Appended corrole manganese complexes: Catalysis and axial-ligand effect

Article abstract of DOI:10.1016/j.cclet.2014.01.027

A series of N-base appended corroles and their manganese complexes were synthesized and their binding constants with three different nitrogenous ligands, triethylamine, N-methylimidazole and pyridine, were evaluated by spectroscopy. Kinetic studies indica

Full text of DOI:10.1016/j.cclet.2014.01.027

Chinese Chemical Letters 25 (2014) 571–574
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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Appended corrole manganese complexes: Catalysis and
axial-ligand effect
a
a
Nga-Chun Ng a, Mian HR Mahmood b, Hai-Yang Liu a,b, , Fei Yam , Lam-Lung Yeung ,
*
Chi-K. Chang a,c,
*
a Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, Hong Kong
University of Science and Technology, Hong Kong, China
b Department of Chemistry, South China University of Technology, Guangzhou 510641, China
c Department of Chemistry, Michigan State University, E. Lansing, MI 48824, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of N-base appended corroles and their manganese complexes were synthesized and their
binding constants with three different nitrogenous ligands, triethylamine, N-methylimidazole and
pyridine, were evaluated by spectroscopy. Kinetic studies indicated that the presence of appended N-
donor ligands may cause a significant enhancement of the rate of oxygen atom transfers (OAT) from
(oxo)manganese(V) corrole to alkene, and the stronger axial ligand binding has impact on the rate of the
oxidation reaction. Turnover frequency (TOF) for the catalytic oxidation of alkenes by appended
manganese corroles varies with the following ligand order: acetamido < pyridyl < imidazolyl. The
influence of the external axial ligands on the catalytic epoxidation was investigated by using appended
acetamido manganese corrole as catalyst, with the results revealing that N-methylimidazole gave the
best enhancement on the yields of total oxidation products among the investigated nitrogenous ligands.
ß 2014 Hai-Yang Liu and Chi-K. Chang. Published by Elsevier B.V. on behalf of Chinese Chemical
Society. All rights reserved.
Received 10 October 2013
Received in revised form 29 December 2013
Accepted 30 December 2013
Available online 24 January 2014
Keywords:
Manganese
Corrole
Catalysis
Axial-ligand
Oxygen atom transfer
1. Introduction
[14]. It has recently been reported that mild and room temperature
oxidation of electron rich alcohols to carbonyl compounds may be
Corrole is a ring-contracted porphyrin analog having one direct
pyrrole–pyrrole link [1]. It represents a distinctive family of
macrocyclic compounds with unique structural, chemical, and
photophysical properties [2–5]. Unlike porphyrin [6], corrole
contains three protons in the inner core of the ring which renders it
a tri-anionic ligand capable of stabilizing metals in high oxidation
states [7]. This feature is mainly responsible for metallocorroles
acting as potent catalysts in a variety of chemical reactions, such as
epoxidation, cyclopropanation, asymmetric sulfoxidation, aziridi-
nation, and oxygen atom transfer reactions [8].
Manganese complexes of corroles and related macrocycles are
typically employed in various oxygenation reactions to get insights
intothe reaction mechanism owingto their efficiencyandspecificity
[8–12]. Manganese corroles are found to be effective catalysts for
selective epoxidation of olefins at less-substituted double bond [13],
as well as selective oxidation of unactivated C–H bonds in alkanes
achieved through manganese corroles bearing electronegative
substituents [15]. Significantly less attention, however, has been
paid to the catalytic effects of axial ligation in metallomacrocycles.
The catalytic activity and selectivity of metalloporhyrins and related
transition metal complexes in biomimetic oxidation of cytochrome
P-450 is substantially affected by the fifth coordination [16]. The N-
donor axial ligands are the most common ones used in the
metalloporphyrin catalyzed oxidation systems. Importantly, the
rate of oxidation reactions catalyzed by metalloporphyrins could be
stronglyenhancedbytheadditionofanaxialligand, suchaspyridine
or imidazole bases [17,18]. For asymmetric epoxidation, addition of
axial ligands to catalytic reaction systems is a convenient and
efficient way to enhance enantioselectivity [19,20]. Metallopor-
phyrins with covalently linked axial ligand ‘‘tails’’ are often used in
the biomimetic modeling [21]. The use of a covalently linked axial
ligand may avoid the problem of exogenous ligand oxidation
competing with the substrate oxidation. Despite the significant
effect of axial ligation in metalloporphyrin catalyzed systems,
similar investigations on metallcorroles are scarce, and to the best of
our knowledge, only one such report is found in the literature [22]
and, in addition, no report exists for the catalysis by ligand-
appendedmetallocorroles.Herein,wewishtoreportthesynthesisof
*
Corresponding authors at: Department of Chemistry and Open Laboratory of
Chemical Biology of the Institute of Molecular Technology for Drug Discovery and
Synthesis, Hong Kong University of Science and Technology, Hong Kong, China.
E-mail addresses: chhyliu@scut.edu.cn (H.-Y. Liu), changc@msu.edu
(C.-K. Chang).
1001-8417/$ – see front matter ß 2014 Hai-Yang Liu and Chi-K. Chang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2014.01.027

