Spectroelectrochemical characterization of meso triaryl-substituted Mn(IV), Mn(III) and Mn(II) corroles. Effect of solvent and oxidation state on UV-visible spectra and redox potentials in nonaqueous media

Article abstract of DOI:10.1142/S1088424614501119

Two series of substituted manganese triarylcorroles were synthesized and characterized as to their electrochemical and spectroelectrochemical properties in CH₂Cl₂, CH₃CN and pyridine. The investigated compounds are represented as (YPh)₃CorMn^III and (YPh)₃CorMn^IVCl, where Cor is a trianion of the corrole and Y is a Cl, F, H or CH₃ para-substituent on the three phenyl rings of the macrocycle. Each neutral Mn(III) corrole exists as a four-coordinate complex in CH₂Cl₂ and CH₃CN and as a five-coordinate species in pyridine. (YPh)₃CorMn^III undergoes two oxidations to stepwise generate a Mn(IV) corrole and a Mn(IV) μ-cation radical. It also undergoes one reduction to generate a Mn(II) corrole in CH₂Cl₂ or CH₃CN. In contrast, the reduction of (YPh)₃CorMn^III leads to a Mn(III) corrole μ-anion radical in pyridine. One oxidation is observed for (YPh)₃CorMn^IVCl in CH₂Cl₂ and CH₃CN to generate a Mn(IV) corrole μ-cation radical while Mn(III) and Mn(II) corroles are stepwise formed after reduction of the same compound. The second reduction of (YPh)₃CorMn^IVCl in pyridine gives a Mn(III) μ-anion radical as opposed to a Mn(II) corrole with an unreduced μ-ring system. The neutral, reduced and oxidized forms of each corrole were characterized by electrochemistry and UV-visible spectroelectrochemistry and comparisons are made between the UV-visible spectra and redox potentials of the compounds in different central metal oxidation states. An overall reduction/oxidation mechanism in the three solvents is proposed.

Full text of DOI:10.1142/S1088424614501119

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2014; 18: 1–14
DOI: 10.1142/S1088424614501119
Published at http://www.worldscinet.com/jpp/
Spectroelectrochemical characterization of meso
triaryl-substituted Mn(IV), Mn(III) and Mn(II) corroles.
Effect of solvent and oxidation state on UV-visible spectra
and redox potentials in nonaqueous media
Bingbing Gaoa, Zhongping Ou*a◊, Xueyan Chena, Shi Huanga, Bihong Lia,
Yuanyuan Fangb◊◊ and Karl M. Kadish*b◊
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
b Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA
Received 7 September 2014
Accepted 21 September 2014
ABSTRACT: Two series of substituted manganese triarylcorroles were synthesized and characterized
as to their electrochemical and spectroelectrochemical properties in CH2Cl2, CH3CN and pyridine.
The investigated compounds are represented as (YPh)3CorMnIII and (YPh)3CorMnIVCl, where Cor is
a trianion of the corrole and Y is a Cl, F, H or CH3 para-substituent on the three phenyl rings of the
macrocycle. Each neutral Mn(III) corrole exists as a four-coordinate complex in CH2Cl2 and CH3CN and
as a five-coordinate species in pyridine. (YPh)3CorMnIII undergoes two oxidations to stepwise generate
a Mn(IV) corrole and a Mn(IV) p-cation radical. It also undergoes one reduction to generate a Mn(II)
corrole in CH2Cl2 or CH3CN. In contrast, the reduction of (YPh)3CorMnIII leads to a Mn(III) corrole
p-anion radical in pyridine. One oxidation is observed for (YPh)3CorMnIVCl in CH2Cl2 and CH3CN
to generate a Mn(IV) corrole p-cation radical while Mn(III) and Mn(II) corroles are stepwise formed
after reduction of the same compound. The second reduction of (YPh)3CorMnIVCl in pyridine gives a
Mn(III) p-anion radical as opposed to a Mn(II) corrole with an unreduced p-ring system. The neutral,
reduced and oxidized forms of each corrole were characterized by electrochemistry and UV-visible
spectroelectrochemistry and comparisons are made between the UV-visible spectra and redox potentials
of the compounds in different central metal oxidation states. An overall reduction/oxidation mechanism
in the three solvents is proposed.
KEYWORDS: manganese(IV) and manganese(III) corroles, synthesis, electrochemistry, spectroelectro-
chemistry, effect of solvent and oxidation state.
state of the central metal ion in manganese corroles can
INTRODUCTION
vary from +3 to +6 [8]. The formal oxidation state of the
Manganese corroles have attracted much interest
[1–11] due to their possible applications in the fields of
catalysis [12–17], ion-selective electrodes [18], single-
chain magnets [19] and medicine [20, 21]. The oxidation
four-coordinate compound is +3 while the five-coordinate
complex with a single anionic or s-bonded axial ligand
is assigned as having a formal metal oxidation state of
+4. Six-coordinate manganese corroles have also been
characterized with central metal ion oxidation states of
◊SPP full and ◊◊ student member in good standing
+5 and +6 [1, 2, 6, 8].
There have been numerous electrochemical studies of
*Correspondence to: Zhongping Ou, email: zpou2003@yahoo.
