Some novel manganese(III) porphyrins with catalytic properties

Article abstract of DOI:10.1080/00958972.2018.1511779

A series of manganese(III) porphyrins with?4-methylimidazole have been prepared. These are high-spin complexes having general formula [Mn^III(THMPP)X(4-MeIm)], where THMP = 5,10,15,20-tetra(4-hydroxy-3-methoxyphenyl)porphine ligand, X = Cl^?

Full text of DOI:10.1080/00958972.2018.1511779

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Manganese(III) porphyrins with oxidation
properties
Niharika Anand & Sudha Yadava
To cite this article: Niharika Anand & Sudha Yadava (2018): Manganese(III) porphyrins with
oxidation properties, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2018.1511779
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Manganese(III) porphyrins with oxidation properties
NIHARIKA ANAND and SUDHA YADAVA*
Department of Chemistry, D.D.U. Gorakhpur University, Gorakhpur – 273009, India
A series of manganese(III) porphyrins with 4-methylmidazole have been prepared. These are
III
high-spin complexes having general formula [Mn (THMPP)X(4-MeIm)], where THMP =
ˉ
ˉ
ˉ
ˉ
5
4
,10,15,20-tetra(4-hydroxy-3-methoxyphenyl)porphine ligand, X = Cl , Br , NCS or N and
3
-MeIm =ꢀ4-methylimidazole. All the complexes have been characterized by UV-visible, FT-IR,
ESI-MS spectra, elemental analyses and magnetic susceptibility measurements. These
manganese(III) porphyrins oxidize aromatic alcohols to aldehydes. The oxidation reactions have
been carried out at room temperature in the presence of oxidants such as NaIO , H O and
4
2
2
NaOCl. The comparative studies proved that NaIO behaves as the most efficient oxidant in
4
these oxidative transformation reactions.
Keywords: Manganese(III) porphyrins; 5,10,15,20-Tetrakis(4-hydroxy-3-methoxyphenyl)
porphine; 4-Methylimidazole; Benzyl alcohol; Catalyst
1
. Introduction
Metalloporphyrins have been widely studied in order to understand the biological activity of
natural compounds [1, 2]. Porphyrins form complexes with manganese(II), manganese(III) and
manganese(IV) [3] among which manganese(III) porphyrins are most stable [4]. Therefore, these
complexes can be used as better catalytic option for various enzymatic redox reactions. Synthetic
manganese(III) porphyrins show versatile catalytic behavior and are mainly used as models for
biological mono oxygen atom transport [5, 6] and metalloenzymes [7, 8]. Manganese porphyrins
are efficient catalysts of olefin epoxidations [9-13] and alkane hydroxylations [14-16].
Manganese porphyrin derivatives have also been used in the oxidative transformation of non-
degradable pollutants, discharged from paper mills, into degradable forms [17].
Due to these applications, we have explored a few manganese(III) porphyrins with
different axial ligands previously in our laboratory [18, 19]. The utility of manganese(III)
*Corresponding author. Email: dr_sudhayadava@yahoo.co.in

