β-Nitro-substituted free-base, iron(III) and manganese(III) tetraarylporphyrins: Synthesis, electrochemistry and effect of the NO 2 substituent on spectra and redox potentials in non-aqueous media

Article abstract of DOI:10.1142/S1088424613500612

Two free-base and four metal derivatives of substituted tetraarylporphyrins containing a nitro-substituent on the β-pyrrole position of the macrocycle were synthesized and characterized by UV-vis, FTIR, ¹H NMR and mass spectrometry as well as e

Full text of DOI:10.1142/S1088424613500612

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2013; 17: 1–13
DOI: 10.1142/S1088424613500612
Published at http://www.worldscinet.com/jpp/
b-Nitro-substituted free-base, iron(III) and manganese(III)
tetraarylporphyrins: synthesis, electrochemistry and
effect of the NO2 substituent on spectra and redox
potentials in non-aqueous media
ShuiboYanga, Bin Suna, Zhongping Ou*a◊, Deying Menga, Guifen Lua◊◊,
Yuanyuan Fangb and Karl M. Kadish*b◊
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China
b Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA
Dedicated to Professor Evgeny Luk’yanets on the occasion of his 75th birthday
Received 21 March 2013
Accepted 18 April 2013
ABSTRACT: Two free-base and four metal derivatives of substituted tetraarylporphyrins containing
a nitro-substituent on the b-pyrrole position of the macrocycle were synthesized and characterized by
UV-vis, FTIR, 1H NMR and mass spectrometry as well as electrochemistry and spectroelectrochemistry
in non-aqueous media. The porphyrins are represented as (NO2TmPP)M and (NO2TdmPP)M, where
M = 2H, FeIIICl or MnIIICl, m is a CH3 group on the para-position of the four meso-phenyl rings of
the tetraphenylporphyrin (TPP) and dm represents two OCH3 substituents on the meta-positions of
each phenyl ring of the TPP macrocycle. UV-visible spectra of the nitro-substituted porphyrins exhibit
absorption bands which are red-shifted by 4–11 nm as compared to bands of the same substituted
tetraarylporphyrins lacking a nitro substituent. Three or four reductions are observed for each iron and
manganese nitroporphyrin, the first of which is metal-centered, leading to formation of an Fe(II) or Mn(II)
complex. Further reduction at the metal center occurs for the iron porphyrins but this reaction proceeds
via an Fe(II) p anion radical in the case of the two nitro-substituented derivatives. The b-nitro-substituted
porphyrins are easier to reduce and harder to oxidize than the corresponding compounds lacking a nitro
group. The effect of NO2 substituent on reduction/oxidation potentials and the site of electron transfer
was also discussed.
KEYWORDS: iron and manganese porphyrins, b-nitro-substituent, synthesis, electrochemistry, effect
of substituent.
coordinating ability of the central metal ion. There
INTRODUCTION
are many ways to “tune” or change the properties of a
Metalloporphyrins and their analogues have attracted
porphyrin, the most common of which involves changes in
considerable attention for use in a broad range of
the peripheral functionalization, i.e. introducing electron-
applications [1–3] because of their versatile properties
withdrawing or electron-donating substituents onto the
derived from the characteristic p-ring system and
meso and/or b-pyrrole positions of the macrocycle.
Theadditionofoneormorehighlyelectron-withdrawing
◊SPP full and ◊◊ student member in good standing
NO2 groups to a porphyrin will significantly modify
its chemical and physical properties, with the type and
magnitude of the effect being dependent in large part on the
position of the substituents. Four types of nitro-substituted
porphyrins have been extensively examined in the last
*Correspondence to: Zhongping Ou, email: zpou2003@yahoo.
com, tel/fax: +86 511-8879-1800; Karl M. Kadish, email:
kkadish@uh.edu, tel: +1 (713)-743-2740, fax: +1 (713)-743-
2745
Copyright © 2013 World Scientific Publishing Company

2
S. YANG ET AL.
three decades [4–52]. These are: (i) meso-NO2 substituted
octaethylporphyrins [36–40], (ii) b-pyrrole substituted
porphyrins [4–35], (iii) meso-phenyl substituted derivatives
[41–47] and (iv) porphyrins containing one or more NO2
substituents on a b,b′-fused group(s) of the macrocycle
as in the case of nitro-substituted quinoxalinoporphyrins
[48–51].
effect is to shift E1/2 for the oxidation and reduction.
In a few cases, the addition of NO2 to the porphyrin
macrocycle has been shown to change the site of electron
transfer from metal to macrocycle, one example being
given in the case of b-pyrrole substituted [(NO2)P]
AuIIIPF6 [31], where P = 5,10,15,20-tetrakis(3,5-di-tert-
butylphenyl)porphyrin. Finally, it should be pointed out
that NO2 substitution at the b-pyrrole or meso-position
of a porphyrin ring has a much smaller effect on E1/2
for oxidation than for reduction, and this has been well-
documented in the literature for a variety of TPP and
OEP type complexes [52].
