Significant effect of 5,10,15,20-meso-tetraarylporphyrinatoiron(III) chloride/triflate and acidic/neutral/basic imidazolium ionic liquids in catalytic oxidation of phenols

Article abstract of DOI:10.1016/j.molliq.2018.03.017

The influence of acidic, neutral and basic ionic liquids and their binary mixture with dichloromethane on the reactivity of iron(III)porphyrins was investigated during oxidation of phenols with hydrogen peroxide catalysed by 5,10,15,20-tetraarylporphyrinatoiron(III) chloride and 5,10,15,20-tetraarylporphyrinatoiron(III) triflate. The generation of different intermediates of iron(III) porphyrin in different ILs was studied through viscosity, density, UV–Vis and ¹H NMR spectroscopy. The heterolytic cleavage efficiency of (TAP)Fe^III-OOH and formation of quinone using iron(III)porphyrin (TAP)Fe^IIICl with Cl atom as an axial ligand, is influenced by the structure of imidazolium moiety and the counteranion following the order [(CH₂)₄SO₃HMIm]CF₃COO > [Hmim]CF₃COO > [bmim]TFA ?? negligible amount in [bmim]CF₃SO₃, [Hmim]CF₃SO₃, [bmim]BF₄, [bmim]PF₆ and [bmim]Cl. On the other hand, the heterolytic cleavage efficiency of (TAP)Fe^III-OOH with iron(III)porphyrin (TAP)Fe^IIICF₃SO₃ with triflate as an axial ligand, was found in the following order [(CH₂)₄SO₃HMIm]CF₃SO₃ > [Hmim]CF₃SO₃ > [(CH₂)₄SO₃HMIm]CF₃COO > [Hmim]CF₃COO > [bmim] CF₃COO > [bmim]PF₆ ≈ [bmim]BF₄ ≈ [bmim]CF₃SO₃, while epoxidation and polymerization were mainly observed in basic and neutral ILs. The reactive intermediates formed by the reaction of monooxygen donors with (TAP)Fe^IIICl varied with ILs, as (TAP)^+?Fe^IV = O intermediate was dominated in acidic ILs, while (TAP)Fe^IV = O was formed in neutral ILs and (TAP)Fe^III-OO^– was formed in basic ILs.

Full text of DOI:10.1016/j.molliq.2018.03.017

Accepted Manuscript
Significant
effect
of
5,10,15,20-meso-
tetraarylporphyrinatoiron(III) chloride/triflate and acidic/neutral/
basic imidazolium ionic liquids in catalytic oxidation of phenols
Sohail Ahmad, Renu Gautam, Anchal Singhal, S.M.S. Chauhan
PII:
DOI:
Reference:
S0167-7322(17)35450-8
doi:10.1016/j.molliq.2018.03.017
MOLLIQ 8791
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
12 November 2017
31 January 2018
3 March 2018
Please cite this article as: Sohail Ahmad, Renu Gautam, Anchal Singhal, S.M.S. Chauhan
, Significant effect of 5,10,15,20-meso-tetraarylporphyrinatoiron(III) chloride/triflate and
acidic/neutral/basic imidazolium ionic liquids in catalytic oxidation of phenols. The
address for the corresponding author was captured as affiliation for all authors. Please
check if appropriate. Molliq(2017), doi:10.1016/j.molliq.2018.03.017
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ACCEPTED MANUSCRIPT
Significant effect of 5,10,15,20-meso-
tetraarylporphyrinatoiron(III) chloride/triflate and
acidic/neutral/basic imidazolium ionic liquids in catalytic
oxidation of phenols
Sohail Ahmad, Renu Gautam, Anchal Singhal* and S. M. S. Chauhan*
Department of Chemistry, University of Delhi, Delhi 110007, India
*Corresponding authors email: smschauhanchemistry@gmail.com, singhal.anchal@gmail.com
Abstract: The influence of acidic, neutral and basic ionic liquids and their binary mixture
with dichloromethane on the reactivity of iron(III)porphyrins was investigated during
oxidation of phenols with hydrogen peroxide catalyzed by 5,10,15,20-
tetraarylporphyrinatoiron(III) chloride and 5,10,15,20-tetraarylporphyrinatoiron(III)
triflate. The generation of different intermediates of iron(III) porphyrin in different ILs
1
was studied through viscosity, density, UV-Vis and H NMR spectroscopy. The
heterolytic cleavage efficiency of (TAP)FeIII-OOH and formation of quinone using
iron(III)porphyrin (TAP)FeIIICl with Cl atom as an axial ligand, is influenced by the
structure of imidazolium moiety and the counteranion following the order
[(CH2)4SO3HMIm]CF3COO > [Hmim]CF3COO > [bmim]TFA »» negligible amount in
[bmim]CF3SO3, [Hmim]CF3SO3, [bmim]BF4, [bmim]PF6 and [bmim]Cl. On the other
hand, the heterolytic cleavage efficiency of (TAP)FeIII-OOH with iron(III)porphyrin
(TAP)FeIIICF3SO3 with triflate as an axial ligand, was found in the following order
[(CH2)4SO3HMIm]CF3SO3
>
[Hmim]CF3SO3
>
[(CH2)4SO3HMIm]CF3COO
>
[Hmim]CF3COO > [bmim] CF3COO > [bmim]PF6 ≈ [bmim]BF4 ≈ [bmim]CF3SO3,
while epoxidation and polymerization were mainly observed in basic and neutral ILs. The
reactive intermediates formed by the reaction of monooxygen donors with (TAP)FeIIICl

