Nitrothienylporphyrins: Synthesis, crystal structure and, the effect of position and number of nitro groups on the spectral and electrochemical properties

Article abstract of DOI:10.1016/j.saa.2011.11.049

This article describes the investigation on the effect of orientation of the meso thienyl groups of porphyrins in deciding the site of nitration. The thienyl rings present at the meso position is found to be more susceptible for electrophilic nitration reaction than the pyrrole β-position in the molecules where there is a better conjugation between the thienyl rings and the porphyrin π-system. Signal corresponding to the imino hydrogens in the proton NMR spectrum of meso nitrothienylporphyrins get shifted to upfield with increase in the number of nitro groups on the porphyrin. This is also due to the extended conjugation of the porphyrins π-system with the meso thienyl rings. The above observations are also supported by the redox potentials of those compounds.

Full text of DOI:10.1016/j.saa.2011.11.049

Spectrochimica Acta Part A 87 (2012) 258–264
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
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Nitrothienylporphyrins: Synthesis, crystal structure and, the effect of position
and number of nitro groups on the spectral and electrochemical properties
R. Prasatha, R.J. Butcherb, P. Bhavanaa,∗
a Department of Chemistry, Birla Institute of Technology and Science (BITS), Pilani – K. K. Birla Goa Campus, Zuarinagar 403 726, Goa, India
b Department of Chemistry, Howard University, 525 College Street NW, Washington, DC 20059, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
This article describes the investigation on the effect of orientation of the meso thienyl groups of porphyrins
in deciding the site of nitration. The thienyl rings present at the meso position is found to be more
susceptible for electrophilic nitration reaction than the pyrrole ␤-position in the molecules where there
is a better conjugation between the thienyl rings and the porphyrin ␲-system. Signal corresponding to
the imino hydrogens in the proton NMR spectrum of meso nitrothienylporphyrins get shifted to upfield
with increase in the number of nitro groups on the porphyrin. This is also due to the extended conjugation
of the porphyrins ␲-system with the meso thienyl rings. The above observations are also supported by
the redox potentials of those compounds.
Received 30 May 2011
Received in revised form
18 November 2011
Accepted 24 November 2011
Keywords:
Nitrothienylporphyrin
Imino hydrogen
© 2011 Elsevier B.V. All rights reserved.
Nitration
Extended conjugation
Unsymmetrical porphyrins
1. Introduction
and asymmetric porphyrins through Suzuki cross coupling, Sono-
gashira coupling and Heck reactions [17–19].
Porphyrins are the class of compounds that have gained interest
in research for decades owing to their existence in variety of biolog-
ical systems and due to their application in the field of biomimetics,
catalysis, medicine and sensors [1]. Thienylporphyrins, a type of
heteroaromatic porphyrins, are of growing interest because they
form ultra thin films [2,3] and also due to the electron and energy
transfer properties [4] of their derivatives. Because of these remark-
able electrical and optical properties, various research groups
have investigated oligothiophene and polythiophene appended
porphyrins [5,6]. Meso thienylporphyrins show attractive photo-
physical [7], structural [8–10], electrochemical [7,11] and magnetic
[12] properties. Functionalization of the five membered thienyl
ring, that can come in conjugation with the porphyrin ␲-system due
to its small size [13,14], with groups of different electronic behav-
ior can modulate the properties of the macrocycle. These groups
may also act as the site for further reaction. Bromine is one such
substituent which alters the electronic [15,16] as well as the stereo-
chemical [16] properties of the porphyrins. Brominated porphyrins
are good precursors in the synthesis of many highly functionalized
In an attempt to prepare the asymmetric thienylporphyrins
for the synthesis of other highly functionalized and asymmetric
porphyrins, we were successful in synthesizing ␤-nitrated 5-halo
(chloro-or bromo-)thienylporphyrins[20]. Photophysical[21], cat-
alytic [22] and nonlinear optical properties [23] together with the
role of nitro group in nucleophilic substitution [24] and addition
[25] reactions in the synthesis of other asymmetric porphyrinic
molecules for therapeutic applications [26] make nitroporphyrins
very attractive class of compounds in research. Cupric nitrate tri-
hydrate, which is a mild nitrating agent, generally introduces the
nitro group at the ␤-position of the porphyrin, especially when the
central metal ion is Cu(II) or Ni(II) [27,28]. The hydrogen on the
carbon next to sulfur in five membered heterocyclic compounds is
prone to electrophilic substitution reactions. This led us to study,
in the present work, the effect of cupric nitrate trihydrate on meso
5,10,15,20-tetrathienylporphyrinato nickel (II), NiThP, in which the
site next to sulfur on the thienyl group is available for substitu-
tion. To investigate the position of nitration with conformation
of the molecule, nickel derivative of tetrakis(5ꢀ-methylthien-2ꢀ-
yl)porphyrin (Ni5MeThP) in which the fifth position on the thienyl
ring is blocked and nickel derivative of tetrakis(3ꢀ-methylthien-2ꢀ-
yl)porphyrin (Ni3MeThP) in which the fifth position on the thienyl
ring is available for substitution, also have been nitrated under
similar conditions. The molecular structure of various porphyrins
synthesized in this study along with their abbreviation is given
below (Fig. 1).