In the present work, six b-pyrrole nitro-substituted
porphyrins with redox active central metal ions were
synthesized and characterized as to their electrochemical
and spectroelectrochemical properties in non-aqueous
media. The investigated compounds are represented
as (NO2TmPP)M and (NO2TdmPP)M, where M = 2H,
FeIIICl or MnIIICl, m is a CH3 group located at the para-
position of the four meso-phenyl rings and dm represents
two OCH3 substituents located at the 3,5 (meta)-positions
of the phenyl rings of the TPP macrocycle (Chart 1). The
effect of the b-nitro group on the UV-visible spectra and
Previous
electrochemical
characterization
of
nitroporphyrins with redox inactive central metal ions
have shown that the presence of one or more electron-with-
drawing NO2 groups at the meso- or b-pyrrole positions
of the conjugated macrocycle will have a large effect
on the E1/2 for electroreduction, shifting the reversible
potentials in a positive direction by 230–380 mV per
nitro group in the case of b-pyrrole substituted (TPP)M
and by 450–550 mV in the case of meso-substituted (OEP)
M type derivatives [52]. The NO2 group on the phenyl ring
of a TPP type complex [52, 53] or the quinoxaline group
of a quinoxalinoporphyrin [51] may itself be involved in
an electroreduction. However, a direct reduction of the
NO2 group does not occur in non-aqueous media when
this electron-withdrawing group is attached at the meso
or b-pyrrole position of the porphyrin ring and the main
CH3
CH3
(a)
NO2
N
N
N
N
N
N
N
N
CH3
M
CH3
M
H3C
H3C
M = 2H
1a
M = 2H
1b
M = FeIIICl 2a
M = MnIIICl 3a
M = FeIIICl 2b
M = MnIIICl 3b
CH3
CH3
(NO2TmPP)M
(TmPP)M
(b)
H3CO
OCH3
H3CO
OCH3
NO2
OCH3
OCH3
OCH3
OCH3
H3CO
H3CO
N
N
N
N
N
N
N
N
M
M
H3CO
H3CO
M = 2H
1c
M = 2H
1d
M = FeIIICl 2c
M = MnIIICl 3c
M = FeIIICl 2d
M = MnIIICl 3d
H3CO
OCH3
H3CO
OCH3
(NO2TdmPP)M
(TdmPP)M
Chart 1. Structures of tetraarylporphyrins and b-NO2 substituted tetraarylporphyrins
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 2–13

b-NITRO-SUBSTITUTED FREE-BASE, FeIII AND MnIII TETRAARYLPORPHYRINS
3
reduction/oxidation potentials is discussed in terms of
(s, 24H, -OCH3), -2.83 (s, 2H, pyrrole-NH). MS (MALDI-
TOF): m/z 856.85 (calcd. for [M + 2H]+ 855.95).
comparisons with the non-nitro substituted porphyrins
having the same central metal ions. Comparisons are also
made to previously investigated porphyrins and b-nitro
substituted corroles, both the free-base derivatives [54]
and corroles containing transition metal ions such as iron
[55], copper [56], germanium [56] and silver [57].
2-Nitro-5,10,15,20-tetrakis(3,5-dimethoxyphenyl)-
porphyrin copper(II), (NO2TdmPP)Cu. A mixture of
(TdmPP)H2 (320 mg, 0.375 mmol), copper(II) acetate
monohydrate(460mg,2.304mmol),chloroform(120mL),
glacial acetic acid (3 mL) and methanol (3 mL) was heated
at 50–60 °C for 3 h until the (TdmPP)H2 was not detected
by silica TLC.After that, copper nitrate trihydrate (214 mg,
0.885 mmol) and acetic anhydride (7 mL) was then
poured into the mixture, and, continue to stir for 2.5 h at
40–50 °C. The reaction mixture was allowed to cool and
washed with water, ammonia, water, dried over anhydrous
MgSO4. The residue was passed through a plug of alumina
using dichloromethane as eluent. The main fraction was
collected and the solvent completely removed to give a
red-purple solid (300 mg, yield 83%). UV-vis (CH2Cl2):
EXPERIMENTAL
Materials and instrumentation
Reagents and solvents (Sigma-Aldrich, Fluka or
Sinopharm Chemical Reagent Co.) for synthesis and
purification were of analytical grade and used as received.
Dichloromethane (CH2Cl2, 99.8%) was purchased from
Sinopharm Chemical Reagent Co. and fresh distilled
before use. Tetra-n-butylammonium perchlorate (TBAP)
was purchased from Sigma Chemical or Fluka Chemika
Co., recrystallized from ethyl alcohol, and dried under
vacuum at 40 °C for at least one week prior to use.
1H NMR spectra were recorded in a CDCl3 solution
at 400 MHz using a Bruker Advance 400 spectrometer
at 25 °C. Chemical shifts (ppm) were determined with
TMS as the internal reference. MALDI-TOF mass
spectra were carried out on a Bruker BIFLEX III
ultrahigh resolution.
Cyclic voltammetry was carried out at 298 K using
a CHI-730C Electrochemical Workstation. A homemade
three-electrode cell was used for cyclic voltammetric
measurements and consisted of a glassy carbon working
electrode, a platinum counter electrode and a homemade
saturated calomel reference electrode (SCE). The SCE
was separated from the bulk of the solution by a fritted
glass bridge of low porosity which contained the solvent/
supporting electrolyte mixture.
l
max, nm 427, 548, 591. MS (MALDI-TOF): m/z 961.34
(calcd. for [M + 2H]+ 961.24).
2-Nitro-5,10,15,20-tetrakis(3,5-dimethoxyphenyl)-
porphyrin, (NO2TdmPP)H2 1d. (NO2TdmPP)Cu
(200 mg, 0.208 mmol) was slowly added to a concentrated
sulfuric acid (15 M, 4 mL) and stirred vigorously. After
15 min of stirring, the reaction mixture was neutralized
with ammonia and extracted with chloroform. The extract
was dried over sodium sulfate and evaporated to dryness.