Mn(III) corroles [8–12, 22, 23], but only two papers have
been published on the electrochemistry of stable Mn(IV)
corroles [9, 24] and no systematic comparisons have
com, tel: +86 511-88791800, fax: +86 0511-88791800; Karl
M. Kadish, email: kkadish@uh.edu, tel: +1 (713) 743-2740,
fax: +1 (713) 743-2745
Copyright © 2014 World Scientific Publishing Company

2
B. GAO ET AL.
been made between the redox potentials and spectra of
neutral, reduced and oxidized triaryl-substituted Mn(III)
and Mn(IV) corroles containing the same series of
electron-donating or electron-withdrawing substituents
on the macrocycle.
Instrumentation
Cyclic voltammetry was carried out at 298 K using a
CHI-730E Electrochemistry Workstation. A homemade
three-electrode cell was used for cyclic voltammetric
measurements and consisted of glassy carbon work
electrode, a platinum counter electrode and a homemade
saturated calomel reference electrode (SCE). The SCE
is separated from the bulk of the solution by a fritted
glass bridge of low porosity which contained the solvent/
supporting electrolyte mixture.
Thin-layer UV-visible spectroelectrochemical experi-
ments were performed with a home-built thin-layer
cell which has a light transparent platinum networking
electrode. Potentials were applied and monitored
with an EG&G PAR Model 173 potentiostat or BiStat
electrochemistry station. Time-resolved UV-vis spectra
were recorded with a Hewlett-Packard Model 8453
diode array spectrophotometer. High purity N2 were used
to deoxygenate the solution and kept over the solution
during each electrochemical and spectroelectrochemical
experiment.
This is done in the present paper which examines two
series of substituted triarylcorroles containing Mn(III)
and Mn(IV) as the central metal ion. The first series of
compounds is represented as (YPh)3CorMnIII and the
second as (YPh)3CorMnIVCl, where Cor is a trianion of
the corrole and Y is a Cl, F, H or CH3 substituent on the
para-position of each phenyl ring of the macrocycle.
The structures of these corroles are shown in Chart 1. The
neutral, reduced and oxidized forms of each manganese
corrole were electrochemically and spectroscopically
examined in CH2Cl2, CH3CN and pyridine containing
0.1 M TBAP and a comparison is then made between
the UV-visible spectra and redox potentials for each
complex generated in a given oxidation state by chemical
synthesis or by electrogeneration under the application of
an applied oxidizing or reducing potential.
MALDI-TOF mass spectra were taken on a Bruker
BIFLEX III ultra-high resolution instrument using
alpha-cyano-4-hydroxycinnamic acid as the matrix and
the spectra obtained for examined corroles are shown in
Figs S1–S8 (Supporting information). Infrared spectra
were recorded with a Nicolet Nexus 470 IR spectrometer.
EXPERIMENTAL
Chemicals
Dichloromethane (CH2Cl2) was dried over molecular
sieves (4 Å), stored under argon and distilled over CaH2
prior to use. Acetonitrile (CH3CN) was purchased
from Aldrich Chemical Co., distilled over P2O5 under
vacuum prior to use. Anhydrous pyridine (99.8%) from
Sinopharm Chemical Reagent Co. or Aldrich Chemical
Co., was used without further purification. Tetra-n-butyl
ammonium perchlorate (TBAP) was purchased from
Sigma Chemical or Fluke Chemika Co., recrystallized
from ethyl alcohol and dried under vacuum at 40°C for
at least one week prior to use.
Synthesis
Mn(III) corroles 1–4. The corresponding free-base
corroles (~40 mg, 64 mmol) were synthesized according
to procedure described in literature [25] and dissolved in
30 mL N,N ′-dimethylformamide containing 5 equival-
ents of Mn(OAc)2·4H2O. The solution was heated to
reflux under N2 for 10 min and then cooled to room
temperature, after which it was diluted by CHCl3 and
washed thoroughly with aqueous Na2CO3 to remove
DMF. The organic layer was collected and dried with
Y
Y
Cl
N
N
N
N
MnIII
MnIV
Y
Y
Y
Y
N
N
N
N
Y = Cl, (ClPh)3CorMnCl 5
Y = F, (FPh)3CorMnCl 6
Y = H, (Ph)3CorMnCl 7
Y = Cl, (ClPh)3CorMn 1
Y = F, (FPh)3CorMn 2
Y = H, (Ph)3CorMn 3
Y = CH3, (CH3Ph)3CorMnCl 8
Y = CH3, (CH3Ph)3CorMn 4
Chart 1. Structures of investigated Mn(III) and Mn(IV) triarylcorroles
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 2–14

SPECTROELECTROCHEMICAL CHARACTERIZATION OF MESO TRIARYL-SUBSTITUTED
3
anhydrous MgSO4. The solvent was then removed and
1452, 1371, 1295, 1175, 1120, 1059, 1026, 799, 723,
624. UV-vis (CH3CN): lmax, nm (log e) 315 (4.21), 364
(4.29), 439 (4.47). MS (MALDI-TOF): m/z calcd. for
C40H29N4ClMn 656.076; found 621.261.
the residue chromatographed on neutral alumina (200–
300 mesh) using CH2Cl2/hexane or CH2Cl2 as eluent. The
green fraction was collected and evaporated to dryness.
(ClPh)3CorMnIII 1. 35 mg, yield 14%. IR (KBr): n,
cm-1 3092, 2919, 1642, 1586, 1490, 1382, 1299, 1217,
1087, 1050, 1018, 801, 714, 492. UV-vis (CH3CN):
RESULT AND DISCUSSION
l
max, nm (log e) 405 (4.61), 433 (4.58), 654 (4.00). MS
(MALDI-TOF): m/z calcd. for C37H20N4Cl3Mn: 681.879;
found 679.314.