∗
Corresponding author. Tel.: +91 832 2580156; fax: +91 832 2557033;
mobile: +91 9421249883.
E-mail addresses: prasad24487@yahoo.co.in (R. Prasath),
rbutcher99@yahoo.com (R.J. Butcher), juliebhavana@gmail.com (P. Bhavana).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.11.049

R. Prasath et al. / Spectrochimica Acta Part A 87 (2012) 258–264
259
e
purged with nitrogen and 0.1 M TBAPF6 was used as the supporting
electrolyte. Electrospray mass spectra were recorded on a THERMO
Finnigan LCQ Advantage max ion trap mass spectrometer.
d
S
f
m
2.3. Synthesis
2.3.1. Synthesis of
c
5-(5ꢀ-nitrothien-2ꢀ-yl)-10,15,20-tris(thien-2ꢀ-yl)porphyrinato
nickel (II), NiNThP
g
h
N
N
N
b
S
M
Nitration of NiThP was carried out using cupric nitrate trihy-
drate [20] in acetic acid/acetic anhydride medium. To a solution of
NiThP (0.17 g, 0.25 mmol) in 50 mL of chloroform and 3 mL of acetic
acid, a solution of Cu(NO3)2·3H2O (0.06 g, 0.25 mmol) in 15 mL of
acetic anhydride was added dropwise over a period of 20 min. The
reaction mixture was stirred under refluxing conditions for 25 min.
At the end of this period, the reaction mixture was treated with
water (3 times) followed by 100 mL of 20% aqueous NaHCO3 (2
times) and the organic layer was dried over anhydrous Na2SO4. The
solvent was removed by distillation and the product was purified
by column chromatography using 3:2 chloroform and hexanes as
the eluent. Unreacted NiThP, which was present in trace amounts
and eluted first on the column, was collected separately. The yield
of NiNThP was 70% (0.13 g). The compound was characterized by
UV–vis, 1H NMR and mass spectral techniques. UV–vis and 1H NMR
spectral details are given in Tables 1 and 2, respectively. ESI-MS for
NiNThP: m/z [M]+ Calcd. 740.52. Found: 740.9.
S
a
N
i
l
S
j
k
MThP
NiNThP
:M = 2H or Ni(II); a,b,c,d,e,f,g,h,i,j,k,l,m = H
:M = Ni(II); b,c,d,e,f,g,h,i,j,k,l,m = H; a = NO2
M5MeThP
:M = 2H or Ni(II); b,c,e,f,h,i,k,l, m = H; a,d,g,j = CH3
Ni4N5MeThP :M = Ni(II); c,e,f,h,i,k,l,m = H; b = NO2; a,d,g,j = CH3
M3MeThP
NiN3MeThP
:M = 2H or Ni(II); a,b,d,e,g,h,j,k,m = H; c,f,i,l = CH3
:M = Ni(II); a,b,d,e,g,h,j,k = H; m = NO2; c,f,i,l = CH3
2.3.2. Synthesis of 5-(4ꢀ-nitro-5ꢀ-methylthien-2ꢀ-yl)-10,15,20-
tris(5ꢀ-methylthien-2ꢀ-yl)porphyrinato nickel (II),
Ni4N5MeThP
Nitration of Ni5MeThP was carried out using cupric nitrate tri-
hydrate as given in the case of NiNThP. The reaction mixture was
refluxed for 30 min after the addition of nitrating agent. The yield
was 77%. The compounds were characterized by UV–vis, 1H NMR
and mass spectral techniques. UV–vis and 1H NMR spectral details
are given in Tables 1 and 2, respectively. ESI-MS for Ni4N5MeThP:
m/z [M]+ Calcd. 796.63. Found: 797.1.