The residue was dissolved in dichloromethane and eluted
with the same solvent through a column of alumina. The
major eluted fraction was again evaporated to dryness
and afforded a purple solid (NO2TdmPP)H2 (140 mg,
yield 75%). UV-vis (CH2Cl2): lmax, nm 432, 528, 597,
1
668. H NMR (400 MHz, CDCl3): dH, ppm 9.18 (s, 2H,
pyrrole-H), 9.02–9.05 (d, 3H, pyrrole-H), 8.83 (s, 2H,
pyrrole-H), 7.39–7.46 (m, 8H, ph-H), 6.91–6.93 (m, 4H,
ph–H), 4.0 (s, 24H, -OCH3), -2.67 (s, 2H, pyrrole-NH).
MS (MALDI-TOF): m/z 901.24 (calcd. for [M + 2H]+
900.95).
Thin-layer UV-visible spectroelectrochemical experi-
ments were performed with a home-built thin-layer
cell which has a light transparent platinum net working
electrode. Potentials were applied and monitored with a
CHI-730C Electrochemical Workstation. Time-resolved
UV-visible spectra were recorded with a Hewlett-Packard
Model 8453 diode array spectrophotometer. High purity
N2 was used to deoxygenate the solution and a stream
of nitrogen gas was kept over the solution during each
electrochemical and spectroelectrochemical experiment.
2-Nitro-5,10,15,20-tetrakis(p-methylphenyl)por-
phyrin(NO2TmPP)H21b.Thesynthesisof(NO2TmPP)H2
was carried out as described for (NO2TdmPP)H2.UV-vis
1
(CH2Cl2): lmax, nm 426, 530, 601, 668. H NMR (400
MHz, CDCl3): dH, ppm 8.76–9.09 (m, 7H, pyrrole-H)
8.09–8.18 (m, 8H, ph-H), 7.54–7.60 (m, 8H, ph-H),
2.70–2.73 (m, 12H, -CH3), -2.57 (s, 2H, pyrrole-NH).
MS (MALDI-TOF): m/z 717.01 (calcd. for [M + 2H]+
716.85).
Iron(III) and manganese(III) porphyrins were synthe-
sized by reaction of the corresponding free-base
porphyrin and a metal salt according to the route shown
in Scheme 1.
Synthesis
5,10,15,20-Tetrakis(3,5-dimethoxyphenyl)-
porphyrin, (TdmPP)H2 1c. (TdmPP)H2 was synthe-
sized by the reaction of pyrrole and 3,5-dimethoxy-
benzaldehyde according to a procedure in the literature
[58]. UV-vis (CH2Cl2): lmax, nm 420, 514, 549, 589,
(TmPP)FeCl 2a.Yield 87%. UV-vis (CH2Cl2): lmax, nm
386, 418, 512. IR (KBr): n, cm-1 998 (nN-Fe). MS (MALDI-
TOF): m/z 726.23 (calcd. for [M - Cl + 2H]+ 726.68).
(NO2TmPP)FeCl 2b. Yield 62%. UV-vis (CH2Cl2):
l
max, nm 382, 428, 518. IR (KBr): n, cm-1 1525, 1331
1
644. H NMR (400 MHz, CDCl3): dH, ppm 8.93 (s, 8H,
(nNO2), 999 (nN-Fe ). MS (MALDI-TOF): m/z 770.99 (calcd.
for [M - Cl + 2H]+ 771.68).
pyrrole-H), 7.4 (s, 8H, ph-H), 6.9 (s, 4H, ph-H), 3.96
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 3–13

4
S. YANG ET AL.
R
R
NO2
N
N
N
N
NH
N
N
(i) Cu(OAc)2, CHCl3, AcOH
(ii) Cu(NO3)2, (Ac)2O
Cu
R
R
R
R
HN
R
R
R
R
NO2
NO2
NH
N
N
N
N
N
N
FeCl2 or Mn(OAc)2
DMF, rf
M
R
R
R
R
R
HN
R
OCH3
R =
CH3
or
OCH3
M = Fe(III)Cl or Mn(III)Cl
Scheme 1. Synthesis routes of the b-nitro-substituted porphyrins
(TdmPP)FeCl 2c.Yield 85%. UV-vis (CH2Cl2): lmax, nm
370, 422, 511. IR (KBr): n, cm-1 1001(nN-Fe). MS (MALDI-
TOF): m/z 910.03 (calcd. for [M - Cl + 2H]+ 910.79).
(NO2TdmPP)FeCl 2d. Yield 65%. UV-vis (CH2Cl2):
The copper porphyrins were not separated from the
solution and used directly for the following nitration step
to synthesis of (NO2TRPP)Cu [58]. The b-pyrrole NO2
substituted copper porphyrins were purified by a plug of
alumina using dichloromethane as eluent and demetalated
in concentrated sulfuric acid to give the corresponding
b-pyrrole NO2 substituted free-base porphyrins. The
(NO2TRPP)M complexes (M = FeCl or MnCl) were
then synthesized via a reaction between the free-base
nitroporphyrin (NO2TRPP)H2 and the corresponding
metal salt in DMF (see Scheme 1). The purity of the
final synthesized porphyrins was confirmed by 1H NMR,
UV-vis spectroscopy and mass spectrometry.
l
max, nm 374, 433, 521. IR (KBr): n, cm-1 1522, 1348
(nNO2), 1006 (nN-Fe). MS (MALDI-TOF): m/z 955.15
(calcd. for [M - Cl + 2H]+ 955.78).
(TmPP)MnCl 3a. Yield 89%. UV-vis (CH2Cl2): lmax
,
nm 379, 480, 622. IR (KBr): n, cm-1 1008 (nN-Mn). MS
(MALDI-TOF): m/z 725.12 (calcd. for [M - Cl + 2H]+
725.78).
(NO2TmPP)MnCl 3b. Yield 81%. UV-vis (CH2Cl2):
l
max, nm 382, 484, 630. IR (KBr): n, cm-1 1524, 1338
(nNO2), 1009 (nN-Mn). MS (MALDI-TOF): m/z 769.99
(calcd. for [M - Cl + 2H]+ 770.77).
