5.2 Results and discussion
5.2.4 Cyclic voltammetric analysis
Table 5.6
The cyclic voltammetry data of ferrocenylimidazolium salts showing the oxidation (anodic) and reduction (cathodic) peak potentials (Epa and Epc, respectively) and peak currents (ipa and ipc, respectively).
Compound Epa/V Epc/V E1/2/V ∆E (Epc – Epa) /V ipcµ/A ipaµ/A ipc/ipa
Ferrocene 0.479 0.393 0.436 0.086 10.7 8.88 1.20
5.2 0.577 0.506 0.542 0.071 6.53 5.77 1.13
5.3 0.631 0.553 0.592 0.078 8.93 10.5 0.85
5.4 0.601 0.512 0.557 0.089 2.47 2.29 1.08
5.5 0.621 0.540 0.581 0.081 4.39 2.68 1.64
5.6 0.637 0.553 0.595 0.084 8.90 8.10 1.10
5.7 0.619 0.547 0.583 0.072 5.59 3.63 1.54
5.8 0.518 0.446 0.482 0.072 4.33 3.74 1.16
5.9 0.565 0.494 0.529 0.071 6.86 7.93 0.87
5.10 0.524 0.458 0.491 0.066 0.94 1.55 0.61
5.11 0.536 0.458 0.497 0.078 0.79 0.80 0.99
5.12 0.547 0.458 0.503 0.089 0.39 0.37 1.06
In addition, the data in Table 5.6 show a variation of the redox potentials of the ferrocenyl with methyl (5.2-5.7) or phenyl (5.8-5.12) linkers to the imidazolium moiety. The methyl linkers exhibited a higher formal electrode potential than the phenyl ones. This can be attributed to the delocalization of electrons in the phenyl ring, which decreases the electron-withdrawing effect of the imidazolium salts. This renders the E1/2 value in compounds 5.8-5.12 less positive than those of compounds 5.2-5.7. Batterjee and co-workers have reported a similar effect in which the electron potential shifted to a lesser positive potential as a result of the introduction of a double bond in their compound II as compared to compound I (from 0.317 V in I to 0.302 V in II).43
Furthermore, as the ionic size increases, oxidation becomes more difficult. Thus, a shift to higher positive potential was observed. This trend occurred with both the methyl (5.3 and 5.4) and phenyl (5.9 and 5.10) spacers in the ferrocenylimidazolium salts on changing their counter-ions from bromide to iodide (see Table 5.6). A possible explanation could be that as the anion size increases, there is a relatively larger charge distribution and the withdrawing ability of the imidazolium salt
increases. As a result, a lower electron density is experienced by the ferrocenyl centre, thus oxidation becomes difficult. We have in the past probed the effect of increasing the anionic size using 1H NMR spectroscopy and have found that as the anionic size of the ferrocenylimidazolium salt decreases, the chemical shift of the resonances in the 1H NMR spectrum shifted downfield.38 This confirmed that the anion size has an influence on the electrostatic interaction between the imidazolium moiety and the ferrocenyl substituent.
Figure 5.6 Comparison of the cyclic voltammograms of compounds 5.2, 5.4, 5.5 and 5.6 with that of ferrocene. Arrows (→ or ←) indicate the scan direction.
5.2 Ferrocene
← Cathodic
→ Anodic 5.6
5.5
5.4
Figure 5.7 Comparison of the cyclic voltammograms of ferrocenyl compounds 5.8, 5.10, 5.11 and 5.12 with that of ferrocene. Arrows (→ or ←) indicate the scan direction.
