The homogeneous catalysts were immobilized by ionic interactions onto the commercially available cation-exchange resins H+ DOWEX 50WX2-100. This is a conventional ion exchange resin, which consists of a cross-linked polymeric construction and is functionalised with sulfonic acid groups (-SO3H), and is the solid equivalent of p- toluene sulfonic acid. The chemical properties of the resin are expressed in a number of ways and can be controlled to individual wishes. The capacity expresses the total number of sites available for exchange; this is expressed on a dry weight. The particle size and swelling properties are expressed in the solvent uptake and as a result determined in its wet weight. All these properties depend on the polymeric backbone and the amount of cross linkages. The swelling properties can influence catalytic behaviour dramatically, and as a consequence can improve the accessibility of the reactants to the catalytic active centre. Therefore catalysts immobilised onto a polymeric support matrix can profit from the swell ability, different to other more conventional solid supports.
The metal content of the individual heterogenised catalyst resins was determined by Energy Dispersive X-ray Spectrometry (EDX) (details see experimental III). This was done by calculating the average of at least 2 different resin-beads, from which a minimum of 3 different areas on the resin was examined. Mapping, to confirm the evenly dispersion of the catalyst on the resins was also carried out for all different types of resin (experimental III). The metal content was also determined for selected resins by Inductively Coupled Plasma optical emission spectrometry (ICP-OES) and Inductively Coupled Plasma Atomic emission spectrometry (ICP-AES).
3.3.1 Heterogenisation of homogeneous rhodium complexes
The commercially available and strong cation exchange, gel-type resin H+ DOWEX 50WX2-100 was treated as described in the experimental III prior to use. The dried resin was weighed into a Schlenk flask, placed under nitrogen atmosphere and left in MeOH for swelling. Shortly after that, the homogeneous catalyst was also dissolved in MeOH and the solution added to the resin. The resin catalyst solution was gently stirred or shaken overnight at room temperature (Scheme 38).
Scheme 38 Immobilisation of homogeneous rhodium phanephos derived catalysts onto an acidic ion exchange resin H+ DOWEX.
Image 1 shows the empty H+ DOWEX resin in MeOH (left side) and the amine- tethered rhodium complex onto the cation-exchange resin (right side), after immobilisation of the homogeneous catalyst (77) after 24 hours.
empty resin in MeOH resin containing the Rh catalyst
Image 1 (Left) H+ DOWEX resin in MeOH; (right) after addition of homogeneous catalyst (77)and stirring for 24 hours. The immobilised catalyst changes the resin’s colour to an intense red.
The EDX confirmed that Rh was indeed on the polymeric resin. Furthermore mapping confirmed that the Rh was evenly dispersed on the surface of the ion exchange resin. The amount of rhodium metal observed is displayed in Table 33 (after EDX and ICP measurements). The obtained numbers assure the effective heterogenisation of the complex onto the resin. ICP-AES confirmed the Rh content and the results are summarised in Table 33.
Table 33 Immobilisation of rhodium complexes on H+ DOWEX 50WX2 resin.[a] Homogeneous catalyst Tethered catalyst Rh loading
(w/w)[b] (%) Rh EDX (%) (R)-[Rh(COD)(44)]BF4 (74) DOWEX-74-COD-Rh 0.84 0.70 (R)-[Rh(NBD)(44)]BF4 (75) DOWEX-75-NBD-Rh n.d. 0.86 (S)-[Rh(COD)(39)]BF4 (76) DOWEX-76-COD-Rh 3.35 1.30 (S)-[Rh(NBD)(39)]BF4 (77) DOWEX-77-NBD-Rh n.d. 0.75 (R)-[Rh(COD)(44)]BF4 (74) Li-DOWEX-74-COD-Rh n.d. 0.35
[a] The resin was put into MeOH (10 ml) under inert atmosphere and a solution of the individually described homogeneous catalysts was added (4 ml). The solution was stirred for 24 hours and the resin was washed and dried as described in the experimental III. [b] The ICP-AES, average value over three samples.
The immobilised Rh catalyst will throughout the thesis be assigned with the resin name first, then the compound number of the tethered ligand, the used cyclodiene of the rhodium precursor and finally the metal (DOWEX-LIGAND-DIENE-Rh).
As mentioned in chapter I, lithiated resins have been reported to give enhanced metal loading onto the resin when non-tagged ligands are used. Barbaro and co-workers found the Li+ DOWEX resin to give higher metal loading of the Rh catalysts by ion- exchange.[5] To investigate the exchange capacity with lithiated resins catalyst (74) was immobilised onto both the lithiated Li+ DOWEX and the protonated H+ DOWEX resin. The anchoring of the homogeneous catalyst (74) onto the lithiated resin was observed to be much lower, with a Rh loading of 0.35 % (according to EDX), 0.35 % lower than with the protonated resin. In the case of tertiary amine functionalised ligands, protonation is most likely quantitative and leads to a strong interaction with the support, whereas coordination of the amine to lithium is a far weaker interaction.