MSNThP
:M = 2H or Ni(II); b,c,d,e,f,g,h,i,j,k,l,m = H; a = NO2
:M = 2H; b,c,d,e,f,h,i,j,k,l,m = H; a,g = NO2
:M = 2H; b,c,e,f,g,h,i,j,k,l,m = H; a,d = NO2
:M = 2H; b,c,e,f,h,i,j,k,l,m = H; a,d,g = NO2
:M = 2H; b,c,e,f,h,i,k,l,m = H; a,d,g,j = NO2
MTDNThP
MCDNThP
MTiNThP
MTaNThP
Fig. 1. Chemical structures of nitro substituted thienylporphyrins.
2. Experimental
2.1. Materials
2.3.3. Synthesis of
2-nitro-5,10,15,20-tetrakis(3ꢀ-methylthien-2ꢀ-yl)porphyrinato
nickel (II), NiN3MeThP
CH2Cl2 (SD Fine Chemicals, India) was refluxed and distilled
over CaH2 and stored over 4 A molecular sieves. Pyrrole, various
˚
Nitration of Ni3MeThP was carried out using cupric nitrate tri-
hydrate as given in the case of NiNThP. The reaction mixture was
refluxed for 20 min after the addition of nitrating agent. The yield
was 71%. The compounds were characterized by UV–vis, 1H NMR
and mass spectral techniques. UV–vis and 1H NMR spectral details
are given in Tables 1 and 2, respectively. ESI-MS for NiN3MeThP:
m/z [M]+ Calcd. 796.63. Found: 796.9.
thiophenecarboxaldehydes and tetrabutylammonium hexafluo-
rophosphate (TBAPF6) were procured from Sigma–Aldrich. Other
required chemicals were purchased from SD Fine Chemicals, India.
Free base tetrakis(2ꢀ-thienyl)porphyrin, H2ThP [29] and its methyl
derivatives, H25MeThP and H23MeThP [30] were prepared by fol-
lowing the procedure reported. The products were characterized
by spectral methods and the data confirmed the formation of the
compounds. For preparing Ni(II) derivatives of above porphyrins,
nickel acetate in dimethylformamide was used as the metal ion
carrier [31].
2.3.4. Synthesis of
5-(5ꢀ-nitrothien-2ꢀ-yl)-10,15,20-tris(thien-2ꢀ-yl)porphyrin,
H2SNThP
This porphyrin was synthesized using a mixture of thienylalde-
hydes by following the procedure by Lindsey et al. with modifica-
tions in it [32]. To a mixture of 5-nitro-2-thiophenecarboxaldehyde
(0.6 g, 3.82 mmol) and thiophene-2-carboxaldehyde (1.08 mL,
11.4 mmol) in the ratio 1:3 in 600 mL of dichloromethane, pyr-
role (1 mL, 15.2 mmol) was added under N2 atmosphere. To this,
BF3 etherate (0.62 mL, 5.06 mmol) was added and the reaction
mixture was stirred for 1 h at room temperature. At the end of
the period, p-chloranil (3.73 g, 15.2 mmol) was added and stirring
was continued for another half an hour. Following this, triethy-
lamine (0.7 mL, 5.06 mmol) was added and stirred for another
1 h. The reaction mixture was then adsorbed on 200 g of sil-
ica gel (230–400 mesh) and the mixture of porphyrins was
eluted using chloroform. The solution was then concentrated
2.2. Measurements
1H NMR spectra were recorded on a DRX-500 spectrometer
in deuteriated chloroform using tetramethylsilane as the internal
standard. Optical absorption spectra were recorded on a JASCO
V-570 model UV/VIS/NIR spectrophotometer using quartz cells
of 1 cm path length. BAS EPSILON model electrochemical system
was employed for the cyclic voltammetric measurements. The
electrochemical cell consists of three electrode cell assembly and
the electrodes used were platinum working electrode, Ag/AgCl
reference electrode and platinum wire auxiliary electrode. The
concentrations of all the porphyrins employed were ∼1 mM. All
the measurements were performed in CH2Cl2 solution which was

260
R. Prasath et al. / Spectrochimica Acta Part A 87 (2012) 258–264
Table 1
UV–vis absorption spectral dataa of nitro substituted thien-2-ylporphyrins in CH2Cl2.