5.2.5 Catalytic transfer hydrogenation
The application of the ferrocenylimidazolium salts was investigated in transfer hydrogenation of both saturated and unsaturated ketones. In an exploratory experiment, acetophenone was used as substrate and 0.05 mol% of the ferrocenylimidazolium compounds were used as catalysts. The solvent, propan-2-ol, was also the H-transfer agent and the reaction mixture was refluxed at 82 oC for 12 h. Compounds 5.2-5.12 showed high activity towards the conversion of acetophenone to 1- phenylethanol as shown in Table 5.7. A turnover number (TON) up to 1880 was observed, which was comparable to some data for precious metal catalysed reactions reported earlier.44 The initial encouraging results prompted us to extend the catalytic system to other ketones in order to establish its scope and limits.
5.12 Ferrocene
5.8 5.10
5.11
→ Anodic
← Cathodic
For this purpose compounds 5.6 and 5.12 were selected since they gave the highest conversion which is attributable to the shift to more positive potentials than their counterparts (see Table 5.6).
They are almost similar in structure, apart from their linkers i.e. methyl and phenyl linkers of the ferrocenyl to the imidazole respectively. Table 5.8 shows the results obtained. By introducing an electron-donating group to the ortho-position of cyclohexanone, the % conversion obtained was drastically reduced (Table 5.8, entry 2). A better conversion was obtained, when the same electron- donating group was moved to the para-position. An increase in the % conversion was obtained by changing the electron-donating group from the para- to the ortho- position by Enthaler and co- workers.45 Introduction of an electron-withdrawing group to acetophenone brought about a reduction in % conversion (entry 9), which was even more pronounced when a more electron- withdrawing group was attached to the para-position of acetophenone (entry 8). The cyclic aliphatic ketones gave a better % conversion than the acyclic aliphatic ketones. This can be attributed to an easy access to the C=O bond in cyclic aliphatic ketones as compare to more sterically hindered acyclic aliphatic ketones, thus making reduction difficult.
Table 5.7
Transfer hydrogenation of acetophenone catalysed bycompounds 5.2-5.12.
O OH
KOH, 5.2-5.12 i-PrOH, 82 oC, 12 h
Compound Conversion (%)a TOFb TONc
5.2 63 105 1260
5.3 72 120 1440
5.4 66 110 1320
5.5 70 117 1400
5.6 80 133 1600
5.7 74 123 1480
5.8 57 95 1140
5.9 68 113 1356
5.10 61 102 1220
5.11 69 115 1380
5.12 94 157 1880
aConversion was determined by GC analysis
bTurnover frequency (TOF) = mol product/(mol catalyst x time), determined after time t
cTurnover number (TON) = mol product/mol catalyst
Furthermore, the investigation was extended to three aliphatic unsaturated ketones. Two of the α,β-unsaturated ketones were not converted to the corresponding alcohols, instead they were reduced to saturated ketones at high yields (entries 10 and 11). Noteworthy, 5-hexen-2-one was significantly converted to the corresponding alcohol, showing the selectivity of the catalyst system.
Overall, compound 5.12 gave the best conversion in comparison to 5.6. The high activity exhibited by 5.12 can be attributed to its molecular structure, which also contributed to the high melting point, shift to higher resonance of the imidazole carbon in the 1H NMR spectrum and shift to more positive potentials in cyclic the voltammetry.
Table 5.8
Transfer hydrogenation of saturated and unsaturated ketones catalysed by compound 5.6 and 5.12.
R2 R1
O
R2
R1 OH
KOH, 5.6 or 5.12 i-PrOH, 82 oC, 12 h
Entry Ketones %Conversiona
(5.6)
%Conversiona (5.12)
TONb (5.6)
TONb (5.12)
1
O 76 83 1520 1660
2
O 50 20 1000 400
3 O 62 93 1240 1860
4
O 9 6 180 120
5
O
9 24 180 480
6
O
13 11 269 220
7
O
10 5 200 100
8
O
F 21 58 420 1160
9
O
Cl 31 62 620 1240
10[c]
O
68 57 1360 1140
11[c]
O
89 85 1780 1700
12
O
30 48 600 960
aConversion was determined by GC analysis
bTurnover number (TON) = mol product/mol catalyst
cConverted to saturated ketone