Electrochemistry in CH3CN and CH2Cl2
(FPh)3CorMnIII 2. 37 mg, yield 14.5%. IR (KBr): n,
cm-1 3102, 2922, 1644, 1595, 1506, 1460, 1368, 1298,
1224, 1154, 1053, 807, 719, 606, 523. UV-vis (CH3CN):
The investigated corroles undergo three types of redox
processes in CH3CN and CH2Cl2. One is a ring-centered
oxidation involving conversion of a MnIV corrole to a
MnIV p-cation radical and the other two involve metal-
centered processes, MnIII/MnII and either MnIII/MnIV or
MnIV/MnIII depending upon the initial oxidation state
of the central metal ion in 1–8. Examples of cyclic
voltammograms are shown in Fig. 1 for (ClPh)3CorMnIII
1 and (ClPh)3CorMnIVCl 5 and a summary of half-wave
or peak potentials for each redox process of the eight
compounds is given in Table 1 which compares the
ring- and metal-centered redox processes for 1–4 and
5–8 as a function of the specific electron transfer process
which occurs in each solvent, i.e. formation of a corrole
p-cation radical or oxidation/reduction of the corrole
central metal ion.
l
max, nm (log e) 402 (4.69), 431 (4.60), 650 (4.10). MS
(MALDI-TOF): m/z calcd. for C37H20N4F3Mn 632.515;
found 631.370.
Ph3CorMnIII 3. 39 mg, yield 15%. IR (KBr): n, cm-1
3049, 2920, 1635, 1590, 1503, 1454, 1299, 1217, 1170,
1060, 1024, 786, 707. UV-vis (CH3CN): lmax, nm (log e)
403 (4.51), 431 (4.47), 656 (3.98). MS (MALDI-TOF):
m/z calcd. for C37H23N4Mn 578.543; found 577.583.
(CH3Ph)3CorMnIII 4. 41 mg, yield 16%. IR (KBr):
n, cm-1 3017, 2918, 2854, 1595, 1509, 1451, 1367,
1300, 1263, 1217, 1179, 1054, 1024, 799, 713. UV-vis
(CH3CN): lmax, nm (log e) 404 (4.29), 438 (4.30), 657
(3.74). MS (MALDI-TOF): m/z calcd. for C40H29N4Mn
620.623; found 619.389.
As seen in the table and figure, the MnIII/MnIV reaction
of 1–4 is characterized by two reversible redox processes
in CH3CN or CH2Cl2. The first occurs at E1/2 values
between 0.08 and 0.24 V and the second at E1/2 = 0.31
to 0.45 V vs. SCE. Both oxidations are reversible and the
current is smaller for the first process than for the second.
Similar redox behavior was previously reported
for other manganese(III) triarylcorroles under similar
solution conditions [10] and the “split” oxidation
processes were assigned as resulting from two different
MnIV corrole products, one which was ion-paired with the
ClO4- from the supporting electrolyte, the other existing
in its dissociated cationic form, i.e. (YPh)3CorMnIVClO4
and [(YPh)3CorMnIV]+, respectively.
Mn(IV) corroles 5–8. The manganese(III) corroles
(15 mg for compounds 1–3, 20 mg for compound
4) were dissolved in chloroform or dichloromethane
and then washed with a 10% HCl solution. After the
color of the solution changed from green to brownish
yellow, the organic layer was collected and evaporated
to dryness. The residue was dissolved in CH2Cl2 and
chromatographed using silica gel (CHCl3 as eluent).
The brown-yellow fraction corresponding to the Mn(IV)
corrole was collected and evaporated to dryness.
(ClPh)3CorMnIVCl 5. 13.6 mg, yield 90%. IR (KBr):
n, cm-1 3095, 2922, 2853, 1588, 1486, 1352, 1300, 1165,
1089, 1054, 1022, 803, 721, 492. UV-vis (CH3CN):
In contrast to the two MnIII/MnIV oxidations in
CH3CN and CH2Cl2, only a single MnIV/MnIII transition
is exhibited for 5–8 in these two solvents (see Fig. 1b).
The reversible E1/2 values for the MnIV/MnIII reductions of
(YPh)3CorMnCl range from 0.07 to 0.23 V, values which
are virtually identical to E1/2 values for the first metal-
centered oxidation of the neutral Mn(III) corroles in the
same solvents. For example, E1/2 = 0.24V for oxidation of
the para-chlorophenyl complex 1 in CH3CN and 0.23 V
for the reduction of the para-chlorophenyl complex 5 in
the same solvent. Additional comparisons can be seen in
Table 1 where in each case the E1/2 values for reduction
of MnIV in 5–8 are within experimental error of the
measured potentials for the first MnIII oxidation of 1–4.
As will be demonstrated, the one-electron reduction
of (YPh)3CorMnIVCl leads to a stable anionic MnIII
corrole with an axially coordinated chloride anion,
l
max, nm (log e) 313 (4.39), 363 (4.48), 431 (4.70). MS
(MALDI-TOF): m/z calcd. for C37H20N4Cl4Mn 717.332;
found 681.281.
(FPh)3CorMnIVCl 6. 13.9 mg, yield 91%. IR (KBr):
n, cm-1 3100, 2921, 2853, 1596, 1504, 1412, 1355, 1301,
1228, 1154, 1030, 805, 723, 606, 524. UV-vis (CH3CN):
l
max, nm (log e) 313 (4.53), 358 (4.58), 431 (4.84). MS
(MALDI-TOF): m/z calcd. for C37H20N4F3ClMn 667.968;
found 633.246.