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varied with ILs, as (TAP)+•FeIV=O intermediate was dominated in acidic ILs, while
(TAP)FeIV=O was formed in neutral ILs and (TAP)FeIII-OO- was formed in basic ILs.
Keywords: Porphyrins, ionic liquids, catalysis, phenols
Introduction
Ionic liquids (ILs) are excellent green reaction media for synthesis, where the reaction
rates are significantly altered as compared to organic solvents that makes them greener
replacements for traditional volatile organic solvents in a variety of fields such as
chemical synthesis,1 catalytic chemistry,1 electrochemical devices,2 separation and
biomass dissolution3 due to their unique physicochemical properties. The ILs have been
classified in various ways depending upon their properties such as acidic, basic and
neutral, depending upon the organic cations and anions. Different properties can be
achieved by tuning the coupling of cations and anions, and hence ILs are considered as
designable solvents. This makes it possible to introduce ILs into the organic
transformations,4 enzyme stablizing,5a protein folding-defolding5b, polymer synthesis6
and solubilize organic7a and inorganic complexes.7b The viscosity of an ionic liquid has
remarkable influence on the course of chemical reactions and influences the selectivity
and solubility of products. Most ionic liquids have undesirable high viscosities; one
possible way to lower their viscosities is by addition of water or some other organic
solvents. Various studies have been done in binary mixture of ILs and organic solvent
and demonstrated that the addition of cosolvents like water, methanol, etc. can strongly
affect the physical and chemical properties of ionic liquids such as viscosity, electrical
conductivity and reactivity as well as solvation and solubility properties.8 It has been
reported that the rates of Diels-Alder reaction decreased by gradually increasing the
volume fraction of ionic liquids in binary mixture of ILs and organic solvents.9 In
engineering, low viscosities are preferred as operational costs such as stirring, mixing,
and pumping can be reduced in practice.
In metalloporphyrin catalyzed oxidation reactions,10 similar to cytochrome P450
catalyzed metabolic reactions in biological systems, various electrophilic and
nucleophilic catalytic intermediate species such as ferric hydroperoxy complex

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(TAP)FeIII–OOH), ferric peroxy anions (TAP)FeIII–OO–) and oxoferrylporphyrin π-cation
radicals (TAP)FeIV=O•+), have been postulated. The peroxidase and catalase-like activity
of 5,10,15,20-tetraarylporphyrinatoiron(III) chlorides (TAP)FeIIICl have been extensively
studied with various oxidants such as peroxyacids and hydroperoxides,10 with the
intention of elucidating the mechanisms of O-O bond activation and oxygen atom transfer
reactions. The meso substituents and axial ligands of metalloporphyrins influence the
oxidative potential of the reactive intermediates formed by the reaction of monooxygen
donors with (TAP)FeIIICl in different reaction conditions.11 The nature of the added axial
ligand greatly influenced the product distribution, the rate of overall reaction and the
stability of the catalyst toward degradation.12 The conventional axial ligands like
pyridine/imidazole or their derivatives could stabilize metalloporphyrins/non-porphyrin
and promote the catalytic reaction in a selective and desirable manner.13 Nature of solvent
comprises the major factor for deciding the catalyst stability and recovery of valuable
metalloporphyrin in the recycling procedures and minimized intermolecular self-
aggregation due to π-π stacking.14 Hence, many chemical and biochemical
transformations catalysed by metalloporphyrins have been developed in ionic liquids and
reported in high yields and increased selectivity15 with good activity and the improved
stability.
Herein, we have systemically investigated the significant effect of tuning of
5,10,15,20-meso-tetraarylporphyrinatoiron(III) chloride/triflate, acting as catalyst in pure
neutral, acidic and basic imidazolium ionic liquids with different cations and anions and
their binary mixture in dichloromethane on the formation of products during the reaction
of phenols with hydrogen peroxide. The coordination of ILs and generation of different
intermediates of iron(III) porphyrin in different ILs were studied through viscosity,
density, UV-Vis and 1H NMR spectroscopy.
Results and discussion
Reaction of phenols with hydrogen peroxide catalysed by iron(III) porphyrins in binary
mixture of acidic ILs (2a-2f) and dichloromethane.
The reaction of 2-methoxyphenol 3a with H2O2 catalysed by 5,10,15,20-tetra(2’,6’-
dichlorophenyl)porphyrinatoiron(III) chloride (Cl8TPPFe(III)Cl, 1a, Figure 1) in

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dichloromethane (DCM) gave minor amount of 2-methoxy-1,4-benzoquinone 4a and
major amount of polymeric products (polyguaiacol 6) (entry 1, Table 1, Scheme 1). The
addition of acidic ionic liquid [(CH2)4SO3HMIm]CF3COO 2a (Figure 2) to DCM
markedly enhanced the yield of 4a. The reaction of 3a catalysed by 1a with H2O2 in 3:
0.2 binary mixture of DCM and IL 2a possessing viscosity of about 17.3 cP, gave 4a and
6 in 16% and 48% yields respectively (entry 2, Table 1). Further addition of IL 2a to
DCM dramatically increases the viscosity (17.3 to 69 cP) and density of the solution, thus
increases the yield of the oxygenated product 4a (entry 3-6, Table 1). The reaction of 3a
in 3: 1 binary mixture of DCM and IL 2a possessing viscosity of about 69 cP gave 4a in
89% yield (entry 6, Table 1), whereas the yield of oxygenated product 4a was found to
decrease very sharply in the ionic liquids possessing viscosities of about 120-178 cP
(entry 7-9, Table 1). Thus, it can be demonstrated that very low and very high viscosity
of the reaction mixture is not favourable for the formation of oxidation product 4a.
1H NMR is an important technique to understand the mechanistic aspects of the
reaction while exchange of axial ligand of iron(III) porphyrin with IL. The 1H NMR of 1a
1
was recorded in binary mixtures of IL and DCM of different viscosities. The H NMR
spectrum of 1a in binary mixtures of dichloromethane and acidic ionic liquid 2a (3: 1, 3:
0.8, 3: 0.6, 3: 0.4, 3: 0.2 and equimolar mixture of 1a and 2a) without stirring showed no
change in chemical shifts of β-pyrrolic and aromatic protons of 1a, but splitting in β-
1
pyrrolic proton was observed. While in the H NMR spectrum of IL 2a, broadening was
appeared in peaks with shifting in C-2 hydrogen of imidazole. There is a possibility that
either CF3COO- counteranion of IL coordinated at 6th position of 1a and holds
responsibility for splitting in β-pyrrolic proton, or C-2 hydrogen of imidazolium IL
coordinated to ortho disubstituted chloride of 1a and disturbed the chemical environment
of 1a which resulted in the splitting of β-pyrrolic proton. The later possibility was ruled
out by changing the IL 2a with bmimBF4 which contains weak nucleophilic counteranion
-
BF4 and catalyst 1a with 5,10,15,20-tetraphenylporphyrinatoiron(III) chloride
(TPPFeIIICl) which does not contain ortho substituted chloride to avoid the interference
of ortho (2 and 6) disubustituted chloride of 1a. IL bmimBF4 contains weak nucleophilic
-
counteranion BF4 which does not show strong coordination with C-2 hydrogen of
1
imidazole,16a thus the interference in the reaction medium is diminished. The H NMR

