Solvent, anion, and structural effects on the redox potentials and UV-visible spectral properties of mononuclear manganese corroies

Article abstract of DOI:10.1021/ic8007415

A series of manganese(III) corroles were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous solvents. Up to three oxidations and one reduction were obtained for each complex depending on the solvents. The main compound discussed in this paper is the meso-substituted manganese corrole, (Mes₂PhCor)Mn, and the main points are how changes in axially coordinated anion and solvent will affect the redox potentials and UV-vis spectra of each electrogenerated species in oxidation states of Mn(III), Mn(IV), or Mn(II). The anions OAc^-, Cr^-, CN^-, and SCN^- were found to form five-coordinate complexes with the neutral Mn(III) corrole while two OH^- or F^- anions were shown to bind axially in a stepwise addition to give the five- and six-coordinate complexes in nonaqueous media. In each case, complexation with one or two anionic axial ligands led to an easier oxidation and a harder reduction as compared to the uncomplexed four-coordinate species.

Full text of DOI:10.1021/ic8007415

Inorg. Chem. 2008, 47, 7717-7727
Solvent, Anion, and Structural Effects on the Redox Potentials and
UV-visible Spectral Properties of Mononuclear Manganese Corroles
Jing Shen,† Maya El Ojaimi,‡ Mohammed Chkounda,‡ Claude P. Gros,‡ Jean-Michel Barbe,‡
Jianguo Shao,† Roger Guilard,*,‡ and Karl M. Kadish*,†
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, and UniVersite´ de
Bourgogne, ICMUB (UMR 5260), 9 AVenue Alain SaVary BP 47870, 21078 Dijon Cedex, France
Received April 24, 2008
A series of manganese(III) corroles were investigated as to their electrochemistry and spectroelectrochemistry in
nonaqueous solvents. Up to three oxidations and one reduction were obtained for each complex depending on the
solvents. The main compound discussed in this paper is the meso-substituted manganese corrole, (Mes2PhCor)Mn,
and the main points are how changes in axially coordinated anion and solvent will affect the redox potentials and
UV-vis spectra of each electrogenerated species in oxidation states of Mn(III), Mn(IV), or Mn(II). The anions
OAc-, Cl-, CN-, and SCN- were found to form five-coordinate complexes with the neutral Mn(III) corrole while
two OH- or F- anions were shown to bind axially in a stepwise addition to give the five- and six-coordinate
complexes in nonaqueous media. In each case, complexation with one or two anionic axial ligands led to an easier
oxidation and a harder reduction as compared to the uncomplexed four-coordinate species.
Introduction
part to the fact that multiple metal oxidation states are
possible ranging from +5 in the case of Fe and Mn to +1
in the case of iron and cobalt. Corroles with manganese,
cobalt, and iron central metal ions have also been used as
catalysts in a variety of reactions.21,27–36 For example,
A number of different metal ions have been incorporated
into corroles and examined as to their electrochemical and
spectroscopic properties since the mid-1990s.1–5 The most
studied of these compounds have been the transition metal
derivatives of manganese,6–13 cobalt,14–22 and iron23–26 due
in part to the biological relevance of these metal ions and in
(11) Kadish, K. M.; Adamian, V. A.; Caemelbecke, E. V.; Gueletii, E.;
Will, S.; Erben, C.; Vogel, E. J. Am. Chem. Soc. 1998, 120, 11986–
11993.
* To whom correspondence should be addressed. E-mail: roger.guilard@
u-bourgogne.fr (R.G.), kkadish@uh.edu (K.M.K.).
(12) Bro¨ring, M.; Cordes, M.; Ko¨hler, S. Z. Anorg. Allg. Chem. 2008, 634,
125–130.
†
University of Houston.
Universite´ de Bourgogne.
(13) Gross, Z.; Gray, H. B. Comments Inorg. Chem. 2006, 27, 61–72.
(14) Kadish, K. M.; Koh, W.; Tagliatesta, P.; Sazou, D.; Paolesse, R.;
Licoccia, S.; Boschi, T. Inorg. Chem. 1992, 31, 2305–2313.
(15) Adamian, V. A. Electrochem. Soc. Interface 1995, 4, 57–58.
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Kadish, K. M. Inorg. Chem. 1996, 35, 5577–5583.
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Kadish, K. M. C. R. Acad. Sci. Paris, Chimie/Chemistry 2001, 4, 245–
254.
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Fischer, J.; Weiss, R.; Kadish, K. M. Inorg. Chem. 2001, 40, 4856–
4865.
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F.; Bolze, F.; Burdet, F.; Guilard, R. Inorg. Chem. 2002, 41, 3990–
4005.
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Guilard, R. Inorg. Chem. 2003, 42, 4062–4070.
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Burdet, F.; Gros, C. P.; Barbe, J.-M.; Guilard, R. J. Am. Chem. Soc.
2005, 127, 5625–5631.
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Gros, C. P.; Guilard, R. Inorg. Chem. 2005, 44, 9023–9038.
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‡
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Guilard, R. , Eds.; Academic Press: Burlington, MA, 2000; Vol. 2,
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10.1021/ic8007415 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 17, 2008 7717
Published on Web 07/31/2008

Shen et al.
Chart 1
Mn(III) tris(pentafluorophenyl)corrole has been shown to be
a mild epoxidation catalyst37 and also a direct precursor for
the preparation of oxomanganese(V), nitridomanganese(V),
and even nitridomanganese(VI) derivatives.37,38 Langmuir-
Blodgett (LB) films of a manganese(III) corrole have also
been reported39 and, in the field of molecular materials, this
might be a useful approach to develop quartz microbalances
with improved and very particular sensing properties.39
Our own interest in manganese corroles has been in
elucidating their electrochemical and spectroscopic proper-
ties,6,11 some of which closely resemble the properties of
porphyrins,40 phthalocyanines,41,42 and related macrocy-
cles43–45 which have been extremely well-characterized over
the years. In each of these related manganese compounds,
the central ion can exist in oxidation states of +5, +4, +3,
or +2, although in the case of corroles, Mn(II) derivatives
have never been formally identified, in part because the
reduction of Mn(III) occurs at very negative potentials and
in part because a Mn(III) π-anion radical may be the product
of the first reduction.6,11
This is investigated in the present study where we have
electrochemically reduced and oxidized the series of Mn(III)
corroles shown in Chart 1 and then selected (Mes2PhCor)Mn
1 as a representative compound to characterize the high and
low oxidation state products in different solvents and when
coordinated with different anionic axial ligands.