572
N.-C. Ng et al. / Chinese Chemical Letters 25 (2014) 571–574
manganese corroles with covalently attached nitrogenous ligands
and their catalytic activities. The catalytic epoxidation of alkenes
revealed that the presence of appended N-donor ligands will
significantly enhance the rate of OAT from (oxo)manganese(V)
corroles to alkenes.
Kb8 > Kb9 > Kb10 (Table S1 in Supporting information). The
binding constant for pyridine and 8 is 19.4 L/mol, which is
comparable to the binding between pyridine and corrole (OMC)Mn
[25] (where OMC = octamethylcorrole). The binding between
pyridine and 9, 10 was too weak to be measured reliably. The
binding constant for triethylamine and 8, 9 was determined to be
1.08 ꢀ 102 L/mol and 2.20 L/mol, respectively, and the binding
between triethylamine and 10 was too weak to be measured. It is
obvious that these appended manganese(III) corroles exhibit good
binding tendency with N-methylimidazole among the examined
N-donor ligands, and the binding ability of triethylamine is
stronger than pyridine. Manganese(III) corroles are generally four-
coordinated, as in the case of the complex 8, where the
coordination of acetamido-ligand is not possible with the
2. Experimental
The appended corroles were prepared as shown in Scheme 1
starting from 5-(pentafluorophenyl)-dipyrromethane 1 [23]. The
trans-A2B corroles 2 and 3 were prepared by our previously reported
procedure [24]. The synthetic detail, including synthesis, character-
ization and catalysis data, are given in Supporting information.
manganese center. However, complex
8 will turn to five-
3. Results and discussion
coordinated state in the presence of the external axial ligand.
The presence of the appended ligand in complexes 9 and 10 will
increase the coordination number from four to five, and only one
All synthesized appended corroles and their manganese com-
plexes were characterized by spectroscopic methods. The Soret-
band of appended manganese corroles 8, 9 and 10 appeared at about
400 nmand 420 nm(Fig. S1inSupporting information), whilethe Q-
band of 9, 10 exhibited a significant red shift due to the coordination
between the tailed, appended ligand and the manganese. The most
definitive difference upon appended ligand coordination is the
dramatic increase in the absorbance at about 490 nm.
Axial ligation exerts a profound effect on the physicochemical
properties of metallocorroles similar to metalloporphyrins [19,20].
The spectral changes in manganese(III) corrole 8 on addition of N-
methylimidazole is shown in Fig. S2 in Supporting information.
Excellent isosbestic points were observed, indicating a clean
transformation of 8 to its axial coordinated product. Quantitative
analysis of the spectroscopic titration data revealed that 8 formed a
1:1 complex with N-methylimidazole, and the binding constant
was found to be 1.87 ꢀ 104 L/mol. The complexes 9 and 10 also
formed a 1:1 complex with N-methylimidazole, but their binding
constants were two orders of magnitude less than that of 8. This is
due to the coordination between the appended nitrogenous ligand
and the core metal in 9 and 10. The binding constants between
Mn(III) corrole and N-methylimidazole follows the order
nitrogenous ligand can coordinate with Mn(III) to form
maximum of five-coordinated complex.
a
High valent (oxo)manganese(V) corroles are the active oxidants
in the catalytic cycle of manganese corroles-catalyzed oxidation
reactions of alkenes [13,26] and sulfides [27]. In order to
investigate the effects of a tailed axial ligand on the reactivity of
the (oxo)manganese(V) corroles, we prepared (oxo)manganese(V)
corroles 11, 12 and 13 by using PhIO as oxidant [26] and performed
kinetic studies on their reactions with styrenes. The UV–vis spectra
of these (oxo)manganese(V) corroles resemble those found in
literatures [8,27]. At room temperature, these metal-oxo species
decompose gradually and eventually return to the corresponding
manganese(III) corroles as indicated by UV–vis data. Addition of
styrene can significantly speed up the reduction of these
(oxo)manganese corroles. The UV–vis changes of 11 upon mixing
with styrene in CH2Cl2 is shown in Fig. S3 in Supporting
information. With excess styrene, the reactions followed pseudo-
first order kinetics. The rate constants (kobs) and the pseudo-first
order reaction activation energy (EA) of the OAT can be derived
according to the rate law and Arrhenius equation. The rate
Cl
O
Br
(a)
O
6 a) X = Br
NH2
NO2
N
H
SnCl2/HCl
X
Na(OAc)
7
N
b) X =
NH
N
(b)
NH
Na2CO3
3
2
1
(from and
o-nitro benzaldehyde)
Na(OAc)
Cl
X
O
(c) M = Mn(III)
X =
X=
CH3
N
8
9
O
O
N
X =
X=
CH3
4
N
H
X
N
H
X
X=
10
N
5
N
*M
(c) Mn(OAc)2
(d) PhIO
M = Mn(V)oxo
(d)
X =
X=
CH3
N
11
12
N
F
F
F
F
F
F
F
F
N
NH
F
F
X=
13
N
NH HN
*Metal insertion will replace the inner three hydrogens
Scheme 1. Synthesis of appended free base corroles and their manganese complexes.

