4.4 Results and Discussion
4.4.3 Oxidation of styrene
Table 4.5. π…π interactions of complexes 1f and 2f continued…
π…π (Cg…Cg) Distance (Å) ARU
Cg(2)…Cg(4) 5.154(2) -X,2-Y,1-Z
Cg(3)…Cg(2) 4.898(3) X,Y,Z
Cg(3)…Cg(4) 4.979(2) -X,2-Y,1-Z
Cg(3)…Cg(5) 4.194(3) -X,1/2+Y,1/2-Z
Cg(4)…Cg(2) 5.436(2) X,Y,Z
Cg(4)…Cg(2) 5.154(2) -X,2-Y,1-Z
Cg(4)…Cg(3) 5.203(2) X,Y,Z
Cg(4)…Cg(3) 4.979(2) -X,2-Y,1-Z
Cg(4)…Cg(5) 5.207(2) X,Y,Z
Cg(4)…Cg(6) 5.164(2) -X,1-Y,1-Z
Cg(5)…Cg(3) 4.195(3) = -X,-1/2+Y,1/2-Z
Cg(5)…Cg(4) 5.274(2) -X,1-Y,1-Z
Cg(5)…Cg(6) 5.078(2) X,Y,Z
Cg(6)…Cg(1) 5.858(2) 1-X,1-Y,1-Z
Cg(6)…Cg(2) 5.457(2) X,Y,Z
Cg(6)…Cg(4) 5.363(2) X,Y,Z
complexes. The highest activities were exhibited by catalysts 1e (88%) and 2e (83%), with the chlorine substituted phenyl group on the nitrogen atom. Catalysts 1c and 2d were least active. Comparable activity between catalysts 1 and 2 of the cyclohexyl (a) substituted and functionalized phenyl groups are noted (e and f). The difference in the activity of the catalysts bearing the different substituents could be attributed to their basicity.18 The basic nature of the ligand is attributed to the substituent on the nitrogen atom, which lowers the activity with an increase in basicity. This basic substituent is likely to increase the electron density at the metal center, which makes it easier for the oxidation process to occur, where the oxidation state of the metal goes from a M(I) to M(III) species. However, the formation of the super oxo species becomes more difficult since the high basic nature of the complex stabilizes the MIII oxidation state. Thus, less basic and electron withdrawing substituents such as 1e and 2e are more active in comparison to 1c and 2c, where the alkyl substituent increases the basic nature of the complex.
Figure 4.5. Conversion of styrene over catalysts 1 and 2 in MeCN.
Conditions: Catalyst:styrene (1:100); Styrene: TBHP (1:2.5); Temperature: 80 °C.
The yields to benzaldehyde for both catalysts are comparable, however, catalysts 1 are more selective to styrene oxide than catalysts 2 (Fig. 4.6). The TONs towards benzaldehyde for catalysts 2 are slightly greater than catalysts 1, however, the TONs towards styrene oxide are higher for catalysts 1 (Table 4.6). This is an indication that deeper oxidation is more prevalent when using catalysts 1, which also reflects their high activity. The catalyst with the chlorine substituted phenyl group (1e) is most selective to styrene oxide, also giving the highest yield (28%) and TON (23). The catalysts with the methoxy substituent (f) and the unsubstituted phenyl group (d) exhibit similar activity and selectivity. Catalysts with the cyclohexyl (a) and isopropyl (b) substituted nitrogen atoms are comparable in terms of activity and yield to
0 1 2 3
0 20 40 60 80 100
a b c d e f
Rate of reaction /mol s-1
% Conversion
Catalysts
Catalysts 1- Conversion Catalysts 2- Conversion Catalysts 1- Reaction Rate Catalysts 2- Reaction Rate
benzaldehyde and styrene oxide and this is also noted in chromium catalysts bearing the same functional groups in ethylene oligomerisation.18
Figure 4.6. Yield to benzaldehyde and styrene oxide over catalysts 1 and 2 in MeCN.
Conditions: Catalyst:styrene (1:100); Styrene: TBHP (1:2.5); Temperature: 80 °C.
This is due to these ligands having similar basic properties. It has been reported that increasing the chain length by at least four carbons improves the selectivity, which accounts for the good selectivity to benzaldehyde and poor selectivity to styrene oxide by catalyst 1c.18 This could be due to the basic nature of catalyst 1c, where the increase in carbon chain length increases the basic nature of the catalyst. This controls the reaction whereby the metal goes from a M(I) to M(III) species, thus controlling the selectivity to one product (benzaldehyde).