The electrochemistry of manganese corroles was first
described in the literature for Mn(III) and Mn(IV) derivatives
of octaethylcorrole (OEC), examples being (OEC)MnIII,11
(OEC)MnIII(py),6 (OEC)MnIVCl,6 and (OEC)Mn(C6H5),6 and
this was followed several years later by studies of meso-
substituted corroles with Mn(III) and Mn(IV) central
ions,7,10,30,46,47 two examples being tris(pentafluorophenyl)-
corrole and meso-trisphenylcorrole. The air stable Mn(III)
corroles could be reversibly oxidized to their Mn(IV) form
while the air stable Mn(IV) derivatives were readily reduced
to a Mn(III) form of the compound. Additional oxidations
to give Mn(IV) π-cation radicals and dications or reduction
to give a Mn(III) π-anion radical or formal Mn(II) species
have also been reported and these electrode reactions are
given by eqs 1–4 where Cor represents a general corrole
macrocycle and the central metal ions are shown without
axial ligands.
(24) Fukuzumi, S. Kagaku 2006, 61, 31–32.
(25) Van Caemelbecke, E.; Will, S.; Autret, M.; Adamian, V. A.; Lex, J.;
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Gross, M.; Vogel, E.; Kadish, K. M. J. Am. Chem. Soc. 1994, 116,
9141–9149.
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Gross, Z. J. Phys. Chem. B 2002, 106, 4772–4778.
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2007, 63, 1987–1994.
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Guilard, R. J. Inorg. Biochem. 2006, 100, 858–868.
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559.
III
II -
[
(
)
(
)
Cor Mn + e h Cor Mn ]
(1)
(36) Simkhovich, L.; Gross, Z. Tetrahedron Lett. 2001, 42, 8089–8092.
(37) Gross, Z.; Golubkov, G.; Simkhovich, L. Angew. Chem., Int. Ed. 2000,
39, 4045–4047.
III
Cor Mn h Cor MnIV + + e
(2)
(3)
(4)
[
]
(
)
(
)
h Cor MnIV 2+ + e
h Cor MnIV]3+ + e
[
IV +
·
[
]
[( ) ]
(
)
Cor Mn
(38) Golubkov, G.; Gross, Z. J. Am. Chem. Soc. 2005, 127, 3258–3259.
(39) Paolesse, R.; Di Natale, C.; Macagnano, A.; Sagone, F.; Scarselli,
M. A.; Chiaradia, P.; Troitsky, V. I.; Berzina, T. S.; D’Amico, A.
Langmuir 1999, 15, 1268–1274.
·
IV 2+
[(
)
]
(
)
Cor Mn
The electron transfer process in eq 2 has in the past been
investigated either as an oxidation6,11,46,47 or as a reduc-
tion7,10,12,30,46 depending in large part upon the synthetic
method of generating the starting compound. In some cases
a four-coordinate Mn(III) species is the product of the
synthesis6 and in others it is a five-coordinate Mn(IV)
(40) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R.,
Eds.; Academic Press: Burlington, MA, 2000.
(41) The Porphyrin Handbook, Kadish, K. M., Smith, K. M., Guilard, R.,
Eds.; Academic Press: Boston, MA, 2003.
(42) Phthalocyanines Properties and Applications; Leznoff, C. C., Lever,
A. B. P., Eds.; VCH Publishers, Inc.: New York, NY, 1993.
(43) Kerber, W. D.; Goldberg, D. P. J. Inorg. Biochem. 2006, 100, 838–
857.
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Tondello, E.; Boschi, T. Inorg. Chem. 1997, 36, 1564–1570.
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Hahn, J.; Zvyagin, S. A.; Telser, J.; Krzystek, J.; Zhan, R.; Ou, Z.;
Kadish, K. M.; Zakharov, L.; Rheingold, A. L.; Goldberg, D. P. Inorg.
Chem. 2005, 44, 4485–4498.
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DiBilio, A. J.; Gross, Z. Angew. Chem., Int. Ed. 2001, 40, 2132–
2134.
(47) Fryxelius, J.; Eilers, G.; Feyziyev, Y.; Magnuson, A.; Sun, L.; Lomoth,
R. J. Porphyrins Phthalocyanines 2005, 9, 379–386.
7718 Inorganic Chemistry, Vol. 47, No. 17, 2008

Mononuclear Manganese Corroles
derivative such as (OEC)MnCl.11 The half-wave potentials
for reduction of (TPFC)MnIVCl differ by 300 mV from E1/2
values for the oxidation of (TPFC)MnIII in solutions contain-
ing 0.1 M TBAP which was one evidence for metal-centered
processes at these potentials.46 This is addressed in the
present paper where we demonstrate how changes in axially
coordinated anion, the solvent, and the structure of corrole
will affect the redox potentials and UV-vis spectra of each
electrogenerated species in reactions 1–4. Previous spectral
characterization of electroreduced MnIII corroles (eq 1) is
limited to a single study of (OEC)Mn(py) in pyridine11 while
nothing at all is known about UV-vis spectra of the doubly
oxidized Mn(III) corroles (eq 3). The results of our present
study should be of importance to better understand and
control the chemical reactivity of mononuclear manganese
corroles as well as dyads containing mono- or bis-Mn corrole
macrocycles.
Synthesis of Manganese Derivatives. The structures of the
investigated manganese corroles are given in Chart 1. The spectral
characterization of each compound is in agreement with the
proposed structure. FAB or MALDI-TOF mass spectra of com-
plexes 1-5 show in each case the molecular peak which is consistent
with the observed stability of the manganese(III) complexes.
Two main strategies can generally be used to obtain manganese
corroles;1,2 one involves cyclization of a,c-biladiene in the presence
of a manganese salt49 and the other involves a direct reaction of
the free-base corrole with the manganese salt.1,2,37,50 In the present
study, the manganese complexes were prepared in good yields
starting from the free-base corroles and either Mn2(CO)10 in toluene
or Mn(OAc)2 ·4H2O in a dichloromethane/methanol mixture, both
solutions being under argon. The free-base corroles (Me2Et2Ph4-
Cor)H3, (Me4Ph5Cor)H3, (Me4PhTp-OCH3PCor)H3, (Me2Ph7Cor)H3,
and (Mes2PhCor)H3 were synthesized as previously reported.51,52
(5,15-Dimesityl-10-Phenylcorrole)manganese(III)
(Mes2-
PhCor)Mn (1). solution of 60 mg (0.098 mmol) of
A
(Mes2PhCor)H3 and 96 mg of Mn(OAc)2 ·4H2O in 30 mL of DCE/
MeOH 75/25 was heated under nitrogen at 85 °C for 5 h. The
solvent was removed in vacuum and the residue passed through a
silica column, first using methylene chloride and then methylene
chloride/methanol (90/10) as eluents. After evaporation, the com-
pound was obtained as a blackish-brown powder in 84% yield (55
mg). UV-visible (CH2Cl2): λmax (ε, M-1 cm-1) ) 401 (27 000),
435 (25 000), 498 (9 000), 586 (6 000), 648(7 000). MS (MALDI-
Experimental Section
Instrumentation. Cyclic voltammetry was carried out with an
EG&G model 173 potentiostat/galvanostat. A homemade three-
electrode electrochemistry cell was used and consisted of a platinum
button or glassy carbon working electrode, a platinum wire counter
electrode, and a 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. All potentials are referenced to the SCE.