UV-visible spectra
(TdmPP)MnCl 3c. Yield 82%. UV-vis (CH2Cl2):
max, nm 380, 481, 615. IR (KBr): n, cm-1 1024 (nN-Mn).
UV-visible spectra of the porphyrins were measured
in CH2Cl2. The lmax values of each compound are
summarized in Table 1 and examples of UV-visible
spectra for the TmPP and TdmPP derivatives are
illustrated in Fig. 1. As seen in the figure and table,
the Soret bands of (TmPP)H2 1a and (TdmPP)H2 1c,
(TmPP)FeCl 2a and (TdmPP)FeCl 2c or (TmPP)MnCl 3a
and (TdmPP)MnCl 3c are located at almost identical
values of lmax. This result indicates that changing
substituents on the four meso-phenyl rings of the
porphyrin from one para-CH3 group to two OCH3
substituents at the meta-positions of the phenyl ring has
only a negligible effect on UV-visible spectra.
l
MS (MALDI-TOF): m/z 909.15 (calcd. for [M - Cl +
2H]+ 909.88).
(NO2TdmPP)MnCl 3d. Yield 51%. UV-vis (CH2Cl2):
l
max, nm 384, 490, 623. IR (KBr): n, cm-1 1525, 1349
(nNO2), 1026 (nN-Mn). MS (MALDI-TOF): m/z 954.22
(calcd. for [M - Cl + 2H]+ 954.88).
RESULTS AND DISCUSSION
Synthesis
To the first step, the (TRPP)H2 was treated with
copper(II) acetate in chloroform to produce (TRPP)Cu.
In contrast, significant differences are seen between
UV-visible spectra of the NO2 substituted porphyrins
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b-NITRO-SUBSTITUTED FREE-BASE, FeIII AND MnIII TETRAARYLPORPHYRINS
5
Table 1. UV-visible spectral data (lmax, nm) of the neutral and reduced porphyrins in CH2Cl2, 0.1 M TBAP
M
Compound
b-sub
meso-suba
Soret region
Visible region
2H
1a
H
NO2
H
CH3
CH3
419
426
420
432
515
530
514
528
512
518
511
521
582
586
581
594
540
541
540
544
578
590
576
590
577
541
570
544
552
601
549
597
590
668
589
668
651
644
1b
1c
OCH3
OCH3
CH3
1d
NO2
H
Fe(III)
Mn(III)
Fe(II)
2a
386
382
370
374
379
382
378
390
418
428
420
433
403
408
404
410
417
420
420
426
418
428
422
433
480
484
481
490
445
444
445
446
445
450
446
457
443
444
445
446
2b
NO2
H
CH3
2c
OCH3
OCH3
CH3
2d
NO2
H
3a
622
630
616
640
3b
NO2
H
CH3
3c
OCH3
OCH3
CH3
3d
NO2
H
[2a]-
[2b]-
[2c]-
[2d]-
[3a]-
[3b]-
[3c]-
[3d]-
[2a]2-
[2b]2-
[2c]2-
[2d]2-
NO2
H
CH3
OCH3
OCH3
CH3
NO2
H
Mn(II)
Fe(I)b
619
650
616
635
618
616
614
620
NO2
H
CH3
OCH3
OCH3
CH3
NO2
H
NO2
H
CH3
690
695
OCH3
OCH3
NO2
a Substituents on the meso-phenyl rings of the porphyrin macrocycle. b Fe(II) p-anion radical is the major product
upon the second reduction of the b-NO2 substituted compound.
and their unsubstituted parent compounds. The Soret
bands of the nitro-porphyrins are broader and red-shifted
by 4–11 nm as compared to lmax of the unsubstituted
compounds. The largest red-shift (Dl = 11 nm) is seen
upon going from (TdmPP)H2 1c to (NO2TdmPP)H2
1d and the smallest (Dl = 4 nm) upon going from
(TmPP)MnCl 3a to (NO2TmPP)MnCl 3b.
two nitroporphyrins is not reduced, consistent with what
occurs for other b-pyrrole NO2 substituted porphyrins
[23]. However, as expected, half-wave potentials of
the free-base nitroporphyrins are shifted positively by
110–130 mV for the first oxidation and by 280–290 mV
for the first reduction, as compared to E1/2 values of
the parent porphyrins lacking an NO2 group (Table 2).
Similar behavior was reported for the oxidation and
reduction of (TPP)H2 and (NO2TPP)H2 [23].
Electrochemistry
Iron porphyrins. The two non-nitro iron porphyrins
(2a and 2c) undergo three one-electron reductions in
CH2Cl2 containing 0.1 M TBAP while four reductions are
observed for the two nitro-substituted Fe(III) compounds
(2b and 2d) under the same solution conditions. In addi-
tion, the nitro-substituted Fe(III) porphyrins are easier to
reduce and harder to oxidize than the unsubstituted TmPP
or TdmPP Fe(III) complexes. For example, the first
two reductions and two oxidations of (NO2TdmPP)FeCl
2d in CH2Cl2 are located at E1/2 = -0.16, -0.79 V. 1.23
and 1.41 V as compared to the corrosponding redox of
The unsubsituted and nitro-substituted TmPP and
TdmPP complexes were examined as to their electro-
chemistry and spectroelectrochemistry in CH2Cl2 contai-
ning 0.1 M TBAP. A summary of the measured half-wave
and peak potentials for each redox process is given in
Table 2.
Free-base porphyrins. Each currently examined free-
base porphyrin undergoes two reversible one-electron
reductions and two reversible one-electron oxidations in
CH2Cl2 containing 0.1 M TBAP. The NO2 group on the
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S. YANG ET AL.