Ph3CorMnIVCl 7. 13.4 mg, yield 90%. IR (KBr): n,
cm-1 3051, 2925, 1602, 1507, 1427, 1351, 1301, 1158,
1058, 1027, 988, 845, 787, 757, 710, 664. UV-vis
(CH3CN): lmax, nm (log e) 312 (3.89), 357 (3.98), 430
(4.22). MS (MALDI-TOF): m/z calcd. for C37H23N4Mn
613.996; found 579.076.
(CH3Ph)3CorMnIVCl 8. 18.4 mg, yield 92%. IR
(KBr): n, cm-1 3112, 2925, 2860, 1729, 1560, 1513,
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 3–14

4
B. GAO ET AL.
CH3CN or CH2Cl2. This electrochemical EC mechanism
is shown in Equation 1.
-Cl-
ꢂꢀꢀꢀ
III
[(YPh)3 CorMnIIICl]-
(YPh) CorMn
ꢀꢀꢀꢁ
3
+e
II
-
ꢀꢀꢀꢁ
[(YPh) Mn ]
ꢂ ꢀ ꢀꢀ
3
(1)
A more complex situation is seen in CH2Cl2 for the
MnIII/MnII process which occurs in two steps as seen
in Fig. 1. In brief, the first reduction of MnIII at -1.38
to -1.47 V generates a MnII corrole which reacts with
the solvent to give a second MnIII corrole that is then
irreversibly reduced to MnII at a peak potential of -1.53
to -1.72 V (see Ep values in Table 1). This mechanism has
been examined in detail for related MnIII octaethylcorroles
and is described in the literature [9]. It is not a subject of
the current publication since the same chemical reactions
occur for 1–4 and 5–8 and these are not related to the
initial oxidation state of the corrole central metal ion.
In summary, the two metal-centered electrode
reactions of (YPh)3CorMnIII and (YPh)3CorMnIVCl in
CH3CN and CH2Cl2 are proposed to occur as shown in
Scheme 1 where only the first of two MnIII/II processes is
illustrated.
Electrochemistry in pyridine
MnIII corroles have been shown to axially coordinate
one pyridine [10, 11, 26] and this is also the case for the
currently examined MnIII corroles 1–4 which exist as
five-coordinate complexes in pyridine containing 0.1 M
TBAP. Cyclic voltammograms of (YPh)3CorMnIII(py)
1–4 under these solution conditions are shown in
Fig. 2a. Each corrole undergoes a well-defined reversible
reduction at -1.30 to -1.37 V and a more complicated
MnIII/IV oxidation which involves non-coupled oxidation/
reduction peaks as seen in the figure.
The mechanism for oxidation of related MnIII corroles
in pyridine has been described in a previous publication
as involving an electrochemical “box scheme” [10]
and the same sequence of steps is proposed to occur
for oxidation of the five-coordinate (YPh)3CorMn(py)
derivatives 1–4 in this solvent. This prevailing
mechanism is given in the top part of Scheme 2 where
(YPh)3CorMnIII(py) 1–4 (shown in blue and bold) is
initially oxidized to [(YPh)3CorMnIV(py)]+ at E1/2 = 0.45–
0.53 V. This electrochemical reaction is then followed
by rapid addition of a second pyridine molecule to give
[(YPh)3CorMnIV(py)2]+ whichisre-reducedat0.30–0.38V
on the reverse scan to give a transient six-coordinate
(YPh)3CorMnIII(py)2, after which one pyridine dissociates
to give back the original five-coordinate complex.
The main MnIV species in solution after oxidation of
[(YPh)3CorMnIV(py)]+ is [(YPh)3CorMnIV(py)2]+ but
because the oxidized five-coordinate complex is easier
to re-reduce than the oxidized bis-pyridine derivative,
a dissociation of one bound py ligand occurs prior to
electron transfer (an electrochemical CE mechanism)
Fig. 1. Cyclic voltammograms of (a) (ClPh)3CorMnIII 1 and
(b) (ClPh)3CorMnIVCl 5 in CH3CN and CH2Cl2 containing
0.1 M TBAP. Scan rate = 0.10 V/s
i.e. [(YPh)3CorMnIIICl]-, as opposed to (YPh)3CorMnIII.
However, the potentials for reduction of the five-
coordinate MnIII corroles electrogenerated from 5–8 are
virtually identical to the E1/2 values for reduction of the
neutral four-coordinate MnIII corroles 1–4 which lack an
anionic ligand.These values are given in Fig. 1 andTable 1
where E1/2 varies with the phenyl-ring substituent and
ranges from -1.33 to -1.40 V in CH3CN.