Porphyrin
Soret band (nm)
Q bands (nm)
FWHM (nm)
H2ThP
426(5.59)
426 (5.51)
428(5.43)
429(5.43)
431(5.42)
432(5.50)
423(5.26)
423(5.29)
428 (5.44)
428(5.29)
420 (5.55)
434(5.46)
523(4.30), 560 (4.02), 597 (3.88), 661 (3.86)
522 (4.30), 562 (4.02), 596 (3.90), 658 (3.48)
523(4.30), 563 (4.05), 596 (3.90), 658 (3.55)
523(4.35), 562 (4.08), 596 (3.99), 658 (3.54)
523(4.34), 562 (4.04), 596 (3.95), 658 (3.39)
522(4.45), 560 (4.12), 596 (4.05), 656 (3.46)
537(4.12), 628 (3.03)
538(4.27)
540 (4.34), 628 (3.64)
540 (4.27)
534(4.38), 626 (3.45)
16.56
21.46
26.11
26.77
29.45
30.73
26.73
33.30
28.22
31.67
24.34
35.43
H2SNThP
H2TDNThP
H2CDNThP
H2TiNThP
H2TaNThP
NiThP
NiNThP
Ni5MeThP
Ni4N5MeThP
Ni3MeThP
NiN3MeThP
546(4.39), 588 (4.20)
aThe values in parenthesis refer to logε values, ε in dm3 mol−1 cm−1
.
(4 mg, 0.5%). The compounds were characterized by UV–vis, 1H
NMR and mass spectral techniques. UV–vis and 1H NMR spec-
tral data are given in Tables 1 and 2, respectively. ESI-MS: m/z
[M]+ H2SNThP (Calcd. 683.84. Found: 684.3), H2TDNThP (Calcd.
728.84. Found: 729.2), H2CDNThP (Calcd. 728.84. Found: 729.2),
H2TiNThP (Calcd. 773.84. Found: 774.1), H2TaNThP (Calcd. 818.84.
Found: 819.1).
by rotary evaporation and the porphyrins were separated by
column chromatography using chloroform containing 0.5–3% of
methanol as the eluent for various nitro derivatives. The yield
of the required product H2SNThP was 202 mg (7.8%). Other por-
phyrins isolated from the reaction mixture were obtained in the
following yield: H2ThP (92 mg, 5.1%), H2TDNThP (39 mg, 2.8%),
H2CDNThP (36 mg, 2.6%), H2TiNThP (20 mg, 2.1%) and H2TaNThP
Table 2
1H NMR spectral dataa,b of nitroporphyrins in deuteriated CHCl3 (ppm).
Porphyrin
H2ThP
Imino H
Pyrrole ␤-H
Thienyl H
Methyl H
–
−2.64, s, 2H
9.07, s, 8H
7.51, dd, 4H (b,e,h,k)
7.86, d, 4H (a,d,g,j)
7.92, d, 4H (c,f,i,l)
–
–
8.884–9.005, sm, 8H
7.410–7.437, asm, 3H (e,h,k)
7.672–7.682, d, 1H (b)
7.734–7.745, d, 3H (d,g,j)
7.764–7.785, d, 3H (f,i,l)
8.223–8.233, d, 1H (c)
NiNThP
−2.61, s, 2H
9.09, s, 8H
7.14–7.15, d, 4H (b,e,h,k)
7.66–7.67, d, 4H (c,f,i,l)
2.81, s, 12H
H25MeThP
–
8.704–8.716, d, 2H
8.996–9.023, asm, 6H
7.044–7.052, asm, 3H (e,h,k)
7.468–7.486, asm, 3H (f,i,l)
7.702–7.704, s, 1H (c)
2.728–2.737, asm, 12H
Ni4N5MeThP
−2.62, s, 2H
–
8.91–9.09, m, 8H
7.30–7.32, dd, 4H (b,e,h,k)
7.71–7.72, dd, 4H (a,d,g,j)
2.09–2.17, m, 12H
H23MeThP
NiN3MeThP
H2SNThP
8.737–9.06, asm, 7H
9.005–9.111, sm, 8H
7.146–7.226, asm, 4H (b,e,h,k)
7.651–7.66, asm, 4H (a,d,g,j)
2.169, s, 12H
–
−2.658, s, 2H
7.511–7.53, asm, 3H (e,h,k)
7.866–7.887, asm, 4H (b,d,g,j)
7.926–7.934, d, 3H (f,i,l)
8.32–8.33, d, 1H (c)
–
–
–
–
−2.704, s, 2H
−2.692, s, 2H
−2.743, s, 2H
−2.82, s, 2H
9.02–9.13, sm, 8H
9.02–9.13, sm, 8H
9.04–9.15, sm, 8H
9.12, s, 8H
7.52–7.54, dd, 2H (e,k)
7.87–7.88, d, 2H (b,h)
7.89–7.91, dd, 2H (d,j)
7.93–7.95, dd, 2H (f,l)
8.33–8.34, d, 2H (c,i)
H2TDNThP
H2CDNThP
H2TiNThP
7.52–7.54, dd, 2H (h,k)
7.87–7.88, d, 2H (b,e)
7.89–7.90, dd, 2H (g,j)
7.93–7.94, dd, 2H (i,l)
8.33–8.34, d, 2H (c,f)
7.554–7.563, dd, 1H (k)
7.892–7.905, asm, 3H (b,e,h)
7.92–7.936, dd, 1H (j)
7.953–7.964, dd, 1H (l)
8.34–8.36, asm, 3H (c,f,i)
7.91, d, 4H (b,e,h,k)
8.39, d, 4H (c,f,i,l)
H2TaNThP
a
