Synthesis, spectral and electrochemical redox properties of N-methyl fused nickel(II) porphyrin

Article abstract of DOI:10.1142/S1088424618501109

N-methyl fused nickel(II) porphyrin was synthesized by a facile synthetic route in excellent yield. The effect of the electron-donating methyl group on spectral and electrochemical redox properties was analyzed by comparing the electrochemistry with that of its precursors. N-methylated fused nickel(II) porphyrin exhibited a red-shifted absorption spectrum (δλmax = 6-13 nm) and a 180mV anodic shift in the first ring oxidation as well as a 210mV shift in reduction with respect to its Ni(II)-fused porphyrin precursor (NiII-(NH)TPP). However, the absorption spectral features and redox potentials of N-methyl fused nickel(II) porphyrin are marginally shifted as compared to its immediate precursor, β-formyl Ni(II)-fused porphyrin. Notably, Ni(II)(N-CH3)(CHO)TPP exhibited a third oxidation at 1.51mV, corresponding to oxidation of Ni(II) to Ni(III) due to the presence of push-pull β substituents.

Full text of DOI:10.1142/S1088424618501109

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2018; 22: 1106–1110
DOI: 10.1142/S1088424618501109
Published at http://www.worldscinet.com/jpp/
Synthesis, spectral and electrochemical redox properties
of N-methyl fused nickel(II) porphyrin
Tawseef Ahmad Dar, Mandeep and Muniappan Sankar*◊
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee — 247667, India
Received 16 October 2018
Accepted 4 November 2018
ABSTRACT: N-methyl fused nickel(II) porphyrin was synthesized by a facile synthetic route in excellent
yield. The effect of the electron-donating methyl group on spectral and electrochemical redox properties
was analyzed by comparing the electrochemistry with that of its precursors. N-methylated fused nickel(II)
porphyrin exhibited a red-shifted absorption spectrum (∆lmax = 6–13 nm) and a 180 mV anodic shift in
the first ring oxidation as well as a 210 mV shift in reduction with respect to its Ni(II)-fused porphyrin
precursor (NiII-(NH)TPP). However, the absorption spectral features and redox potentials of N-methyl
fused nickel(II) porphyrin are marginally shifted as compared to its immediate precursor, b-formyl Ni(II)-
fused porphyrin. Notably, Ni(II)(N-CH3)(CHO)TPP exhibited a third oxidation at 1.51 mV, corresponding
to oxidation of Ni(II) to Ni(III) due to the presence of “push–pull” b substituents.
KEYWORDS: synthesis of fused porphyrins, N-methylation, spectral and electrochemical properties.
therapy (PDT). p-Extended porphyrins formed by dimer-
ization using different linkers were known as early as
1970 [21]. Since then, such porphyrin dimers/polymers
have been widely explored [22, 23]. Fused porphyrins
with polycyclic aromatic hydrocarbons are one of the
most promising candidates in the field of molecular elec-
tronics and molecular nanotechnology since they contain
heteroatoms and can mimic doped graphene. To date,
a number of systems have been prepared by fusion of
benzene, naphthalene, pyrene, azulene, anthracene, cor-
annulene and other aromatic moieties to the meso- and
b-positions of the porphyrin macrocycle [24–26]. Fusion
of aromatic moieties with porphyrin core leads to exten-
sive p-delocalization, leading to enhanced absorption and
efficient emission in the near IR region [27]. Different
approaches have been used to synthesize fused porphy-
rins. For example, oxidative ring closure is particularly
important for fusion of aromatic moieties [28]. Ziessel
et al. reported the formation of triphenylene-fused por-
phyrins using the same protocol. Similarly, Osuka et al.
have reported the formation of highly planar diarylamine-
fused porphyrins with highly stable radical cations [29].
Quinone-fused porphyrins have also been reported for
clinical applications for photoacoustic imaging of tumors
and cardiovascular tissues [30]. In our quest to develop
novel porphyrins for PDT and DSSC applications, we
INTRODUCTION
Porphyrins are aromatic, highly conjugated molecules
with high thermal and chemical stabilities [1–3]. They
have many intriguing properties, and hence nature has cho-
sen them for many important life processes and functions
[4, 5]. Porphyrinoids play important roles in oxygen trans-
port [6] and photosynthesis [7, 8] and also act as cofactors to
many enzymes like cytochromes [9] and methyl coenzyme
reductase [10]. They have also been exploited by scientists
for use in different material and medicinal applications such
as catalysis [11], artificial light harvesting [12], supramo-
lecular assemblies [13], sensing [14], nonlinear optics [15]
and drugs [16]. The properties of porphyrins can be tailored
for specific functions by appending appropriate meso and/
or b substituents [17, 18]. The nature and number of the
substituents affect the physicochemical and redox proper-
ties of the porphyrins [19]. Their properties can also be
changed by extending the p-conjugation in various ways
such as fusion, extension and dimerization [20].
p-Extended porphyrins have useful applications in
the fields of solar energy harvesting and photodynamic
◊ꢀ
SPP full member in good standing
*Correspondence to: Muniappan Sankar, tel.: +91-1332-
284753, fax: +91-1332-273560, email: sankafcy@iitr.ac.in.
Copyright © 2018 World Scientific Publishing Company