Fig. 1. Comparisons between UV-visible spectra of b-NO2 substituted porphyrins (-------) and porphyrins without NO2 group (_______),
Dl is the difference in wavelength of the Soret bands between the two types of porphyrins
Table 2. Half-wave potentials (V vs. SCE) of porphyrins in CH2Cl2 containing 0.1 M TBAP
M
Compound b-sub meso-suba Ring-oxidation
MIII/MII Ring-reductionb
2nd
1st
1st
2nd
2H
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
H
NO2
H
CH3
CH3
1.15
1.25
1.24
1.44
1.35
1.38
1.32
1.41
1.42
1.56
1.48
1.50
0.95
1.06
1.02
1.15
1.07
1.14
1.14
1.23
1.08
1.18
1.14
1.26
-1.19
-0.81
-1.12
-0.83
-1.04
-0.81d -1.29c
-1.01
-1.71c
-1.70c
-1.60c
-1.60c
-1.57
OCH3
OCH3
CH3
NO2
H
Fe(III)
-0.27
-0.18
-0.28
-0.16
-0.31
-0.15
-0.29
-0.10
-1.71c
NO2
H
CH3
OCH3
OCH3
CH3
NO2
H
-0.79d -1.31c
-1.62c
Mn(III)
NO2
H
CH3
-1.14
-1.60c
-1.10
-1.73c
OCH3
OCH3
NO2
-1.66c
a Substituents on the four phenyl rings of porphyrin macrocycle (see Chart 1). b The second
reduction of the Fe(III) porphyrin was metal-centered electron transfer process for 2a and 2c.
c Irreversible peak potential at a scan rate of 0.1 V/s. d Additional reduction observed at Epc = -1.13
(2b) and -0.99 V (2d) (see Fig. 2 and discussion in text).
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b-NITRO-SUBSTITUTED FREE-BASE, FeIII AND MnIII TETRAARYLPORPHYRINS
7
under these two solution conditions is illustrated
by the cyclic voltammograms in the middle and
bottom sections of Fig. 3.
As seen in Fig. 3a, a lowering of the tempe-
rature from room temperature to -60 °C results
in a decrease in current for the peak labeled as
Process II and an increase in that for Process II’.
This behavior is consistent with the reduction
mechanism shown in Scheme 2 where the initial
(NO2TRPP)FeIIICl reduction product is assigned
as [(NO2TRPP)FeIICl]- after the addition of one
electron (Process I). The electrogenerated
Fe(II) porphyrin with an unreduced macrocycle
can lose Cl- to give (NO2TRPP)FeII as shown
in the scheme and then be further reduced to
[(NO2TRPP)FeI]- via Process II. Alternatively,
the Fe(II) porphyrin with bound Cl- ion be
reduceddirectlyto[(NO2TRPP)FeIICl]2- atamore
Fig. 2. Cyclic voltammograms of (a) (TdmPP)FeCl 2c and (b)
(NO2TdmPP)FeCl 2d in CH2Cl2 containing 0.1 M TBAP. Scan rate at
0.10 V/s
negative potential (Process II′ꢀ) before being
converted to (NO2TRPP)FeI after loss of
the axial Cl- ligand. In this latter process,
there would be a transfer of charge from the
conjugated macrocycle to the iron center. In
(TdmPP)FeCl 2c which are located at E1/2 = -0.28, -1.01 V,
1.14 and 1.32 V in the same solvent (see Fig. 2). Similar
positive shifts in potential are also seen for the redox reac-
tions of (NO2TmPP)FeCl 2b as compared to (TmPP)FeCl
2a, with the exact potentials being given in Table 2.
either sequences of steps, [(NO2TRPP)FeI]- would be
the final product of the overall two electroreductions
of (NO2TRPP)FeIIICl. The third reduction of the two
nitroporphyrins is quasireversible by cyclic voltammetry
and this is shown in Fig. 2b for the case of (NO2TdmPP)
FeCl. This one electron addition is assigned as involving a
conversion of the iron(I) porphyrin to its iron(I) porphyrin
p-anion radical form, as was earlier reported for (TPP)
FeCl [59, 61].
The first reduction of each iron porphyrin, both the
nitro and non-nitro derivatives, is reversible and involves
a conversion of Fe(III) to Fe(II). This process occurs
at -0.18 V for the room temperature reduction of
(NO2TmPP)FeCl 2b in CH2Cl2, 0.1 M TBAP and
is labeled as Process I in Fig. 3a. The second room
temperature reduction of (NO2TmPP)FeCl occurs at
E1/2 = -0.81 V and is labeled as Process II. The first
two one-electron reductions of (NO2TmPP)FeCl
involve well-defined peaks with about equal currents
and these reactions are followed at more negative
potentials by a third reduction peak at Ep = -1.13 V
which has a much smaller peak current. This third peak
is labeled as Process II’ in Fig. 3a. A fourth reduction is
located at Ep = -1.29 V (see Table 2) and is not shown in
the figure. This peak can be compared to the process at
Ep = -1.31 V in Fig. 2 for reduction of compound 2d.
We previously demonstrated that the first one-electron
reduction of b-pyrrole brominated iron porphyrins,
(BrxTPP)FeCl (x = 0–8), results in a mixture of [(BrxTPP)-
FeIICl]- and (BrxTPP)FeII [59] and a similar equilibria is
proposed to exist between the chloride-bound and non-
chloride bound forms of the two electrogenerated Fe(II)
nitroporphyrins in CH2Cl2. In order to examine this
proposed equilibrium, two experiments were carried out,
one involving the low temperature electrochemistry of
(NO2TmPP)FeCl in 0.1 M TBAP and the other involving
measurements of the same porphyrin at room temperature
in solutions containing 0.1 M tetrabutylammonium chlo-
ride (TBACl). The electrochemistry of (NO2TmPP)FeCl
The Fe(II) product of the first one-electron reduction
contains bound Cl- and is assigned as [(NO2TRPP)FeIICl]-.