UV-visible spectroelectrochemical experiments were performed
with an optically transparent platinum thin-layer electrode of the
type described in the literature.48 Potentials were applied with an
EG&GModel173potentiostat/galvanostat.Time-resolvedUV-visible
spectra were recorded with a Hewlett-Packard Model 8453 diode
array rapid-scanning spectrophotometer.
Microanalyses were performed at the Universite´ de Bourgogne
on a Fisons EA 1108 CHNS instrument. Mass spectra were obtained
either with a Kratos Concept 32 S spectrometer in FAB mode (m-
nitrobenzyl alcohol as matrix) or on a Bruker Ultraflex II instrument
in MALDI-TOF reflectron mode using dithranol (1,8-dihydroxy-
9[10H]-anthracene) as a matrix, or on a Bruker MicroToFQ
instrument in ESI mode.
TOF): m/z ) 662.057 [M]+·, ESI-HRMS: m/z ) 662.22576 [M]+·
,
662.2242 calculated for C43H35N4Mn.
(8,12-Diethyl-7,13-dimethyl-2,3,17,18-tetraphenylcorrole)man-
ganese(III) (Me2Et2Ph4Cor)Mn (2). A solution of 150 mg (0.22
mmol) of (Me2Et2Ph4Cor)H3 and 141 mg (0.33 mmol) of Mn2(CO)10
in 30 mL of toluene was heated under argon at 118 °C for 4 h. The
solvent was then evaporated in vacuum. The residue was first passed
through a column of alumina using methylene chloride/heptane (70/
30), then neat methylene chloride, and finally methylene chloride/
methanol (99.5/0.5). After crystallization from methylene chloride/
methanol 1/1, the title compound 2 was obtained as a black powder
in 18% yield (30 mg). UV-visible (CH2Cl2): λmax (ε, M-1 cm-1
)
) 394 (39 000), 456 (2 000), 484 (1 800), 579 (1 600), 604 (1 800).
MS (FAB): m/z (%) 738 [M]+·, 738.25 calculated for C49H39N4Mn.
Elemental analysis: Calcd for C49H39N4Mn,MeOH: C 77.91, H 5.62,
N 7.27. Found: C 77.65, H 6.06, N 7.47.
Chemicals and Reagents. Pyridine (py, 99.8%), dimethylsul-
foxyde (DMSO, g 99.9%), tetra-n-butylammonium chloride (TBACl,
g 99%), tetra-n-butylammonium fluoride (TBAF, 1.0 M solution
in THF), tetra-n-butylammonium cyanide (TBACN, 95%), tetra-
n-butylammonium thiocyanate (TBASCN, 98%), tetra-n-butylam-
monium hydroxide (TBAOH, 1.0 M solution in methanol) were
obtained from Sigma-Aldrich Chemical Co. and used without
further purification. Benzonitrile (PhCN) was purchased from
Aldrich Chemical Co. and distilled over P2O5 under vacuum prior
to use. Absolute dichloromethane (CH2Cl2, 99.8%) was received
from EMD Chemicals Inc. and used without further purification.
Tetra-n-butylammonium perchlorate (TBAP, g 99%) and tetrabu-
tylammonium acetate (TBAOAc, g 99%) were purchased from
Fluka Chemical Co. and used as received. Neutral alumina (Merck;
usually Brockmann grade III, i.e., deactivated with 6% water) were
used for column chromatography. Analytical thin layer chroma-
tography was performed using Merck 60 F254 neutral Aluminum
oxide gel (precoated sheets, 0.2 mm thick) or Merck 60 F254 silica
gel (precoated sheets, 0.2 mm thick). Reactions were monitored
by thin layer chromatography and spectrophotometry.
(7,8,12,13-Tetramethyl-2,3,10,17,18-pentaphenylcorrole)man-
ganese(III) (Me4Ph5Cor)Mn (3). This corrole was similarly
prepared in 50% yield (55 mg) from 100 mg (0.11 mmol) of
(Me4Ph5Cor)H3 and 80 mg (0.20 mmol) of Mn2(CO)10. UV-visible
(CH2Cl2): λmax (ε, M-1 cm-1) ) 394 (53 000), 412 (48 000), 461
(27 000), 491 (24 000), 582 (22 000), 609 (21 000). MS (FAB):
m/z (%) 786 [M]+·, 786.25 calculated for C53H39N4Mn. Elemental
analysis: Calcd for C53H39N4Mn, MeOH: C 79.20, H 5.29, N 6.84.
Found: C 79.03, H 4.98, N 6.71.
(2,3,17,18-Tetra(p-anisil)-7,8,12,13-tetramethyl-9-phenylcor-
role)manganese(III) (Me4PhTp-OCH3PCor)Mn (4). This corrole
was similarly prepared in 30% yield (60 mg) from 200 mg (0.24
mmol) of (Me4PhTp-OCH3PCor)H3 and 160 mg (0.4 mmol) of
Mn2(CO)10. UV-visible (CH2Cl2): λmax (ε, M-1 cm-1) ) 402
(49) Licoccia, S.; Paolesse, R. Metal Complexes with Tetrapyrrole Ligands
III; Springer-Verlag: Berlin and Heidelberg, 1995.
(50) Bendix, J.; Gray, H. B.; Golubkov, G.; Gross, Z. Chem. Commun.
2000, 1957–1958.
(51) Guilard, R.; Gros, C. P.; Bolze, F.; Je´roˆme, F.; Ou, Z.; Shao, J.; Fischer,
J.; Weiss, R.; Kadish, K. M. Inorg. Chem. 2001, 40, 4845–4855.
(52) Gros, C. P.; Brisach, F.; Meristoudi, A.; Espinosa, E.; Guilard, R.;
Harvey, P. D. Inorg. Chem. 2007, 46, 125–135.
(48) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498–1501.
Inorganic Chemistry, Vol. 47, No. 17, 2008 7719

Shen et al.