N.-C. Ng et al. / Chinese Chemical Letters 25 (2014) 571–574
573
Table 1
Catalytic epoxidation of alkenes by appended manganese(III) corroles in the presence of PhIO.
a
Entry
Substrate
Catalyst
Yield (%)
AAb
TOF (hꢁ1
)
BZc
Epoxide
Total
1
8
9
1.8
3.2
1.9
0.6
3.2
5.5
5.5
3.6
7.5
7.9
10.0
14.9
47
54
90
10
2
8
9
–
–
–
–
–
–
19.6 (cis/trans = 1.3)
21.0 (cis/trans =1.4)
35.7 (cis/trans = 1.5)
–
–
–
117
126
214
10
a
Turnover frequency (TOF) was calculated according to reaction rate of the first 10 min.
b
c
Arylacetaldehyde.
Benzaldehyde.
Alkene
Epoxide
Mn(III)
a
R
R
R
X
O
.
Mn(III)
+
I
b
O
O
R
O
PhIO
Mn(V)
Mn(IV)
Mn(IV)
c
Mn(III)
O2
+
d
O
R
O
CHO
.
O
O
O
R
O
e
Mn(IV)
Mn(V)
+
O
O2
f
R
(radical chain propagation)
R
R
carbonyl compounds
+
Mn(III)
+
+
O
O
Scheme 2. Proposed mechanism for the catalytic oxidation of alkenes by appended Mn(III) corroles.
constants and activation energy data are summarized in Table S2 in
Supporting information. The results reveal that manganese
corroles with appended pyridine and imidazole ligands can
enhance the OAT process. The half-lives of the acetamido-, pyridyl-
and imidazolyl-appended (oxo)manganese(V) corroles 11, 12 and
13 in the presence of styrene (7.28 ꢀ 10ꢁ2 mol/L) are 150 min,
80 min and 7 min at 25 8C and the activation energies of the
pseudo-first order reaction is 27.8 kcal/mol, 21.8 kcal/mol and
14.3 kcal/mol, respectively, which indicate that N-donor type axial
ligands will provide enhancement on the rate of OAT by lowering
the activation energy. The OAT rate follows the order:
kobs13 > kobs12 > kobs11. By comparing the rate and the activation
energy between the pyridyl-appended and the imidazolyl-
appended (oxo)manganese(V) corroles (12, 13), it is evident that
a stronger binding may contribute significant enhancement on the
rate of an oxidation reaction. The binding of manganese(III)
corroles with imidazole is found to be stronger than with pyridine
(Table S1). The coordination of nitrogenous ligand at the position
trans to the manganese-oxo bond could induce the elongation of
the Mn–O bond distance and leads to destabilization of the high
valent (oxo)manganese(V) intermediate and thus increases the
rate of reduction of (oxo)manganese corroles [22,28].
and benzaldehyde. The turnover frequency (TOF) for the catalytic
oxidation decreased in the order 10 > 9 > 8. The pyridyl appended
Mn(III) corrole 9 exhibited only a small improvement in the catalytic
activityascomparedwithacetylappendedMn(III)corrole8, whilein
the case of imidazole appended Mn(III) corrole 10, a significant
increase in the yields of total oxidation products was observed. The
productionoftrans-epoxidefromcis-b-methylstyreneindicatesthat
theintermediatewhichleadstothedirectformationofepoxidemust
undergo a cis/trans inversion, and this suggests that the mechanism
may involve radical intermediates, as observed in manganese
porphyrin catalyzed oxidation of alkenes [29]. The proposed
mechanism for catalytic oxidation of alkenes by Mn(III) corroles is
depictedinScheme2. Collmanetal. [30]performedaMn(III)corrole-
catalyzed competitive epoxidation of styrene and cyclooctene and
demonstrated that there is at least one other active intermediate in
addition to (oxo)manganese(V) corrole, and suggested that PhIO-
Mn(IV)corrole adduct may also be an active intermediate (Scheme 2,
path a). Recent studies on manganese porphyrins and related
macrocyclic systems also demonstrate the existence of multiple
active oxidants in the oxygenation reactions [31].
The effect of external axial ligands on appended acetamido
manganese corrole 8 catalyzed epoxidation of styrene is shown in
Fig. 1. Pyridine, triethylamine and N-methylimidazole were used
as axial ligands. In case of triethylamine, as axial ligand, which uses
sp3-N for donating electrons, the yield of total oxidation products
was observed to decrease sharply with increasing ligand concen-
The catalytic oxidation data of styrene and cis-b-methylstyrene
by appended Mn(III) corroles is summarized in Table 1. Epoxides
were the major products in all cases as indicated by GC analysis.
Catalytic oxidation produced styrene epoxide, arylacetylaldehyde