This anionic species appears to be stable in solution
at room temperature in the absence of an applied
potential. Its direct reduction by one electron at more
negative potentials leads to a transient doubly reduced
Fe(II) porphyrin which also contains axially ligated
chloride ion, but this species is rapidly converted to its
Fe(I) form upon a loss of Cl- as shown in Scheme 2.
An earlier study of (F20DPP)FeCl suggested a similar
electron transfer mechanism for (F20DPP)FeIIICl which
was reduced to [(F20DPP)FeIICl]2- prior to the loss of
axially bound chloride ion, with generation of an Fe(I)
porphyrin as the final two-electron reduction product [60].
Additional proof for assignment of the reactant and
product in Process II of Scheme 2 comes from cyclic
voltammograms obtained in CH2Cl2 solutions containing
added chloride ion in the form of TBACl. An example
of the cyclic voltammogram under these conditions is
shown in Fig. 3b for the case of (NO2TmPP)FeCl.As seen
in this figure, the reduction peak II′ in CH2Cl2 containing
0.1 M TBACl is well-defined and has almost the same
maximum peak current as for the reduction process I. The
cathodic reduction peak for II′ is located at -1.00 V for a
scan rate of 0.1 V/s and the anodic reoxidation is located
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S. YANG ET AL.
Fig. 3. Cyclic voltammograms showing the reductions of
(NO2TmPP)FeCl 2b in CH2Cl2 (a) containing 0.1 M TBAP at
room and low temperature and (b) containing 0.1 M TBACl at
room temperature
Fig. 4. Thin-layer UV-visible spectral changes of (a)
(TmPP)FeIIICl 2a and (b) (NO2TmPP)FeIIICl 2b during
controlled potential reductions in CH2Cl2 containing 0.1 M
TBAP
at Epa = -0.78 V. An anodic peak at the same potential is
seen for reoxidation of electroreduced (NO2TmPP)FeCl
at room temperature or at -60 °C as seen in Fig. 3.
The investigated iron(II) porphyrins each exhibit
a split Soret band between 418 and 446 nm and one
Q-band at 540–544 nm (see Table 1). Similar spectral
patterns have been reported for other Fe(II) porphyrins in
nonaqueous media [59–64].
Quite different UV-visible spectral patterns are seen
during the second controlled potential reduction of
(TmPP)FeCl and (NO2TmPP)FeCl in a thin-layer cell
(Rxn II in Fig. 4). The reductions of both porphyrins
lead ultimatly to an Fe(I) porphyrin as a final product
but two different mechanisms are proposed as shown in
Scheme 2. A direct reduction of Fe(II) to Fe(I) occurs for
(TmPP)FeCl and (TdmPP)FeCl, as is also the case for
OEP [65, 66] and TPP complexes [59, 61]. In contrast,
the nitro-substituted Fe(II) porphyrins both undergo
an initial reduction at the conjugated p-system to give
an Fe(II) p-anion radical and this form of the reduced
porphyrin is then converted to an Fe(I) porphyrin product
after loss of the Cl- axial ligand.
and the fluorinated dodecaporphyrin contain highly
electron-withdrawing substituents, as is also the case for
the currently investigated nitroporphyrins.
In summary, the overall electroreduction mechanism
of (NO2TmPP)FeCl and (NO2TdmPP)FeCl in CH2Cl2,
0.1 M TBAP is proposed to occur as shown in Scheme 2.
A dissociation of the chloride axial ligand from the
electrogenerated Fe(II) porphyrins is favored after a one
electron reduction of (TmPP)FeCl and (TdmPP)FeCl
in CH2Cl2. However, the electron density at the iron(II)
center of the porphyrin decreases when an NO2 group is
introduced into the macrocyclic p-ring system as in the
case of (NO2TmPP)FeCl and (NO2TdmPP)FeCl. This
results in a strengthening of the Fe–Cl bond for the singly
reduced b-nitro substituted porphyrins. The retaining
of a chloride axial ligand results in a sufficent shift in
electron density at the iron center such that reduction
occurs at the macrocycle instead of the metal.
Consequently, the second reduction of (NO2TmPP)FeCl
and (NO2TdmPP)FeCl occurs via processes II′ to generate
an Fe(II) porphyrin p-anion radical prior to formation of
an Fe(I) porphyrin as the ultimate reduction product (see
Fig. 3). Further reduction of the Fe(I) porphyrin by one
electron yields the Fe(I) p anion radical.
In a previous study of [(CN)4TPP]FeCl, an equilibrium
between an iron(I) porphyrin and an iron(II) porphyrin
p-anion radical was proposed, with the iron(II) radical
anion assigned as the major contributor to the resonance
formofthecompound[61].Asimilarequilibriumbetween
an Fe(I) porphyrin and an Fe(II) porphyrin p-anion
radical was also proposed after the two electroreductions
of (F20DPP)FeCl [60]. Both the tetra-cyano porphyrin
Controlled potential oxidations of each iron porphyrin
were also carried out in a thin-layer cell in CH2Cl2 contai-
ning 0.1 M TBAP and the spectral changes are illustrated
in Fig. 5 for (TmPP)FeIIICl and (NO2TmPP)FeIIICl.