Table 1. Half-Wave Potentials (V vs SCE) of Monomeric Mn(III)
Corroles in Different Nonaqueous Solvents Containing 0.1 M TBAP
oxidation
reduction
1st
cpd
solvent 3rd 2nd 1st
(Mes2PhCor)Mn (1)
CH2Cl2
1.00 0.32a
0.95 0.37
0.93 0.37
0.90 0.32
0.96 0.36
(Me2Et2Ph4Cor)Mn (2)
(Mes4Ph5Cor)Mn (3)
(Me4PhTp-OCH3PCor)Mn (4)
(Me2Ph7Cor)Mn (5)
(OEC)Mnb
PhCN
1.56c 0.93 0.36 -1.58
1.83c 1.07 0.45d -1.36, -1.64
1.60c 1.02 0.47 -1.39
1.62c 0.95 0.41 -1.43
1.63c 0.91 0.38 -1.42
1.66c 1.06 0.46 -1.34
(Mes2PhCor)Mn (1)
(Me2Et2Ph4Cor)Mn (2)
(Mes4Ph5Cor)Mn (3)
(Me4PhTp-OCH3PCor)Mn (4)
(Me2Ph7Cor)Mn (5)
(OEC)Mnb
pyridine
0.32 -1.66
0.42 -1.46
0.50 -1.43
0.47 -1.41
0.44 -1.47
0.49 -1.37
(Mes2PhCor)Mn (1)
(Me2Et2Ph4Cor)Mn (2)
(Mes4Ph5Cor)Mn (3)
(Me4PhTp-OCH3PCor)Mn (4)
(Me2Ph7Cor)Mn (5)
a Extra peak was obtained at E1/2 ) 0.03 V. b Data were taken from refs
6 and 11. c Peak potential at a scan rate of 0.1 V/s. d Extra peak was observed
at E1/2 ) 0.12 V (see Figure 1c).
Figure 1. Cyclic voltammograms of (a) (OEC)Mn, (b) (Me4Ph5Cor)Mn
(3), and (c) (Mes2PhCor)Mn (1) in PhCN containing 0.1 M TBAP.
reactive in CH2Cl2, and meaningful half-wave potentials
could not be obtained because of a rapid reaction of the
electrogenerated anionic species with the solvent.
(45 000), 415 (14 000), 464 (25 000), 489 (20 000), 585 (17 000),
607 (17 000). MS (FAB): m/z (%) 906 [M]+·, 906.29 calculated
for C57G47N4O4Mn. Elemental analysis: Calcd for C57H47N4O4Mn,
MeOH: C 74.19, H 5.47, N 5.97. Found: C 74.01, H 5.17, N 5.69.
It is well-documented in the literature that metalloocta-
ethylporphyrins are easier to oxidize and harder to reduce
by 200-250 mV than the same metal complex of meso-
substituted tetraphenylporphyrins.53,54 A 200-250 mV po-
tential difference in E1/2 for oxidation or reduction of OECs
and meso-substituted triarylcorroles with the same metal ion
is often observed although this will depend upon the specific
electrode reaction and specific central metal ion.1 For
example, a comparison between (OEC)Mn and (Mes2Ph-
Cor)Mn 1 (Figure 1) shows that the latter compound is 90
mV harder to oxidize and 220 mV easier to reduce in PhCN
than the OEC derivative with the same metal ion. The
absolute potential difference in reversible half-wave poten-
tials for oxidation and reduction of (OEC)Mn is 1.94 V in
PhCN (see Table 1), a larger value than for compounds 1-5
in the same solvent where the ∆E1/2 values range from 1.80
to 1.86 V and show little variation with changes in the type
or position of substituents on the conjugated macrocycle. This
difference may suggest that at least one (and probably both)
of the redox processes for cpds 1-5 are metal- rather than
macrocycle-centered.
The reductions of 1-5 are easier than (OEC)Mn by
150-250 mV in PhCN or py (see Table 1) but much smaller
differences in E1/2 values are seen for the oxidation where
the ∆E1/2 between (OEC)Mn and the newly investigated
compounds are less than 100 mV and close to zero in the
case of compound 4.
Because of the lack of a significant structural effect on
the redox potentials of 1-5, we have selected one repre-
sentative corrole to elucidate the effect of solvent and anion
binding on the electrochemistry and UV-vis spectra of the
(8,12-Dimethyl-2,3,7,10,13,17,18-heptaphenylcorrole)manga-
nese(III) (Me2Ph7Cor)Mn (5). This corrole was similarly prepared
in 23% yield (20 mg) from 100 mg (0.11 mmol) of (Me2Ph7Cor)H3
and 68 mg (0.16 mmol) of Mn2(CO)10. UV-visible (CH2Cl2): λmax
(ε, M-1 cm-1) ) 394 (53 000), 412 (48 000), 460 (25 000), 491
(24 000), 582 (22 000), 609 (21 000). MS (FAB): m/z (%) 910
[M]+·, 910.29 calculated for C63H43N4Mn. Elemental analysis: Calcd
for C63H43N4Mn, MeOH: C 81.51, H 5.02, N 5.94. Found: 81.36,
H 4.74, N 5.79.
Results and Discussion
Electrochemistry of 1-5 in CH2Cl2, PhCN and Pyri-
dine. Cyclic voltammograms of (OEC)Mn,6,11 (Mes2Ph-
Cor)Mn 1, and (Me4Ph5Cor)Mn 3 in PhCN, 0.1 M TBAP
are shown in Figure 1, and a summary of redox potentials is
given in Table 1 for (OEC)Mn and the five corroles
investigated in the current study. A third irreversible oxida-
tion of 1-5 also occurs in PhCN and is not illustrated in the
figure. This oxidation is located at peak potentials between
1.56 and 1.83 V versus SCE for a scan rate of 0.1 V/s (see
exact value in Table 1).
The electrochemistry of compounds 1-5 is similar to that
of (OEC)Mn6,11 and other earlier characterized manganese
corroles7,10,30,46,47 in nonaqueous media. Each compound
exhibits 1, 2, or 3 single electron abstractions in CH2Cl2,
py, and PhCN, the exact number of which depends upon
the positive potential limit of the solvent in addition to the
electron-donating effect of the methyl, ethyl, or phenyl
substituents. With one exception (cpd 1 in PhCN), a single
one-electron reversible reduction is observed in py and
PhCN, these reactions occurring over a narrow potential
range of -1.34 to -1.47 V for compounds 1-5. The
electroreduced forms of the Mn(III) corroles are highly
(53) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: Burlington, MA, 2000; Vol. 8, pp 1-114.
(54) Kadish, K. M. In Prog. Inorg. Chem.; Lippard, S. J., Ed.; John Wiley:
New York, 1986; Vol. 34, pp 435-605.