sm, symmetric multiplet like; asm, asymmetric multiplet like; s, singlet; d, doublet; dd, doublet of doublet; m, multiplet.
Position of proton in parenthesis, refer Fig. 1.
b

R. Prasath et al. / Spectrochimica Acta Part A 87 (2012) 258–264
261
Table 3
layer chromatography (TLC) and the formation of the product was
indicated by the appearance of red colored band on the TLC plate.
The reaction was quenched by adding water when it indicated
completion on the TLC plate and the product was purified by col-
umn chromatography.
Summary of crystallographic details of NiN3MeThP.
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C40H27N5NiO2S4
796.62
296(2) K
˚
1.54178 A
Monoclinic
P 1 21/c 1
The UV–vis spectrum of the above nitrated porphyrin did
not show any shift in the Soret band compared to that of
parent compound, NiThP (Table 1). However, on mononitration
using cupric nitrate, tetrabromothienyl- and tetrachlorothienyl-
porphyrins showed a shift of ∼14 nm of the Soret band [20] of
the porphyrin (␤-nitration took place). This led us to synthesize
H2SNThP, for a comparison, by following a variant of the procedure
reported by Lindsey et al. using a mixture of aldehydes; thiophene-
2-carboxaldehyde and 5-nitrothiophene-2-carboxaldehyde [32].
H2SNThP (second fraction from the reaction mixture eluted on the
column) showed a similar UV–vis pattern as that of H2ThP. Also,
the nickel incorporated H2SNThP (i.e. NiSNThP) showed similar
spectral pattern as that of the mononitrated sample using cupric
nitrate. This shows that the nitration has taken place on the meso
thienyl ring (NiNThP) and not at the generally expected pyrrole
␤-position while using Cu(NO3)2 as the nitrating agent. In order to
confirm the position of nitration, 1H NMR spectra were recorded for
both H2SNThP (modified Lindsey procedure) and the nitrated sam-
ple, NiNThP (Fig. 2). Both spectra showed similar spectral pattern
(with metallated sample showing relatively upfielded signals). This
indicated the formation of meso 5-(5ꢀ-nitrothien-2ꢀ-yl)-10,15,20-
tris(thien-2ꢀ-yl)porphyrinato nickel (II), NiNThP on nitrating NiThP
with Cu(NO3)2 (Table 2). i.e. it is found that NiSNThP and NiNThP
obtained from different synthetic routes are the same.
a = 24.7587(9) A
˛ = 90◦
ˇ = 96.575(3)◦
ꢀ = 90◦
˚
Unit cell dimensions
˚
b = 11.4258(4) A
˚
c = 12.6142(4) A
˚
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
3544.9(2) A3
4
1.493 Mg/m3
3.354 mm−1
1640
Crystal size
0.52 mm × 0.35 mm
× 0.12 mm
Theta range for data collection
Index ranges
5.24–75.46◦
−31 ≤ h ≤ 29, −14 ≤ k < = 9,
−15 ≤ l < = 13
Reflections collected
13468
Independent reflections
Completeness to theta = 67.50◦
Absorption correction
7108 [R(int) = 0.0472]
100.0%
Semi-empirical from
equivalents
1.00000 and 0.26805
Full-matrix least-squares
on F2
7108/130/513
1.073
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2 sigma(I)]
R indices (all data)
R1 = 0.0763, wR2 = 0.2076
R1 = 0.0942, wR2 = 0.2226
0.653 and −0.517 e A−3
˚
Largest diff. peak and hole
In order to see the effect of orientation of the thienyl rings at
the meso position on the site of nitration, 5ꢀ- and 3ꢀ-methyl deriva-
tives of tetrathienylporphyrins, Ni5MeThP and Ni3MeThP have
been mononitrated under similar conditions using cupric nitrate.