porphyrins in depolymerization of coal using humic acid as the coal model has also been tested
20]. As an extension of this work, we have synthesized and characterized a new series of
[
manganese(III) porphyrins with 4-methylimidazole as an axial ligand. The catalytic behavior of
these complexes in the oxidation of aromatic alcohols to aldehydes is also reported.
2
2
4
. Experimental
.1. Materials
-Methylimidazole and 4-methoxybenzyl alcohol were purchased from the Chemical Centre,
Mumbai. Vanillin i.e. 4-hydroxy-3-methoxybenzaldehyde, benzyl alcohol, sodium periodate and
hydrogen peroxide were purchased from s.d.fine-chem limited, Mumbai. Sodium hypochlorite
was purchased from Merck, Mumbai. All chemicals were of A.R. grade. Pyrrole (Merck,
Germany) and acetylacetone (s.d. fine-chem limited, Mumbai) were freshly distilled before use.
Milli-Q water has been used wherever required from our laboratory (Waters Millipore).
2
.2. Physical measurements
FT-IR spectra were recorded on a Perkin Elmer spectrum II spectrophotometer using potassium
-1
bromide pellets (4000-400 cm ). Electronic spectra were recorded in our laboratory on a Hitachi
Japan) spectrophotometer model U-2900 from 190-900 nm using methanol as solvent. ESI-MS
(
spectra were recorded on a Waters UPLC-TQD. FT-IR and ESI-MS data were provided by
SAIF, CDRI, Lucknow. Elemental analyses were carried out on an Elementar Vario EL III by
SAIF, STIC, Cochin. Magnetic susceptibility measurements were obtained at room temperature
on a Vibrating Sample Magnetometer from IIT, Kanpur. Electrical conductivity data were
-3
recorded on a TOSHCON Auto ranging Conductivity Meter TCM 15+ by preparing 10 molar
solution in ethanol at room temperature.
2
2
3
.3. Synthesis
.3.1. Preparation of porphyrin [H THMPP]. For the synthesis of 5,10,15,20-tetra(4-hydroxy-
2
-methoxyphenyl) porphine [H THMPP], Adler’s method had been used with some
2
modifications [21] according to which 5.6 mL (0.08 mol) of freshly distilled pyrrole and 12.16 g
0.08 mol) of vanillin were added in 300 mL of boiling propionic acid. The reaction mixture was
refluxed for 2 h and then cooled, followed by 12 h of refrigeration. The tarry reaction mixture
(

was filtered and washed thoroughly with methanol until the filtrate became colorless. Shining
purple crystals of porphyrin were obtained, dried in a vacuum desiccator and recrystallized with
1
:1 ratio of chloroform and dichloromethane.
III
2
.3.2. Preparation of manganese(III) porphyrins [Mn (THMPP)X]. For preparation of
III
manganese(III) porphyrin complexes, first Mn (acac) X was synthesized, where acac =
2
ˉ
ˉ
ˉ
ˉ
acetylacetonate, X = Cl , Br , NCS or N , that act as intermediates during the syntheses. These
3
bis(acetylacetonato) complexes of manganese(III) combine with porphyrin ligand to give
manganese(III) porphyrin complexes [22-24].
III
Equimolar ratio (0.003 mol) of Mn (acac)X and H THMPP were taken in a 500 mL
2
round bottom flask containing 150 mL of glacial acetic acid and 45 mL acetic anhydride. The
whole reaction mixture was refluxed for 4 h. After removing the solvent by evaporation on a
steam-bath, bluish-green solid complexes were obtained that were recrystallized with 1:1 ratio of
chloroform and methanol. All these complexes are crystalline solids and non-hygroscopic.
2
.3.3. Preparation of manganese(III) porphyrin complexes with 4-methylimidazole
III
III
[
Mn (THMPP)X(4-MeIm)]. 1 mmol of Mn (THMPP)X was dissolved in 8 mL of methanol
and 10 mmol of 4-methylimidazole added. The whole mixture was stirred for 10 minutes
followed by reflux of 22-25 h in respective cases. The reaction mixture after cooling at room
temperature for 1 h was filtered. Evaporating the filtrate and drying under vacuum over P O
2
5
resulted into the formation of greenish-black sticky solid in all four cases.
2
.4. Determination of Catalytic Activity of the complexes
The catalytic activities of the complexes for the conversions of aromatic alcohols to their
corresponding aldehydes were done by taking 5 mL acetonitrile, 0.5 mL alcohol, 0.01 g of
complex and adding 2 mmol of oxidant in 5 mL water. The solution was stirred using a magnetic
stirrer at room temperature. 1 mL aliquots of the reaction mixture were withdrawn at intervals of
1
0 minutes and UV-visible spectra were recorded using a UV-visible spectrophotometer Hitachi
(Japan) model U-2900.