The similar potentials for reduction of the two
compounds in each series (1/5, 2/6, 3/7 and 4/8) suggests
that there is a fast dissociation of the Cl- axial ligand
on the electrogenerated [(YPh)3CorMnIIICl]- complexes
prior to further electroreduction at -1.33 to -1.40 V in
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 4–14

SPECTROELECTROCHEMICAL CHARACTERIZATION OF MESO TRIARYL-SUBSTITUTED
5
Table 1. Half-wave potentials (V vs. SCE) for redox reactions of (YPh)3CorMnIII 1–4 and (YPh)3CorMnIVCl 5–8 in CH2Cl2, CH3CN
and pyridine containing 0.1 M TBAP
Solvent
Compound
Corrole ring ox
Metal rxn
Metal red, MnIII/II,a
cpd 1–4
cpd 5–8
MnIV red (5–8) MnIII ox (1–4)
cpd 1–4
cpd 5–8
CH3CN Y = Cl 1, 5
Y = F 2, 6
1.02
0.99
0.96
0.92
1.05
1.03
1.02
0.97
0.23
0.21
0.18
0.14
0.24, 0.45
0.23, 0.42
0.21, 0.39
0.18, 0.35
-1.34
-1.36
-1.38
-1.39
-1.33
-1.36
-1.38
-1.40
Y = H 3, 7
Y = CH3 4, 8
CH2Cl2 Y = Cl 1, 5
Y = F 2, 6
1.03
1.02
0.99
0.94
1.07
1.05
1.03
0.97
0.17
0.14
0.09
0.07
0.19, 0.42
0.16, 0.39
0.12, 0.32
0.08, 0.31
-1.38b (-1.54b)d
-1.40b (-1.66b)d
-1.46b (-1.70b)d
-1.47b (-1.72b)d
-1.40b (-1.53b)d
-1.42b (-1.66b)d
-1.44b (-1.70b)d
-1.46b (-1.72b)d
Y = H 3, 7
Y = CH3 4, 8
Pyridine Y = Cl 1, 5
Y = F 2, 6
0.49b,c, 0.32b
0.49b,c, 0.25b
0.46b,c, 0.22b
0.42b,c, 0.22b
0.38, 0.53
0.37, 0.52
0.33, 0.49
0.30, 0.45
-1.30
-1.33
-1.34
-1.37
-1.31
-1.32
-1.34
-1.37
Y = H 3, 7
Y = CH3 4, 8
a The MnIII reduction is assigned as metal-centered in CH3CN and CH2Cl2 and ring-centered in pyridine; see discussion in text.
b Irreversible peak potential at a scan rate of 0.10 V/s. c Re-oxidation peak potential at a scan rate of 0.1 V/s. d The second reduction
in parentheses involves product of the MnII chemical reaction.
Scheme 1. Proposed redox mechanism for the metal-centered reactions of (YPh)3CorMnIII and (YPh)3CorMnIVCl in CH3CN and
CH2Cl2 containing 0.1 M TBAP
and both MnIV/III re-reductions are detected on the cyclic
voltammetry timescale, one at E1/2 = 0.45–0.53 V and the
other at Ep = 0.30–0.38 V for a scan rate of 0.10 V/s.
A similar “box mechanism” also occurs for conversion
of (YPh)3CorMnIVCl 5–8 to its MnIII form (Scheme 2,
bottom). The chloride bound compound in solution is
irreversibly reduced at a peak potential of 0.22–0.32 V in
pyridine to give [(YPh)3CorMnIIICl]- as seen in Fig. 2b.
This anionic product is not stable in pyridine and
undergoes a replacement of bound Cl- for a py molecule,
leading to the formation of neutral (YPh)3CorMnIII(py)
which is re-oxidized at 0.42–0.49 V to reform the
initial compound after a Cl- for py exchange of axial
ligand as shown in Scheme 2. The opposite ligand
exchange reaction, a conversion of (YPh)3CorMnIVCl
to [(YPh)3CorMnIV(py)]+, does not occur, even in
neat pyridine, and the UV-visible spectrum of neutral
(YPh)3CorMnIVCl in pyridine is the same as in CH2Cl2 or
CH3CN (see later discussion).
The reduction of MnIII corroles 1–4 in pyridine is
assigned to a corrole ring-centered reaction to generate a
MnIII p-anion radical and no MnIII/II process is observed
in this solvent. Almost exactly the same potential is
seen for the second reduction of the structurally similar
MnIV corroles 5–8 (Fig. 2b) as for the first reduction of
compounds 1–4 (see Fig. 2a). For example, the reversible
potential for the first reduction of (CH3Ph)3CorMnIII 4 is
located at E1/2 = -1.37 V and the same half-wave potential
is seen for the second reduction of (CH3Ph)3CorMnIVCl
8. Therefore, it is reasonable to conclude that the electron
transfer mechanism for the second one-electron reduction
of the MnIV corroles 5–8 is the same as that for the first
one-electron reduction of the MnIII corroles 1–4, namely,
formation of a MnIII corrole p-anion radical.
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 5–14

6
B. GAO ET AL.
Fig. 2. Cyclic voltammograms of (a) (YPh)3CorMnIII 1–4 and (b) (YPh)3CorMnIVCl 5–8 in pyridine containing 0.1 M TBAP. Scan
rate = 0.10 V/s
measured in volt, represents the sensitivity of the given
redox reaction to the change of substituent. In the case of
para-substituted tetraphenylporphrins (TPP) the value of
r generally ranges from 50 to 100 mV, depending upon
solvent, the central metal ion and the site of electron
transfer [30]. The larger the value of r, the greater is the
effect of the substituent on the specific redox reaction. In
general, oxidations or reductions at the porphyrin p-ring
system will be more sensitive to para-phenyl substituents
than metal-centered reactions and this fact has sometimes
Scheme 2. Proposed “box mechanism” for redox reactions of
been used to assign the site of electron transfer [27, 30].