7720 Inorganic Chemistry, Vol. 47, No. 17, 2008

Mononuclear Manganese Corroles
Figure 3. UV-visible spectra of (Mes2PhCor)Mn 1 in different nonaqueous
solvents.
split Soret band at 398 to 407 and 427 to 442 nm are reversed
in py and DMSO as compared to CH2Cl2 and PhCN, and
the second, that the intensities of the two other major bands
at 491 to 510 nm and 645 to 667 nm are also larger in py
and DMSO than in the other two more weakly coordinating
solvents (see Figure 3 and Supporting Information, Table
S1). An increased intensity visible band in the spectrum of
(OMC)Mn in the presence of pyridine was earlier assigned
to a formation of a five-coordinate complex in the bonding
solvent,44 and this also seems to be the case in the present
study where the spectroscopically observed form of the
corrole is assigned as (Mes2PhCor)Mn(py) in pyridine,
(Mes2PhCor)Mn(DMSO) in DMSO, and (Mes2PhCor)Mn in
CH2Cl2 or PhCN. Under these conditions the first oxidation
of (Mes2PhCor)Mn in PhCN or CH2Cl2 is proposed to occur
as shown in eq 2 while in py or DMSO, the prevailing
electrooxidation can be described by eq 5 where S is a
solvent molecule.
Figure 2. Cyclic voltammograms of (Mes2PhCor)Mn 1 in (a) CH2Cl2, (b)
PhCN, (c) py, and (d) DMSO containing 0.1 M TBAP.
electrogenerated products in reactions 1–4. As seen in Figure
1c, there are two “extra” redox processes obtained for cpd 1
in PhCN, one at E1/2 ) 0.12 V and another at -1.64 V. These
extra processes are not seen for the other corroles and cpd 1
then was therefore selected for further detailed studies on
anion and solvent effect since these “extra” processes seemed
to be associated with different forms of axial ligand binding
under the given electrochemical conditions. This is clearly
demonstrated by the cyclic voltammograms in Figure 2. As
seen in the figure, compound 1 undergoes a reversible one-
electron reduction in py (-1.46 V vs SCE) or DMSO (-1.34
V), an irreversible reduction in CH2Cl2 (because of a rapid
chemical reaction with the solvent), and two reversible
reductions in PhCN (-1.36 and -1.64 V) which will be
shown to correspond to two different axially coordinated
forms of MnIII. The formal MnIII/MnII and MnIII/MnIV
processes in each solvent are described by eqs 1 and 2 and
notated as “first red” and “first ox” in the figure. The second
major oxidation at 1.00 V (CH2Cl2) or 1.07 V (PhCN) is
notated as “second ox”, while the redox process with smaller
currents at E1/2 ) 0.03 V in CH2Cl2 and 0.12 V in PhCN
are assigned to a rival MnIII/MnIV couple (first ox) and that
at -1.64 V to a different MnIII/MnII couple (first red) based
on experiments described in later sections of the manuscript.
Each redox process of compound 1 was characterized by
spectroelectrochemistry before and after electrooxidation and
electroreduction as described on the following pages.
UV-vis Spectra of Neutral Compound in Different
Solvents. The spectrum of 1 in neat py or neat DMSO differs
from the spectrum obtained in PhCN or CH2Cl2 (see Figure
3 and Supporting Information, Table S1). There are two key
differences. The first is that the ratio of intensities for the
Mes PhCor MnIII S h Mes PhCor Mn
S
+ + e (5)
( )
IV( )
[
]
(
)
(
)
2
2
Spectroelectrochemistry of 1. The first oxidation of
manganese corroles has been described as a metal-centered
reaction in the literature,1,6,7,30,46,47 and this is what is
observed for (Mes2PhCor)Mn 1 after the abstraction of one
electron in PhCN or DMSO (see UV-vis spectra in Figures
4a and Supporting Information, S1a). Further oxidation of
the singly oxidized species leads to a decreased intensity
Soret band and the appearance of a broad NIR band centered
at 750-900 nm as shown in Figures 4b and Supporting
Information, S1b. The latter spectra are assigned to a MnIV
corrole π-cation radical55–57 and is followed by a third
irreversible reaction leading to a species assigned as a MnIV
dication (spectrum not shown). These oxidations are de-
scribed by eqs 2–4.
The single reduction of (OEC)Mn in pyridine was pre-
viously assigned as a macrocycle-centered reaction6 but this
seems not to be the case for (Mes2PhCor)Mn in PhCN whose
spectral changes during the first reduction at an applied
potential of -1.50 V are shown in Figure 5a. The spectrum
(55) Fuhrhop, J. H. Struct. Bonding (Berlin) 1974, 18, 1–67.
(56) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138–163.
(57) Perrin, M. H.; Gouterman, M.; Perrin, C. L. J. Chem. Phys. 1969, 50,
4137–4150.
Inorganic Chemistry, Vol. 47, No. 17, 2008 7721

Shen et al.
5b). The final product at this controlled potential has bands
at 460 (sh), 481 and 677 nm and also looks like a metal-
rather than macrocycle-centered reduced species. Similar
spectral changes are seen during controlled potential reduc-
tions in DMSO and these changes are shown in Supporting
Information, Figure S2.
In the case of manganese porphyrins, the chemically or
electrochemically generated Mn(II) form of the compound
generally has a sharper Soret band with a larger molar
absorptivity than the initial Mn(III) compound before reduc-
tion. Thus, the two electroreduction products in Figures 5a
and 5b might at first be simply assigned to two different
axially coordinated forms of the electrogenerated Mn(II)
corrole. This is not the case, however, as will become evident
after examining the UV-visible spectrum of the corrole in
the presence of different axially coordinating anions. This
is described on the following pages.
UV-visible Spectra of (Mes2PhCor)Mn 1 in PhCN
Containing Different Anions. The UV-visible spectral
changes obtained upon addition of OAc- or CN- to
(Mes2PhCor)Mn 1 in PhCN are shown in Figure 6. Com-
pound 1 before ligand addition has a complex spectral pattern
with absorption bands at 308, 374 (sh), 407, 431, 510, 557,
601, and 652 nm but after addition of the anion in the form
of TBAX the Soret band, initially at 407 nm, decreases in
intensity and shifts to 388 nm (OAc-) or 401 nm (CN-)
while the band at 431 nm changes only slightly in intensity
as new bands grow in at 482 and 650 nm (OAc-) or 509
and 681 nm (CN-). The same spectral pattern before and
after addition of CN- or OAc- is obtained in PhCN solutions
Figure 4. UV-visible spectral changes of (Mes2PhCor)Mn 1 during the
(a) first and (b) second oxidations in PhCN containing 0.1 M TBAP.
-
containing 0.1 M TBAP, and this suggests that ClO4 , present
as a supporting electrolyte in the electrochemical studies,
does not coordinate to the Mn(III) center as is the case for
the other anions.