3
. Results and discussion
The newly synthesized complexes of manganese(III) were soluble in methanol, ethanol, acetone,
acetonitrile and chloroform. The molar conductance values (in ethanol) of these complexes show
their non-electrolytic nature [25, 26]. The physical properties and analytical data of these
complexes are given in table 1.
3
.1. UV-visible spectral studies
The most advantageous technique to study the porphyrins and metalloporphyrins is UV-visible
spectroscopy. The normal metalloporphyrin spectrum shows an intense B (Soret) band at
~400 nm and several weaker Q bands at ~480-700 nm. These spectral absorptions arise from
π-π* transitions of the aromatic porphyrin ligand [27, 28]. The B band and Q bands of newly
synthesized complexes are shown in table 2. Mn(THMPP)X exhibits four Q bands. Out of these
four bands, two bands disappear after coordination with 4-methylimidazole. Further, the B band
and Q bands of the complexes i.e. Mn(THMPP)X(4-MeIm) show red shift and blue shift,
respectively, while comparing with spectra of their parent complexes i.e. Mn(THMPP)X. These
shifts in the wavelength arise because of axial ligation in the complexes [29].
Ligand field parameters i.e. 10Dq, B and β have also been calculated (table 3). The
values of λmax at maximum absorbance have been used for calculation of 10Dq. Racah
interelectronic repulsion parameter (B) has been calculated using a Tanabe-Sugano diagram
whereas β values were calculated by using the formula:
β = Bin complex/Bfree ion
3
+
-1
where Bfree ion for Mn = 1140 cm [30]. The value of covalency factor β is less than 1, which
suggests that the metal-ligand bond is covalent in these complexes.
3
.2. FT-IR spectral studies
The significant absorption bands of Fourier Transform Infrared Spectroscopy for all the newly
III
-1
synthesized Mn complexes are listed in table 4. A medium band at 669 cm is due to ring
-
1
deformation out-of-plane bending of 4-methylimidazole. Another weak band at ~1522 cm
shows in-plane stretching mode of 4-methylimidazole [31]. Strong bands at 755-772 cm
-1
-1
represent ν(C-H) bending mode and at 1066-1083 cm represent ν(C-H) rocking mode of

-1
ˉ
pyrrole. A medium peak at ~1264 cm suggests coordination of N [32]. Stretching frequencies
3
-1
-1
at ~1404 cm and ~1638 cm correspond to C=N and C=C bonds of pyrrole rings. Another
-1
ˉ
characteristic peak at 2062 cm confirms the coordination of NCS [33]. The occurrence of
-1
broad bands at 3396-3401 cm indicates the presence of O-H bonds.
3
.3. Magnetic susceptibility measurements
Magnetic susceptibility measurements give the values of magnetic momentum of the complexes.
The magnetic momentum values of newly synthesized complexes are 4.76-4.95 B.M., which
indicate the presence of four unpaired electrons and thus confirm the syntheses of high-spin
manganese(III) complexes [34-36].
3
.4. ESI-MS spectral studies
Electrospray-ionization mass spectrometry is the most reliable tool for the study of
macromolecules. This technique is helpful in determining molecular weight as well as giving
structural information of the complex.
The spectra of Mn(THMPP)Br(4-MeIm) shows m/z values at 851, 852, 853 and 931 for
+
+
+
+
[
C H N O Mn] , [C H N O Mn + H] , [C H N O Mn + 2H] and [C H N O BrMn]
48 36 4 8 48 36 4 8 48 36 4 8 48 36 4 8
ions, respectively. The m/z values of Mn(THMPP)Cl(4-MeIm) spectra are 851, 852, 853 and
+
+
+
9
33, which correspond to [C H N O Mn] , [C H N O Mn + H] , [C H N O Mn + 2H] and
48 36 4 8 48 36 4 8 48 36 4 8
+
[
C H N O Mn] ions, respectively. For Mn(THMPP)NCS(4-MeIm), the m/z values are
52 42 6 8
+
obtained at 851, 852, 853 and 932, which correspond to [C H N O Mn] , [C H N O Mn +
48
36
4
8
48 36
4
8
+
+
+
H] , [C H N O Mn + 2H] and [C H N O SMn + Na] ions, respectively. The spectra of
48
36
4
8
49 36
5
8
+
Mn(THMPP)N (4-MeIm) gives m/z values at 821, 851, 853 and 885 for [C H N O Mn + H] ,
3
47 33
4
7
+
+
+
[
C H N O Mn] , [C H N O Mn + H] and [C H N O Mn] , respectively.
48 36 4 8 48 36 4 8 51 38 6 6
3
.5. Structure
On the basis of the above data and available literature [37, 38], an octahedral geometry has been
suggested for the complexes with 4-methylimidazole. The tentative structure is shown in
figure 1.