MnIII corroles 1–4 and MnIV corroles 5–8 in pyridine containing
Triaryl-substituted corroles have been shown to
0.1 M TBAP. The initial compounds are shown in bold blue
have similar substituent effects as tetraaryl-substituted
color
porphyrins[10,31,32],withtherelevantlinearfreeenergy
equation being DE1/2 = 4sr in the case of porphyrins with
four meso substituted-phenyl groups and DE1/2 = 3sr in
the case of corroles with three meso substituted-phenyl
Effect of substituents on redox potentials
The effect of substituents on redox potentials for a
given set of structurally related porphyrins or corroles
with different electron-donating or electron-withdrawing
substituents can often be quantitated by the linear free
energy relationship E1/2 = Ssr [27–29], where E1/2 is
the half-wave potential for each redox process of the
compound, s is the Hammett substituent constant and r,
groups. Figure 3 illustrates examples of these plots
for (YPh)3CorMnIII 1–4 and (YPh)3CorMnIVCl 5–8 in
CH3CN and pyridine. Linear relationships are observed
between E1/2 and 3s for each electrode reaction, with the
slope (r) of the straight line ranging from 43 to 85 mV.
In pyridine the oxidation and reduction have similar
r values which are little affected by the initial metal
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SPECTROELECTROCHEMICAL CHARACTERIZATION OF MESO TRIARYL-SUBSTITUTED
7
Fig. 3. Substituent effects on half-wave potentials in different solvents. Plots of measured potentials for oxidation and reduction of
1–4 and 5–8 vs. the sum of the Hammett substituent constants 3s. Values of s taken from reference 28
oxidation state on the compound or the site of electron
transfer (p-ring system upon reduction and central metal
ion upon oxidation). A more significant difference is
seen in CH3CN where r for the MnIII/MnII reaction is
smaller than for the metal- and ring-centered oxidations.
There are no unexpected trends in the data of Fig. 3 and
linear free energy relationships are observed for all of the
compounds in the two series of corroles.
shown in Fig. 4a and are consistent with what was earlier
reported for other MnIII triarylcorroles which easily bind
one pyridine molecule under the same solution conditions
[10, 26].
The chloride bound MnIV corroles 5–8 do not bind
pyridine as demonstrated for (FPh)3CorMnIVCl 6 where
an identical spectrum is observed in all three solvents
(Fig. 4b). As can be seen in this figure, the electronic
absorption spectrum of (FPh)3CorMnIVCl 6 is quite
different from that of (FPh)3CorMnIII in CH3CN or
CH2Cl2 or (FPh)3CorMnIII(py) in pyridine, consistent
with two different oxidation states of the manganese
central ion.
UV-visible spectra of neutral corroles 1–8
UV-visible spectra of (YPh)3CorMnIII 1–4 and
(YPh)3CorMnIVCl 5–8 were measured in CH3CN, CH2Cl2
and pyridine prior to oxidation and reduction. Examples
of the spectra for (FPh)3CorMnIII 2 and (FPh)3CorMnIVCl
6 in the three solvents containing 0.1 M TBAP are
illustrated in Fig. 4.
UV-visible spectra of oxidized and reduced corroles
Compounds 1–8 were examined by thin-layer
UV-visible spectroelectrochemistry during controlled
potential oxidation or reduction and examples of the
spectral changes obtained for (ClPh)3CorMnIII 1 and
(ClPh)3CorMnIVCl 5 in CH2Cl2 are shown in Fig. 5.
(FPh)3CorMnIII 2 has the same UV-visible spectrum in
CH2Cl2 and CH3CN but a different spectrum is exhibited
in pyridine due to formation of the five-coordinate
(FPh)3CorMnIII(py) in this solvent. These spectra are
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B. GAO ET AL.
Fig. 4. UV-visible spectra of neutral (a) (FPh)3CorMnIII 2 and (b) (FPh)3CorMnIVCl 6 in CH2Cl2, CH3CN and pyridine containing
0.1 M TBAP
Fig. 5. Thin-layer UV-visible spectral changes of (a) (ClPh)3CorMnIII 1 and (b) (ClPh)3CorMnIVCl 5 during the controlled potential
reduction and oxidation in CH2Cl2 containing 0.1 M TBAP
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SPECTROELECTROCHEMICAL CHARACTERIZATION OF MESO TRIARYL-SUBSTITUTED
9
The spectrum obtained after reduction of 1 in the thin-
layer cell at a controlled potential of -1.50 V has bands
at 428, 479, and 663 nm (top spectra in Fig. 5a), and
the product is assigned as a MnII corrole rather than a
corrole p-anion radical. Similar spectral changes result
during the first reduction of compounds 2–4 in CH2Cl2
and CH3CN as well as during the second reduction of 5
(top spectra in Fig. 5b). These results are consistent with
a MnIII/MnII transition in each case [9–11].
The first oxidation of structurally related MnIII corroles
has been described as a metal-centered reaction to
generate a MnIV corrole [2, 9–12, 33, 34] and this is what
is observed for (ClPh)3CorMnIII 1 after the abstraction of
one electron in CH2Cl2 at a controlled potential of 0.60 V
(see middle spectra in Fig. 5a). Further oxidation of the
MnIV corrole at 1.20 V (bottom spectra of Fig. 5a) leads
to an intensity decreased Soret band and the appearance
of new bands at 480 and ~580 nm. The final spectrum
is assigned as a MnIV corrole p-cation radical, consistent
with earlier assignments in the literature [9–11, 35–37].
The same types of spectral changes are observed for the
second oxidation of compounds 2, 3 and 4 in CH2Cl2
and for the first oxidation of 5 under the same solution
conditions (Fig. 5b, bottom).