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b-NITRO-SUBSTITUTED FREE-BASE, FeIII AND MnIII TETRAARYLPORPHYRINS
9
+e
+e
[(NO2TRPP)FeIICl]-
[(NO2TRPP.-)FeIICl]2-
(NO2TRPP)FeIIICl
II'
I
(-0.18 V)
(-1.13 V)
+ Cl-
-Cl-
-Cl-
+e
+e
III
(NO2TRPP)FeII
[(NO2TRPP)FeI]-
[(NO2TRPP)FeI]2-
II
(-0.81V)
(-1.29 V)
Scheme 2. Proposed reduction mechanism of iron nitroporphyrins in CH2Cl2, 0.1 M TBAP, where the potentials in scheme are those
for (NO2TmPP)FeCl
Fig. 5. Thin-layer UV-visible spectral changes of (a) (TmPP)FeIIICl 2a and (b) (NO2TmPP)FeIIICl 2b during controlled potential
oxidations in CH2Cl2 containing 0.1 M TBAP
As seen in the figure, the Soret band continuously
decreases in intensity upon the first and second
oxidations for each compound, thus indicating that
Fe(III) p-cation radical and dianions are formed as
two electrons are stepwise removed from the porphyrin
macrocycle.
Manganese porphyrins. The electrochemical behavior
of the two b-nitro substituted manganese porphyrins
differs from what is seen for the unsubstituted compounds,
(TmPP)MnCl and (TdmPP)MnCl, in nonaqueous media.
The half-wave and peak potentials of each Mn(III)
porphyrin are given in Table 2 while examples of the cyclic
voltammograms of (TmPP)MnCl and (NO2TmPP)MnCl
in CH2Cl2 containing 0.1 M TBAP are shown in Fig. 6.
The highly electron-withdrawing nitro group on
(NO2TmPP)MnCl and (NO2TdmPP)MnCl leads to a
substantial positive shift in redox potentials, with three
reductions being observed for the b-NO2 substituted
compounds in CH2Cl2 as compared to the parent
compounds, (TmPP)MnCl and (TdmPP)MnCl which
exhibit only two reductions under the same solution
conditions (Table 2). These results are consistent with
what was previously reported for other b-NO2 substituted
manganese porphyrins [33].
Fig. 6. Cyclic voltammograms shown the first reduction and
first oxidation of (a) (TmPP)MnCl 3a and (b) (NO2TmPP)-
MnCl 3b in CH2Cl2 containing 0.1 M TBAP
Each current investigated manganese(III) porphyrin
in CH2Cl2 exhibits a split Soret band, located between
379 and 490 nm. There are also two Q-bands at 581–594
and 616–640 nm (Table 1). Significant spectral changes
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10
S. YANG ET AL.
and 616–650 nm (Table 1). This electrode reduction
has generally been assigned as a MnIII/MnII conversion
[63, 67]. The reductions which occur at more negative
potentials are assigned to porphyrin ring-centered
electron transfer processes to give Mn(II) p-anion
radicals and dianions.
Figure 8 illustrates of the spectral changes which
occur for (TdmPP)MnCl and (NO2TdmPP)MnCl during
controlled potential oxidation in CH2Cl2 containing
0.1 M TBAP. Similar spectral changes are seen for
these two compounds upon the first and second one-
electron abstractions, suggesting that the NO2 substituted
and unsubstituted porphyrins both follow the
same electron transfer mechanism under the given
experimental conditions. As seen in Fig. 8, the Soret
band at 481 nm for (TdmPP)MnCl 3c and at 490 nm for
(NO2TdmPP)MnCl 3d both decrease in intensity while
a broad visible band grows in at 660 nm (3c) or 695 nm
(3d). Mn(III) p-cation radicals are generated during the
first one-electron oxidation of these compounds and the
second oxidation is then assigned to the formation of a
Mn(III) dication in CH2Cl2.
Factors affecting the magnitude of the NO2 substi-
tuent effect. As mentioned earlier, the magnitude of the
NO2 substituent effect on redox potentials for reduction
or oxidation of a nitro-porphyrin will depend not only
on the specific placement of the nitro substituent on
the compound but also on the site of electron transfer
(oxidation or reduction, metal or macrocycle), and the
nature of the central metal ion. This is seen by the data
in Table 3 which compares substituent effects for adding
a single nitro group directly onto the meso-position of a
porphyrin with that of a porphyrin or corrole containing
one b-pyrrole substituted nitro group.
Fig. 7. Thin-layer UV-visible spectral changes of (a)
(TdmPP)MnCl 3c and (b) (NO2TdmPP)MnCl 3d during the
controlled potential reductions in CH2Cl2 containing 0.1 M
TBAP
are observed during the controlled potential reduction in
CH2Cl2 containing 0.1 M TBAP, with examples being
given in Fig. 7 for (TdmPP)MnCl and (NO2TdmPP)MnCl.