SYNTHESIS, SPECTRAL AND ELECTROCHEMICAL REDOX PROPERTIES OF N-METHYL FUSED NICKEL(II) 1107
CH3
N
NH
NH
CHO
CHO
N
N
N
N
N
N
N
N
N
N
N
N
Ni
Ni
Ni
Ni(II)(NH)TPP
Ni(II)(NH)(CHO)TPP
Ni(II)(N-CH3)(CHO)TPP
Chart 1. Molecular structures of the synthesized porphyrins
NO2
NH
P(OEt)3, o-DCB
155 oC, 2 h
N
N
N
N
N
N
N
N
Ni
Ni
DMF/POCl3
CH2Cl2
CH3
NH
N
CHO
CHO
CH3I, K2CO3
DMF
N
N
N
N
N
N
Ni
Ni
N
N
Scheme 1. Synthesis of N-methylated porphyrin Ni(II)(N-CH3)(CHO)TPP
hereby report the synthesis of an N-methylated fused
MALDI-TOF mass spectroscopic techniques (Fig. 1 and
Ni(II) porphyrin (Ni(II)(N-CH3)(CHO)TPP) along with
comparing its electrochemical properties with its parent
porphyrin derivatives (Chart 1).
Figs S1–S3, SI).
Absorption and electrochemical properties
The UV-vis absorption spectra of Ni(II)(N-CH3)
(CHO)TPP and its precursors Ni(II)(NH)TPP and Ni(II)
(NH)(CHO)TPP were measured in CHCl3 at 298 K, and
the corresponding spectral data is presented in Table 1.
The Soret band was observed in the range of 420–455 nm
and Q bands in the range of 550–635 nm. A marginal red
shift (∆lmax = 6–13 nm) was observed when the methyl
group was introduced at the nitrogen of Ni(II)(NH)
(CHO)TPP to obtain Ni(II)(N-CH3)(CHO)TPP which
RESULTS AND DISCUSSION
N-methylated nickel(II) porphyrin, Ni(II)(N-CH3)
(CHO)TPP was synthesized in 94% yield using a modi-
fied literature method [31]. The final step involved
N-methylation in DMF using CH3I as the methylating
agent and K2CO3 as base (Scheme 1). Ni(II)(N-CH3)
(CHO)TPP was characterized by UV-vis, NMR and
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J. Porphyrins Phthalocyanines 2018; 22: 1107–1110