ESEM image
Figure 40 EDX spectrum (left) of DOWEX- 50XWX2 resin with heterogenised catalyst (74). and (right) ESEM image (backscattered electrons, 679 magnifications, 25KeV, 1 torr).
An ESEM image of the DOWEX-74-COD-Rh resin was obtained, presenting the dried charged resin. Theoretically 200 mg resin are used, the amount of exchangeable –SO3H groups available on the resin would be 0.96 mmol for 200 mg resin (4.8 meq/g –SO3H groups on the resin). If each N on catalyst (74) exchanges with one -SO3H group and each Rh cation exchanges as well, a total consumption of 5 x -SO3H groups/catalyst would exchange. This means a maximum exchange amount of 0.192 mmol of each Rh complex onto the resin would be theoretically possible. A maximum catalyst loading of 0.016 mmol of the homogeneous catalyst was observed onto the resin. Therefore considering each of this catalysts used 5 available -SO3H groups, this leads to a maximum usage of 8.3 % -SO3H groups only. This degree of attachment leaves many available -SO3H groups on the resin, which therefore can have in some cases important influence onto the catalytic activity of the resin.
3.3.2 Heterogenisation of homogeneous palladium complexes
The heterogenisation and characterisation of a neutral chiral palladium complex (53)and(61) onto the commercially available ion-exchange resin is described below. For comparison studies, the homogeneous catalyst (48), with no nitrogen moiety was also heterogenised. The same procedures used for the Rh catalyst were also applied for the Pd catalysts. The H+ DOWEX resin was left swelling in MeOH and the homogeneous catalyst dissolved in MeOH was added. After careful stirring or shaking of the solution, the resin was washed thoroughly as reported for the Rh resin and was subsequently dried under argon or nitrogen overnight (experimental II).
Scheme 39 Immobilisation of a neutral palladium complex on acidic ion-exchange resin. Reaction conditions: The resin was left for swelling in MeOH before the homogeneous catalyst, dissolved in MeOH, was added; washing and drying of the catalyst resulted the desired heterogenised catalyst.
The ability of the resin to immobilise this catalyst is merely through ion-exchange of the nitrogen lone pair on the catalysts. However, during catalysis heterogenisation via
protonation of PdCl2 by the strong acidic groups (-SO3H), will most likely result in the formation of a cationic Pd-H species and therefore ionic interactions with the resulting cationic [Pd-H]+ species cannot be entirely excluded. It is also possible that the anion of the resin could replace chloride (with formation of HCl) as another mechanism for immobilisation.
Tanaka and co-workers carried out methoxycarbonylation reactions of styrene and reported the recycling of the Pd catalyst by using a acidic polymeric Wang type resins.[6] Anchoring of the catalyst was purely by ionic [Pd-H]+ SO3-PS interactions while recycling was indeed possible, it was undertaken for 4 cycles only and Pd leaching was not determined. Further details are reported in 3.4.2.
Figure 41 EDX spectrum (left) and ESEM images, image backscattered electrons, 679 magnifications, 25KeV, 1torr (right) of DOWEX-53-Pd of the catalyst bead.
The amount of palladium onto the resins was determined by EDX measurements and ICP-AAS was carried out to verify the accurate amount of palladium on the resin. Both, methods EDX and ICP measurements were used to confirm the total amount of metal. Numbers determined by ICP analysis were in reasonable agreement with the numbers obtained by EDX. The ESEM picture of the DOWEX-53-Pd resin is displayed in Figure 41.
The image of the resin differs slightly from the DOWEX-74-COD-Rh shown in Figure 40; this could be possibly due to inefficient drying of the resin. The metal content of the immobilised homogeneous catalysts (48), (53) and (61) onto the ion-exchange resin are summarised in Table 34.
Table 34 Palladium metal content on the H+DOWEX resins with different catalyst systems.[a]
Homogeneous catalyst Tethered catalyst Pd loading (w/w)[b] (%) Pd EDX[c] (%) (R)- (53) DOWEX-53-Pd 0.29 0.35 (R)- (61) DOWEX-61-Pd 0.60 1.36 (S)- (48) DOWEX-48-Pd 2.14 1.35
[a] The resin was put into MeOH (10 ml) under inert atmosphere and a solution of the individually described homogeneous catalysts was added (4 ml). The solution was stirred for 24 hours and the resin was washed and dried as described in the experimental III. [b] The ICP- AES, average value over three samples. [c] EDX measurements carried out on SEM.
Catalyst (61) is troubled by solubility problems, which made characterisation difficult. Despite low solubility of complex (61) heterogenisation was carried out following the same procedure as for all other catalysts. (61) was partially dissolved in MeOH, the protonated resin added and the mixture carefully stirred for 18 hours. The resin was washed and dried following the same procedure as described before. When EDX of this resin catalyst was carried out, the metal loading observed was 5 times higher than obtained for the heterogenised catalyst (53). A higher catalyst loading was expected however this result was very surprising. In the EDX the resins were checked for possible unusual “lumps” visible on the resin itself, which possibly due to (61)’s insolubility could not be washed away. Investigations by EDX of the resin did not show any signs of “lumps”, however to preclude the presence of any smaller clusters on the resin would require further tests.