Similar spectral changes were seen upon addition of Cl-
or SCN- to (Mes2PhCor)Mn 1 in PhCN, and Hill-plots were
used to analyze the data as a function of added anion to
calculate the number of axially coordinated ligands and anion
binding constants. The diagnostic log-log plots are shown
in Figure 6 and indicate that one and only one OAc- or CN-
axial ligand binds to the Mn(III) center under the given
solution conditions. The spectrum of the five-coordinate
corrole with bound OAc- has major absorption bands at 388,
431, 482, and 650 nm (Figure 6a), and a binding constant
of logK ) 4.5 is calculated for the reaction given in eq 6.
Figure 5. UV-visible spectral changes of (Mes2PhCor)Mn during the (a)
first and (b) second reductions in PhCN containing 0.1 M TBAP.
Mes PhCor Mn + X- f Mes PhCor MnX - (6)
(
)
[(
)
]
2
2
The five-coordinate CN- adduct, [(Mes2PhCor)Mn(CN)]-,
has bands at 401, 440, 509, and 681 nm (Figure 6b) and a
logK of 4.7 was calculated for this ligand addition reaction.
Similar spectral changes are seen when SCN- or Cl- are
added to 1 in PhCN (Supporting Information, Figure S3)
and again the log-log plot indicates the addition of a single
anion to give [(Mes2PhCor)MnX]- where logK ) 4.8 (Cl-)
or 4.2 (SCN-).
of the corrole electrogenerated at this potential has bands at
424, 480, and 658 nm, and the overall spectral changes are
as would be expected for a metal-centered reaction rather
than for formation of a corrole π-anion radical where
significant decreases in all absorption intensities would be
observed.6 The applied potential was then shifted from -1.50
to -1.80 V, and the 481 nm band was seen to increase even
more in intensity as the 424 nm band disappeared (Figure
7722 Inorganic Chemistry, Vol. 47, No. 17, 2008

Mononuclear Manganese Corroles
Figure 6. UV-visible spectral changes of (Mes2PhCor)Mn 1 in PhCN during addition of (a) TBAOAc or (b) TBACN to solution.
Figure 7. UV-visible spectral changes of (Mes2PhCor)Mn 1 during addition of (a) TBAF or (b) TBAOH to PhCN.
Unlike OAc-, Cl-, CN-, or SCN-, the addition of F- or
OH- to 1 in PhCN results in more than one set of spectral
changes, each with well-defined isosbestic points as shown in
Figure 7. The first change is complete after addition of 1.9 equiv
F- or 2.6 equiv OH- and the second after addition of 65 equiv
F- or 31 equiv OH-. A third and still unexplained reaction is
also obtained after further addition of OH- ion.
The first product formed after addition of OH- or F- to 1
in PhCN (Figure 7) has a UV-visible spectrum similar to
that obtained upon addition of OAc-, Cl-, CN-, or SCN-
to 1 in the same solvent (see Figures 6 and Supporting
Information, Figure S3). These ligand addition reactions are
described by eq 6 where the corrole is represented as
uncomplexed in PhCN. The second set of spectral changes
Inorganic Chemistry, Vol. 47, No. 17, 2008 7723

Shen et al.
Table 2. UV-visible Spectral Data of (Mes2PhCor)Mn Complexes 1 in
or are less intense in the spectrum of the monoadduct. The
latter type of spectrum is identical to the UV-visible
spectrum generated after reduction of 1 at -1.50 V in PhCN
(Figure 5a), thus suggesting the two species present in
solution are the same under the different experimental
conditions.
PhCN Containing Different Anions
λ, nm (ε × 10-4 M-1 cm-1
)
formula
X-
I
II
IIIa
IV
b
(Cor)MnIII
none
407 (3.3) 431 (2.9) 510 (1.0) 652 (0.8)
[(Cor)MnIIIX]-
OAc- 388 (2.1) 431 (2.5) 482 (3.3) 650 (1.0)
Cl-
393 (2.4) 435 (2.8) 489 (3.2) 656 (1.1)
As indicated above, the first electroreduction product of
1 in PhCN (see Figure 5a) has a UV-visible spectrum almost
identical to that of the bis-OH- species formed in the absence
of an electrochemical reaction (see Figure 7). A comparison
of the two spectra is shown in Chart 2 where the only
difference between the two solution conditions is the presence
of the non-coordinating 0.1 M TBAP under the electro-
chemical conditions. The similarity between the two spectra
leaves no doubt that the same form of the corrole is present
under both experimental conditions. In the case of Mn(III)
porphyrins, hydroxide ion is known to catalytically generate
the Mn(II) form of the compound,58,59 but this seems highly
unlikely in the present case given the extremely negative
potential for the first corrole reduction in PhCN, namely
-1.36 V versus SCE, 1100 mV more negative than E1/2 for
reduction (TPP)MnIIICl under the same solution conditions.60
A more likely sequence of events involves generation of
the bis-OH- Mn(III) corrole after reaction of the singly
reduced corrole with trace water in PhCN. The proposed
mechanism for this reaction is shown in Scheme 1 where
the first electron transfer at -1.36 V is a Mn(III)/Mn(II)
reaction of the uncoordinated compound in solution and the
second at -1.64 V involves reduction of the homogeneously
generated [(Mes2PhCor)MnIII(OH)2]2- to its Mn(II) form at
the electrode surface. A similar sequence of steps has
previously been observed upon reduction of SiIV or GeIV
porphyrins in nonaqueous media61–63 and was confirmed in
the present study by monitoring the electroreduction of 1 in
PhCN containing 2 equiv of TBAOH, the same conditions
for generation of the spectrum in the lower part of Chart 2.
In this solution, only a single reduction is observed at a
potential close to that of -1.64 V. Thus, the behavior of
SCN- 402 (2.9) 440 (3.0) 499 (1.9) 660 (0.9)
CN-
F-
401 (2.9) 440 (2.7) 509 (2.4) 681 (1.1)
400c 438c 508c 678c
OH-
398 (2.9) 437 (2.7) 508 (2.1) 681 (0.9)
[(Cor)MnIIIX2]2- F-
OH-
421 (2.5) 472 (5.8) 647 (1.7)
422 (3.1) 478 (4.4) 656 (1.5)
a Bands in bold print are designated as “marker bands” for axial
coordination. b Other bands located at 308, 374sh, 557 and 601 nm (see
Figure 3). c Transient species, ε cannot be determined.
in Figure 7 are described by eq 7 and occur only for X )
OH- and F-.
Mes PhCor MnX -+X- f Mes PhCor MnX
]
(7)
2-
[
(
)
]
[
(
)
2
2
2
The spectra of the six-coordinate bis-hydroxide and bis-
fluoride products in reaction 6 are similar in that each has
an intense band at 472-478 nm and two weaker bands at
421-422 and 647-656 nm. The exact values of λnm and ε
are listed in Table 2 along with spectral data for the
monoadducts, [(Mes2PhCor)MnF]- and [(Mes2PhCor)Mn-
(OH)]-, both of which closely resemble the spectrum of
[(Mes2PhCor)Mn(CN)]- displayed in Figure 6b.