1108 T. AHMAD ET AL.
Fig. 1. 1H NMR spectrum of Ni(II)(N-CH3)(CHO)TPP in CDCl3 at 298 K
Table 1. UV-vis absorption spectral data of fused porphyrins in CHCl3 at 298 K
B and Q bands, lmax, nm
Porphyrin
Ni(II)(NH)TPP
421 (7.07 × 104), 554 (6.02 × 103), 588 (8.71 × 103), 625 (1.51 × 104)
448 (1.20 × 105), 561 (1.00 × 104), 627 (1.58 × 104)
454 (1.14 × 105), 574 (7.41 × 103), 633 (1.38 × 104)
Ni(II)(NH)(CHO)TPP
Ni(II)(N-CH3)(CHO)TPP
-1
Values in parentheses refer to ε in Lmol-1 cm .
.
exhibited the electron releasing effect of methyl group as
shown in Fig. S1, SI.
anodically shifted as compared to the porphyrin devoid
of the formyl group, Ni(II)(NH)TPP. This is attributed
to the electron withdrawing effect of the formyl group.
Similarly, Ni(II)(NH)(CHO)TPP was 220 mV harder
to reduce compared to Ni(II)(NH)TPP. The difference
between the first oxidation potentials of Ni(II)(NH)
(CHO)TPP and Ni(II)(N-CH3)(CHO)TPP was only
30 mV. Similarly the difference between their first reduc-
tion potentials was only 10 mV. Notably, Ni(II)(N-CH3)
(CHO)TPP exhibited a third oxidation potential at 1.51V,
corresponding to metal centered oxidation of Ni(II) to
Ni(III) due to the presence of push–pull b substituents.
1H NMR and 13C NMR of Ni(II)(N-CH3)(CHO)TPP
recorded in CDCl3 are shown in Fig. 1 and Fig. S2, SI.
The signals obtained match with the proposed structure.
The peaks around 9.5 ppm and 4.6 ppm correspond to
aldehyde and N-methyl functionalities. The MALDI
mass spectrum of Ni(II)(N-CH3)(CHO)TPP is shown in
Fig. S3, SI. The obtained molecular ion peak matches
with the proposed structure.
Electrochemical redox potentials of the porphyrins
were recorded in CH2Cl2 containing 0.1M TBAPF6 as the
supporting electrolyte at 298 K. The comparative cyclic
voltammograms are shown in Fig. 2 and the relevant data
is given in Table 2. The first oxidation potential of the
porphyrins varies in the range of 0.57 to 0.78 V, whereas
the first reduction potential varied from -0.95 to -1.17 V.
The first ring oxidation potential of CHO-appended
N-fused porphyrin Ni(II)(NH)(CHO)TPP was 210 mV,
EXPERIMENTAL SECTION
Chemicals and instrumentation
Pyrrole,benzaldehyde,propionicacid,Ni(OAc)2•4H2O,
Cu(NO3)2•3H2O and CH3I were purchased from HiMedia,
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J. Porphyrins Phthalocyanines 2018; 22: 1108–1110

