Ionic liquids ligand effects

In conclusion, many methods are available for the immobihzation of metal complexes with diazaligands. The biphasic liquid methods seem to be more versatile - especially in the case of ionic liquids, where modification of the ligands is not needed. Very good results can also be obtained with solid supports, although the surface effects, which can be positive or negative. 

Olefin dimerisation with Ni-NHC complexes became a topic of interest following reports of Ni(II) phosphine complexes being employed in imidazolium-based ionic liquid solvents [23, 24]. It had previonsly been established that aIkyl-Ni(II) complexes containing NHC ligands can rapidly decompose via imidazolium formation (Scheme 4.1) [5], and it was thus of interest to explore the effect that an excess of the imidazolinm cation would have on this reaction. 

Besides the advantage of recyclability, reactions in ionic liquids are generally faster and are run under milder conditions than reactions with conventional solvents. Further activation may come from ultrasonic agitation.520 Since the majority of ionic liquids used are imidazolium salts, the effect of these solvents can be at least partly attributed to the in situ formation of carbene complexes (Section 9.6.3.4.10).521 Cross-coupling of ArB(OH)2 can also be efficiently performed in ionic liquids based on long-chain tetraalkylphosphonium salts, in which case aryl bromides and some aryl chlorides can be processed in the presence of the trivial ligand PPh3.522... 

For Rh-catalysed hydroformylation the role of the ionic liquid as an innocent solvent is by far the most important. To our knowledge, none of the published research in this area claims special chemistry. The selectivity found with the different Rh-ligand complexes corresponds in most cases to the values obtained in traditional organic solvent or water (with the surprisingly low selectivity of TPPTS ligands in ionic liquids being a remarkable exception). Overall activities were found to be very comparable if mass transfer effects between the gas phase and the two immiscible liquid phases were overcome by proper stirring. 

Since the solubility of long-chain alkenes is higher in ionic liquids than in water, there is much interest in finding effective ionic liquid catalysts for the hydroformylation reaction. Bis-phosphines have proved to be particularly useful in hydroformylation reactions and a bis-phosphine with a charged cobaltocenium backbone, analogous to l,l/-bis(diphenylphosphino)ferrocene (dppf), has been developed specifically for use in ionic liquid hydroformylation reactions [26], In combination with [Rh(CO)2(acac)], the 1, I -bis(diphenylphosphino)cobaltocenium hexafluorophosphate ligand dissolved in [bmim][PF6] effectively catalyses the hydroformylation of 1-octene as shown in Scheme 8.11. 

In basic tetraalkylammonium chloride ionic liquids, the active catalyst was suggested to form from the dissociation of the chloride ligand of RuCl2(PPh3)3 in the base. The effect of the cation became evident as the catalyst in tetraalkylammonium chloride was much more active than that in [BMIMJCl. It is known that the bulky tetraalkylammonium cation is weaker in its association with the chloride anion than a planar [BMIM] cation. Therefore, it was concluded that the ionic liquids giving the best catalytic activity appeared to be tetraalkylammonium hydroxide, which melts at approximately room temperature. 

Okubo et al. (260) reported that Pd(II)/Si02 was a more effective catalyst than Pd/C when two equivalents of triethylamine base were added to the same ionic liquid one equivalent was not sufficient. The reaction was carried out without phosphine ligands. The unreduced Pd(II)/Si02 catalyst with two equivalents of base in [BMIM]PF6 was more active than the supported palladium catalysts in DMF. Furthermore, the stability of [BMIM]PF6 also improved with the addition of triethylamine. 

The Heck reaction is a C-C coupling reaction where an unsaturated hydrocarbon or arene halide/triflate/sulfonate reacts with an alkene in presence of a base and Pd(0) catalyst so as to form a substituted alkene. Kaufmann et al. showed that the Heck reaction carried out in presence of ILs such as tetra-alkyl ammonium and phosphonium salts without the phosphine ligands, resulted in high yields of product. They attributed the activity to the stabilizing effect of ammonium and phosphonium salts on Pd(0) species. Carmichael et al. used ionic liquids containing either A,A -dialkylimidazolium and A-alkylpyridinium cations with anions such as halide, hexafluorophosphate or tetrafiuoroborate to carry out reactions of aryl halide and benzoic anhydride with ethyl and butyl acrylates in presence of Pd catalyst. An example of iodobenzene reacting with ethyl acrylate to give trans-et vy cinnamate is shown in Scheme 14. 

For the above reasons, the IFCC recommendations on activity coefficients [19] and the measurement of and conventions for reporting sodium and potassium [21] and chlorides [25] by ISEs were developed. At the core of these recommendations is the concept of the adjusted active substance concentration (mmol/L), as well as a traceable way to remove the discrepancy between direct and indirect determinations of these electrolytes in normal sera. Extensive studies of sodium and potassium binding to inorganic ligands and proteins, water binding to proteins, liquid-junction effects and the influence of ionic strength have demonstrated that the bias between sodium and potassium reports obtained from an average ISE-based commercial... 

A series of different nucleophiles was tested in the reaction with 3-acetoxy-l,3-diphenylprop-l-ene in [C4Ciim][BF4] with Pd(OAc)2 as catalyst precursor.11291 As with molecular solvents, the presence of an excess of phosphine ligand was found to be necessary for the reaction to take place. The effect of employing different phosphines has been studied and it was found that the reaction rate is greatly enhanced with electron-rich or o-donating phosphines.11301 With 7t-accepting phosphines, little or no reaction took place. As can be seen from Table 6.8, the reaction proceeds faster in the ionic liquid than in THF and a wider range of phosphines can be applied. 

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