We had expected ␤-nitration on Ni5MeThP as the fifth position
on the thienyl ring is blocked in this molecule. At the same time,
nitration at the fifth position of the thienyl ring in Ni3MeThP was
expected as this position is available. Also, this position is prone
to electrophilic substitution reactions in five membered hetero-
cyclic compounds. Therefore, a high red shift of the Soret band for
Ni5MeThP and no significant shift of the same for Ni3MeThP were
expected on mononitration. Interestingly, UV visible spectral data
showed a different result. There was a significant red shift of Soret
band (14 nm) on introducing a nitro group on Ni3MeThP whereas
no shift for the same in Ni5MeThP (Table 1). The electronic spec-
tral behavior of Ni5MeThP on mononitration is similar to that of
NiThP where there was no shift of Soret band on introducing the
nitro group on the thienyl ring. The electronic communication [36]
due to the nitro group is more for Ni3MeThP as evidenced from full
width at half maximum (FWHM) values.
2.4. X-ray crystallography
The dark brown colored irregular plate shaped single crys-
tals of NiN3MeThP suitable for the X-ray structure analysis was
obtained by the diffusion of methanol into a solution of the
compound in chloroform. X-ray diffraction results were obtained
with an Oxford Diffraction Gemini-R diffractometer equipped
with graphite-monochromated Mo K␣ radiation. The structure
was solved by the direct method and refined by full-matrix
least-squares analysis using anisotropic thermal parameters for
nonhydrogen atoms with the SHELXL-97 program [33,34]. Details
of the crystal parameters, data collection, and refinements are sum-
marized in Table 3. The crystallographic data of NiN3MeThP was
deposited with the Cambridge Crystallographic Data Centre and
supplementary publication number is CCDC 840586.
3. Results and discussion
The above electronic spectral results can be explained on com-
paring the 1H NMR spectra of the synthesized compounds. From
the 1H NMR spectrum, it is found that in Ni3MeThP (in which
the methyl groups are pointing towards the porphyrin core and
also hindering the thienyl groups to come in conjugation with the
porphyrin ␲-system), the nitration has taken place at the pyrrole ␤-
position (Table 2). But in the case of Ni5MeThP (in which the methyl
groups are pointing away from the porphyrin core and not hinder-
ing the thienyl rings to come in conjugation with the porphyrin
␲-system), the 1H NMR spectrum (Fig. 3 and Table 2) indicated
that the mononitration has taken place on one of the thienyl rings
(at the 4ꢀ-position). These results show that the orientation of the
thienyl ring is the deciding factor of position of nitration. In the case
where no extended conjugation is possible for the thienyl ring with
the porphyrin ␲-system, nitration has taken place at the pyrrole
␤-position (NiN3MeThP). But in Ni5MeThP, where better conjuga-
tion is possible both due to the absence of hindering group at the
As the a2u orbital has electron density contribution from imino
nitrogen and meso carbon, and a1u has the same from ␤-carbon,
it is found that nitration of the Cu(II) and Ni(II) derivatives of
porphyrin using cupric nitrate generally ensures nitration at the
pyrrole ␤-position [27,28]. In the case of meso 2ꢀ-thienylporphyrins,
susceptibility for ␤-nitration is even higher owing to the pres-
ence of electron withdrawing thienyl groups at the meso position
which can further lift the degeneracy of the HOMO [35]. The
same was observed in the nitration of meso tetrakis(5ꢀ-halothien-2ꢀ-
yl)porphyrins [20]. In the present work, in an effort to understand
the position of nitration on the thienylporphyrins, the compound
NiThP (in which the carbon next to sulfur on the thienyl group can
be substituted) has been nitrated using one equivalent of cupric
nitrate trihydrate in acetic anhydride. The reagent was added drop-
wise over a period of 20 min and the nitration took place only
under refluxing conditions. The reaction was monitored by thin

262
R. Prasath et al. / Spectrochimica Acta Part A 87 (2012) 258–264
Fig. 2. 1H NMR spectra of (a) meso mononitrothienylporphyrin, H2SNThP (modified Lindsey method) and (b) nickel derivative of mononitrated meso nitrothienylporphyrin,
NiNThP (this work).