SYNTHESIS, SPECTRAL AND ELECTROCHEMICAL REDOX PROPERTIES OF N-METHYL FUSED NICKEL(II) 1109
measurements were carried out using a CH instrument
(CH 620E). A three-electrode assembly consisting of a
platinum working electrode, Ag/AgCl as a reference
electrode and Pt-wire as a counter electrode was used.
Concentration of all the porphyrins was maintained at
1 mM during the electrochemical studies. All measure-
ments were performed in triple-distilled CH2Cl2 contain-
ing 0.1 M TBAPF6 as the supporting electrolyte, which
was degassed by argon gas purging.
Synthesis of Ni(II)(N-CH3)(CHO)TPP. 50 mg
(0.069 mmol) of Ni(II)(NH)(CHO)TPP was taken in a
250 mL round-bottomed flask containing 15 mL of DMF.
K2CO3 (200 mg, 20 equiv.) and 25 μL of methyl iodide
(20 equiv.) were added to the above solution. The result-
ing solution was stirred for 5 h. After completion of the
reaction, 100 mL of distilled water was added. The pre-
cipitated porphyrin was filtered through a G4 crucible.
Recrystallization was carried out using a CHCl3/MeOH
mixture (1:4, v/v). Yield of the porphyrin was found
to be 94% (50 mg). UV-vis (CHCl3): lmax in nm (ε in
Lmol-1cm-1): 454 (1.14 × 105), 574 (7.41 × 103), 633
(1.38 × 104); 1H NMR: d (ppm): 9.23, 8.76 (2d, 1 + 1H, J =
4Hz, pyrrole), 8.55 (AB, 2H, J = 4Hz, pyrrole), 8.50 (AB,
2H, J = 4Hz, pyrrole), 8.79 (dd, 1H, J = 8Hz, cyclized
phenyl), 8.19 (dd, 1H, J = 8Hz, cyclized phenyl), 8.04–
7.92 (m, 6H, Hortho), 7.77–7.64 (m, 11H, 9Hmeta+para + 2H
Fig. 2. Comparative cyclic voltammograms of fused porphyrins
in CH2Cl2 at 298 K
cyclized phenyl), 4.62 (s, 3H, CH3), 9.58 (s, 1H, CHO);
13C NMR (100 MHz; CDCl3): 186.00, 144.63, 143.80,
141.71, 140.40, 138.00, 133.46, 128.44, 127.13, 122.35,
116.80, 114.10, 113.03, 29.73 ppm; MALDI-TOF-MS
Table 2. Electrochemical redox potentials (in V vs. Ag/AgCl)
of fused porphyrins in CH2Cl2 at 298 K
(m/z): found 726.23 [M]+, calcd. 726.44: Anal. Calcd
.
for C46H29N5NiO 0.5CHCl3: C, 76.05; H, 4.02; N, 9.64.
Porphyrin
Oxidation (V) Reduction (V) ∆E
Found: C, 75.93; H, 3.86; N, 8.99.
(V)
I
II III
I
II
Ni(II)(NH)TPP
0.57 1.05
—
—
-0.95
-1.17
-1.21 1.52
-1.48 1.95
-1.51 1.91
CONCLUSIONS
Ni(II)(NH)(CHO)TPP 0.78 1.12
N-methyl fused porphyrin, (Ni(II)(N-CH3)(CHO)TPP),
was synthesized in excellent yield and its spectral and
electrochemical redox properties compared to known
fused porphyrins, Ni(II)(NH)(CHO)TPP and Ni(II)(NH)
TPP. Ni(II)(N-CH3)(CHO)TPP exhibited considerable
red shift (∆lmax = 12–33 nm) in its absorption spectrum
as compared to Ni(II)(NH)TPP, whereas a marginal red
shift (∆lmax = 6 nm) was observed with respect to Ni(II)
(NH)(CHO)TPP. Ni(II)(N-CH3)(CHO)TPP exhibited a
180 mV anodic shift in the first oxidation as compared to
Ni(II)(NH)TPP due to the presence of electron-withdraw-
ing b-formyl functionality, but a 30 mV cathodic shift
with respect to Ni(II)(NH)(CHO)TPP due to the electron-
donating nature of N-methyl substituent. The redox tun-
ability was achieved due to ‘push–pull’ b substituents at
the porphyrin periphery. Notably, Ni(II)(N-CH3)(CHO)
TPP exhibited a third oxidation at 1.51 mV, correspond-
ing to oxidation of Ni(II) to Ni(III) due to the presence
of ‘push–pull’ b substituents. The utilization of Ni(II)
(N-CH3)(CHO)TPP in photodynamic therapy (PDT), non-
linear optics (NLO) and sensor applications is in progress.
Ni(II)(N-CH3)(CHO) 0.75 1.07 1.51 -1.16
TPP
Scan Rate = 0.1V/s. Pt working, Pt wire counter electrode and Ag/AgCl
reference electrode were used.
India and used as received. DMF, NaHCO3 and K2CO3
were purchased from Rankem, India and used as received.
POCl3, P(OEt)3, Acetic Anhydride were received from
Thomas Baker, India. Glacial Acetic acid was received
from Merck, India. TBAPF6 was recrystallized twice with
ethanol and vacuum dried before use. Different solvents
were dried over P2O5 prior to use.
Optical absorption spectra were recorded using an
Agilent Cary 100 spectrophotometer using a pair of
quartz cells of 3.5 mL volume and 10 mm path length. 1H
NMR spectra were recorded using a JEOL ECX 400 MHz
spectrometer using CDCl3 as solvent. Mass spectra were
measured using a Bruker Ultraflextreme-TN MALDI-
TOF-MS spectrometer using HABA, [2-(4′-hydroxy-
benzeneazo)benzoic acid] as matrix. Electrochemical
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J. Porphyrins Phthalocyanines 2018; 22: 1109–1110

1110 T. AHMAD ET AL.
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