The spectrum of the singly reduced Mn(III) porphyrins
exhibits an intense Soret band at 445–447 nm, a shoulder
at 403–410 nm and two visible bands between 576–590
Fig. 8. Thin-layer UV-visible spectral changes of (a) (TmPP)MnCl 3c and (b) (NO2TmPP)MnCl 3d during the controlled potential
oxidations in CH2Cl2 containing 0.1 M TBAP
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b-NITRO-SUBSTITUTED FREE-BASE, FeIII AND MnIII TETRAARYLPORPHYRINS
11
Table 3. Comparison of the first reduction and oxidation potentials for the four types of NO2-substituted porphyrins
Type of Cpd
Macrocyclea
M
#NO2
First oxidation
First reduction
Reference
E1/2, V DE, mVb
E1/2, V DE, mVb
meso-NO2
porphyrin
OEP
2H
0
1
1.04
-1.19
1.23
0.87
1.01
0.81
1.04
1.03
1.15
190
-0.85
-1.47
-1.02
340
39
Zn(II)
0
1
140
130
450
530
39
40
Cu(II)
2H
0
1
-1.37
-0.84
b-NO2
porphyrin
TPP
0
1
-1.01
-0.72
120
290
23
Cu(II)
Zn(II)
2H
0
1
0.69
0.78
-1.64
-1.27
90
130
110
70
370
380
280
90
23
23
tw
tw
tw
0
1
0.57
0.70
-1.67
-1.29
TmPP
TdmPP
TMPCor
0
1
0.95
1.06
-1.19
-0.81
Fe(III)
Mn(III)
2H
0
1
1.07
1.14
-0.27
-0.18
0
1
1.08
1.18
-0.31
-0.15
100
130
90
160
290
120
190
190
0
1
1.02
1.15
-1.12
-0.83
tw
tw
Fe(III)
Mn(III)
Fe(III)
0
1
1.14
1.26
-0.28
-0.16
0
1
1.14
1.26
-0.29
-0.10
120
190
tw
b-NO2
corrole
0
1
0.82
1.01
-0.35
-0.16
55
55
0
1
0.97
1.18
-0.14
-0.05
TNPCor
Fe(III)
Ag(III)
210
240
190
250
0
1
-0.03
1.21
-0.79
-0.54
TtBuPCor
57
56
Cu(III)
0
1
0.71
1.90
-0.17
-0.00
190
170
a
TMPCor and TNPCor are the trianions of the tri-methoxylphenylcorrole and tri-nitrophenylcorrole, respectively,
TtBuPCoristhetrianionsofthetri-butylphenylcorroleandthedatameasuredinpyridineforAg(III)corrolescontaining
0.1 M TBAP. b DE1/2 = the potential difference between E1/2 for the reduction or oxidations of the nitrocorrole and the
parent corrole without the nitro group.
The largest substituent effect upon reduction is seen for
the meso-substituted compounds, as shown in Table 3 for
the free-base, Zn(II) and Cu(II) complexes of substituted
OEP, where DE1/2 varies between 340 and 530 mV
[39, 40]. In contrast, the b-pyrrole substituted TPP
compounds with 2H, Zn(II) or Cu(II) centers show DE1/2
values which range between 290 and 380 mV [23].
The shift in potential between the free-base nitro
and free-base non-nitro substituted TPP complexes was
reported to be 290 mV in CH2Cl2 [23] and a similar shift of
280 and 290 mV is seen for the two currently investigated
free-base porphyrins with OCH3 or CH3 substituents. In
contrast, the Mn(III)/Mn(II) reaction of the currently
studied compounds has a smaller nitro substituent effect
of 160 and 190 mV, and this shift in potential is decreased
even further in the case of the Fe(III)/Fe(II) reactions,
where a DE1/2 of 90 and 120 mV shift is seen for the two
investigated porphyrins.
The above trends in nitro group substituent effects as
a function of changes in the type of central metal ion are
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12
S. YANG ET AL.
consistent with what has been observed for other redox-
active and redox-inactive metalloporphyrins [67], but it
varies substantially from what occurs in the case of the
b-pyrrole substituted corroles. This is evident from the
data in Table 3, where the metal-centered reduction of
the Cu(III), Fe(III) and Ag(III) derivatives to their lower
M(II) oxidation state has a larger substituent effect
ranging from 170 to 250 mV [55–57]. This increased
value of DE1/2 for the the metal-centered reduction of the
corroles is consistent with a larger interaction existing
between the corrole central metal ion and the conjugated
macrocycle than in the case of the porphyrins where the
metal and the macrocycle orbitals are better separated.
Finally, it should be pointed out that the nitro-
substituent effects on oxidation of porphyrins are
substantially smaller than those for reduction but this is
not what occurs for Fe(III), Ag(III) and Cu(III) corroles
where the oxidations and reductions are similarly effected
by the nitro group and may in fact be slightly larger for
oxidation, as seen in Table 3.
3. Handbook of Porphyrin Science: With Applications
to Chemistry, Physics, Materials Science, Engineer-
ing, Biology and Medicine, Vol. 12, Kadish KM,
Smith KM and Guilard R. (Eds.) World Scientific:
Hackensack, NJ, 2010.
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CONCLUSION
In summary, the electron-withdrawing NO2 substi-
tuent on the b-pyrrole position of (NO2TmPP)M and
(NO2TdmPP)M leads to a much easier reduction and a
slightly harder oxidation as compared to the unsubstituted
parentporphyrins.Thereisalsoared-shiftintheUV-visible
spectra of the compounds having the nitro group. The
unsubstituted Fe(III) porphyrin favors formation of an
Fe(I) porphyrin upon reduction in CH2Cl2 but reduction of
the b-pyrrole NO2-substituted porphyrin leads to an Fe(II)
porphyrin p-anion radical under the same experimental
conditions. The addition of one NO2 group to the
b-pyrrole position of the porphyrin has a larger effect on
the redox potentials than for addition to the meso-phenyl
rings of the compound. The observed shifts in reduction/
oxidation potentials for addition of an NO2 group to the
b-pyrrole position of the currently investigated porphyrins
is similar to that which was earlier reported for reduction
and oxidation of b-pyrrole substituted triphenylcorroles
containing the same central metal ions.
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Chem. 1994; 33: 626–621.
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Acknowledgements
We gratefully acknowledge supports from the
Natural Science Foundation of China (No. 21071067,
21001054), Postdoctoral Science Foundation of China
(2012M511203) and Jiangsu Province (1102126C), and
the Robert A. Welch Foundation (K.M.K., Grant E-680).
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