The aversion of the silver molecule to coordinate the CO molecule could be attributed to the coordination of bulky phosphine ligands, which could prevent the coordination of CO ligand to the metal centre, as well as the absence of a strong electron-accepting ligand, for example boron- or nitrogen-containing ligands. Another explanation is the high electron density surrounding the silver atom, which prevents-back bonding from the silver atom to the CO molecule. Kinetics of the exchange rate between coordinated phosphine ligands in these complexes and free phosphine is important, as this exchange rate could have an influence on the coordination of other ligands on the silver atom. The exchange rate was investigated using a NMR technique called magnetic spin transfer, or spin saturation transfer. In this method, the sample is saturated at a specific frequency, and through the relaxation of the peak at that frequency the exchange between free and coordinated phosphine could be established. The sample was investigated for different concentrations, shown in Table 2 with the calculated values of the rate of exchange. a) No e.s.d.'s were obtained from the fitting program, but are estimated to be ca. 10%. The rate of exchange at different concentrations is shown in Figure 1.The observed rate of exchange of free phosphine with coordinated phosphine is fast, in ca. 7 s-1 at -80 °C, with an average value of 29±60 mM-1.s-1 for k1 and 6.9±0.7 s-1 for k-1. The rate of exchange between coordinated and free phosphine has been found to be independent of the concentration of phosphine, indicating a dissociative mechanism. Since no evidence could be obtained of coordination of CO to the metal centre, application of complexes of Ag(I) of the type [AgXLn] (L = tertiary phosphine; n = 1-4; X = coordinating or non-coordinating anion) as hydroformylation catalysts does not seem feasible.

[发布机构] University of the Free State