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spectrum of equimolar mixture TPPFeIIICl and bmimBF4 showed similar spectroscopic
behaviour and no change in chemical shifts of β-pyrrolic and aromatic protons was
observed, whereas the splitting in β-pyrrolic protons (Figure 3) and broadening in IL
peaks along with shifting in C-2 hydrogen (Ha, Figure 4) of imidazole was detected. The
C-2 hydrogen of bmimBF4 was shifted from δ 8.86 to 9.08 ppm. In the reaction medium
(without stirring), counteranion CF3COO- was not able to dissociate Fe-Cl bond of 1a,
but IL showed some coordinating ability with catalyst, either by hydrogen bonded
interaction between imidazolium ring protons of ionic liquid and Cl axial ligand of
metalloporphyrin or by π-π interactions of IL with catalyst. The C-2 hydrogen of
imidazolium ion exhibits acidic character and possesses hydrogen bond donor ability16
and the Cl axial ligand of catalyst acts as hydrogen bond acceptors. Thus, the downfield
shift of the C-2 proton signal and splitting in β-pyrrolic protons is an indication of
formation of hydrogen-bonded network between the C-2 hydrogen of IL and axial ligand
1
of catalyst. The broadening of peaks of catalyst and ILs in H NMR spectrum is due to
the π-π interactions of IL with catalyst. When the 1H NMR spectrum of 1a was recorded
after 10 min stirring in 3: 1 binary mixture, the β-pyrrolic and aromatic protons of 1a
were shifted from δ 80.7, 14.1, 12.8 and 8.3 ppm17 to δ 76.3, 13.5, 12.4 and 8.5 ppm
respectively (Figure 5). The electronic absorption spectra of 1a with IL 2a was recorded
in DCM and it showed a blue shift in Soret and Q band (410.3 nm and 508.4 nm) as
compared to Soret and Q bands of 1a in DCM (417 nm and 510.3 nm) ESI†. It clearly
suggests the dissociation of Fe-Cl bond of (TAP)FeIIICl by CF3COO- counteranion of IL
and generation of (TAP)FeIIIOOCCF3 species, which are in good agreement with the
values for 5,10,15,20-tetra(2’,6’-dichlorophenyl)porphyrinatoiron(III) triflouroacteate.17
It is observed that the iron(III)porphyrin 1a in 3: 0.8, 3: 0.6, 3: 0.4 and 3: 0.2 binary
mixtures of dichloromethane and 2a after 10 min stirring, undergo slower exchange
(50%, 30%, 15% and 5% respectively) as compared with 3: 1 binary mixture (Figure 5).

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Figure 1. Structure and symbol of iron(III) porphyrins
Figure 2. Structure and symbol of acidic ionic liquids (ILs)
Scheme 1 Catalytic oxidation of phenol

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Table 1. Reaction of 3a with H2O2 catalyzed by 1a in acidic imidazolium ionic liquids (2a) and
their binary mixturea
Entry
Substrate
Catalyst
Dichlorometahne: 2a
Densityc
(g mL-1)
1.318
Viscosityc
(cP)
4ab
6b
Yield (%) Yield (%)
3: 0
3: 0.2
3: 0.4
3: 0.6
3: 0.8
3: 1
1
2
3
4
5
6
7
8
9
0.393
3
56
48
43
43
41
0
3a
3a
3a
3a
3a
3a
3a
3a
3a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1.316
1.315
1.318
1.321
1.333
1.346
1.353
1.364
17.37
26.95
31.01
48.81
69.04
120.7
178.7
5402.1
16
38
41
69
89
88
76
71
3: 2
0
3: 3
0
0: 3
0
a Reaction condition ratio, substrate: catalyst: H2O2 = 1: 0.005: 1; reaction was checked after 30 minutes. b
The yield of 4a and 6 are based on the substrate 3a. c at 25 °C
Scheme 2 Formation of hydrogen-bonded ring cluster of iron(III)porphyrin and ionic liquid

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The above results may be explained by ion-pair formation and supramolecular
aggregates formation of cations and anions of imidazolium IL in DCM18-20. At lower
concentration of IL in DCM, aggregates such as triple ions, contact ion pairs and solvent-
separated ion pairs are dominated,19 which greatly affect the exchange of axial ligand of
1a with counteranion of IL. In IL-DCM mixture of 69 cP viscosity, iron(III)porphyrin Cl
axial ligand preferentially participated in hydrogen bonded ionic liquid network and
formed stable aggregate rather than π-π stacking interactions among the imidazolium
rings and 1a. The exchange was completely observed in 3: 1 binary mixture (viscosity =
69 cP), which showed higher yield and selectivity of 4a than any other combination of
DCM and IL. Dissociation of Fe-Cl bond of (TAP)FeIIICl was suggested by the initial
coordination of axial ligand with IL and formation of hydrogen-bonded ring cluster
(Scheme 2), which was responsible to activate the breaking of Fe-Cl bond. Due to the
polar nature of ionic liquids, it is also accepted that the activation barrier of the reactions
can be reduced by the stabilization of the polar transition states or reaction intermediates
in polar ionic liquids.21 The viscosity effects on the activation energy of bond breaking in
the transition state have been reported in the reaction of bromination of alkynes in ionic
liquids.22 However, the dissociated [(TAP)Fe]+ species were stabilized by IL and existed
as ring cluster species. This is in accordance with the interaction of methanol with iron-
porphyrins.23 At higher concentration of IL (more viscous), the polymeric supramolecular
structure of IL was predominated and therefore, the reaction of 3a in higher concentration
of IL (3: 2 – 0: 3) was slowed down,19 as it might be difficult for reactants to interact with
each other due to dense solvent environment.
1
Figure 3. H NMR spectra from 0 to 100 ppm. A - TPPFe(III)Cl (1a) 0.023 Mm and [bmim]BF4 0.023
Mm. B - 1a, 0.023 M

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Figure 4. Selected region of 1H NMR spectra from 0 to 15 ppm. A - TPPFe(III)Cl (1a) 0.023 Mm and
[bmim]BF4 0.023 Mm B - 1a, 0.023 Mm and C - [bmim]BF4, 0.023 Mm
Figure 5. 1H NMR spectra of 1a in presence of 2a in CDCl3.
(A) -1a. (B) - 1a in 3: 0.8 mixture of
DCM and 2a stirred 10 min, dried and subjected to NMR (C ) - 1a in 3: 1 mixture of DCM and 2a stirred
10 min, dried and subjected to NMR.