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tration. For pyridine and N-methylimidazole, however, the yield
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[Mn(III) corrole] < ꢂ50. Further increase in the ligand concentra-
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the more effective oxidation of the axial ligand leading to a
decrease in the turnover rate. This indicates that N-oxidation of the
nitrogenous ligand becomes important at higher concentration.
Moreover, the addition of N-methylimidazole may induce a greater
improvement on the yields of total oxidation products than
pyridine, which may be attributed to the relatively less steric effect
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Insummary,wereportthesynthesisandthefirstcatalyticactivity
of N-base appended manganese corrole complexes. The appended
manganese(III) corroles normally form 1:1 complexes with N-donor
axial ligands, and give enhancement on the rate of oxygen atom
transfer from the corresponding (oxo)manganese(V) corrole to the
substrate. The advantage of ligand appended manganese(III) corrole
in the catalytic oxidation of substrate is to provide a donor ligand
without the drawback of competitive oxidation of the excess ligand,
and demonstrates the convenience of using easily available organic
bases,suchaspyridineandN-methylimidazole,toenhancetheyields
of catalytic oxidation products.
of
(7,13-dimethyl-2,3,8,12,17,18-hexaethylcorrolato)manganese(III),
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Acknowledgments
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This work was supported by National Natural Science Founda-
tion of China (Nos. 21171057, 21371059). Research Grant Council
of Hong Kong under project HKUST6182/99p and Area of
Excellence Scheme (No. AoE/P-10/01).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2014.01.027.
´
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