3
.6. Oxidation properties
Scheme 1. Oxidation of alcohols to aldehydes catalyzed by Mn(THMPP)X(4-MeIm).
ˉ
ˉ
ˉ
ˉ
3
Mn(THMPP)X(4-MeIm), where X = Cl , Br , NCS or N , oxidized benzyl alcohol into
benzaldehyde and 4-methoxybenzyl alcohol to 4-methoxy benzaldehyde and further oxidation of
aldehydes into carboxylic acid does not occur (scheme 1). In these conversions, different
oxidants such as NaIO , H O and NaOCl were used as oxygen source. During oxidation the
4
2
2
Soret band of the complex disappears slowly, which confirms the consumption of complex with
time. A typical record of reaction solution showing spectral changes of the complex with time is
shown in figure 2. The product was characterized by UV-visible, TLC and qualitative test of
benzaldehyde. As the complex has been consumed with time, it has no repeatability. The results
of transformation of aromatic alcohols to their corresponding aldehydes are summarized in
tables 5 and 6.
In the blank experiments i.e. same experimental conditions but without complex, only
trace amount of products were obtained.
4
. Conclusion
This communication reports synthesis and characterization of manganese(III) porphyrin
complexes with 4-methylimidazole. The applications of these complexes in the conversion of
benzyl alcohol and 4-methoxybenzyl alcohol to their corresponding aldehydes have also been
demonstrated. Different oxidants such as NaIO , H O and NaOCl have been used as oxygen
4
2
2
source. The results showed that the most potent oxidant was NaIO , which gave highest product
4
yield in these oxidative transformation reactions. The newly synthesized manganese porphyrin
complexes are more effective in comparison with previously reported catalysts for oxidation of

alcohols since they require milder reaction conditions (room temperature) and take less time
40 min only).
(
Acknowledgement
Niharika Anand acknowledges the financial support of UGC, New Delhi in the form of Rajiv
Gandhi National Fellowship.
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[
1]
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[
2]
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5]
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7]
8]
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(1998).
[
[
[
[
9]
T.G. Traylor, A.R. Miksztal. J. Am. Chem. Soc., 111, 7443 (1989).
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2
17, 9 (2004).
[
[
[
[
[
[
[
13] G. Karimipour. Int. J. Chem. Eng. Appl., 5, 194 (2014).
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[
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[
[
22] E.B. Fleischer, J.M. Palmer, T.S. Srivastava, A. Chatterjee. J. Am. Chem. Soc., 93, 3163
(
1971).
23] B.R. Stults, V.W. Day, E.L. Tassett, R.S. Marianelli. Inorg. Nucl. Chem. Lett., 9, 1259
1973).
(
[
[
24] B.R. Stults, V.W. Day, R.S. Marianelli. Inorg. Chem., 14, 722 (1975).
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Press, London (1975).
[
[
[
[
[
26] M.H.A. Al-Amery. IOSR-JAC, 4, 31 (2013).
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29] L.J. Boucher. J. Am. Chem. Soc., 90, 6640 (1968).
30] J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic Chemistry, Principles of Structure and
Reactivity, 4th Edn., p. 445, Pearson Education, Singapore (2005).
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Prentice-Hall of India Private Limited, New Delhi (1969).
[
[
[
[
[
[
[
[
33] R.J.H. Clark, C.S. Williams. Spectrochim. Acta, 22, 1081 (1966).
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Lett., 11, 505 (1975).