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Table 2. Reaction of 3a with H2O2 catalysed by 1(a-c) in acidic imidazolium ionic liquids 2(a-f)a
4ac
6ac
Entry
Acidic Ionic Liquids
Catalyst
Yield (%)
Yield (%)
1aa
1aa
1aa
1aa
1aa
1aa
1bb
1bb
1bb
1bb
1bb
1bb
1ca
1ca
1ca
1ca
1ca
1ca
1
2
0
3
9
2
2b
2c
2d
2e
2f
3
0
11
9
4
4
5
73
95d
88
2
3
6
0
2a
2a
2b
2c
2d
2e
2f
7
0
8
12
0
9
94
0
10
11
12
13
14
15
16
17
18
6
90
79
56
0
2
3
11
1
2a
2b
2c
2d
2e
2f
2
1
0
2
2
3
23
3
aReaction was performed in mixture of dichloromethane and IL of 69 cP viscosity. bReaction was
performed in mixture of dichloromethane and IL of 31 cP viscosity. All reactions are monitored after 30

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c
minutes. Reaction condition ratio, 3a: catalyst: H2O2 = 1: 0.005: 0.5. at 25 °C. The yield of 4a and 6a are
d
based on the substrate 3a. C6H5IO is used as oxidant. Reaction condition ratio, 3a: catalyst: PhIO = 1:
0.005: 2.
With the optimized reaction conditions in hand, we further investigated the role of
different cations and anions of ILs and the scope of the method by generating the solvent
mixture consisting of an ionic liquid and dichloromethane of different viscosities with a
view to understand the effect of viscosity of ionic liquids on the formation of products
during the reactions. The best results were obtained in 3: 1 binary mixture of 2a and
dichloromethane with viscosity of 69 cP. Hence, acidic ILs having different cations and
anions in the binary mixture of dichloromethane with 69 cP were selected as optimum
reaction medium for the further study and results are given in Table 2.
From the results, it was observed that the formation of oxygenated product decreased
-
while using Cl- and CF3SO3 counteranions with same imidazolium cation
[(CH2)4SO3HMIm]+ (2b and 2c) or with different imidazolium cation [Hmim]+ (2d and
2e) (entry 1-4, Table 2). It indicates the remarkable effect of anions of ILs on the activity
1
of catalyst. In the titration studies with 2b and 2d ILs, no change in the H NMR
spectrum of 1a was observed. It could be explained due to the presence of same
counteranion (Cl-) and axial ligand (Cl) which led to the formation of Fe-Cl bond after
1
dissociation of Fe-Cl bond in 1a. Similarly, no change in the H NMR spectrum of 1a
with 2c and 2e ILs was observed neither in binary mixtures nor in pure ILs. The more
-
abundance of CF3SO3 in ILs compared to the Cl- dissociated from the (TAP)FeIIICl (1a),
directed the system towards ligand exchange process by preferential interaction of
-
-
CF3SO3 with [(TAP)Fe]+, but the interaction between CF3SO3 and [(TAP)Fe]+ was
-
relatively very weak due to low nucleophililicity of CF3SO3 as compared to Cl- anion.
1
Therefore, the H NMR spectrum of 1a isolated from the reaction mixture having IL 2c
and 2e showed only the presence of 1a even after 24 h. On the other hand, the reaction of
3a with hydrogen peroxide in IL 2f gave 4a in good yield (73%) (entry 5, Table 2). In the
1H NMR spectrum, the β-pyrrolic protons of 1a in the presence of 2f gradually shifted
and finally matched with the 1H NMR spectrum of Cl8TPPFeIIITFA.

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Scheme 3 Reaction mechanism of oxidation of phenol
The reaction of 3a with iodosylbenzene (C6H5IO) catalyzed by iron(III)porphyrin 1a
gave 4a in 95% yield (entry 6, Table 2). The formation of quinone 4a may be explained
by the formation of high valent-oxo-iron(IV) radical cation from C6H5IO/m-CPBA or
H2O2 as monooxygen donors in IL 2a.24,25 Hydrogen peroxide was coordinated to the
exchanged axial ligand iron(III) porphyrin 7a (Scheme 3) from IL 2a and generated 8a.
The acidic IL 2a was involved in the protonation of terminal oxygen atom of 8a26 and
resulted in the heterolytic cleavage of O-O bond to afford 9a [TFA(TAP)+•Fe=O].25a,27
Figure 6. UV-vis spectral changes upon the addition of m-CPBA to a reaction solution containing
Cl8TPPFe(III)Cl (1a) in the presence of 2a. (a) A mixture of 1a (4.3 ×10-3 mmol) and 2a (binary mixture
3: 1) was stirred and an aliquots of the reaction mixture was taken out immediately after 15 min intervals,

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100 µL of was added to a UV cell containing DCM (2 mL). (b) m-CPBA (1.5 ×10-4 mmol, diluted in 50
µL of DCM) was added to a UV cell containing mixture of 1a and 2a (100 µL) in DCM (2 mL), at -70 °C.
The reactive intermediate 9a reacted with 3a and resulted in the formation of 4a by
initial hydroxylation followed by oxidation mechanism (ipso).10,28 The reaction
mechanism was supported by UV-Vis spectroscopic analysis. On the addition of one
equivalent oxidant (m-CPBA) to mixed solution of 1a-2a in CH2Cl2, the formation of a
new green species was observed that showed a Soret band at 391 nm and a broad Q-band
at 678.3 nm (Figure 6). The UV–visible spectrum of the green species was characteristic
for oxoiron(IV) porphyrin radical cation and was quite similar to the spectra of several
oxoiron(IV) porphyrin radical cation species (of cytochrome P450 Compound I)
generated by oxidizing ferric porphyrins containing weak axial ligands at lower
temperature.29
On the basis of the above results, IL 2a was found to be superior than 2b and 2c in
terms of activity and selectivity. The higher catalytic activity of 1a in IL 2a could be
explained by the generation of more electrophilic intermediate 9a in the reaction,26c
which showed greater selectivity towards electron rich phenols. The iron(III) porphyrin
with the stronger electron donors counteranion (Cl-) yielded low or no oxygenated
products in the reactions with H2O2.30 While, IL 2f (same CF3COO- counteranion) was
found less active than IL 2a and that can be explained on the basis of terminal SO3H
group. The SO3H proton of the side chain in IL 2a could easily approach and protonate
(TAP)FeIII–OOH intermediate as compared to IL 2f and therefore, exhibited better
catalytic activity. Similar protonation have been reported in cyto P-450, where the proton
is donated from a threonine unit on the enzyme. This process is also facilitated by several
other amino acid derivatives on the enzyme.25a,25b,31 Thus, the reactivity of the catalyst 1a
in acidic ILs followed the order in terms of cation skeleton and anion basicity as
[(CH2)4SO3HMIm]CF3COO (2a) > [Hmim]CF3COO (2f) »» negligible amount in
[(CH2)4SO3HMIm]Cl (2b), [(CH2)4SO3HMIm]CF3SO3 (2c), [Hmim]Cl (2d) and
[Hmim]CF3SO3 (2e).
5,10,15,20-tetra(2,6-dichlorophenyl)porphyrinatoiron(III)
triflate
(Cl8TPPFe(III)O3SCF3, 1b) catalyst containing CF3SO3 axial ligand was found to be
-
easily exchangeable (31 cP) with CF3COO-, Cl- and CF3SO3 counteranions in most of