Table 4.6. Turnover numbers for catalysts 1 and 2 towards benzaldehyde and styrene oxide.
Turnover number (TON)
Catalysts 1 2
Benzaldehyde Styrene Oxide Benzaldehyde Styrene oxide
a 23 19 25 15
b 24 15 27 10
c 25 5 27 12
d 26 12 23 7
e 18 23 22 17
f 24 12 27 9
Unlike other studies reported, such as styrene oxidation carried out by iridium cyclopentadienyl half sandwich complexes using PhIO as an oxidant where low yields to
- 5 10 15 20 25 30 35
a b c d e f
% Yield
Catalysts
Catalysts 1-Benzaldehyde Catalysts 2-Benzaldehyde Catalysts 1-styrene oxide Catalysts 2-Styrene oxide
benzaldehyde (6-11%) and no selectivity to styrene oxide was found, these systems achieve a relatively good yield to both benzaldehyde and styrene oxide. In comparison, bis(pyridylimino)isoindolato-iridium complexes gave a 55% conversion over a 48 h period with 50% yield to the epoxide.56 Furthermore, triphenylphosphine complexes of Ir and Rh used in the oxidation of styrene gave low yields to styrene oxide.29,30. Using a PNNP system, Stoop et al. also reported low conversions in the epoxidation of styrene.57
Figure 4.7. 31P NMR of the recovered and fresh catalysts 1c and 2c.
Figure 4.8. Conversion of styrene by recovered catalysts over 1c and 2c over three cycles in MeCN.
Conditions: Catalyst:styrene (1:100); Styrene: TBHP (1:2.5); Temperature: 80 °C.
0 10 20 30 40 50 60 70 80
- 5 10 15 20 25 30 35
1 2 3
% Conversion
% Yield
Cycle
Catalysts 1c- Benzaldehyde Catalyst 2c- Benzaldehyde Catalyst 1c- Styrene oxide Catalyst 2c- Styrene oxide
Conversion 1c Conversion 2c
Recovered
Fresh
1c 2c
34.1885 ppm34.1915 ppm 67.8052 ppm 67.6078 ppm67.8157 ppm 67.0709 ppm
The recovery of the catalyst was investigated. The used catalytic mixtures were evaporated to dryness to which diethyl ether was added. This caused the precipitation of the used catalysts.
The recovered catalysts were washed several times with diethyl ether and reused. It has been reported that when using phosphine based ligands the catalysts are destroyed due to progressive oxidation of the ligands.58 The used catalysts were recovered and characterized and the melting points (catalyst 1c recovered melting point: 263-264 °C) and NMR (Fig. 4.7) are comparable to those of the fresh catalysts. Furthermore, the recovered catalysts 1c and 2c were re-used over 3 cycles (Fig 4.8). However, the amount of catalyst recovered decreased over time, due to mechanical loss of the small quantities involved. The conversion over catalyst 1c after cycle 1 decreased slightly from the original run (53% to 41%) and thereafter increased to be essentially constant at 49% in cycle 2 and 48 % in cycle 3, results of which are probably within experimental error. Also, the repeat reactions were slightly more dilute.
The yield to benzaldehyde is comparable to the first run, however, the yield to styrene oxide increases from cycle 1 (2%) to cycle 3 (6%). The recycled catalyst 2c showed a decrease in conversion compared to the first run, but significantly increased in cycle 3 (from 45 % to 65
%). This could be due to concentration effects. The yield to benzaldehyde is comparable to the first run, however, the yield to styrene oxide decreased (from 15% to 5 %).
To elucidate the mechanism by which these two products form, styrene oxide was used as a substrate with catalyst 1 under conditions. The reaction was monitored over 9 h, during which time which no conversion took place. When benzaldehyde was used as a substrate, under the same conditions, the deeper oxidized product, benzoic acid and the cleaved product benzene formed. On the basis of this experimental work and the mechanism proposed by Li et al. on a cobalt system, a probable mechanism is proposed in Scheme 4.1.8 The metal complex (LM(I)) can activate and bind to oxygen from the oxidant (t-BuOOH) forming a peroxo LM(III) species (A) (eqn 1) which then reacts with styrene to form the intermediate (B) eqn (2).1,59-61 Rearrangement of intermediate B to form C (eqn 3). The formation of benzaldehyde and styrene oxide via two different pathways occurs through D and generation of the catalyst (eqn 4).1,18,59-61
Scheme 4.1. Proposed mechanism for the oxidation of styrene by complexes 1 and 2.