3ꢀ-position of thienyl ring and due to the smaller size of the thienyl
ring (compared to that of phenyl group in tetraphenylporphyrin),
the nitration had taken place at the 4ꢀ-position of one of the thienyl
rings (Ni4N5MeThP). This shows that the electronic requirement
for the electrophilic substitution by nitro group on thienyl ring is
satisfied in Ni5MeThP due to its relatively better conjugation with
porphyrin core when compared to that in Ni3MeThP. The effect
due to the methyl groups is expected to be the same on both com-
pounds.
To confirm the site of nitration by X-Ray diffraction technique,
we tried to crystallize the compounds. We were successful in
solving the crystal structure of NiN3MeThP. In the structure of
NiN3MeThP, the usual ruffling of the saddle type [37–39] as indi-
cated by the deviations of C5, C10, C15 and C20 from the NiN4 plane
of nitrothienyl groups (Table 2). This indicates that, as the number
of nitro groups increases, there is an increased ring current (possi-
bly due to the lesser dihedral angle of the thienyl groups with the
porphyrin core) that makes the NH to resonate at a higher field.
This chemical shift is nearly linear with respect to the number of
nitrothienyl groups (Fig. 5). The NH signals of cis and trans did not
show a noticeable difference in their spectrum. The signal corre-
sponding to the ␤-pyrrole protons of the mono- (Fig. 2a), di- [both
trans- (Fig. 6a) and cis- (Fig. 6b)] and tri- (Fig. 6c) meso nitrothienyl
substituted compounds were symmetric multiplet like in nature
which indicated the symmetry in the molecule. Free base of all meso
nitrothienylporphyrins showed typical porphyrinic UV–vis spec-
trum comprising of four Q bands with decreasing order of intensity
and an intense Soret band. There is a gradual increase in the
˚
˚
˚
˚
[C5 (0.481 A), C10 (−0.485 A), C15 (0.496 A), C20 (−0.497 A)] was
observed (Fig. 4). Ni-N bond lengths are in the normal range for low
˚
˚
spin Ni(II) porphyrins ranging from 1.918(3) A to 1.934(3) A [40].
While the structure contains two disordered thiophene rings (as is
typically found for such substituents [41,42]), there is no disorder
in the rest of the molecule and the position of the nitro substituent
in the molecule is clearly established.
The 1H NMR spectra of the nitrothienylporphyrins obtained
from the synthesis using mixture of thiophenealdehydes were
also compared to see the effect of number of nitro groups on
the electronic properties of the molecules. A downfield shift of
NH resonance was expected with the increase in the number of
nitrothienyl rings at the porphyrin periphery (meso position). Inter-
estingly, the imino hydrogen signals of the thienylporphyrins are
shifted towards the shielded region with increase in the number
Fig. 3. Aromatic region in the 1H NMR spectrum of Ni4N5MeThP recorded in deu-
Fig. 4. Molecular structure of NiN3MeThP at 50% thermal ellipsoidal probability.
teriated CHCl3.

R. Prasath et al. / Spectrochimica Acta Part A 87 (2012) 258–264
263
quasireversible in nature. As the number of nitrothienyl groups
at the meso position of porphyrin increases, the oxidation poten-
tial also increases. The magnitude of the reduction potentials was
decreased with the increase in the number of nitrothienyl groups;
for H2TDNThP, H2TiNThP and H2TaNThP, the first reduction was
irreversible. As for NiThP, electron deficiency of the porphyrin core
due to the presence of Ni (II) is reflected in the difference in redox
potentials of H2SNThP (modified Lindsey’s procedure) and NiNThP
(nitration by cupric nitrate).
From Table 4, it is clear that among Ni3MeThP and Ni5MeThP,
oxidation of Ni5MeThP is easier. This can be due to the enhanced
conjugation of the 5ꢀ-methylthienyl rings with the porphyrin core.