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the ionic liquids. The oxygenation reactions carried out with catalyst 1b in binary mixture
of IL 2a and dichloromethane (31 cP), gave quinone in 88% yield (entry 7, Table 2,
-
Scheme 4) whereas with ILs 2c and 2e (CF3SO3 anion) under similar reaction conditions,
quinone was obtained 94% and 90% yield respectively (entry 9 and 11). This
demonstrated the higher catalytic efficiency of 2c and 2e compared to 2a. Due to lower
-
nucleophilicity of CF3SO3 anion in 2c and 2e than CF3COO- anion of 2a, it generates
more electrophilic intermediate 9 in the reaction. However, when identical reactions were
carried out with 1b in binary mixture of dichloromethane- 2b or with dichloromethane-
2d of 31 cP, only trace amount of oxygenated products were obtained (Table 2, entry 8
and 10). The coordination of phenol to iron(III) porphyrin (1b) was carefully examined
during the reaction by UV-Vis and NMR spectroscopy and no exchange of axial ligand
by phenol was observed in acidic ILs. Thus, the reactivity of the catalyst 1b in acidic ILs
can be established to follow the order in term of cation skeleton and anion basicity as
[(CH2)4SO3HMIm]CF3SO3 (2c) > [Hmim]CF3SO3 (2e) > [(CH2)4SO3HMIm]CF3COO
(2a) > [Hmim]CF3COO (2f). »» negligible amount in [(CH2)4SO3HMIm]Cl (2b) and
[Hmim]Cl (2d).
It was observed that the presence of peripheral electron withdrawing groups on the
iron(III) porphyrin ring (1a and 1b) enhance the heterolytic cleavage of 8a as compared
to electron donating group on the periphery of iron(III) porphyrin (1c) (entry 13 and 14,
Table 2).26c Thus, it is demonstrated that the catalytic efficiency was decreased for
porphyins containing electron donating substituents.
Scheme 4 Effect of reaction conditions on the formation of product

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Reaction of phenols with hydrogen peroxide catalysed by iron(III) porphyrins in binary
mixture of neutral ILs (2g-2k) and dichloromethane.
The aprotic ionic liquids with imidazolium cations (Figure 7) were then investigated
for the oxidation of phenol. The reaction of 3a with H2O2 catalysed by 1a in binary
mixture of dichloromethane and 2g (0.39 to 23 cP) gave higher amount of polymeric
products (6) and negligible amount of 4a (entry 1-3, Table 3). The polymeric products (6)
were characterized using GPC and found to have moderate molecular weights (Table 3)
with low polydispersity index (PDI) (ESI†) The yield of 6 increased further (65%-79%)
as the 2g content (27.5 to 65 cP) increased in binary mixture (entry 4-6, Table 4). The
reaction of 3a in pure 2g (233 cP) gave 6 in 88% yield and negligible amount of 4a and
5a (entry 7, Table 3). The homolytic cleavage of hydroperoxy intermediate
((TAP)FeIII•OOH) in dichloromethane is favored,32 which is responsible for formation of
polymeric products. It was observed that the gradual increase of IL in binary mixture of
dichloromethane increased the yield of polymeric products as well the polymeric chain
length (Mw) with low PDI (~1) as compared to use of pure DCM. This type of increased
electron transfer in ionic liquid as compared to molecular solvent has been previously
reported.20,9
Figure 7. Structure and symbol of neutral ionic liquids (ILs)
Similarly, the reactions of 3a in different binary mixture of dichloromethane with
[bmim]PF6 (2h)/[bmim]CF3SO3 (2i) were carried out and high yield of polymeric

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products were resulted along with small amounts of 4a and 5a. Also, the similar reaction
of 3a in pure IL 2h and 2i gave 6 in 94% and 95% yield respectively (entry 8-9, Table 3).
This result can be manifested in terms of nucleophilicity of counteranions of ILs. The
-
-
-
counteranions such as PF6 , BF4 and CF3SO3 are weakest nucleophiles and hence, were
not able to dissociate Fe-Cl bond of metalloporphyrin (1a). This is supported by the
observation that no change in chemical shifts of β- pyrrolic and aromatic protons was
observed (Table 4).
Once hydrogen peroxide is coordinated to unexchanged axial ligand iron(III)
porphyrin (1a) in neutral ILs and form the intermediate 8b, it resulted in the homolytic
cleavage of the O-O bond to afford 10b (Cl(TAP)Fe(IV)=O) and hydroxyl radical (Path
a, Scheme 5).32 The reaction mechanism was supported by UV-Vis spectroscopic
analysis in the presence of neutral IL (2g). On the addition of one equivalent oxidant (m-
CPBA) to mixed solution of 1a-2g in DCM, the formation of a new Q-band at 545 nm in
the UV–visible spectrum is observed which is characteristic for oxoiron(IV) porphyrin
(TAP)FeIV=O intermediate, obtained by homolysis of O–O bond (TAP)FeIII•OOH
(Figure 8).30 The oxoiron(IV) porphyrin complexes were found to be incapable of
hydroxylating phenol when oxygen transfer to the substrate is concerned. 32a While the
reaction of 3a with H2O2 catalysed by 1a in binary mixture of dichloromethane and
[bmim]CF3COO 2k (69 cP, exchange of axial ligand was completely observed at 69 cP)
gave 4a, 5a and 6 in 63%, 8% and 21% yields, respectively (entry 11, Table 3, Path b,
Scheme 5).
Table 3. Reaction of 3a with H2O2 catalysed by 1(a-b) in neutral imidazolium ionic liquids and
their binary mixturea
4ab
5ab
6b
Entry
Ionic
Catalyst Dichlorometahne: Densityc Viscosityc
Liquids
IL
(g mL-1)
(cP)
Yield
(%)
Yield
(%)
Yield
(%)
Mwt.
Neutral
2g
1
2
3
3: 0
1.318
1.321
1.318
0.393
20.36
23.01
4
4
4
0
0
0
54
56
55
300-600
330-615
348-681
1a
1a
1a
3: 0.4
3: 0.8
2g
2g