In summary, two types of UV-visible spectral changes
are obtained upon addition of anionic axial ligands to
(Mes2PhCor)Mn 1 in PhCN. The first is for SCN-, CN-,
Cl-, and OAc- which forms only monoadducts with
(Mes2PhCor)Mn and gives the five-coordinate complex. The
second is for F- and OH- where a stepwise formation of
mono- and bis-adducts occurs. The five-coordinate complex
has four bands at 388-407, 431-440, 482-509, and
650-681 nm while the six-coordinate species has three bands
at 421-422, 472-478, and 647-656 nm which are not seen
Chart 2
7724 Inorganic Chemistry, Vol. 47, No. 17, 2008

Mononuclear Manganese Corroles
Scheme 1
compound 1 in PhCN is remarkable not only by the sequence
of steps shown in Scheme 1 but also by the fact that this is
the first direct evidence for generation of a Mn(II) corrole
as opposed to a Mn(III) corrole π-anion radical.
The electrogenerated Mn(II) species is extremely reactive
in its uncomplexed form, not only in PhCN but also in
CH2Cl2 where a rapid reaction with the chlorinated solvent
is shown to occur. This contrasts with the enhanced stability
of the electroreduced species in pyridine where reduction is
proposed to occur at the conjugated macrocycle. A well-
defined cyclic voltammogram with only a single reduction
process is obtained (Figure 2) and the UV-visible spectrum
after reduction at -1.80 V (Figure 8) is quite different than
that in PhCN (Figure 5) or DMSO (Supporting Information,
Figure S2). The strong binding of pyridine to (Mes2PhCor)-
Mn leads not only to a change in the site of reduction but
also prevents a chemical reaction of the electrogenerated
species because of a blocking of the axial positions on the
manganese center.
Anion Effect on the Electrochemistry in PhCN and
Pyridine. As described earlier in the manuscript, Cl-, OAc-,
CN-, and SCN- all form monoadducts with the MnIII form
of corrole and have binding constants between 104.1 and 104.8
for the reaction given in eq 6. Thus, the electroactive species,
represented as [(Mes2PhCor)MnIIIX]- after coordination, is
negatively charged and should be harder to reduce and easier
to oxidize than the corrole not having a bound anion, either
(Mes2PhCor)Mn in PhCN and CH2Cl2 or (Mes2PhCor)Mn(S)
in pyridine and DMSO. This is exactly what is observed as
shown in Figure 9a which compares cyclic voltammograms
of millimolar solutions of 1 in PhCN with and without added
chloride ion in solution. The first oxidation in PhCN
containing 1.0 M TBACl occurs at E1/2 ) 0.10 V as
compared to E1/2 ) 0.45 V in PhCN, 0.1 M TBAP, a 350
Figure 9. Cyclic voltammograms of (Mes2PhCor)Mn 1 (1.7 × 10-3 M) in
(a) PhCN containing (i) 0.1 M TBAP, (ii) 0.1 M TBAP and 0.3 equiv
TBACl, and (iii) 1.0 M TBACl and (b) the same concentration of the corrole
in pyridine with 0.1 M TBAP and added TBACl.
mV negative shift of potential. At the same time the first
reduction in PhCN with 0.1 M TBACl occurs at E1/2 ) -1.56
V as compared to -1.36 V when no Cl- is present, a
negative shift of 200 mV in half-wave potential.
After formation of the five-coordinate compound, the E1/2
for oxidation is independent of additional chloride added to
solution (see Figure 9a) and this would only occur if the
number of axially bound anions on the initial MnIII corrole
and singly oxidized MnIV corrole were identical; otherwise
one would expect a 60 mV shift in E1/2 per each 10-fold
(58) Arasasingham, R. D.; Bruice, T. C. Inorg. Chem. 1990, 29, 1422–
1427.
(59) Ramirez-Gutierrez, O.; Claret, J.; Ribo, J. M. J. Porphyrins Phtha-
locyanines 2005, 9, 436–443.
(60) Autret, M.; Ou, Z.; Antonioni, A.; Boschi, T.; Tagliatesta, P.; Kadish,
K. M. J. Chem. Soc., Dalton Trans. 1996, 2793–2797.
(61) Guilard, R.; Barbe, J.-M.; Boukhris, A.; Lecomte, C.; Anderson, J. E.;
Xu, Q. Y.; Kadish, K. M. J. Chem. Soc., Dalton Trans. 1988, 1109–
1113.
(62) Kadish, K. M.; Xu, Q. Y.; Barbe, J.-M.; Anderson, J. E.; Wang, E.;
Guilard, R. Inorg. Chem. 1988, 27, 691–696.
Figure 8. UV-visible spectral changes during controlled potential reduction
of (Mes2PhCor)Mn (1) in pyridine, 0.1 M TBAP at an applied potential of
-1.80 V.
(63) Kadish, K. M.; Xu, Q. Y.; Barbe, J.-M.; Guilard, R. Inorg. Chem.
1988, 27, 1191–1198.
Inorganic Chemistry, Vol. 47, No. 17, 2008 7725

Shen et al.
change in log[Cl-].64 Thus, the invariance of E1/2 with added
Cl- after the reversible oxidation potential has shifted from
0.45 to 0.10 V indicates that the prevailing electrode reaction
on the time scale of the electrochemical measurement can
be described by eq 8 in solutions of PhCN containing >0.3
equiv Cl-. Similar arguments were used for identifying46
the redox-active site of the (TPFC)MnX/(TPFC)Mn couple
as MnIV/MnIII and that of the (TPP)FeX/(TPP)Fe64 as FeIII/
FeII.
Scheme 2
Mes PhCor MnIIICl - h Mes PhCor MnIVCl + e (8)
[
(
]
)
(
)
2
2
Scheme 3
The binding constant for addition of one Cl- ligand to
(Mes2PhCor)MnIII was calculated as 104.8 from spectroscopic
data of the type shown in Figure 6 for CN- and OAc- (see
Supporting Information, Figure S3 for Cl- titration results)
and thus a combination of eq 8 with eq 2 enables calculation
of the chloride binding constant to the Mn(IV) form of the
corrole, that is, [(Mes2PhCor)MnIV]+. The relevant electro-
64
chemical equation is ∆E1/2 ) 0.059 log(KMn(IV)/KMn(III)
)
which leads to a KMn(IV)/KMn(III) ratio of 105.9 and a logKMn(IV)
of 1010.7 for the chloride binding reaction given in eq 9.
reactions being given by eq 8. A six-coordinate MnIV species
with bound OAc- or OH- is the final electroxidation product,
and both corroles can be reduced back to Mn(III) at similar
potentials of -0.06 and -0.08 V. [(Mes2PhCor)Mn(OH)]2-
is stable and can be reversibly oxidized and reduced but not
the electrogenerated [(Cor)Mn(OAc)]2- which rapidly loses
one axial ligand, giving the irreversible redox process shown
by the cyclic voltammogram in Figure 10b.