Reduction potentials also supported the same reason. The nitra-
tion at the 4ꢀ-position of the thienyl ring (which is generally not
the preferred position in the electrophilic substitution reactions of
five membered heterocyclic rings) can be ascribed to this electronic
conjugation which satisfied the electronic requirement of nitration.
Though the number of nitro group is same on the molecules, the
electronic nature of Ni4N5MeThP and NiN3MeThP is very different.
The presence of electron withdrawing nitro group at the pyrrole
␤-position made NiN3MeThP very electron deficient. The differ-
ence in the oxidation potentials of NiN3MeThP and Ni4N5MeThP is
306 mV. Compared to the respective parent compounds, the influ-
ence of nitration is more on the properties of NiN3MeThP (267 mV)
than on Ni4N5MeThP (124 mV). The higher electron deficiency of
the porphyrin core of NiN3MeThP can be due to the combined effect
of the position of the nitro group (at pyrrole ␤-) and the lack of con-
jugation of the thienyl ring due to the hindering nature of methyl
groups at the 3ꢀ-position. Effect of conjugation on the electronic
properties was also evident from the NMR study that as the num-
ber of 5ꢀ-nitrothienyl groups at the meso position increases, there
was an upfield shift in the imino hydrogen signals.
Fig. 5. Plot of variation of the chemical shift of imino hydrogens in the 1H NMR
spectra vs number of nitrothienyl groups present on meso nitrothienylporphyrins.
Fig. 6. 1H NMR signal of pyrrole ␤-protons of (a) H2TDNThP, (b) H2CDNThP, and (c)
H2TiNThP.
4. Conclusions
wavelength of the Soret band absorption on increasing the num-
ber of nitrothienyl groups, with isomers H2TDNThP and H2CDNThP
absorbing at nearly same wavelength. The electronic communica-
tion was also found to be increasing with increasing the number of
nitrothienyl groups at the meso position.
For delineating the effect of number as well as the position
of nitro groups on the electronic properties of thienylporphyrins,
cyclic voltammetric studies had been carried out and the data
is summarized in Table 4. For comparison, the data of the half
wave redox potentials of MThP is also given. Porphyrins generally
show two, one electron oxidations and two, one electron reduc-
tions which are reversible and ligand based in nature. In nitrated
thienylporphyrins, the redox potentials are either irreversible or
As a part of the study on the properties of unsymmetrical
and highly functionalized tetrathienylporphyrins, various nitro
substituted thienylporphyrins have been synthesized and their
properties have been investigated electrochemically and spec-
trophotometrically. The method adopted in the present study is a
convenient method for the synthesis of meso nitrothienylporphyins
in high yield, compared to the conventional method of condens-
ing pyrrole with a mixture of aldehydes. The redox potentials of
Ni3MeThP and Ni5MeThP showed the role of ␲-conjugation in
deciding the site of electrophilic substitution on the porphyrin. An
X-ray crystal structure has clearly established the position of nitro
substitution in NiN3MeThP. NMR study also indicated the role of
␲-conjugation in explaining the observed upfield shift of the imino
hydrogen signal with the increase in number of nitrothienyl rings.
These results show the potential use of nitrothienylporphyrins
in the synthesis of materials for nonlinear optical applications.
The study of functionalizing the meso thienyl groups with various
synthetically potential substituents other than nitro groups is in
progress in our laboratory.
Table 4
Half-wave redox potentialsa (mV) of nitro substituted tetrathienylporphyins in
CH2Cl2 using 0.1 M TBAPF6 at 25 ◦C.
Porphyrin
III
II
I
I
II
III
H2ThP
NiThP
NiNThP
1130
1280
1361
1082
1218
1353
890
−1090
−1083i
−818
−1420
1058
1168
939
−1231
−1132i
−1063
−1196
−874
−1243
Acknowledgements
Ni5MeThP
Ni4N5MeThP
Ni3MeThP
NiN3MeThP
H2SNThP
H2TDNThP
H2CDNThP
H2TiNThP
H2TaNThP
1063
1102
1369
1057i
1210i
1254i
1335i
1442i
PB gratefully acknowledges Department of Science and Tech-
nology, India for research grant (SR/FTP/CS-57/2007). RJB wishes
to acknowledge the NSF-MRI program (Grant CHE-0619278) for
funds to purchase the diffractometer.
−1173
−1536b
−1133
−921
−1775i
1508i
1330i
1493i
1584i
1570i
−853
−943i
−777
−1427
−1109
−1379
−1340i
−837i
−675i
−1185i
−843
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