Figure 2.6.1 shows the effect of changing the initial diiodomethane (substrate) concentration upon the catalyst TON. It is observed that higher concentrations of diiodomethane are advantageous to catalyst stability. A potential explanation of this effect is discussed in detail in chapter three. An increase in diiodomethane concentration would increase the rate of the first step in the catalytic cycle causing an advantageous shift in the equilibrium position (according to Le Chatelier’s principle) in the final step of the proposed mechanistic cycle (section 3.13).
15 - 1 0- 5 - 1500 500 1000 2000 2500 Initial [C H ^y/m M Figure 2.6.1: Graph showing TON vs, initial diiodomethane concentration.
2 .7 Effects of changing the phosphine ligand
A range of phosphines and a phosphite were investigated as possible catalyst
precursor units in systems \^ re the catalyst was allowed to form in-situ from ^ ^ [Rh2(OAc)2] / 6 PR3. These results are presented in table 2.7.1.
Table 2.7.1: Showing the effect of a variety of phosphine ligands upon the reaction.
Y ie ld /
Run P hosphine DEM C H2(O E t)2 EtC0 2Et TON Selectivity
81 PMeg 4.6 1.6 - 14.4 74% 33 PEt3 5.9 1.3 <0.1 18.5 82% 75 P'Pr3 2 .2 0 .6 - 6.9 79% 36 P(OEt)2 6 .2 1.9 <0.1 19.4 77% 80 PPh3 7.1 1.0 <0.1 22.3 88% 59 dmpe 2 .6 0 .6 - 8 .2 81%
a) P:Rh = 3:1 for all reactions except for dmpe P:Rh 2:1
As table 2.7.1 shows, a range of phosphine ligands can form active catalytic species. The majority of the ligands result in very similar yields of DEM, with no obvious trend related to the electronic character of the ligand being observed. The use of either dmpe or P^Prg leads to significantly lower yields than the other ligands. The steric
lowering the DEM yield. Recent work from within our group on the reaction of R h C ls with dmpe and CO has shown that the 5 co-ordinate species [RhCl(CO)2(dmpe)] forms in preference to [RhCl(CO)(dmpe)].^^^ This 5 co-ordinate
18 e species would have to lose a CO molecule before any oxidative addition reaction could occur. This different behaviour (compared to [Rh(X)CO(PR3)2]) probably explains the poor catalytic performance of the dmpe derived species.
2.8 Effects of varying the carbon monoxide partial pressure
The mechanism proposed for this reaction in chapter three includes a competition step between CO insertion and catalyst decomposition. This step is illustrated in figure 2.8. 1.
O, QO ^ O E t
CO, T / P H 3 I— R b ^ (-o --- I— Rh— CO — I
^ E t s P ^ I ° “ D p E t 3
^
EtsPI
^ \ o E tI
R h — CO EtgP^I
I Figure 2.8.1With the proposed existence of a competition step, such as shown above, a direct relationship between CO pressure and catalyst TON would be expected. The observed effect of CO pressure on TON is shown in figure 2.8.2 and the predicted trend is indeed observed.
If the trend observed in figure 2.8.2 could be extrapolated to much higher pressures, complete conversion of the diiodomethane substrate by this catalyst may be expected at approximately 700 atm CO. In order to test this hypothesis a series of reactions were carried out at 300-350 atm CO using ultra-high pressure equipment in the labs of Prof.
W Keim in Aachen, Germany. Sadly the yields of DEM achieved at 300-350 atm CO are similar to those achieved at 40 atm CO under the same conditions.
I
30 - 20 — 1 0- 0 25 50 75 100 pCO / atm Figure 2.8.2: Graph showing effect of pCO on Catalyst TON.Two potential explanations for this are; i) the displacement of the phosphine ligands from the metal centre by CO given the high CO concentrations achieved at these pressures and ii) competition between CH2I2 and CO for co-ordination sites on the initial Rh(I) species. The second explanation has also been proposed by Yamamoto^^ for the decrease in rate for the double carbonylation of PhBr at elevated CO pressures. Although evidence for the displacement of PEtg by CO at high CO pressures could potentially be obtained by HPIR, independent work^^^ on the reactions of [Rh(I)CO(PEt3)2] under CO from within our group has not implied the occurrence of such a phenomenon at up to 100 atm. This does not rule out the possibility of this displacement occurring during reactions carried out at 300-350 atm. A reversible reaction between [Rh(Cl)CO(PEt3)2] and CO has been observed^^^ and this is shown in figure 2.8.3. Cl— R h — GO E(3P Cl— Rh E tsP^ PEt3 CO CO Cl OC — Rh’ EtaP^ PEt3 CO
In light of the work of Yamamoto and other work within our group the blocking of the reaction of [Rh(X)CO(PEt3)2] with CH2I2 by CO co-ordination appears, at present, to be the best explanation for low DEM yields at 350 atm CO.
2 .9 Study of catalvst stability and effects of catalyst reducing agents
The low yields of DEM observed could conceivably be due to one of two reasons: i) Catalyst decomposition; ii) the reaction reaching an unfavourable equilibrium limiting the yield of DEM. If catalyst decomposition is occurring within 4 hours then increasing the reaction time beyond 4 hours should not increase the yield of DEM. An increase in the catalyst concentration should increase the yield of DEM if the catalyst's lifetime is lower than 4 hours. If it is an unfavourable equilibrium which is limiting the DEM yield, altering the concentration of the catalyst should not alter the position of the equilibrium and the DEM yield should not change. It was observed that the yield of DEM did not increase from its value at 4 hours after a 21 hour reaction time and that increasing the catalyst concentration increases the yields of all the products. These results of changing the reaction time and catalyst concentration demonstrate that it is catalyst stability that is the limiting factor in this reaction. Further evidence confirming catalyst decomposition comes from 'snapshots’ of the rhodium phosphine species present in solution at different reaction times. These data are discussed in chapter four (section 4.6).
The major catalyst decomposition step has been shown to be the competition reaction illustrated in figure 2.8.1 above (see also chapter four). It was thought that this inactive Rh(III) species might be reduced in-situ and thus the metal centre re introduced into the catalytic cycle. An analogous reduction is achieved in the Monsanto process using hydrogen.
[ R h l 4 ( C O ) 2 r + H 2 --- ^ [ R h l 2 ( C 0 ) 2 ] - + 2 H I Figure 2.9.1
A comparison of two nearly identical catalytic runs, one with pCO = 40 atm and pH2 = 40 atm (run 149) and one with pCO = 40 atm and no hydrogen shows that the hydrogen has no effect on the catalyst stability - see table 2.9.1
Table 2.9.1: Showing the effect of hydrogen as a potential reductant upon the TON.
Y ie ld / %
1
R un No. Gas used DEM CH2(OEt)2 EtC02Et TON
33 CO only 5.9 1.3 <0.1 19
149 CO/H2 4.6 1.0 0 .2 14
146 CO only^ 9.3 1.0 trace 29
a) Mg amalgam added as the reducing agent.
Although the route which allows hydrogen to reduce [Rhl4(CO)2]" does not operate with [Rhl3(CO)(PEt3)2], the effect of the Mg amalgam suggests that catalyst oxidation is responsible for its loss of activity.