The relevant electron transfer mechanism in the case of
OH- is given in Scheme 3 where E1/2 ) 0.10 V for the
reversible oxidation of [(Cor)Mn(OH)]- to (Cor)MnIV(OH)
and then, as [(Cor)MnIII(OH)2]2- is formed at higher con-
centrations of OH-, the prevailing redox process switches
Mes PhCor MnIV ++Cl- h Mes PhCor MnIVCl (9)
[
]
(
)
(
)
2
2
The large chloride binding constant to the MnIV corrole
in PhCN suggests that Cl- might also bind to the singly
oxidized species in pyridine and indeed this is the case as
demonstrated by the cyclic voltammograms in Figure 9. The
first oxidation is reversible in PhCN and pyridine but not in
pyridine containing low concentrations of added TBACl
where a classic box mechanism is observed. Here the MnIII/
MnIV process is described by two related electrochemical
EC mechanisms, each of which involves an electron transfer
followed by a fast chemical reaction. The initial oxidation
involves a one electron conversion of (Cor)Mn(py) to
[(Cor)Mn(py)]+ followed by the rapid formation of (Cor)MnCl
at the electrode surface. The halide bound species is harder
to reduce than the uncoordinated species and after conversion
of (Cor)MnCl to [(Cor)MnCl]-, the axial chloride ion
dissociates giving again (Cor)Mn at the electrode surface
and in solution. These reactions occur at the noncoupled peak
potentials of 0.41 to 0.44 V for oxidation and 0.18 V for
reduction as seen in the lower portion of Figure 9.
This mechanism for oxidation of (Mes2PhCor)Mn 1 under
the different solution conditions is summarized in Scheme
2 where the reversible electrode reactions are observed in
neat PhCN (0.45 V), neat pyridine (0.42 V), and pyridine
containing >225 equiv of added TBACl.
Acetate and Hydroxide Effect on Electrooxidation in
PhCN. The effect of adding acetate or hydroxide ions to 1
in PhCN is similar to what is seen upon addition of Cl- or
CN-. In each case the anionic [(Cor)MnX]- species in
solution is easier to oxidize than uncharged (Cor)Mn, these
reactions occurring at E1/2 ) 0.30 V in the case of OAc-
and 0.04 V in the case of CN-, the relevant electron transfer
(64) Kadish, K. M. In Iron Porphyrins II; Lever, A. B. P., Gray, H. B.,
Eds.; Addison-Wesley Publishing Company, Inc.: Canada, 1983, pp
161-249.
Figure 10. Cyclic voltammograms of (Mes2PhCor)Mn 1 in PhCN with
(a) 0.1 M ClO4-, (b) 0.1 M OAc-, and (c) added OH-.
7726 Inorganic Chemistry, Vol. 47, No. 17, 2008

Mononuclear Manganese Corroles
Scheme 4
Table 3. E1/2 Values of (Mes2PhCor)Mn 1 Complexes in PhCN
Containing added Anions
first ox
MnIII/IV
first red
formula
X- (conc.)
none
MnIII/II
(Mes2PhCor)Mn
0.45
-1.36
[(Mes2PhCor)MnX]-
CN- (1.6 equiv)
Cl- (1.0 M)
0.04
0.10
0.10
0.30a
-1.46
-1.56
-1.66
-1.43
OH- (1.9 equiv)
OAc- (0.1 M)
to an E1/2 of -0.06 V, consistent with an easier oxidation of
the dianionic species. At the same time, the electrogenerated
[(Cor)MnIV(OH)2]2- is also easier to oxidize, and the E1/2
for this process shifts from 1.07 V in solutions with 0.1 M
TBAP and 0.5 equiv OH- to 0.17 V in PhCN containing
0.1 M TBAP and 160 equiv TBAOH. This is a remarkable
shift in potential, a high stability of the doubly oxidized
corrole, and is similar to what was previously reported after
chloride titrations of a similar compound in CH2Cl2.33
As mentioned above, the oxidation of [(Cor)MnIII(OAc)]-
to (Cor)MnIV(OAc) is followed by addition of a second axial
ligand in solutions with high acetate concentrations, and the
bis-acetate adduct formed as a result of this electrochemical
EC mechanism can then be reduced to [(Cor)MnIII(OAc)2]2-,
followed by loss of one OAc- ligand giving again the original
five-coordinate species by a second EC mechanism. This
classic electrochemical box mechanism is shown in Scheme
4 where the reversible electron transfer reactions occur at
0.30 and -0.08 V prior to the addition or loss of an axial
ligand.
Finally, the half-wave potentials for each examined mono-
and bis-adduct in PhCN are summarized in Table 3. The
monoadducts can all be reversibly oxidized at E1/2 valves of
0.04 to 0.30 V and reversibly reduced at potentials of -1.43
to -1.66 V, the latter potential being for the mono-OH-
complex. Reductions could not be observed in solutions with
high concentrations of the anionic ligand, perhaps because
this reaction would occur at very negative potentials if the
axial ligand were lost after reduction, the expected shift in
[(Mes2PhCor)MnX2]2-
OAc- (0.1 M)
-0.08a
-0.06
OH- (400 equiv)
a Peak potential at a scan rate of 0.1 V/s.
E1/2 from the uncomplexed species amounting to -60 to
-120mVpereach10-foldchangeinaxialligandconcentration.
In summary, a series of meso-substituted manganese(III)
corroles were investigated as to their electrochemical,
spectroelectrochemical, and anion binding properties in
nonaqueous solvents. The Mn(II) and Mn(IV) forms of the
corroles could be electrogenerated by controlled potential
oxidation or reduction. Five- or six-coordinate complexes
were in situ generated depending upon the specific anions
added to solution in the form of a TBA+ salt. Complexation
with one or two anionic axial ligands led to an easier
oxidation and a harder reduction in each case as compared
to the uncomplexed four-coordinated species.
Acknowledgment. The support of the Robert A. Welch
Foundation (K.M.K., Grant E-680) and the French Ministry
of Research (MENRT), CNRS (UMR 5260) are gratefully
acknowledged.
Supporting Information Available: Table of UV-visible
spectra of neutral compound, (Mes2PhCor)Mn, in different solvents,
spectra of SCN- and Cl- bound species in PhCN and spectroelec-
trochemistry data in DMSO. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC8007415
Inorganic Chemistry, Vol. 47, No. 17, 2008 7727