Fig. 6. UV-visible spectral changes during titration of
(ClPh)3CorMnIII 1 with TBACl in CH3CN. The insert shows
the diagnostic log-log plot for analyzing the number of bound
chloride ions and the formation constant
compound 1 (two middle spectra in Figs 5a and 5b).
The product electrochemically generated at -0.05 V
from 5 is assigned as a MnIII corrole with a bound
chloride ion, i.e. [(ClPh)3CorMnIIICl]-. This five-
coordinate species could also be generated in solution
by a titration of TBACl into a CH3CN solution of
neutral (ClPh)3CorMnIII. The final chloride bound MnIII
species has bands at 398, 432, 483 and 650 nm as seen
It should be pointed out that the final spectrum of
the MnIII product obtained after the first reduction
of compound 5 differs from that of the MnIII neutral
Fig. 7. UV-visible spectra of neutral (YPh)3CorMnIII 1–4 in CH3CN, the [(YPh)3CorMnIIICl]- generated after addition of TBACl into
the solution of compounds 1–4 in CH3CN and the [(YPh)3CorMnIIICl]- electrogenerated after the first controlled potential reduction
of compounds 5–8 in CH3CN containing 0.1 M TBAP
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B. GAO ET AL.
Table 2. Comparison of the UV-vis spectral data (lmax, nm) between electrogenerated and chemically synthesized
MnIV corroles, [(YPh)3CorMnIV]+, in CH2Cl2, CH3CN and pyridine containing 0.1 M TBAP
Solvent
CH2Cl2
Compound
Electrogenerated
Compound
Synthesized
(YPh)3CorMnIVClO4
(YPh)3CorMnIVCl
1, Y = Cl
2, Y = F
316, 391, 438, 612
318, 383, 435, 609
313, 362, 434, 606
320, 387, 444, 600
5, Y = Cl
6, Y = F
315, 366, 437, 601
314, 360, 435, 602
312, 358, 434, 600
318, 361, 441, 601
3, Y = H
4, Y = CH3
7, Y = H
8, Y = CH3
CH3CN
1, Y = Cl
2, Y = F
312, 361, 433, 598
311, 370, 429, 601
312, 362, 429, 610
317, 363, 439, 620
5, Y = Cl
6, Y = F
313, 363, 431, 596
313, 358, 431, 595
312, 357, 430, 600
315, 364, 439, 625
3, Y = H
4, Y = CH3
7, Y = H
8, Y = CH3
Pyridinea
1, Y = Cl
2, Y = F
320, 402, 443, 505, 677
315, 402, 441, 504, 672
318, 405, 442, 501, 681
316, 397, 441, 507, 689
5, Y = Cl
6, Y = F
318, 365, 443, 600
319, 363, 442, 592
317, 362, 439, 670
320, 361, 445, 665
3, Y = H
4, Y = CH3
7, Y = H
8, Y = CH3
a The singly oxidized species from the MnIII corroles 1–4 is assigned to the (YPh)3CorMnIV(py)2 in pyridine, see
discussion in text.
Table 3. Comparison of the UV-vis spectral data (lmax, nm) between the chemically synthesized and electrogenerated MnIII
corroles in CH2Cl2, CH3CN and pyridine containing 0.1 M TBAP
Solvent
CH2Cl2
Compound
Synthesized (YPh)3CorMnIII
Compound
Electrogenerated [(YPh)3CorMnIIICl]-
1, Y = Cl
2, Y = F
312, 406, 436, 663
314, 405, 435, 656
312, 407, 433, 659
308, 406, 445, 668
5, Y = Cl
6, Y = F
398, 436, 488, 659
395, 435, 488, 656
391, 435, 484, 656
395, 438, 488, 656
3, Y = H
4, Y = CH3
7, Y = H
8, Y = CH3
CH3CN
1, Y = Cl
2, Y = F
308, 405, 433, 654
311, 402, 431, 650
309, 403, 431, 656
309, 404, 438, 657
5, Y = Cl
6, Y = F
398, 432, 483, 650
402, 432, 484, 651
398, 434, 482, 654
402, 436, 486, 655
3, Y = H
4, Y = CH3
7, Y = H
8, Y = CH3
Pyridinea
1, Y = Cl
2, Y = F
317, 405, 445, 505, 669
315, 403, 443, 504, 668
317, 405, 444, 505, 670
316, 404, 445, 506, 673
5, Y = Cl
6, Y = F
318, 406, 444, 504, 671
319, 405, 443, 504, 669
317, 400, 440, 501, 668
320, 405, 445, 508, 674
3, Y = H
4, Y = CH3
7, Y = H
8, Y = CH3
a The neutral compounds 1–4 and the singly reduced species of the compounds 5–8 are assigned as MnIII(py) in pyridine.
in Fig. 6 which includes a diagnostic plot of log((Ai –
A0)/(Af – Ai)) vs. log[TBACl]. The slope of the plot is
1.0 and the calculated binding constant is logK = 4.8
for the reaction given in Equation 2.
A comparison of UV-visible spectra for the
chemically generated MnIII corroles 1–4 in CH3CN with
a bound chloride ion with spectra of the electrogenerated
[(YPh)3CorMnIIICl]- species from 5–8 is given in Fig. 7.