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4
5
3: 1
3: 2
1.314
1.303
1.291
1.123d
1.361d
1.220d
1.281
1.212
1.313
1.311
1.313
1.310
27.47
42.29
65.35
233d
452d
90d
4
4
0
0
1
2
0
0
0
8
4
5
5
6
65
71
79
88
94
95
86
21
8
1100
1312
2314
8702
8930
7284
2651
1231
1336
1614
1341
1354
2g
2g
2g
2g
2h
2i
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
6
3: 3
6
7
0: 3
6
8
0: 3
3
9
0: 3
4
10
11
12
13
14
15
3: 1.5
0.2: 3
3: 1.2
3: 0.9
0.9: 3
0.8: 3
68
4
2j
69
63
54
51
52
62
2k
2g
2h
2i
31
31
6
31
4
31
19
2k
a ,b as Table 1. c at 25 °C. d ref. 6b
Table 4. 1H NMR spectral positions of 1a and 1b complexes with ILs (Solvent CDCl3; Reference
Me4Si at 25 °C)
δ (ppm)
Catalyst
Pyrrole-H
Phenyl-H
Refrence
15a
Meta
Para
80.7
12.8
14.1
8.3
1a

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63
13.1
-
8.7
-
1b
1a +
80.6
12.7
14.1
8.5
15a
(2b, 2c, 2d, 2e,
2g, 2h, 2i, 2j)
(1a, 1b) + (2a,
2f, 2k)
76
12.4
12.9
13.1
13.1
13.2
9.0
8.6
8.7
8.4
15
-
62.4
60.7
63.1
-
-
-
1b + 2g
1b + 2h
1b + 2i
-
-
1b + (2g, 2h, 2i)
+ 2-
78.3
12.6
13.4
8.3
-
Methoxyphenol*
* -130 ppm (Phenol ortho-H); 108.8 ppm, 97.6 ppm (Phenol meta-H)
Scheme 5 Mechanistic steps for the synthesis of 4a and 5a in different reaction conditions

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Figure 8. (a) 1a-2g in DCM, (b) 1a-2g on addition of m-CPBA
The reaction of 3a with H2O2 catalysed by 1b in binary mixtures of 2g, 2h, 2i and 2k
ILs in 31 cP gave 4a in 54%, 51%, 52% and 62% yields, respectively (entry 12-15, Table
3). In the mixtures of 1b-[bmim]BF4 and 1b-[bmim]PF6, replacement of axial ligand
-
-
(CF3SO3) by PF6 and BF4 counteranions were observed in NMR spectra in which
shifting in β-pyrrolic proton of 1b from δ 63 ppm to δ 60.7 ppm (corresponding to
TPPFeIIIPF6) and δ 62.4 ppm (corresponding to TPPFeIIIBF4) (Table 4) was observed.
Further, it was noticed that the addition of phenol leads to generation of ironIII-phenol
-
-
-
complex (TPPFeIIIOPh) by exchange of PF6 /BF4 /CF3SO3 axial ligands with phenol
1
(Path c, Scheme 5). It was supported by H NMR and UV–Vis spectroscopic titrations.
1
Figure 9 shows the H NMR spectra of 1b in CDCl3, in which upon the addition of 2g,
the upfield shift of the pyrrole signal at 62.4 ppm was observed which indicated the
-
coordination of BF4 counteranion. On the other hand, on the addition of one equivalent
of phenol (3a) in the same mixture of 1b and 2g, a large downfield shift of pyrrole signal
at δ 78.3 ppm along with appearance of ortho-phenolic proton at δ -130 ppm, and the
meta-phenolic proton at δ 108.8 and δ 97.6 ppm were observed which clearly indicated
the coordination of phenol. No such coordination of phenols were observed in the
mixture of 1b-2k.
Above results revealed that the iron(III) porphyrin, either with phenoxide axial ligand13
or with CF3COO axial ligand undergoes heterolytic cleavage in neutral IL to give 4a

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(Path b and c, Scheme 5), but shows less selectivity in comparison to acidic IL 2a. Thus,
the reactivity of catalyst 1a towards heterolytic cleavage in neutral ILs was found to be as
[bmim]CF3COO (2k) »» negligible amount in [bmim]BF4 (2g), [bmim]PF6 (2h),
[bmim]CF3SO3 (2i) and [bmim]Cl (2j). While, the reactivity of the catalyst 1b followed
the order as [bmim]CF3COO > [bmim]PF6 = [bmim]BF4 = [bmim]CF3SO3.
Figure 9. 1H NMR spectra at 25 °C of CDCl3 solutions of (A) 1b, (B) 1b and 2g, (C) mixture of 1b, 2g and
phenol (3a)
Reaction of 3a and 4a with hydrogen peroxide catalysed by iron(III) porphyrins in
binary mixture of basic ILs (2l-2m) and dichloromethane.
The reaction of 3a with H2O2 catalysed by 1a in different binary mixtures (3: 0.1 - 0: 3)
of dichloromethane and basic IL ([bmim]OH, 2l) gave polymeric products in moderate
yields but with low PDI (~1) as shown in GPC results (ESI†) and the formation of 4a or
5a was not detected (entry1-9, Table 5). The exchange of chloride axial ligand of 1a with
hydroxyl anion of 2l to form 7c (TAP)FeIII-OH (Scheme 6) was observed even in lower
viscosity (2.91 cP) binary mixture of IL (3: 0.1). It might be due to the higher
nucleophilicity of hydroxyl anion.33