Table 1. Physical properties and analytical data of the manganese(III) porphyrins.
Elemental analyses
Molar
conducti
Found (Calculated) %
%
Color Yie
ld
Molecular
formula
vity
Complex
-
(
Ω
1
2
C
H
N
O
S
Cl
Br
Mn
cm mol
-
1
)
Mn(THMPP)
Cl(4-MeIm)
C H N O
Blacki 78. 64.8 4.0 9.14 13.3
3.4
1
(3.6
6)
5.1
6
(5.6
7)
10
12
11
8
5
2
42
6
8
8
8
8
ClMn
sh-
green
67
9
6
(8.6
7)
4
(13.
20)
-
-
-
(64. (4.3
43) 7)
1.5 4.2
7
.7
Mn(THMPP)
Br(4-MeIm)
C H N O
72.
5
2
42
6
8
.43
1
BrMn
Blacki 40
sh-
green
6
(8.2 12.5
9)
(7.8 5.5
2
.9
2
6
7
(12.
63)
-
-
-
8)
1
(5.4
2)
9
(3.2
3)
Mn(THMPP)
NCS(4-MeIm)
C H N O
75.
83
5
3
42
7
(
61. (4.1
1) 8)
4.1 4.3
1
0.0
4
SMn
6
-
Blacki
sh-
green
6
(9.8 12.9
8)
5.6
0
(5.5
4)
Mn(THMPP)
N (4-MeIm)
3
C H N O
70.
56
5
2
42
9
1
2
4
(12.
90)
Mn
-
-
(
64. (4.2
7) 7)
64.1 4.4
1
2.9
9
1
Blacki
sh-
(12. 13.0
5.3
6
green
2
5
92)
8
(
0
64. (4.3
0) 4)
(13.
12)
(5.6
3)
Table 2. UV-visible spectroscopic data of manganese(III) porphyrins in methanol.
λ (nm)
Complex
Soret(B) band
Q bands
Mn(THMPP)Cl(4-MeIm)
Mn(THMPP)Br(4-MeIm)
475
470
475
475
570, 610
565, 605
570, 610
570, 605
Mn(THMPP)N (4-MeIm)
3
Mn(THMPP)NCS(4-MeIm)

Table 3. Calculated values of ligand field parameters.
a
Molar absorptivity of
-
1
-1
Complex
10Dq (cm )
B (cm )
β
-
1
-1
λmax (M cm )
5
[
Mn(THMPP)Cl(4-MeIm)]
7.42 × 10
23,148
23,585
23,148
23,148
857
874
857
857
0.75
0.77
0.75
0.75
5
[
Mn(THMPP)Br(4-MeIm)]
7.09 × 10
5
[
Mn(THMPP)N (4-MeIm)]
7.30 × 10
3
5
[
Mn(THMPP)NCS(4-MeIm)]
7.54 × 10
a
-4
c = 2×10 M
-
1
Table 4. Selected vibrational frequencies (cm ) in FT-IR spectra of manganese(III)
porphyrins with 4-methylimidazole.
Mn(THMPP)Cl(4-
MeIm)
Mn(THMPP)Br(4-
MeIm)
Mn(THMPP)N (4-
Mn(THMPP)NCS(4-
MeIm)
Assignment
3
MeIm)
6
7
69
58
669
755
1067
1216
-
669
765
669
767
OPP bend(4-
MeI)
1
1
066
215
-
1083
1216
1271
1404
1519
1639
-
1069
1216
-
ν(C-H) bend
py
ν(C-H) rock
py
ν(C-N) str
νN3
1
1
1
404
522
638
-
1402
1515
1638
-
1402
1523
1630
2069
3020
3401
ν(C=N)py
In plane str (4-
MeI)
3
3
021
396
3021
3398
3021
3396
ν(C=C)py
ν(NCS)
ν(C-H) str
ν(O-H) str

Table 5. Effect of different oxidants on the oxidation of benzyl alcohol catalyzed by
Mn(THMPP)X(4-MeIm).
Entry
Oxidant
Solvent
Aldehyde yield (%) after 40 min
1
2
3
NaIO4
H O
CH CN/H O
87±4
71±4
32±5
3
2
CH CN
2
2
3
NaOCl
CH CN
3
Table 6. Effect of different oxidants on the oxidation of 4-methoxybenzyl alcohol catalyzed
by Mn(THMPP)X(4-MeIm).
Entry
Oxidant
Solvent
Aldehyde yield (%) after 40 min
1
2
3
NaIO4
H O
CH CN/H O
90±4
74±4
36±3
3
2
CH CN
2
2
3
NaOCl
CH CN
3