As seen in this figure, the spectra of the singly reduced
MnIV corroles are identical to spectra for the structurally
related MnIII corroles with added TBACl and both of
III
-
(ClPh)3CorMnIII + Cl-
[(ClPh) CorMn Cl]
3
ꢀꢀꢁ
(2)
ꢂꢀꢀ
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SPECTROELECTROCHEMICAL CHARACTERIZATION OF MESO TRIARYL-SUBSTITUTED
11
Table 4. Comparison of the UV-vis spectral data (lmax, nm) of the singly reduced species from MnIII corroles 1–4 and the
doubly reduced species from MnIV corroles 5–8 in CH2Cl2, CH3CN and pyridine containing 0.1 M TBAP
Solvent
CH2Cl2
Compound
Singly reduced (YPh)3CorMnII
Compound
Doubly reduced (YPh)3CorMnII
1, Y = Cl
2, Y = F
428, 479, 663
425, 478, 659
426, 479, 662
420, 480, 654
5, Y = Cl
6, Y = F
431, 477, 664
426, 477, 661
428, 479, 662
424, 481, 659
3, Y = H
4, Y = CH3
7, Y = H
8, Y = CH3
CH3CN
1, Y = Cl
2, Y = F
428, 476, 660
426, 473, 655
424, 476, 660
424, 478, 661
5, Y = Cl
6, Y = F
426, 476, 662
421, 475, 657
420, 476, 659
423, 477, 661
3, Y = H
4, Y = CH3
7, Y = H
8, Y = CH3
Pyridinea
1, Y = Cl
2, Y = F
317, 400, 433b, 667
315, 400, 430, 665
318, 402, 432, 663
316, 403, 435, 675
5, Y = Cl
6, Y = F
398, 434b, 671
318, 400, 430, 670
319, 398, 430, 669
319, 404, 434, 676
3, Y = H
4, Y = CH3
7, Y = H
8, Y = CH3
a The products are assigned to the MnIII p-anion radicals in pyridine. bA peak at 481 nm can also be observed.
Fig. 8. UV-vis spectra of (FPh)3CorMn 2 and (FPh)3CorMnCl 6 in CH2Cl2 containing 0.1 M TBAP in their MnIV, MnIII and MnII
oxidation states
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B. GAO ET AL.
Fig. 9. UV-vis spectra of (CH3Ph)3CorMn 4 and (CH3Ph)3Cor MnCl 8 in pyridine containing 0.1 M TBAP in their different oxidation
states of the central metal ion with different coordinated axial ligand
these spectra are different from the spectrum of the initial
four-coordinate MnIII complex.
virtually identical spectra are seen for: (a) the MnIV forms
of 1–4 and 5–8 (top spectra in Fig. 8) and (b) the MnII
forms of 1–4 and 5–8 (bottom spectra in Fig. 8). This
is contrast with the MnIII forms of 1–4 and 5–8 which
are different in CH3CN and CH2Cl2 due to different
coordination numbers of the MnIII compounds in the two
series (see middle spectra in Fig. 8).
In pyridine, identical UV-visible spectra are seen
for: (a) the MnIII forms of 1–4 and 5–8 and (b) the MnIII
p-anion radical forms of 1–4 and 5–8. This contrasts with
the MnIV forms of the compounds in the two series which
differ from each other as seen in the top two spectra of
Fig. 9 for compounds 4 and 8. This difference in UV-
visible spectra is due to the different axial coordination
of the MnIV corrole in the two series, Cl- in the case of
5–8 and two pyridine molecules in the case of 1–4. The
exact spectral wavelengths of each compound in the four
different oxidation states are given in the tables.
Comparisons in spectra between the MnIV, MnIII
and MnII oxidation states of 1–4 and 5–8
A summary of the spectral data for the MnIV, MnIII and
MnII corroles is given in Tables 2–4 and a comparison of
spectra for selected compounds in each metal oxidation
state of the (YPh)3CorMnIII 1–4 and (YPh)3CorMnIVCl
5–8 is illustrated in Figs 8 and 9. Figure 8 compares
spectra for compounds 2 and 6 in CH2Cl2 while a
similar comparison is given in Fig. 9 for compounds 4
and 8 in pyridine. The last reduction of 1–8 in CH2Cl2
or CH3CN corresponds to a MnIII/MnII process while
the last reduction of 1–8 in pyridine corresponds to the
conversion of a MnIII corrole to a MnIII corrole p-anion
radical as discussed in earlier sections of the manuscript.
The same spectral patterns were obtained in CH3CN and
CH2Cl2 for each compound in a given oxidation state.
The spectral data in Tables 2–4 and in Figs 8 and 9
can be summarized as follows: in CH3CN and CH2Cl2,
In summary, two related series of manganese
substituted meso-triarylcorroles containing electron-
withdrawing or electron-donating substituents on the
macrocycle were examined as to their electrochemistry
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SPECTROELECTROCHEMICAL CHARACTERIZATION OF MESO TRIARYL-SUBSTITUTED
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and spectroelectrochemistry in CH2Cl2, CH3CN and
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pyridine. One series consisted of neutral four-coordinate
MnIII complexes and the other was comprised five-
coordinate MnIV corroles with a chloride axial ligand.
The effect of solvent, coordination number and metal
oxidation state on the UV-visible spectra and redox
potentials of each compound were examined in the
three solvents and a mechanism for each oxidation/
reduction process was proposed after analysis of the
redox potentials and the spectroelectrochemical data.
The use of spectroelectrochemistry is the key to identify
the electroactive species and the products of each redox
reaction which could not be done by electrochemical
measurements alone.
Acknowledgements
This work was supported by the Natural Science
Foundation of China (Grant No. 21071067) and the
Robert A. Welch Foundation (KMK, E-680).
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