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Table 5. Reaction of 3a and 4a with H2O2 catalysed by 1a in basic imidazolium ionic liquids and
their binary mixturea
4ab
5ab
6b
Entry
Basic Catalyst Substrate DCM: Viscosityc
Yield
(%)
Yield
(%)
Yield
(%)
Mwt.
IL
IL
2l
2l
2l
2l
2l
2l
2l
2l
2l
2l
2l
2l
2l
2l
2l
(cP)
0.391
2.911
9.212
17.87
22.21
36.94
43.11
76.61
-
1
2
3: 0
3: 0.1
3: 0.2
3: 0.4
3: 0.6
3: 0.8
3:1
0
0
0
0
0
0
0
0
0
-
0
0
53
51
49
43
35
31
33
33
26
56
49
41
38
23
11
301-800
423-810
1311
1531
1591
1610
2341
2891
7541
381
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
3a
3a
3a
3a
3a
3a
3a
3a
3a
4a
4a
4a
4a
4a
4a
3
0
4
0
5
0
6
0
7
0
8
3:2
0
9
0:3
0
10
11
12
13
14
15
3: 0
0.389
2.910
9.201
17.91
21.94
36.89
19
23
29
46
71
86
3: 0.1
3: 0.2
3: 0.4
3: 0.6
3: 0.8
-
459
-
1301
1492
1421
1629
-
-
-

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16
17
18
19
20
3: 1
3: 2
43.41
76.70
-
-
-
-
-
-
89
87
87
86
94
09
09
08
05
0
2187
4231
6115
2567
-
2l
2l
1a
1a
1a
1a
1c
4a
4a
4a
4a
4a
0: 3
2l
3: 1.6
3: 1
43.19
43.53
2m
2l
a ,b as Table 1. c at 25 °C
Scheme 6 Reaction mechanism in presence of basic ionic liquid
Figure 10. UV-Vis spectral change upon the addition of H2O2 to a reaction solution containing
Me8TPPFe(III)Cl 1c and BmimOH 2l. (a) 1c (1.1 ×10-3 mmol). (b) 1c and 2l. (c) H2O2 (1.1 ×10-3 mmol)
added to mixture of 1c and 2l UV taken after 15 second. (d) Mixture of 1c, 2l and H2O2 after 30 second.

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The intermediate 7c reacts with H2O2 and form the hydroperoxy intermediate (8c,
Scheme 6) which undergoes homolytic cleavage to form the intermediate 10c and
hydroxyl radical30,31 and is responsible for polymeric products. From the above results, it
is clear that the oxidation (polymerization) of phenol in the presence of basic IL
decreases on increasing the concentration of IL as compared to reaction catalysed in
neutral ILs and the complete consumption of phenol did not take place even after 24 h.
However, when identical reactions were carried out with 4a, the formation of epoxide 5a
was observed in good yield and on the addition of basic IL in the reaction; increase in the
yield of epoxide was observed (entry 10-16, Table 5).
Further, the yield of 5a was observed to be constant in higher ratio of IL (entry
17-18, Table 5) and thus 3: 1 ratio was found suitable for the high yield of 5a. The yield
of epoxide was markedly enhanced when reaction was performed by using catalyst 1c in
basic IL (3:1). The intermediate 8c in the presence of basic IL forms the intermediate
HO(TAP)FeIII-OO- (11c)34,35 which on reaction with 4a gave the epoxide (5a).34 The
formation of 11c was confirmed by the appearance of a Soret band at 431.5 nm and Q
band at 566 nm and disappearance of characteristic bands at 415 and 507 nm in the UV-
visible spectra of 1c on the addition of hydrogen peroxide in the mixture of 1c and 2l
(Figure 10).34,35
Further, the optimized reaction conditions were applied to various phenol derivatives
in different ILs in the presence of 1a to explore the generality of the reaction system and
the results are summarized in Table 6. In acidic ILs, methoxy and hydroxyl substituted
phenols are more prone to oxidation and gave quinone as major product. 2-methylphenol
and 2-methyl-1-naphthol gave 2-methyl-1,4-benzoquinone and 2-methyl-1,4-
naphthoquinone respectively in high yields with trace amount of methyl oxidized
products. Para-substituted phenols (4-nitro-phenol, 4-methoxy-phenol, p-halogenated
phenols) were shown to undergo ipso substitution easily and gave quinone as major
product. Phenols, substituted at aromatic ring through a carbon-carbon bond (such as p-
cresol, etc.), gave lower amount of quinone. It might be due to the stable carbon-carbon
bond, which resists the cleavage. While in neutral ILs, methoxy and hydroxyl substituted
phenols are more prone to oxidation and responsible to give polymer as major product in

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neutral ILs. While electron withdrawing substituted phenols (nitrophenol, chlorophenol)
were less prone to oxidation and gave low amount of polymer.
Reaction of p-halogenated phenol with H2O2 catalysed by iron(III) porphyrins (1a) in
ionic liquid (2a).
p-Halogenated phenol (1.0 mmol) were treated by the same procedure as that used for the
synthesis of 4a, giving 1,4-benzoquinone as yellow solid. Quinone formation decreased
in the order p-fluorophenol > p-chlorophenol > p-bromophenol (Scheme 7). Trend can be
explained on the basis of size of the halogen substituents (F < Cl < Br). The active
species, high valent iron oxocation radical is easily approachable to ipso position if
substituent is smaller in size as it gets more hindered in its approach to the ipso position
by a larger substituent.36
O
X
1a / H2O2
2a
OH
O
x
F
Quinone %
63
48
12
Cl
Br
Scheme 7 Oxidation of p-halogenated phenols

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Table 6. Oxidation of phenols in presence of 1a and different ionic liquids
2a
[bmim]BF4
[bmim]OH
Phenols
Quinone Polymer Quinone Polymer Quinone Polymer
(%)
(%)
(%)
(%)
(%)
(%)
69
0
7
92
0
51
63
41
2
4
2
0
21
28
0
0
42
21
51
54
78
0
0
0
0
2
5
6
1
0
0
0
5
3
5
16
83
71
53
0
0
0
3
1
5
49
54
8
0
0
0
31
19
8
Reaction of p-Hydroxybenzylalcohol and p-Cresol with H2O2 catalysed by iron(III)
porphyrins (1a) in ionic liquid (2a).
The compounds in which substituents are attached to the aromatic ring through a carbon-
carbon bond (such as p-hydroxybenzylalcohol, p-cresol, etc.), gave no yield of quinone,
because the carbon-carbon bond is very stable and has generally been thought to resist

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cleavage (Scheme 8). p-hydroxybenzylalcohol yielded p-hydroxybenzaldehyde in 93%
yield while no product was obtained in case of p-cresol.
CHO
OH
CH2OH
1a / H2O2
2a, 30 min
OH
93%
O
CH3
1a / H2O2
+
24 h
2a,
OH
O
Scheme 8 Selectivity of formation of product
Epoxidation Study
The epoxidation reactions were carried out directly with 1,4-naphthoquinone as substrate
(Scheme 9). In acidic IL using 1a as catalyst, no trace of epoxide was observed. When
similar reactions were carried out in basic ILs ([bmim]OH and [bmim]OAc) using 1
equivalent of hydrogen peroxide, epoxide products were obtained in 79% yield in
[bmim]OH and 71% in [bmim]OAc. While neutral ILs [bmim]BF4 and [bmim]PF6 gave
epoxide in 11% and 23% yields respectively (Table 7).
O
O
OH
1a, H2O2
1a, H2O2
O
[bmim]OH
or
[bmim]BF4
2a
O
O
[bmim]BF4
or
[bmim]OH/[bmim]OAc
No
Quinone and Epoxide
1a, H2O2
1a, H2O2
2a
No Epoxide
Scheme 9 Selective reaction conditions for synthesis of epoxide
Table 7. Epoxidation of 1,4-naphthoquinone using H2O2 in ILsa

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Ionic Liquids
Catalyst
Epoxide b
Yield (%)
[bmim]OH
[bmim]OAc
[bmim]BF4
[bmim]PF6
2a
79
71
11
23
0
1a
1a
1a
1a
1a
1c
[bmim]OH
91
aAll reactions were performed in 0.5 mL of ILs. Reaction condition ratio, 1,4-naphthoquinone: catalyst:
H2O2 = 1: 0.005:1, Reaction checked after 30 minutes. bThe yields are based on the substrate
O
O
O
X-TAPFe(III)-OH
1a / H2O2
[bmim]X
O
O
O-Fe(III)TAP-X
O
O-O-Fe(III)TAP-X
O
O
X = OH-, OAc-
Scheme 10 Epoxidation mechanism of 1,4-naphthoquinone intermediate in basic ILs
Recycling of catalyst and ionic liquids.
In order to demonstrate the recycling of catalyst, the catalyst immobilized in ionic
liquid phase was reused for the oxidation reactions. After the reaction, the cosolvent
DCM was removed under reduced pressure from the mixture of acidic, basic and neutral
Ils and then, on the addition of organic solvent, two phases were formed. The lower phase
was constituted by the IL while the upper layer was comprised of the formed product and
unreacted phenol. The product was easily separated from the IL by simply decanting the
organic phase into another flask. The IL (lower layer) was dried under reduced pressure;
the catalyst remained in IL and was reused for three to five times. The presence of an

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unchanged Soret band in the UV–Vis spectrum of IL containing the catalyst after the
reaction indicated the stability of the catalyst.
Table 8 shows the catalytic efficiency of catalyst 1a in 2a. From entries 1, 2 and 3 of
Table 8, it is clear that the recycling of the catalyst slightly affects the conversion and
yield of 4a, as catalytic activity slightly falls after two cycles. Due to the oxidant nature
of H2O2, the catalyst may suffer from degradation, which might be responsible for the
reduction of catalytic activity. It was noticed that the fresh addition of catalyst after first
and 2nd run (entry 4 and 5) into synthetic system enhanced the yield compared with 2nd
and 3rd run of recyclability process (entry 2 and 3). Even after the addition of fresh
catalyst, reaction did not lead to 100% conversion as compared to fresh cycle. This
indicated that the decrease in yield is not only by the deactivation of catalyst with H2O2
but also by the unexpected removal of ionic liquid during extraction process, resulting in
leaching of the ionic liquid and catalyst.
Table 8. Recycling of 2a and catalyst (1a) in the synthesis of 4a
Entry
Run
Conversion
(%)
4a (%)
1
2
3
1
100
93
84
79
64
2
3
77
IL-catalyst mixture after 1st run
+ fresh catalyst
4
5
95
79
81
75
IL-catalyst mixture after 2nd run
+ fresh catalyst
Experimental section

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General
All phenols were commercially available and used without further purification.
Hydrogen peroxide (30%) was obtained from Rankem Chemical Co., Polystyrene
standards of Mwt. ranging from 2000 to 30000 which was used in GPC analysis, were
purchased from the Sigma-Aldrich USA. The 1H and 13C NMR spectra were measured in
CDCl3 and DMSO-d6 at ambient temperature on a JEOL 400 MHz. The chemical shifts
were given in ppm related to tetramethylsilane (TMS) as internal standard. IR spectra
were recorded on a Perkin-Elmer spectrometer and were expressed in cm-1. UV-visible
spectra were recorded on a Perkin- Elmer Lambda 35 UV/VIS spectrophotometer and
absorption maxima were expressed in nm. Thermal analysis was performed using a
thermogravimetric analyzer (TGA), the flow rate of nitrogen was 50 ml/min and the
heating rate was 10° C/min. The molecular weights of polyguaiacol were measured using
gel permeation chromatography (GPC) with Waters LC module with three linear styergel
columns connected in series using THF as eluent at a flow rate of 1mL/min. Polystyrene
standards were used as molecular weight calibrants. HPLC analyses were performed on
Waters photodiode array detector (PDA) set at 254 nm on C18 column (150 mm×4.6
mm), using methanol and water (35:65, V/V) as eluent at a flow rate of 1 mL/min. The
density measurements (ρ) were performed with an Anton-Paar DMA 4500 M (Austria)
vibrating-tube densimeter, equipped with a built-in solid-state thermostat and a resident
program with accuracy of ± 0.03 K. The ρ values of the samples were measured at 298.15
K under ambient pressure with a precision of ± 0.00005 g cm-3. Proper calibration at each
temperature was achieved with doubly distilled, deionized water and with air as
standards. Vibro viscometer (model SV-10, A&D Company Limited, Japan) was used for
viscosity measurements. The instrument has been provided with two sensor plates of gold
coating. The measurements of η were taken from the digital display device attached to the
vibro viscometer. Viscometer was calibrated with pure water and purified methanol at
various temperatures. A thermostatically controlled, well-stirred circulated water bath
with a temperature controlled to ±0.01 °C was used for all the viscosity measurements.
Conclusion